The Polar Phase of NaNbO3: A Combined Study by Powder Diffraction, Solid-State NMR, and First-Principles Calculations

Metastable layered lithium-rich niobium and tantalum oxides via nearly instantaneous cation exchange

Lithium-rich early transition metal oxides are the source of excess removeable lithium that affords high energy density to lithium-rich battery cathodes. They are also candidates for solid electrolytes in all-solid-state batteries. These highly ionic compounds are sparse on phase diagrams of thermodynamically stable oxides, but soft chemical routes offer an alternative to explore new alkali-rich crystal chemistries. In this work, a new layered polymorph of Li3NbO4 with coplanar [Nb4O16]12- clusters is discovered through ion exchange chemistry. A more detailed study of the ion exchange reaction reveals that it takes place almost instantaneously, changing the crystal volume by more than 22% within seconds. The transformation of coplanar [Nb4O16]12- in L-Li3NbO4 into the supertetrahedral [Nb4O16]12- clusters found in the stable cubic c-Li3NbO4 is also explored. Furthermore, this synthetic pathway is extended to access a new layered polymorph of Li3TaO4. NMR crystallography with 6,7Li, 23Na, and 93Nb NMR, X-ray diffraction, neutron diffraction, and first-principles calculations is applied to A3MO4 (A = Li, Na; M = Nb, Ta) to identify local and long-range atomic structure, to monitor the unusually rapid reaction progression, and to track the phase transitions from the metastable layered phases to the known compounds found using high-temperature synthesis. A mechanism is proposed whereby some sodium is retained at short reaction times, which then undergoes proton exchange during water washing, forming a phase with hydrogen bonds bridging the coplanar [Nb4O16]12- clusters. This study has implications for lithium-rich transition metal oxides and associated battery materials and for ion exchange chemistry in non-framework structures. The role of techniques that can detect light elements, local structure, and subtle structural changes in soft-chemical synthesis is emphasized.

Orthorhombic Polar Phase in Sodium Niobate Nanoribbons

Ferroelectric materials exhibit switchable spontaneous polarization below Curie's temperature, driven by octahedral distortions and rotations, as well as ionic displacements. The ability to manipulate polarization coupled with persistent remanence, drives diverse applications, including piezoelectric devices. In the last two decades, nanoscale exploration has unveiled unique material properties influenced by morphology, including the capability to manipulate polarization, patterns, and domains. This paper focuses on the characterization of nanometric sodium niobate (SN) synthesized from metallic niobium through alkali hydrothermal treatment, utilizing electron microscopy techniques, including high-resolution differential phase contrast (DPC) in scanning transmission electron microscopy (STEM). The material exhibits a nanoribbon structure forming a tree root-like network. The study identifies crystallographic phase, atomic columns displacement directions, and surface features, such as exposed planes and the absence of particular atomic columns. The high sensitivity of integrated DPC images proves crucial in overcoming observational challenges in other STEM modes. These observations are essential for potential applications in electronic, photocatalytic, and chemical reaction contexts.

Physical Properties of CaTiO3-Modified NaNbO3 Thin Films

NaNbO3(NN)-based lead-free materials are attracting widespread attention due to their environment-friendly and complex phase transitions, which can satisfy the miniaturization and integration for future electronic components. However, NN materials usually have large remanent polarization and obvious hysteresis, which are not conducive to energy storage. In this work, we investigated the effect of introducing CaTiO3((1-x)NaNbO3-xCaTiO3) on the physical properties of NN. The results indicated that as x increased, the surface topography, oxygen vacancy and dielectric loss of the thin films were significantly improved when optimal value was achieved at x = 0.1. Moreover, the 0.9NN-0.1CT thin film shows reversible polarization domain structures and well-established piezoresponse hysteresis loops. These results indicate that our thin films have potential application in future advanced pulsed power electronics.

Noncollinear Electric Dipoles in a Polar Chiral Phase of CsSnBr3 Perovskite

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two phases of the 3D perovskite, CsSnBr3, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K < T < 85 K, space group P21/m) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I (T < 77 K, space group P21) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and pronounced electrostriction and negative thermal expansion along the polar axis (Q22 ≈ 1.1 m4 C-2; αb = -7.8 × 10-5 K-1) through the onset of polarization. The structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and 81Br nuclear quadrupole resonance spectroscopies. Preliminary attempts to suppress unintentional hole doping to allow for ferroelectric switching are described. Together, the polar symmetry, small band gap, large spin-orbit splitting of Sn 5p orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr3 or its neighboring solid solutions as candidates for bulk Rashba effects.

Translational Antiphase Boundaries in NaNbO3 Antiferroelectrics

Planar defects are known to be of importance in affecting the functional properties of materials. Translational antiphase boundaries (APBs) in particular have attracted considerable attention in perovskite oxides, but little is known in lead-free antiferroelectric oxides that are promising candidates for energy storage applications. Here, we present a study of translational APBs in prototypical antiferroelectric NaNbO3 using aberration-corrected (scanning) transmission electron microscopy (TEM) techniques at different length scales. The translational APBs in NaNbO3 are characterized by a 2-fold-modulated structure, which is antipolar in nature and exhibits a high density, different from the polar nature and lower density in PbZrO3. The high stability of translational APBs against external electric fields and elevated temperatures was revealed using ex situ and in situ TEM experiments and is expected to be associated with their antipolar nature. Density functional theory calculations demonstrate that translational APBs possess only slightly higher free energy than the antiferroelectric and ferroelectric phase energies with differences of 29 and 33 meV/f.u., respectively, justifying their coexistence down to the nanoscale at room temperature. These results provide a detailed atomistic elucidation of translational APBs in NaNbO3 with antipolar character and stability against external stimuli, establishing the basis of defect engineering of antiferroelectrics for energy storage devices.

Optimized Electrocaloric Refrigeration in Lead-Free NaNbO3-Based Ceramics via AFE ↔ FE Phase Transition Modulation

An outstanding challenge for eco-friendly ferroelectric (FE) refrigeration is to achieve a large adiabatic temperature change within a broad temperature range originating from the electrocaloric (EC) effect, which is expected to be realized in antiferroelectric (AFE) materials owing to the large entropy change during electric field and thermally induced phase transition. In this work, a large EC response over a wide response temperature range can be achieved slightly above room temperature via designing the phase transition of NaNbO3. An irreversible to reversible AFE-FE phase transition on heating induced by the introduction of CaZrO3 into NaNbO3 plays a key role in the optimized electrocaloric refrigeration. Accordingly, accompanying the local structure transformation corresponding to the B-site ions, the transition temperature between the square polarization-electric field (P-E) hysteresis loop (the irreversible AFE-FE phase transition induced by the electric field) and the repeatable double P-E hysteresis loop (the electric field induced reversible AFE-FE phase transition) was tailored to around room temperature, in favor of extending large entropy change to the wide temperature range. This work provides an efficient approach to designing lead-free EC materials with excellent EC performance, promoting the advancement of environmentally friendly solid-state cooling technology.

Superb Energy Storage Capability for NaNbO3 -Based Ceramics Featuring Labyrinthine Submicro-Domains with Clustered Lattice Distortions

Designing superb dielectric capacitors is valuable but challenging since achieving simultaneously large energy-storage (ES) density and high efficiency is difficult. Herein, the synergistic effect of grain refining, bandgap widening, and domain engineering is proposed to boost comprehensive ES properties by incorporating CaTiO3 into 0.92NaNbO3 -0.08BiNi0.67 Ta0.33 O3 matrix (as abbreviated NN-BNT-xCT). Apart from grain refining and bandgap widening, multiple local distortions embedded in labyrinthine submicro-domains, as indicated by diffraction-freckle splitting and ½-type superlattices, produce slush-like polar clusters for the NN-BNT-0.2CT ceramic, which should be ascribed to the coexisting P4bm, P21 ma, and Pnma2 phases. Consequently, a high recoverable ES density Wrec of ≈ 7.1 J cm-3 and a high efficiency η of ≈ 90% at 646 kV cm-1 is achieved for the NN-BNT-0.2CT ceramic. Such hierarchically polar structure is favorable to superb comprehensive ES properties, which provide a strategy for developing high-performance dielectric capacitors.

Rapid and Reversible Lithium Insertion in the Wadsley-Roth-Derived Phase NaNb13O33

The development of new high-performing battery materials is critical for meeting the energy storage requirements of portable electronics and electrified transportation applications. Owing to their exceptionally high rate capabilities, high volumetric capacities, and long cycle lives, Wadsley-Roth compounds are promising anode materials for fast-charging and high-power lithium-ion batteries. Here, we present a study of the Wadsley-Roth-derived NaNb13O33 phase and examine its structure and lithium insertion behavior. Structural insights from combined neutron and synchrotron diffraction as well as solid-state nuclear magnetic resonance (NMR) are presented. Solid-state NMR, in conjunction with neutron diffraction, reveals the presence of sodium ions in perovskite A-site-like block interior sites as well as square-planar block corner sites. Through combined experimental and computational studies, the high rate performance of this anode material is demonstrated and rationalized. A gravimetric capacity of 225 mA h g-1, indicating multielectron redox of Nb, is accessible at slow cycling rates. At a high rate, 100 mA h g-1 of capacity is accessible in 3 min for micrometer-scale particles. Bond-valence mapping suggests that this high-rate performance stems from fast multichannel lithium diffusion involving octahedral block interior sites. Differential capacity analysis is used to identify optimal cycling rates for long-term performance, and an 80% capacity retention is achieved over 600 cycles with 30 min charging and discharging intervals. These initial results place NaNb13O33 within the ranks of promising new high-rate lithium-ion battery anode materials that warrant further research.

Reversible electric-field-induced phase transition in Ca-modified NaNbO3 perovskites for energy storage applications

Sodium niobate (NaNbO₃) is a potential material for lead-free dielectric ceramic capacitors for energy storage applications because of its antipolar ordering. In principle, a reversible phase transition between antiferroelectric (AFE) and ferroelectric (FE) phases can be induced by an application of electric field (E) and provides a large recoverable energy density. However, an irreversible phase transition from the AFE to the FE phase usually takes place and an AFE-derived polarization feature, a double polarization (P)-E hysteresis loop, does not appear. In this study, we investigate the impact of chemically induced hydrostatic pressure (p_chem) on the phase stability and polarization characteristics of NaNbO₃-based ceramics. We reveal that the cell volume of Ca-modified NaNbO₃ [(Ca_xNa_1-2xV_x)NbO₃], where V is A-site vacancy, decreases with increasing x by a positive p_chem. Structural analysis using micro-X-ray diffraction measurements shows that a reversible AFE-FE phase transition leads to a double P-E hysteresis loop for the sample with x = 0.10. DFT calculations support that a positive p_chem stabilizes the AFE phase even after the electrical poling and provides the reversible phase transition. Our study demonstrates that an application of positive p_chem is effective in delivering the double P-E loop in the NaNbO₃ system for energy storage applications.

Size-Induced Ferroelectricity in Antiferroelectric Oxide Membranes

Despite extensive studies on size effects in ferroelectrics, how structures and properties evolve in antiferroelectrics with reduced dimensions still remains elusive. Given the enormous potential of utilizing antiferroelectrics for high-energy-density storage applications, understanding their size effects will provide key information for optimizing device performances at small scales. Here, the fundamental intrinsic size dependence of antiferroelectricity in lead-free NaNbO₃ membranes is investigated. Via a wide range of experimental and theoretical approaches, an intriguing antiferroelectric-to-ferroelectric transition upon reducing membrane thickness is probed. This size effect leads to a ferroelectric single-phase below 40 nm, as well as a mixed-phase state with ferroelectric and antiferroelectric orders coexisting above this critical thickness. Furthermore, it is shown that the antiferroelectric and ferroelectric orders are electrically switchable. First-principle calculations further reveal that the observed transition is driven by the structural distortion arising from the membrane surface. This work provides direct experimental evidence for intrinsic size-driven scaling in antiferroelectrics and demonstrates enormous potential of utilizing size effects to drive emergent properties in environmentally benign lead-free oxides with the membrane platform.

Well-defined double hysteresis loop in NaNbO3 antiferroelectrics

Antiferroelectrics (AFEs) are promising candidates in energy-storage capacitors, electrocaloric solid-cooling, and displacement transducers. As an actively studied lead-free antiferroelectric (AFE) material, NaNbO3 has long suffered from its ferroelectric (FE)-like polarization-electric field (P-E) hysteresis loops with high remnant polarization and large hysteresis. Guided by theoretical calculations, a new strategy of reducing the oxygen octahedral tilting angle is proposed to stabilize the AFE P phase (Space group Pbma) of NaNbO3. To validate this, we judiciously introduced CaHfO3 with a low Goldschmidt tolerance factor and AgNbO3 with a low electronegativity difference into NaNbO3, the decreased cation displacements and [BO6] octahedral tilting angles were confirmed by Synchrotron X-ray powder diffraction and aberration-corrected scanning transmission electron microscopy. Of particular importance is that the 0.75NaNbO3−0.20AgNbO3−0.05CaHfO3 ceramic exhibits highly reversible phase transition between the AFE and FE states, showing well-defined double P-E loops and sprout-shaped strain-electric field curves with reduced hysteresis, low remnant polarization, high AFE-FE phase transition field, and zero negative strain. Our work provides a new strategy for designing NaNbO3-based AFE material with well-defined double P-E loops, which can also be extended to discover a variety of new lead-free AFEs.

Large piezoelectricity in NaNbO3-based lead-free ceramics via tuning oxygen octahedral tilt

The development of high-performance lead-free piezoceramics for replacing Pb-based perovskites has attracted lots of attention, and comparable room-temperature piezoresponse has been realized in (Na,K)NbO₃ and BaTiO₃-based ceramics with local structure heterogeneity via adjusting polymorphic phase boundary. In this work, a new method of oxygen octahedron tilt design is used in NaNbO₃-based lead-free ceramics, which usually shows complex oxygen octahedron tilt information inherited from NaNbO₃. The substitution of Ba(Fe_0.5Nb_0.5)O₃ and BaTiO₃ with high tolerance factor into NaNbO₃ leads to a gradual elimination of anti-parallel cation displacement and anti-phase tilt along the three axes; as a result, a single tetragonal phase with P4bm space group and only in-phase tilt along c axis is achieved in 0.88NaNbO₃-0.04Ba(Fe_0.5Nb_0.5)O₃-0.08BaTiO₃ ceramic. Accompanying the decreased oxygen octahedral tilt degree, a drastically improved d₃₃ up to 367 pC N^-1 (over ten times that of NaNbO₃ ceramic) is obtained, reaching a record high in NaNbO₃-based lead-free ceramics. Together with temperature insensitive piezoresponse originating from thermally stable oxygen octahedral tilt structure, the studied NaNbO₃-based materials show large potential for replacing Pb-based ceramics in some electronic devices. The oxygen octahedron tilt engineering would be used for designing more high-performance lead-free ceramics with high piezoresponse and excellent thermal stability simultaneously.

The role of cations in hydrothermal synthesis of nonlinear optical sodium niobate nanocrystals

The usability of the alkali niobates with their ferroelectric and photorefractive properties could be expanded if the development of synthesis methods would allow to obtain small, preferably monodispersed, crystals in the sub-μm to nanometer regime. Of all the possible synthesis methods, the most reliable is currently hydrothermal synthesis to generate small crystallite sizes of these materials. Although the products of sodium niobate are polydisperse and partially agglomerated, they show a significant SHG signal that is unexpectedly comparable to that of potassium niobate. A view on the hydrothermal synthesis of sodium niobate reveals that the incorporation of cations in the crystalline lattice of the niobium educt plays a part in the formation of the product. The occurrence of distinct different phases, as in the case of potassium niobate, is not observed. Instead, it is shown that a clear assignment of the crystalline phase cannot be made here. This indicates that crystallization of the alkali niobates in hydrothermal synthesis depends on the stoichiometry, the niobium starting material and the cation used.

Impact of Li Doping on the Structure and Phase Stability in AgNbO3

The impact of Li doping on the temperature-induced phase transitions in silver niobates Ag_1-xLi_xNbO₃ has been investigated using a combination of high-resolution powder neutron diffraction and synchrotron X-ray diffraction. Considering both the cell metric and distortions of the NbO₆ octahedra, estimated by Rietveld refinements, it is shown that the sequence of temperature-induced phases in AgNbO₃ is P2₁am → Pcam → Cmcm → P4/mbm → Pm3̅m. This sequence is simpler than that proposed in earlier studies. Evidence is presented for a second-order Jahn-Teller distortion in the Pcam phase. At x > 0.05, Li doping favors the formation of a rhombohedral phase in space group R3c, and such samples display the temperature-induced sequence R3c → Pbnm → Cmcm → P4/mbm → Pm3̅m. Unusual volume changes associated with the phase transitions point to the potential importance of lattice matching in optimizing the properties of thin films of doped AgNbO₃.

Accelerating high-resolution NMR of half-integer quadrupolar nuclei in solids: SPAM-MQMAS and SPAM-STMAS

In solid-state NMR, multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments are well-established techniques to obtain high-resolution spectra of half-integer quadrupolar nuclei. In 2004 and 2005, a soft-pulse-added-mixing (SPAM) concept was introduced by Gan and Amoureux to enhance the S/N ratio of MQMAS and STMAS experiments. Despite their robustness and simplicity, SPAM approaches have not yet been widely applied. Here, we further exploit SPAM concepts for sensitivity enhancement upon acquisition of two-dimensional MQMAS and STMAS spectra and also establish a general procedure upon implementation of SPAM-MQMAS and SPAM-STMAS NMR. Its effectiveness and ease in experimental setup are demonstrated using simulations and experiments performed on I ​= ​3/2 (²³Na, ⁸⁷Rb), 5/2 (²⁷Al, ⁸⁵Rb) and 9/2 (⁹³Nb) nuclei with a variety of quadrupolar coupling constants (C_Q). Compared to the conventional z-filter methods, sensitivity enhancements in between 2 and 4 are achievable with SPAM. We recommend to use SPAM with a ratio of 4:1 for the number of echoes and antiechoes to safely maximize the sensitivity and resolution simultaneously. In addition, a comparison of the experimental approaches is made in the context of SPAM-MQMAS and SPAM-STMAS NMR with respect to repetition delay and spinning frequency, aiming to discuss the precautions upon making a judicious choice of high-resolution NMR methods of half-integer quadrupolar nuclei.

Perovskite Oxides Prepared by Hydrothermal and Solvothermal Synthesis: A Review of Crystallisation, Chemistry, and Compositions

Perovskite oxides with general composition ABO₃ are a large group of inorganic materials that can contain a variety of cations from all parts of the Periodic Table and that have diverse properties of application in fields ranging from electronics, energy storage to photocatalysis. Solvothermal synthesis routes to these materials have become increasingly investigated in the past decade as a means of direct crystallisation of the solids from solution. These methods have significant advantages leading to adjustment of crystal form from the nanoscale to the micron-scale, the isolation of compositions not possible using conventional solid-state synthesis and in addition may lead to scalable processes for producing materials at moderate temperatures. These aspects are reviewed, with examples taken from the past decade's literature on the solvothermal synthesis of perovskites with a systematic survey of B-site cations, from transition metals in Groups 4-8 and main group elements in Groups 13, 14 and 15, to solid solutions and heterostructures. As well as hydrothermal reactions, the use of various solvents and solution additives are discussed and some trends identified, along with prospects for developing control and predictability in the crystallisation of complex oxide materials.

Lead-free (Ag,K)NbO3 materials for high-performance explosive energy conversion

Explosive energy conversion materials with extremely rapid response times have broad and growing applications in energy, medical, defense, and mining areas. Research into the underlying mechanisms and the search for new candidate materials in this field are so limited that environment-unfriendly Pb(Zr,Ti)O3 still dominates after half a century. Here, we report the discovery of a previously undiscovered, lead-free (Ag0.935K0.065)NbO3 material, which possesses a record-high energy storage density of 5.401 J/g, enabling a pulse current ~ 22 A within 1.8 microseconds. It also exhibits excellent temperature stability up to 150°C. Various in situ experimental and theoretical investigations reveal the mechanism underlying this explosive energy conversion can be attributed to a pressure-induced octahedral tilt change from a - a - c + to a - a - c -/a - a - c +, in accordance with an irreversible pressure-driven ferroelectric-antiferroelectric phase transition. This work provides a high performance alternative to Pb(Zr,Ti)O3 and also guidance for the further development of new materials and devices for explosive energy conversion.

Simultaneous tuning of optical and electrical properties in a multifunctional LiNbO3 matrix upon doping with Eu3+ ions

Combined photoluminescence (PL) and dielectric studies have been carried out on both undoped and Eu³⁺ doped LiNbO₃ compounds for their potential application in optical–electrical integration for the first time. Special focus has been given to simultaneously tuning both these physical properties. A PL study reveals that the blank compound is a blue emitting material, while upon doping with Eu³⁺ ions, the emitting color can be tuned from blue to red upon changing the excitation wavelength. Interestingly, the electrical property measurement of this ferroelectric compound showed that upon doping with Eu³⁺ ions, the remnant polarization was increased significantly. Density Functional Theory (DFT) based calculations were carried out to explain both the optical and electrical properties. It has been found that different defect centers are responsible for the bluish host emission while Eu³⁺ ions are energetically preferred to occupy the Nb site and gives rise to red emission. The DFT based results also showed that Eu³⁺ ions induced more distortion into the nearby Nb-site, which is responsible for enhancement of the remnant polarization. Stark-splitting patterns in the PL study also showed that the point symmetry of LiNbO₃ upon Eu³⁺ doping changes from C_6v to D₃, which indicates that the structure becomes less symmetric. Overall, the study presents a novel approach to designing multifunctional materials for optical–electrical integration application and to tuning their physical properties simultaneously in the desired range.

PL and dielectric studies have been carried out on LiNbO₃ and Eu³⁺:LiNbO₃ compounds with a special focus on simultaneous tuning of optical and electrical properties for their potential application in optical–electrical integration.

Achieving Thermo-Mechano-Opto-Responsive Bitemporal Colorful Luminescence via Multiplexing of Dual Lanthanides in Piezoelectric Particles and its Multidimensional Anticounterfeiting

Optical characteristics of luminescent materials, including emission color (wavelength), lifetime, and excitation mode, play crucial roles in data communication and information security. Conventional luminescent materials generally display unicolor, unitemporal, and unimodal (occasionally bimodal) emission, resulting in low-level readout and decoding. The development of multicolor, multitemporal, and multimodal luminescence in a single material has long been considered to be a significant challenge. In this study, for the first time, the superior integration of colorful (red-orange-yellow-green), bitemporal (fluorescent and delayed), and four-modal (thermo-/mechano-motivated and upconverted/downshifted) emissions in a particular piezoelectric particle via optical multiplexing of dual-lanthanide dopants is demonstrated. The as-prepared versatile NaNbO₃ :Pr³⁺ ,Er³⁺ luminescent microparticles shown are particularly suitable for embedding into polymer films to achieve waterproof, flexible/wearable and highly stretchable features, and synchronously to provide multidimensional codes that can be visually read-out using simple and commonly available tools (including the LED of a smartphone, pen writing, cooling-heating stimuli, and ultraviolet/near-infrared lamps). These findings offer unique insight for designing highly integrated stimuli-responsive luminophors and smart devices toward a wide variety of applications, particularly advanced anticounterfeiting technology.

Chemical-bond effect on epitaxial strain in perovskite sodium niobate

Epitaxial films and heterostructures of perovskite oxides attract tremendous scientific interest because of the unique phenomena therein. Especially important is the epitaxial growth of films subjected to substrate-induced misfit strain. We show here that in contrast to conventional misfit-controlled epitaxy, chemical bonds determine the crystal stability and strain in epitaxial films of sodium niobate on different cubic substrates. Strain relaxation in sodium niobate is independent of misfit magnitude and proceeds through perovskite-specific tilting of oxygen octahedra in addition to common defect formation. The observed structural relaxation evidences a major role of a large internal strain that originates from chemical bonds in the perovskite cell. The effect of chemical bonds on film strain is anticipated to also control the epitaxy of other perovskite oxides and related compounds.

Low temperature synthesis route and structural characterization of (Bi0.5A0.5)(Sc0.5Nb0.5)O3 (A = K+ and Na+) perovskites

First report of the phase pure synthesis and crystal structure determination for Bi_0.5A_0.5(Sc_0.5Ta_0.5)O₃ (A = K⁺, Na⁺).

DFT calculations for structural prediction and applications of intercalated lamellar compounds

This review lists some relevant types of intercalated lamellar materials and applications of DFT calculations in these materials.

Investigating FAM-N pulses for signal enhancement in MQMAS NMR of quadrupolar nuclei

Although a popular choice for obtaining high-resolution solid-state NMR spectra of quadrupolar nuclei, the inherently low sensitivity of the multiple-quantum magic-angle spinning (MQMAS) experiment has limited its application for nuclei with low receptivity or when the available sample volume is limited. A number of methods have been introduced in the literature to attempt to address this problem. Recently, we have introduced an alternative, automated approach, based on numerical simulations, for generating amplitude-modulated pulses (termed FAM-N pulses) to enhance the efficiency of the triple- to single-quantum conversion step within MQMAS. This results in efficient pulses that can be used without experimental reoptimisation, ensuring that this method is particularly suitable for challenging nuclei and systems. In this work, we investigate the applicability of FAM-N pulses to a wider variety of systems, and their robustness under more challenging experimental conditions. These include experiments performed under fast MAS, nuclei with higher spin quantum numbers, samples with multiple distinct sites, low-γ nuclei and nuclei subject to large quadrupolar interactions.

Unveiling the correlation between structural order–disorder character and photoluminescence emissions of NaNbO3

Orthorhombic NaNbO₃ particles obtained via the MAH route show photoluminescence emission related to the structural disorder effect at the medium-range.

Reconciling Local Structure Disorder and the Relaxor State in (Bi1/2Na1/2)TiO3-BaTiO3

Lead-based relaxor ferroelectrics are key functional materials indispensable for the production of multilayer ceramic capacitors and piezoelectric transducers. Currently there are strong efforts to develop novel environmentally benign lead-free relaxor materials. The structural origins of the relaxor state and the role of composition modifications in these lead-free materials are still not well understood. In the present contribution, the solid-solution (100-x)(Bi_1/2Na_1/2)TiO₃-xBaTiO₃ (BNT-xBT), a prototypic lead-free relaxor is studied by the combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, dielectric measurements and ab-initio density functional theory (DFT). For the first time it is shown that the peculiar composition dependence of the EFG distribution width (ΔQIS_width) correlates strongly to the dispersion in dielectric permittivity, a fingerprint of the relaxor state. Significant disorder is found in the local structure of BNT-xBT, as indicated by the analysis of the electric field gradient (EFG) in ²³Na 3QMAS NMR spectra. Aided by DFT calculations, this disorder is attributed to a continuous unimodal distribution of octahedral tilting. These results contrast strongly to the previously proposed coexistence of two octahedral tilt systems in BNT-xBT. Based on these results, we propose that considerable octahedral tilt disorder may be a general feature of these oxides and essential for their relaxor properties.

Combining solid-state NMR spectroscopy with first-principles calculations - a guide to NMR crystallography

Recent advances in the application of first-principles calculations of NMR parameters to periodic systems have resulted in widespread interest in their use to support experimental measurement. Such calculations often play an important role in the emerging field of "NMR crystallography", where NMR spectroscopy is combined with techniques such as diffraction, to aid structure determination. Here, we discuss the current state-of-the-art for combining experiment and calculation in NMR spectroscopy, considering the basic theory behind the computational approaches and their practical application. We consider the issues associated with geometry optimisation and how the effects of temperature may be included in the calculation. The automated prediction of structural candidates and the treatment of disordered and dynamic solids are discussed. Finally, we consider the areas where further development is needed in this field and its potential future impact.

Explorations of New SHG Materials in the Alkali-Metal-Nb(5+)-Selenite System

Standard high-temperature solid-state reactions of NaCl, Nb2O5, and SeO2 resulted in two new sodium selenites containing a second-order Jahn-Teller (SOJT) distorted Nb(5+) cation, namely, Na2Nb4O7(SeO3)4 (P1̅; 1) and NaNbO(SeO3)2 (Cmc21; 2). Compound 1 exhibits an unusual 3D [Nb4O7(SeO3)4](2-) anionic network composed of 2D [Nb4O11(SeO3)2](6-) layers which are further bridged by additional SeO3(2-) anions via corner sharing; the 2D [Nb4O11(SeO3)2](6-) layer is formed by unusual quadruple [Nb4O17](14-) niobium oxide chains of corner-sharing NbO6 octahedra being further interconnected by selenite anions via Nb-O-Se bridges. The polar compound 2 features a 1D [NbO(SeO3)2](-) anionic chain in which two neighboring Nb(5+) cations are bridged by one oxo and two selenite anions. The alignments of the polarizations from the NbO6 octahedra in 2 led to a strong SHG response of ∼7.8 × KDP (∼360 × α-SiO2), which is the largest among all phases found in metal-Nb(5+)-Se(4+)/metal-Nb(5+)-Te(4+)-O systems. Furthermore, the material is also type I phase matchable. The above experimental results are consistent with those based on DFT theoretical calculations. Thermal stabilities and optical properties for both compounds are also reported.

Joint Experimental and Computational 17O and 1H Solid State NMR Study of Ba2In2O4(OH)2 Structure and Dynamics

A structural characterization of the hydrated form of the brownmillerite-type phase Ba₂In₂O₅, Ba₂In₂O₄(OH)₂, is reported using experimental multinuclear NMR spectroscopy and density functional theory (DFT) energy and GIPAW NMR calculations. When the oxygen ions from H₂O fill the inherent O vacancies of the brownmillerite structure, one of the water protons remains in the same layer (O3) while the second proton is located in the neighboring layer (O2) in sites with partial occupancies, as previously demonstrated by Jayaraman et al. (Solid State Ionics2004, 170, 25-32) using X-ray and neutron studies. Calculations of possible proton arrangements within the partially occupied layer of Ba₂In₂O₄(OH)₂ yield a set of low energy structures; GIPAW NMR calculations on these configurations yield ¹H and ¹⁷O chemical shifts and peak intensity ratios, which are then used to help assign the experimental MAS NMR spectra. Three distinct ¹H resonances in a 2:1:1 ratio are obtained experimentally, the most intense resonance being assigned to the proton in the O3 layer. The two weaker signals are due to O2 layer protons, one set hydrogen bonding to the O3 layer and the other hydrogen bonding alternately toward the O3 and O1 layers. ¹H magnetization exchange experiments reveal that all three resonances originate from protons in the same crystallographic phase, the protons exchanging with each other above approximately 150 °C. Three distinct types of oxygen atoms are evident from the DFT GIPAW calculations bare oxygens (O), oxygens directly bonded to a proton (H-donor O), and oxygen ions that are hydrogen bonded to a proton (H-acceptor O). The ¹⁷O calculated shifts and quadrupolar parameters are used to assign the experimental spectra, the assignments being confirmed by ¹H-¹⁷O double resonance experiments.

Solid-state (185/187)Re NMR and GIPAW DFT study of perrhenates and Re2(CO)10: chemical shift anisotropy, NMR crystallography, and a metal-metal bond

Advances in solid-state nuclear magnetic resonance (SSNMR) methods, such as dynamic nuclear polarization (DNP), intricate pulse sequences, and increased applied magnetic fields, allow for the study of systems which even very recently would be impractical. However, SSNMR methods using certain quadrupolar probe nuclei (i.e., I > 1/2), such as (185/187)Re remain far from fully developed due to the exceedingly strong interaction between the quadrupole moment of these nuclei and local electric field gradients (EFGs). We present a detailed high-field (B0 = 21.1 T) experimental SSNMR study on several perrhenates (KReO4, AgReO4, Ca(ReO4)2·2H2O), as well as ReO3 and Re2(CO)10. We propose solid ReO3 as a new rhenium SSNMR chemical shift standard due to its reproducible and sharp (185/187)Re NMR resonances. We show that for KReO4, previously poorly understood high-order quadrupole-induced effects (HOQIE) on the satellite transitions can be used to measure the EFG tensor asymmetry (i.e., ηQ) to nearly an order-of-magnitude greater precision than competing SSNMR and nuclear quadrupole resonance (NQR) approaches. Samples of AgReO4 and Ca(ReO4)2·2H2O enable us to comment on the effects of counter-ions and hydration upon Re(vii) chemical shifts. Calcium-43 and (185/187)Re NMR tensor parameters allow us to conclude that two proposed crystal structures for Ca(ReO4)2·2H2O, which would be considered as distinct, are in fact the same structure. Study of Re2(CO)10 provides insights into the effects of Re-Re bonding on the rhenium NMR tensor parameters and rhenium oxidation state on the Re chemical shift value. As overtone NQR experiments allowed us to precisely measure the (185/187)Re EFG tensor of Re2(CO)10, we were able to measure rhenium chemical shift anisotropy (CSA) for the first time in a powdered sample. Experimental observations are supported by gauge-including projector augmented-wave (GIPAW) density functional theory (DFT) calculations, with NMR tensor calculations also provided for NH4ReO4, NaReO4 and RbReO4. These calculations are able to reproduce many of the experimental trends in rhenium δiso values and EFG tensor magnitudes. Using KReO4 as a prototypical perrhenate-containing system, we establish a correlation between the tetrahedral shear strain parameter (|ψ|) and the nuclear electric quadrupolar coupling constant (CQ), which enables the refinement of the structure of ND4ReO4. Shortcomings in traditional DFT approaches, even when including relativistic effects via the zeroth-order regular approximation (ZORA), for calculating rhenium NMR tensor parameters are identified for Re2(CO)10.

Synthesis of a g-C3N4-sensitized and NaNbO3-substrated II-type heterojunction with enhanced photocatalytic degradation activity

A g-C₃N₄-sensitized and NaNbO₃-substrated II-type heterojunction with enhanced photocatalytic activity was synthesized.

Lead-free antiferroelectric: xCaZrO3-(1 − x)NaNbO3 system (0 ≤ x ≤ 0.10)

Crystal chemistry modification stabilized antiferroelectricity in NaNbO₃-based polycrystalline ceramics.

New methods and applications in solid-state NMR spectroscopy of quadrupolar nuclei

Solid-state nuclear magnetic resonance (NMR) spectroscopy has long been established as offering unique atomic-scale and element-specific insight into the structure, disorder, and dynamics of materials. NMR spectra of quadrupolar nuclei (I > (1)/2) are often perceived as being challenging to acquire and to interpret because of the presence of anisotropic broadening arising from the interaction of the electric field gradient and the nuclear electric quadrupole moment, which broadens the spectral lines, often over several megahertz. Despite the vast amount of information contained in the spectral line shapes, the problems with sensitivity and resolution have, until very recently, limited the application of NMR spectroscopy of quadrupolar nuclei in the solid state. In this Perspective, we provide a brief overview of the quadrupolar interaction, describe some of the basic experimental approaches used for acquiring high-resolution NMR spectra, and discuss the information that these spectra can provide. We then describe some interesting recent examples to showcase some of the more exciting and challenging new applications of NMR spectra of quadrupolar nuclei in the fields of energy materials, microporous materials, Earth sciences, and biomaterials. Finally, we consider the possible directions that this highly informative technique may take in the future.

NMR crystallography: Applications to inorganic materials

Current developments of NMR crystallography as well as some recent applications to diamagnetic inorganic solids are presented. First, we illustrate how solid-state NMR data can be used in combination with diffraction data for the determination of the periodic part of the crystal structures, from the space group selection, to the structure determination over the refinement and validation processes. As ss-NMR, contrary to diffraction (powder and single-crystal), is not restricted to periodic boundary conditions, ss-NMR data can be used to further complete the structural description of materials, including studies of local order/disorder, etc. This illustrated through examples, which are shown and discussed in the second part of this review.

Efficient amplitude-modulated pulses for triple- to single-quantum coherence conversion in MQMAS NMR

The conversion between multiple- and single-quantum coherences is integral to many nuclear magnetic resonance (NMR) experiments of quadrupolar nuclei. This conversion is relatively inefficient when effected by a single pulse, and many composite pulse schemes have been developed to improve this efficiency. To provide the maximum improvement, such schemes typically require time-consuming experimental optimization. Here, we demonstrate an approach for generating amplitude-modulated pulses to enhance the efficiency of the triple- to single-quantum conversion. The optimization is performed using the SIMPSON and MATLAB packages and results in efficient pulses that can be used without experimental reoptimisation. Most significant signal enhancements are obtained when good estimates of the inherent radio-frequency nutation rate and the magnitude of the quadrupolar coupling are used as input to the optimization, but the pulses appear robust to reasonable variations in either parameter, producing significant enhancements compared to a single-pulse conversion, and also comparable or improved efficiency over other commonly used approaches. In all cases, the ease of implementation of our method is advantageous, particularly for cases with low sensitivity, where the improvement is most needed (e.g., low gyromagnetic ratio or high quadrupolar coupling). Our approach offers the potential to routinely improve the sensitivity of high-resolution NMR spectra of nuclei and systems that would, perhaps, otherwise be deemed “too challenging”.

Structural instability of epitaxial (001) BiFeO₃ thin films under tensile strain

We explore BiFeO3 under tensile strain using first-principles calculations. We find that the actual structures are more complex than what had been previously thought, and that there is a strong shear deformation type structural instability which modifies the properties. Specifically, we find that normal tensile strain leads to structural instabilities with a large induced shear deformation in (001) BiFeO3 thin films. These induced shear deformations in (001) BiFeO3 thin films under tension stabilize the (001) BiFeO3 thin films and lead to Cc and Ima2 phases that are more stable than the Pmc21 phase at high tensile strain. The induced shear deformation shifts the Cc to Ima2 phase transition towards lower tensile strain region (~1% less), prevents monoclinic tilt and oxygen octahedral tilts, and increases the ferroelectric polarization. The induced shear deformation also strongly affects the electronic structure. The results are discussed in relation to growth of BiFeO3 thin films on cubic and tetragonal substrates involving high levels of tensile strain.

Ferroelectricity in antiferroelectric NaNbO3 crystal

Sodium niobate (NaNbO3, or NNO) is known to be antiferroelectric at temperatures between 45 and 753 K. Here we show experimentally the presence of the ferroelectric phase at temperatures between 100 and 830 K in the NNO crystals obtained by top-seeded solution growth. The ferroelectric phase and new phase transitions are evidenced using a combination of thermo-optical studies by variable angle spectroscopic ellipsometry, Raman spectroscopy analysis, and photoelectron emission microscopy. The possibility for strain-induced ferroelectricity in NNO is suggested.

Endocytic pH-triggered degradation of nanoengineered multilayer capsules

The synthesis of cross-linker free layer-by-layer (LbL) capsules that solely utilize cellular pH variations as a trigger to specifically deconstruct and subsequently release cargo in cells is reported. These capsules demonstrate retention of water-soluble therapeutic molecules as small as 500 Da at extracellular pH. Triggered capsule degradation and release of cargo is observed within 30 min of cell uptake.

Synthesis, structure, and piezoelectric properties of ferroelectric and antiferroelectric NaNbO3 nanostructures

NaNbO₃ nanowires and NaNbO₃ microcubes are indexed to ferroelectric and antiferroelectric structures, respectively.