High-pressure synthesis, crystal structure, and phase stability relations of a LiNbO3-type polar titanate ZnTiO3 and its reinforced polarity by the second-order Jahn-Teller effect

Rapid nucleation and optimal surface-ligand interaction stabilize wurtzite MnSe

Non-native structures (NNS) differ in discrete translational symmetry from the bulk ground state native structure (NS). To explore the extent of deconvolution of various factors relevant to the stabilization of the wurtzite/NNS of MnSe via a heat-up method, we performed experiments using various ligands (oleic acid, oleylamine, octadecylamine, stearic acid, and octadecene), solvents (tetraethylene glycol and octadecene), and precursor salts (manganese chloride and manganese acetate). Experiments suggest that oleic acid in the presence of tetraethylene glycol and oleylamine in the presence of octadecene stabilize wurtzite/NNS. Further, density functional theory (DFT) computations explore the interaction between the functional groups in ligands and the most exposed surfaces of wurtzite/NNS and rocksalt/NS polymorphs. Computations suggest that the interactions between relevant surface facets with carboxylic acid and the double bond functional groups suppress the phase transformation from NNS to NS. In addition, the ionizability of the precursor salt also determines the rate of formation of the metal-ligand complex and the rate of nucleation. Consequently, the formation rate of the Mn-ligand complex is expected to be greater in the case of chloride salt than acetate salt because the chloride salt has higher ionizability in ethylene glycol. From the above, we conclude that the kinetics of the wurtzite/NNS to rocksalt/NS phase transformation depends mainly on two factors: (1) nucleation/growth kinetics which is controlled by the ionizability of the precursor salt, solvent, and stability of the metal-ligand complex, and (2) the activation energy barrier of the NNS to NS conversion which is controlled by surface energy minimization with the ligand.

Synthesis, crystal structure and investigation of ion-exchange possibility for sodium tellurate NaTeO3(OH)

A new sodium tellurate has been hydrothermally synthesized and comprehensively analysed using spectroscopic and thermogravimetric techniques, resulting in the determination of its composition as NaTeO3(OH). The analysis of synchrotron X-ray and neutron diffraction data indicates that NaTeO3(OH) has a crystal structure similar to that of the previously reported tellurate, KTeO3(OH), with the space group P21/a (No. 14). NaTeO3(OH) consists of zigzag one-dimensional chains built by edge-sharing TeO6 octahedra, running parallel to the c-axis and connected to sodium and hydrogen atoms. The hydrogen atoms covalently bond to the terminal oxygen atoms on the one-dimensional chain and also form hydrogen bonds with other terminal oxygen atoms on nearby chains. The structure has been confirmed by optimization using the pseudopotential method and performing Bond Valence Sum (BVS) analysis. Although Li+ ions in LiTeO3(OH) can be exchanged reversibly with H+ ions, no ion exchange behaviour is observed in NaTeO3(OH). The difference is attributed to the size of the alkali ions and their crystal structure.

First-principles study of oxygen vacancies in LiNbO3-type ferroelectrics

LiNbO3-type ferroelectric oxides, as an important class of non-centrosymmetric compounds, have received great attention due to their important and rich properties. Although oxygen vacancies are widely present, studies of them in LiNbO3-type ferroelectric oxides are rare. In this article, we consider three representative LiNbO3-type ferroelectric oxide materials LiNbO3, ZnTiO3 and ZnSnO3 to study the impact of oxygen vacancy doping using first principles calculations. LiNbO3 and ZnTiO3 have ferroelectrically active cations Nb5+ and Ti4+, while ZnSnO3 does not have ferroelectrically active cations. The distribution of the oxygen vacancy induced electrons are quite different in the three materials even though they have similar structures. In oxygen deficient LiNbO3-δ (δ = 0.083/f.u.), electrons are itinerant, while in ZnTiO3-δ and ZnSnO3-δ (δ = 0.083/f.u.) the electrons are localized. These results provide guidance for the application of oxygen vacancies in LiNbO3-type ferroelectric material devices.

Structural Distortion in the Wadsley-Roth Niobium Molybdenum Oxide Phase Triggering Extraordinarily Stable Battery Performance

Wadsley-Roth niobium oxide phases have attracted extensive research interest recently as promising battery anodes. We have synthesized the niobium-molybdenum oxide shear phase (Nb, Mo)13 O33 with superior electrochemical Li-ion storage performance, including an ultralong cycling lifespan of at least 15000 cycles. During electrochemical cycling, a reversible single-phase solid-solution reaction with lithiated intermediate solid solutions is demonstrated using in situ X-ray diffraction, with the valence and short-range structural changes of the electrode probed by in situ Nb and Mo K-edge X-ray absorption spectroscopy. This work reveals that the superior stability of niobium molybdenum oxides is underpinned by changes in octahedral distortion during electrochemical reactions, and we report an in-depth understanding of how this stabilizes the oxide structure during cycling with implications for future long-life battery material design.

Comparison of carrier doping in ZnSnO3 and ZnTiO3 from first principles

Ferroelectric materials have attracted increasing attention due to their rich properties. Unlike perovskite ferroelectric oxides, in the LiNbO3-type ferroelectric oxides of ABO3, ferroelectrically active cations are not necessary. While the effects of carrier doping on perovskite ferroelectric oxides have been extensively studied, the studies on LiNbO3-type ferroelectric oxides are rare. We consider two LiNbO3-type ferroelectric oxides ZnSnO3 and ZnTiO3, where the former has no ferroelectrically active cation and the latter has ferroelectrically active cation Ti4+, and study the effect of carrier doping by performing first-principles calculations. Comparison results indicate that the B-site cation has significant effects on the polar distortion in LN-type ferroelectrics. Our studies show that LN-type materials can maintain the coexistence of ferroelectricity and conductance over a very wide range of concentrations. The polar displacement is even enhanced under hole doping. More importantly, ZnSnO3 can be doped by electrons up to a high level to realize the conducting ferroelectrics of high mobility due to its isolated s conduction band.

Theoretical Justification of Structural, Magnetoelectronic and Optical Properties in QFeO3 (Q = Bi, P, Sb): A First-Principles Study

One of the primary objectives of scientific research is to create state-of-the-art multiferroic (MF) materials that exhibit interconnected properties, such as piezoelectricity, magnetoelectricity, and magnetostriction, and remain functional under normal ambient temperature conditions. In this study, we employed first-principles calculations to investigate how changing pnictogen elements affect the structural, electronic, magnetic, and optical characteristics of QFeO3 (Q = Bi, P, SB). Electronic band structures reveal that BiFeO3 is a semiconductor compound; however, PFeO3 and SbFeO3 are metallic. The studied compounds are promising for spintronics, as they exhibit excellent magnetic properties. The calculated magnetic moments decreased as we replaced Bi with SB and P in BiFeO3. A red shift in the values of ε2(ω) was evident from the presented spectra as we substituted Bi with Sb and P in BiFeO3. QFeO3 (Q = Bi, P, SB) showed the maximum absorption of incident photons in the visible region. The results obtained from calculating the optical parameters suggest that these materials have a strong potential to be used in photovoltaic applications.

Lattice pinning in MoO3 via coherent interface with stabilized Li+ intercalation

Large lattice expansion/contraction with Li+ intercalation/deintercalation of electrode active materials results in severe structural degradation to electrodes and can negatively impact the cycle life of solid-state lithium-based batteries. In case of the layered orthorhombic MoO3 (α-MoO3), its large lattice variation along the b axis during Li+ insertion/extraction induces irreversible phase transition and structural degradation, leading to undesirable cycle life. Herein, we propose a lattice pinning strategy to construct a coherent interface between α-MoO3 and η-Mo4O11 with epitaxial intergrowth structure. Owing to the minimal lattice change of η-Mo4O11 during Li+ insertion/extraction, η-Mo4O11 domains serve as pin centers that can effectively suppress the lattice expansion of α-MoO3, evidenced by the noticeably decreased lattice expansion from about 16% to 2% along the b direction. The designed α-MoO3/η-Mo4O11 intergrown heterostructure enables robust structural stability during cycling (about 81% capacity retention after 3000 cycles at a specific current of 2 A g-1 and 298 ± 2 K) by harnessing the merits of epitaxial stabilization and the pinning effect. Finally, benefiting from the stable positive electrode-solid electrolyte interface, a highly durable and flexible all-solid-state thin-film lithium microbattery is further demonstrated. This work advances the fundamental understanding of the unstable structure evolution for α-MoO3, and may offer a rational strategy to develop highly stable electrode materials for advanced batteries.

BaMo3O10 Polymorphs with Tunable Symmetries and Properties

Polymorphism is a well-known but important phenomenon in the field of solid-state chemistry. Crystalline materials can form various polymorphs and present drastically varied physical and chemical properties. Herein, systematic exploration of the BaO-MoO₃ binary system leads to the discovery of a new barium molybdate, α-BaMo₃O₁₀. The temperature-dependent phase transition from α-BaMo₃O₁₀ to β-BaMo₃O₁₀ is confirmed. The tunable linear and nonlinear optical properties induced by the phase transition are confirmed by both experimental and theoretical approaches. Also, β-BaMo₃O₁₀ is identified as a nonlinear-optical crystal for the first time. The origin of linear- and nonlinear-optical properties of BaMo₃O₁₀ polymorphs is confirmed by the additional theoretical means. This work indicates that a small change in the structure can induce tunable symmetries and thereby widely divergent optical properties.

Second-order Jahn-Teller effect induced high-temperature ferroelectricity in two-dimensional NbO2X (X = I, Br)

Based on the first-principles calculations, we investigated the ferroelectric properties of two-dimensional (2D) materials NbO₂X (X = I, Br). Our cleavage energy analysis shows that exfoliating one NbO₂I monolayer from its existing bulk counterpart is feasible. The phonon spectrum and molecular dynamics simulations confirm the dynamic and thermal stability of the monolayer structures for both NbO₂I and NbO₂Br. Total energy calculations show that the ferroelectric phase is the ground state for both materials, with the calculated in-plane ferroelectric polarizations being 384.5 pC m^-1 and 375.2 pC m^-1 for monolayers NbO₂I and NbO₂Br, respectively. Moreover, the intrinsic Curie temperature T_C of monolayer NbO₂I (NbO₂Br) is as high as 1700 K (1500 K) from Monte Carlo simulation. Furthermore, with the orbital selective external potential method, the origin of ferroelectricity in NbO₂X is revealed as the second-order Jahn-Teller effect. Our findings suggest that monolayers NbO₂I and NbO₂Br are promising candidate materials for practical ferroelectric applications.

First principle studies of TiO2-ZnO alloys under high pressure

The ZnO-TiO₂composite system has been applied as a photocatalyst in the treatment of organic waste and domestic wastewater due to its high separation rate of photogenerated carriers and wide light response range. Using the first-principles approach based on density functional theory, we investigated the crystal structures and the electronic properties of ZnO-TiO₂alloys under high pressure and predicted three stable high-pressure phases (CmcmZnTiO₃,ImmaZn₂TiO₄andCmZnTi₃O₇). Calculations of the phonon spectra and elastic constants showed that the predicted structures are dynamically and mechanically stable. In terms of electronic properties, it was found that the three crystal structures were all semiconductors. With the increase of pressure, the band gap ofCmZnTi₃O₇showed an increasing trend, while the band gap ofCmcmZnTiO₃andImmaZn₂TiO₄gradually decreased. The calculated band structures showed that the band gap first increases nonlinearly and then decreases as the Zn concentration increases. Pressure can regulate the band gap of the above crystals, making them promising for applications in photocatalysis and microwave devices.

Comparative Study on the Magnetic and Transport Properties of B-Site Ordered and Disordered CaCu3Fe2Os2O12

The B-site Fe/Os ordered and disordered quadruple perovskite oxides CaCu3Fe2Os2O12 were synthesized under different high-pressure and high-temperature conditions. The B-site ordered CaCu3Fe2Os2O12 is a system with a very high ferrimagnetic ordering temperature of 580 K having the Cu2+(↑)Fe3+(↑)Os5+(↓) charge and spin arrangement. In comparison, the highly disordered CaCu3Fe2Os2O12 has a reduced magnetic transition temperature of about 350 K. The Cu2+Fe3+Os5+ charge combination remains the same without any sign of changes in the valence state of the constituent ions. Although the average net moments of each sublattice are reduced, the average ferrimagnetic spin arrangement is unaltered. The robustness of the basic magnetic properties of CaCu3Fe2Os2O12 against site disorder may be taken as an indication of the tendency to maintain the short-range order of the atomic constituents.

Suppression of Pressure-Induced Phase Transitions in a Monoclinically Distorted LiNbO3-Type CuNbO3 by Preference for a CuO3 Triangular Coordination Environment

Pressure-induced phase transitions in a monoclinically distorted LiNbO₃-type CuNbO₃ with triangularly coordinated Cu and octahedrally coordinated Nb were experimentally and computationally investigated. Phase transitions into GdFeO₃-type or NaIO₃-type structures generally observed in LiNbO₃-type compounds below 30 GPa were not detected in CuNbO₃ even at the maximum experimental pressure, 32.4 GPa. Our density functional theory calculations revealed that the phase transition is suppressed by the preference for the CuO₃ triangular coordination environment, which reduces the total internal energy. This study clarifies that the change in the coordination environment of given ions can affect the pressure-induced phase transition.

Topochemical Synthesis of LiCoF3 with a High-Temperature LiNbO3-Type Structure

A novel perovskite fluoride, Li_xCoF₃, which has an exceptionally low tolerance factor (0.81), has been synthesized via low-temperature lithium intercalation into a distorted ReO₃-type fluoride CoF₃ using organolithium reagents. Interestingly, this reaction is completed within 15 min at room temperature. Synchrotron X-ray diffractometry and optical second harmonic generation at room temperature have revealed that this compound shows a high-temperature LiNbO₃-type structure (space group: R3̅c) involving Li-Co antisite defects and A-site splitting along the c direction. A-site splitting is consistent with the prediction based on hybrid Hartree-Fock density functional theory calculations. Co-L_2,3 edge X-ray absorption spectroscopy, as well as bond valence sum analysis, has verified the divalent oxidation state of Co ions in the lithiated phase, suggesting that its composition is close to LiCoF₃ (x ≈ 1). This compound exhibits a paramagnetic-to-antiferromagnetic transition at 36 K on cooling, accompanied by weak ferromagnetic ordering. The synthetic route based on low-temperature lithiation of metal fluorides host paves the way for obtaining a new LiNbO₃-type fluoride family.

Chemical design of a new displacive-type ferroelectric

Since the discovery of the ferroelectric perovskite-type oxide BaTiO₃ in 1943, numerous materials have been surveyed as candidates for new ferroelectrics. Perovskite-type materials have played a leading role in basic research and applications of ferroelectric materials since the last century. Experimentalists and theoreticians have developed a new materials design stream for post-perovskite materials. In this stream, we have mainly focused on the role of covalency in the evolution of ferroelectricity for displacive-type ferroelectrics in oxides. This perspective surveys the following topics: (1) crossover from quantum paraelectric to ferroelectric through a ferroelectric quantum critical point, (2) the role of cation-oxygen covalency in ferroelectricity and the crossover to quantum paraelectric in perovskite-type compounds, (3) off-center-induced ferroelectricity in perovskites, (4) second-order Jahn-Teller effect enhancement of ferroelectricity in lithium-niobate-type oxides, (5) the presence of four ferroelectric phases and structural transitions of phases of AFeO₃ with decreasing radius of A (A = La-Al), (6) tetrahedral ferroelectrics of perovskite-related Bi₂SiO₅ and wurtzites, (7) a rare type of polarization switching system in which the coordination number of ions in κ-Al₂O₃ systems changes between 4 and 6, and (8) lone-pair-electron-induced ferroelectrics in langasite-type compounds.

Robust Yellow-Violet Pigments Tuned by Site-Selective Manganese Chromophores

The rational design of multifunctional inorganic pigments relies on the manipulation of ionic valence and local surroundings of a chromophore in structurally and chemically habitable hosts. To date, the development of environmentally benign and intense violet/purple pigments is still a challenge. Here we report a family of A_3-xMn_xTeO₆ and A_3-2xMn_xLi_xTeO₆ (A = Zn, Mg; x = 0.01-0.15) pigments colored by site-selective Mn²⁺O₄ yellow and Mn³⁺O_5-6 violet chromophores. Zn_2.9Mn_0.1TeO₆ is intense bright yellow, comparable with commercial BiVO₄, and has better near-infrared reflectivity (∼89%) in comparison to commercial TiO₂. The codoped Li⁺ "activator" generates holes and charge-balanced Mn³⁺ (Mn³⁺O_5-6), realizing a color transformation from yellow to the bright violet pigments of A_3-2xMn_xLi_xTeO₆. The most vivid Mg_2.8Mn_0.1Li_0.1TeO₆ is probably the best violet pigment known to date, exhibits excellent chemical and thermodynamic stability, and demonstrates pressure-dependent stability up to 5-7 GPa, before a (reversible) phase transition to pink. Theoretical calculations revealed the correlation between site-preference occupancy and chromophore motifs and predicted a wide color gamut of pigments in Zn₃TeO₆-hosted 3d transition-metal ions other than manganese.

Enhanced Sunlight-Driven Reactive Species Generation via Polarization Field in Nanopiezoelectric Heterostructures

Although it is established that the force-induced electric polarization field of piezoelectric semiconductors can be used to tune the transfer rate of photoexcited charge carriers, there is still a lack of successful strategies to effectively improve the photocatalytic reactivity and solar-to-chemical conversion efficiency (SCC) of piezoelectric materials. Here, we are the first to prepare and study a kind of catalyst based on nanopiezoelectric heterostructures of LiNbO₃-type ZnTiO₃·TiO₂ and tetragonal BaTiO₃ with Pt or FeO_x nanoparticle modification (i.e., ZBTO-Pt or ZBTO-FeO_x) for reactive species generation. With respect to the production of ^•OH and ^•O₂^- radicals, higher amounts were observed in piezophotocatalysis relative to those for individual piezo- and photocatalysis. Benefiting from the charge transfer resistance decreases by the deposition of Pt and FeO_x, the amounts of ^•OH radicals formed on ZBTO-Pt and ZBTO-FeO_x were approximately 48 and 21% higher than that on isolated ZBTO during piezophotocatalysis, and for the amounts of ^•O₂^- radicals the enhancements were approximately 11 and 6%, respectively. Furthermore, the concentrations of H₂O₂ formed on ZBTO-Pt and ZBTO-FeO_x under piezophotocatalysis reached approximately 315 and 206 μM after 100 min of reaction (and was still increasing) corresponding to 0.10 and 0.06% SCCs, respectively, which were also much higher than the concentrations and SCCs observed for piezo- and photocatalysis. The enhancements of piezophotocatalytic activities with these piezoelectric materials were related to the mechanical strain exerted on ZBTO, which generated a larger electric polarization field than those on ZnTiO₃·TiO₂ and BaTiO₃ as analyzed by a finite element method. This high-intensity electric polarization field accelerated the separation and transportation of photoexcited charge carriers in the highly sunlight responsive nanopiezoelectric heterostructures based on ZBTO-Pt and ZBTO-FeO_x.

DFT Study of Methylene Blue Adsorption on ZnTiO3 and TiO2 Surfaces (101)

The search for alternative materials with high dye adsorption capacity, such as methylene blue (MB), remains the focus of current studies. This computational study focuses on oxides ZnTiO₃ and TiO₂ (anatase phase) and on their adsorptive properties. Computational calculations based on DFT methods were performed using the Viena Ab initio Simulation Package (VASP) code to study the electronic properties of these oxides. The bandgap energy values calculated by the Hubbard U (GGA + U) method for ZnTiO₃ and TiO₂ were 3.17 and 3.21 eV, respectively, which are consistent with the experimental data. The most favorable orientation of the MB adsorbed on the surface (101) of both oxides is semi-perpendicular. Stronger adsorption was observed on the ZnTiO₃ surface (−282.05 kJ/mol) than on TiO₂ (–10.95 kJ/mol). Anchoring of the MB molecule on both surfaces was carried out by means of two protons in a bidentate chelating (BC) adsorption model. The high adsorption energy of the MB dye on the ZnTiO₃ surface shows the potential value of using this mixed oxide as a dye adsorbent for several technological and environmental applications.

Effect of B-Site Order-Disorder in the Structure and Magnetism of the New Perovskite Family La2MnB'O6 with B' = Ti, Zr, and Hf

In this work, we report the synthesis as well as the structural and magnetic characterization of the three perovskites La₂MnB'O₆ (B' = Ti, Zr, and Hf). Interestingly, only La₂MnTiO₆ crystallizes in the monoclinic double perovskite space group P2₁/n, with a complete rocksalt order of the B-site cations, whereas La₂MnZrO₆ and La₂MnHfO₆ crystallize in the orthorhombic simple perovskite space group Pbnm, with complete disorder in the B site. Moreover, the magnetic susceptibility at low temperatures shows clear antiferromagnetic transitions below 10 K for the three compounds, but only the Ti-based perovskite has long-range magnetic ordering. The latter compound has an antiferromagnetic type-II structure described by the P_S-1 magnetic space group, while the other two have a spin-glass behavior below the transition temperature due to both spin disorder and competing superexchange interactions in the systems. This is the first time that two of the three studied compounds were synthesized (B' = Zr and Hf) and the first time that the whole series is described in thorough detail using symmetry-adapted refinements and magnetic crystallography.

Intrinsic ferromagnetic semiconductors in rhombohedral RMnO3 (R = Sc, Y, and Lu) with high critical temperature and large ferroelectric polarization

Ferromagnetic (FM) semiconductors have been recognized as the cornerstone for next-generation highly functional spintronic devices. However, the development in practical applications of FM semiconductors is limited by their low Curie temperatures (T _C). Here, on the basis of model analysis, we find that the FM super-exchange couplings in the d ⁵ - d ³ system can be significantly strengthened by reducing the virtual exchange gap (G _ex) between occupied and empty e _g orbitals. By first-principle calculations, we predict robust ferromagnetism in three rhombohedral RMnO₃ (R = Sc, Y, and Lu) compounds with the T _C that is as high as ∼1510 K (YMnO₃). The oxygen breathing motions open a band gap and create an unusual Mn²⁺/Mn⁴⁺ charge ordering of the Mn-d electrons, which play an important role in altering the G _ex. Interestingly, the rhombohedral RMnO₃ compounds are also ferroelectric (FE) with a large spontaneous polarization approaching that of LiNbO₃. These results not only deepen the understandings of magnetic couplings in d ⁵ - d ³ system, but also provide a way to design room-temperature FM-FE multiferroics.

Systematic centricity control using a chiral template: novel noncentrosymmetric polar niobium oxyfluorides and tantalum fluorides directed by chiral histidinium cations, [(l-hisH2)NbOF5], [(d-hisH2)NbOF5], [(l-hisH2)TaF7], and [(d-hisH2)TaF7]

Novel noncentrosymmetric polar niobium and tantalum (oxy)fluorides have been systematically synthesized by using a chiral histidinium template.

Surface and morphological studies of LiNbO3: p-type semiconductivity on stoichiometric surfaces

The surface and morphological properties of LiNbO3 surfaces were calculated, and particular semiconductor types for crystal morphologies were found.

High-Pressure Synthesis, Crystal Structures, and Properties of A-Site Columnar-Ordered Quadruple Perovskites NaRMn2Ti4O12 with R = Sm, Eu, Gd, Dy, Ho, Y

The formation of NaRMn₂Ti₄O₁₂ compounds (R = rare earth) under high pressure (about 6 GPa) and high temperature (about 1750 K) conditions was studied. Such compounds with R = Sm, Eu, Gd, Dy, Ho, Y adopt an A-site columnar-ordered quadruple-perovskite structure with the generic chemical formula A₂A'A″B₄O₁₂. Their crystal structures were studied by powder synchrotron X-ray and neutron diffraction between 1.5 and 300 K. They maintain a paraelectric structure with centrosymmetric space group P4₂/nmc (No. 137) at all temperatures, in comparison with the related CaMnTi₂O₆ perovskite, in which a ferroelectric transition occurs at 630 K. The centrosymmetric structure was also confirmed by second-harmonic generation. It has a cation distribution of [Na⁺R³⁺]_A[Mn²⁺]_A'[Mn²⁺]_A″[Ti⁴⁺₄]_BO₁₂ (to match with the generic chemical formula) with statistical distributions of Na⁺ and R³⁺ at the large A site and a strongly split position of Mn²⁺ at the square-planar A' site. We found a C-type long-range antiferromagnetic structure of Mn²⁺ ions at the A' and A″ sites below T_N = 12 K for R = Dy and found that the presence of Dy³⁺ disturbs the long-range ordering of Mn²⁺ below a second transition at lower temperatures. The first magnetic transition occurs below 8-13 K in all compounds, but the second magnetic transition occurs only for R = Dy, Sm, Eu. All compounds show large dielectric constants of a possible extrinsic origin similar to that of CaCu₃Ti₄O₁₂. NaRMn₂Ti₄O₁₂ with R = Er-Lu crystallized in the GdFeO₃-type Pnma perovskite structure, and NaRMn₂Ti₄O₁₂ with R = La, Nd contained two perovskite phases: an AA'₃B₄O₁₂-type Im3̅ phase and a GdFeO₃-type Pnma phase.

Synthesis of the perovskite-type oxyfluoride AgTiO2F: an approach adopting the HSAB principle

A synthetic approach for oxyfluorides involving the hard and soft acids and bases (HSAB) principle was examined. Using AgF composed of the Ag+ ion as a soft acid and F- as a hard base, we attempted to synthesize an oxyfluoride, AgTiO2F, that included the O2- ion as a softer base than the F- ion and the Ti4+ ion as a harder acid than the Ag+ ion. Consequently, a perovskite (Pv)-type oxyfluoride AgTiO2F was synthesized by a solid-state reaction under ambient pressure and under high pressure, and a heat treatment at 1000 °C for 30 min under a pressure of 4 GPa produced the Pv-type phase with minimal impurity phases. The lack of an optical second harmonic generation (SHG) response and the result of Rietveld refinement indicated that the compound is centro-symmetric and shows the anti-phase tilt of Ti(O, F)6 octahedra along the c-axis on the basis of the space group I4/mcm. O/F ordering on the anion site was not clarified. The phase stability of a Pv-type AgTiO2F is discussed in terms of the HSAB principle and the thermodynamics in the formation of the Pv-type phase by comparing KTiO2F and NaTiO2F. AgTiO2F has a light yellow color and a band gap energy of 2.8 eV in the visible region, which originates from the covalent bonding between the soft cation Ag+ and O2- as a softer anion than F-. We propose that the HSAB principle is useful for selecting a cation for the synthesis of oxyfluorides.

Universal A-Cation Splitting in LiNbO3-Type Structure Driven by Intrapositional Multivalent Coupling

Understanding the electric dipole switching in multiferroic materials requires deep insight of the atomic-scale local structure evolution to reveal the ferroelectric mechanism, which remains unclear and lacks a solid experimental indicator in high-pressure prepared LiNbO₃-type polar magnets. Here, we report the discovery of Zn-ion splitting in LiNbO₃-type Zn₂FeNbO₆ established by multiple diffraction techniques. The coexistence of a high-temperature paraelectric-like phase in the polar Zn₂FeNbO₆ lattice motivated us to revisit other high-pressure prepared LiNbO₃-type A₂BB'O₆ compounds. The A-site atomic splitting (∼1.0-1.2 Å between the split-atom pair) in B/B'-mixed Zn₂FeTaO₆ and O/N-mixed ZnTaO₂N is verified by both powder X-ray diffraction structural refinements and high angle annular dark field scanning transmission electron microscopy images, but is absent in single-B-site ZnSnO₃. Theoretical calculations are in good agreement with experimental results and suggest that this kind of A-site splitting also exists in the B-site mixed Mn-analogues, Mn₂FeMO₆ (M = Nb, Ta) and anion-mixed MnTaO₂N, where the smaller A-site splitting (∼0.2 Å atomic displacement) is attributed to magnetic interactions and bonding between A and B cations. These findings reveal universal A-site splitting in LiNbO₃-type structures with mixed multivalent B/B', or anionic sites, and the splitting-atomic displacement can be strongly suppressed by magnetic interactions and/or hybridization of valence bands between d electrons of the A- and B-site cations.

Data-driven computational prediction and experimental realization of exotic perovskite-related polar magnets

Rational design of technologically important exotic perovskites is hampered by the insufficient geometrical descriptors and costly and extremely high-pressure synthesis, while the big-data driven compositional identification and precise prediction entangles full understanding of the possible polymorphs and complicated multidimensional calculations of the chemical and thermodynamic parameter space. Here we present a rapid systematic data-mining-driven approach to design exotic perovskites in a high-throughput and discovery speed of the A 2 BB'O6 family as exemplified in A 3TeO6. The magnetoelectric polar magnet Co3TeO6, which is theoretically recognized and experimentally realized at 5 GPa from the six possible polymorphs, undergoes two magnetic transitions at 24 and 58 K and exhibits helical spin structure accompanied by magnetoelastic and magnetoelectric coupling. We expect the applied approach will accelerate the systematic and rapid discovery of new exotic perovskites in a high-throughput manner and can be extended to arbitrary applications in other families.

Cation- and anion-ordered rutile-type derivative LiTeO3(OH)

The first example of both cation- and anion-ordered rutile-type derivative LiTeO₃(OH) (= HLiTeO₄) has been discovered. LiTeO₃(OH) belongs to a non-centrosymmetric space group P2₁.

First-principles investigation of the ferroelectric, piezoelectric and nonlinear optical properties of LiNbO3-type ZnTiO3

The newly synthesized LN-type ZnTiO₃ (J. Am. Chem. Soc. 2014, 136, 2748) contains cations with the electronic configurations nd¹⁰ (Zn²⁺: 3d¹⁰) along with second-order Jahn-Teller (SOJT) nd⁰ (Ti⁴⁺: 3d⁰) cations. This is different from traditional ferroelectrics with the electric configurations of d⁰ transition metal ions or/and lone pair electrons of ns². Using a first-principles approach based on density functional theory, we investigate the electronic structure, zone-center phonon modes, piezoelectric and nonlinear optical properties of the LiNbO₃-type ZnTiO₃. The electronic structure indicates that this compound is a wide direct-band-gap insulator. The results reveal that this compound is a good ferroelectric material with a large spontaneous polarization of 90.43μC/cm². The Raman scattering peaks of A₁ and E modes are assigned to their zone-center optical modes. Additionally, the large piezoelectric and nonlinear optical susceptibilities reveal that LiNbO₃-type ZnTiO₃ is a high-performance lead-free piezoelectric and nonlinear optical crystal.

Growth and properties of spinel structure Zn1.8Co0.2TiO4 single crystals by the optical floating zone method

The spinel structure Zn_1.8Co_0.2TiO₄ single crystals with 5 mm diameter and 30 mm length were successfully grown by an optical floating zone method. The as-grown crystals were characterized by X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS). Some Zn²⁺ ions at tetrahedral and octahedral sites should be replaced by doped transition metal Co²⁺ ions. The temperature-dependent Raman spectra of spinel Zn_1.8Co_0.2TiO₄ crystals were also described. The optical phonon behaviors of Zn_1.8Co_0.2TiO₄ are stable within the temperature range. The magnetic properties of Zn_1.8Co_0.2TiO₄ were investigated by using Physical Property Measurement System.

The spinel structure Zn_1.8Co_0.2TiO₄ single crystals with 5 mm diameter and 30 mm length were successfully grown by an optical floating zone method.

High-Pressure Synthesis, Crystal Structure, Chemical Bonding, and Ferroelectricity of LiNbO3-Type LiSbO3

A polar LiNbO₃ (LN)-type oxide LiSbO₃ was synthesized by a high-temperature heat treatment under a pressure of 7.7 GPa and found to exhibit ferroelectricity. The crystal structural refinement using the data of synchrotron powder X-ray diffraction and neutron diffraction and the electronic structure calculation of LN-type LiSbO₃ suggest a covalent-bonding character between Sb and O. When comparing the distortion of BO₆ in LN-type ABO₃, the distortions of SbO₆ in LiSbO₃ and SnO₆ in ZnSnO₃, which included a B cation with a d¹⁰ electronic configuration, were smaller than those of BO₆ in LN-type oxides having the second-order Jahn-Teller active B cation, e.g., LiNbO₃ and ZnTiO₃. The temperature dependence of the lattice parameters, second harmonic generation, dielectric permittivity, and differential scanning calorimetry made it clear that a second-order ferroelectric-paraelectric phase transition occurs at a Curie temperature of T_c = 605 ± 10 K in LN-type LiSbO₃. Further, first-principles density functional theory calculation suggested that perovskite-type LiSbO₃ is less stable than LN-type LiSbO₃ under even high pressure, and the ambient phase of LiSbO₃ directly transforms to LN-type LiSbO₃ under high pressure. The phase stability of LN-type LiSbO₃ and the polar and ferroelectric properties are rationalized by the covalent bonding of Sb-O and the relatively weak Coulomb repulsion between Li⁺ and Sb⁵⁺.

First-principles calculations and Raman scattering evidence for local symmetry lowering in rhombohedral ilmenite: temperature- and pressure-dependent studies

ATiO₃-type materials may exist in two different crystalline forms: the perovskite and ilmenite. While many papers have devoted their attention to evaluating the structural properties of the perovskite phase, the structural stability of the ilmenite one still remains unsolved. Here, we present our results based on the lattice dynamics and first-principles calculations (density functional theory) of the CdTiO₃ ilmenite phase, which are confronted with experimental data obtained through micro Raman spectroscopy that is a very good tool to probe the local crystal structure. Additional Raman bands, which are not foreseen from group-theory for the ilmenite rhombohedral structure, appeared in both low temperature (under vacuum condition) and high-pressure (at room temperature) spectra. The behavior can be explained by considering the local loss of inversion symmetry operation which reduces the overall space group from [Formula: see text] ([Formula: see text]) to [Formula: see text] ([Formula: see text]). Our results can also be extended to other ilmenite-type compositions.

Removal of Nitrate by Photocatalytic Denitrification Using Nonlinear Optical Material

Removal of nitrate from water has been receiving growing attention in water treatment. In this study, we report the photocatalytic denitrification (PCDN) by nonlinear optical (NLO) material, i.e. lithium niobate (LiNbO₃). The hydrothermally synthesized LiNbO₃ powder could achieve efficient denitrification in water, evidenced by 98.4% nitrate removal and 95.8% nitrogen selectivity at reaction time of 120 min and pH-neutral condition. Based on the first-order kinetics of PCDN, the kinetic constant for LiNbO₃ is almost three times as that of conventional TiO₂ (P25) under the same conditions. As suggested by the hole scavenger experiments, the LiNbO₃ should proceed with photocatalytic reduction of nitrate through direct heterogeneous interaction with electrons at the conduction band of LiNbO₃. This may represent a different mechanism from P25, where nitrate is mainly reduced by CO₂^•- radicals generated by the holes at the valence band. The unique second harmonic generation (SHG) effects of NLO materials enable them to produce more electrons and minimize the electron-hole recombination, which improves the efficiency and stability of the PCDN process. The current study provides a proof-of-concept demonstration of NLO photocatalytic material for more effective nitrate removal in water treatment.

Low-Temperature Cationic Rearrangement in a Bulk Metal Oxide

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

Comparative studies on the room-temperature ferrielectric and ferrimagnetic Ni3TeO6-type A2FeMoO6 compounds (A = Sc, Lu)

First-principles calculations have been carried out to study the structural, electric, and magnetic properties of Ni₃TeO₆-type A₂FeMoO₆ compounds (A = Sc, Lu). Their electric and magnetic properties behave like room-temperature ferrielectric and ferrimagnetic insulators where polarization comes from the un-cancelled antiparallel dipoles of (A(1), Fe³⁺) and (A(2), Mo³⁺) ion groups, and magnetization from un-cancelled antiparallel moments of Fe³⁺ and Mo³⁺ ions. The net polarization increases with A’s ionic radius and is 7.1 and 8.7 μCcm⁻² for Sc₂FeMoO₆ and Lu₂FeMoO₆, respectively. The net magnetic moment is 2 μ_B per formula unit. The magnetic transition temperature is estimated well above room-temperature due to the strong antiferromagnetic superexchange coupling among Fe³⁺ and Mo³⁺ spins. The estimated paraelectric to ferrielectric transition temperature is also well above room-temperature. Moreover, strong magnetoelectric coupling is also anticipated because the magnetic ions are involved both in polarization and magnetization. The fully relaxed Ni₃TeO₆-type A₂FeMoO₆ structures are free from soft-phonon modes and correspond to stable structures. As a result, Ni₃TeO₆-type A₂FeMoO₆ compounds are possible candidates for room-temperature multiferroics with large magnetization and polarization.

Predicting a ferrimagnetic phase of Zn(2)FeOsO(6) with strong magnetoelectric coupling

Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of practical interest for the development of novel memory devices that allow for electrical writing and nondestructive magnetic readout operation. The great challenge is to create room temperature multiferroic materials with strongly coupled ferroelectric and ferromagnetic (or ferrimagnetic) orderings. BiFeO_{3} is the most heavily investigated single-phase multiferroic to date due to the coexistence of its magnetic order and ferroelectric order at room temperature. However, there is no net magnetic moment in the cycloidal (antiferromagneticlike) magnetic state of bulk BiFeO_{3}, which severely limits its realistic applications in electric field controlled memory devices. Here, we predict that LiNbO_{3}-type Zn_{2}FeOsO_{6} is a new multiferroic with properties superior to BiFeO_{3}. First, there are strong ferroelectricity and strong ferrimagnetism at room temperature in Zn_{2}FeOsO_{6}. Second, the easy plane of the spontaneous magnetization can be switched by an external electric field, evidencing the strong magnetoelectric coupling existing in this system. Our results suggest that ferrimagnetic 3d-5d LiNbO_{3}-type material may therefore be used to achieve voltage control of magnetism in future memory devices.

MnTaO2N: polar LiNbO3-type oxynitride with a helical spin order

The synthesis, structure, and magnetic properties of a polar and magnetic oxynitride MnTaO2N are reported. High-pressure synthesis at 6 GPa and 1400 °C allows for the stabilization of a high-density structure containing middle-to-late transition metals. Synchrotron X-ray and neutron diffraction studies revealed that MnTaO2N adopts the LiNbO3-type structure, with a random distribution of O(2-) and N(3-) anions. MnTaO2N with an "orbital-inactive" Mn(2+) ion (d(5); S=5/2) exhibits a nontrivial helical spin order at 25 K with a propagation vector of [0,0,δ] (δ≈0.3), which is different from the conventional G-type order observed in other orbital-inactive perovskite oxides and LiNbO3-type oxides. This result suggests the presence of strong frustration because of the heavily tilted MnO4N2 octahedral network combined with the mixed O(2-)/N(3-) species that results in a distribution of (super)-superexchange interactions.