Ferroelectric BaTaO2N Crystals Grown in a BaCN2 Flux

Perovskite BaTaO2 N: From Materials Synthesis to Solar Water Splitting

Barium tantalum oxynitride (BaTaO2 N), as a member of an emerging class of perovskite oxynitrides, is regarded as a promising inorganic material for solar water splitting because of its small band gap, visible light absorption, and suitable band edge potentials for overall water splitting in the absence of an external bias. However, BaTaO2 N still exhibits poor water-splitting performance that is susceptible to its synthetic history, surface states, recombination process, and instability. This review provides a comprehensive summary of previous progress, current advances, existing challenges, and future perspectives of BaTaO2 N for solar water splitting. A particular emphasis is given to highlighting the principles of photoelectrochemical (PEC) water splitting, classic and emerging photocatalysts for oxygen evolution reactions, and the crystal and electronic structures, dielectric, ferroelectric, and piezoelectric properties, synthesis routes, and thin-film fabrication of BaTaO2 N. Various strategies to achieve enhanced water-splitting performance of BaTaO2 N, such as reducing the surface and bulk defect density, engineering the crystal facets, tailoring the particle morphology, size, and porosity, cation doping, creating the solid solutions, forming the heterostructures and heterojunctions, designing the photoelectrochemical cells, and loading suitable cocatalysts are discussed. Also, the avenues for further investigation and the prospects of using BaTaO2 N in solar water splitting are presented.

Growth of submillimeter SrTaO2N single crystals by an NH3-assisted SrCl2 flux method

Perovskite-type oxynitrides have recently been highlighted due to their dielectric and photocatalytic properties. Numerous studies have addressed the synthesis and characterization of their nanocrystals and ceramics. However, few research works have considered single-crystal formation in such systems due to difficulties in melt growth. In this study, we explore the crystal growth of perovskite-type oxynitride SrTaO2N by an NH3-assisted SrCl2 flux method. Submillimeter-sized single crystals with lengths of approximately 300 μm were grown at a temperature of 1200 °C for 10 h with a solute concentration of 1.5 mol%. Subsequently, the crystal growth mechanism of SrTaO2N in an SrCl2 flux was studied systematically through experiments with variable holding temperature, holding time, cooling rate, and solute concentration. Our results suggest that SrTaO2N crystal growth is induced by the evaporation of SrCl2 flux.

Highly Electron-Doped TaON Single-Crystal Growth by a High-Pressure Flux Method

Transition-metal oxynitrides have a variety of functions such as visible light-responsive catalysts and dielectric materials, but acquiring single crystals necessary to understand inherent properties is difficult and is limited to relatively small sizes (<10 μm) because they easily decompose at high temperatures. Here, we have succeeded in growing platelet single crystals of TaON with a typical size of 50 × 100 × 10 μm³ under a high pressure and high temperature (6 GPa and 1400 °C) using a LiCl flux. Such a harsh condition, in contrast to powder samples synthesized under mild conditions, resulted in the introduction of a large amount of oxygen vacancies (x = 0.06 in TaO_1-xN) into the crystal, providing a metallic behavior with a large anisotropy of ρ_c/ρ_ab ∼ 10³. Low-temperature oxygen annealing allows for a single-crystal-to-single-crystal transformation to obtain fully oxidized TaON (yellow) crystals. Needle-like crystals can be obtained when NH₄Cl is used as a flux. Furthermore, black Hf₂ON₂ single crystals are also grown, suggesting that the high-pressure flux method is widely applicable to other transition-metal oxynitrides, with extensive carrier control.

LaTiO2N nanopowders (NPs) with low surface defect density via nitridation of flame made NPs retaining simple perovskite structure

This work introduces a novel route to perovskite LaTiO₂N nanopowders (NPs) via nitridation of perovskite LaTiO₃ NPs in an NH₃ gas flow at 1050 °C/NH₃/15 h, in which a simple perovskite structure (ABX₃) is retained during nitridation. The LaTiO₃ NP is formed with a trace of a second phase in a precursor oxide NP (LTO-4/3) produced using liquid-feed flame spray pyrolysis (LF-FSP) of a metallo-organic ethanol solution with La/Ti = 4/3. The characterization of the resulting powders allows for a comparison with LaTiO₂N NPs synthesized by the nitridation of the La₂Ti₂O₇ precursor oxide NP (LTO-1) with a perovskite slab structure (A₂B₂X₇) also prepared by LF-FSP of a La/Ti = 1 solution. Williamson-Hall plots suggest that the as-produced LaTiO₂N from LTO-1 offers a quite small but effective crystallite size of 16-18 nm with almost no lattice spacing fluctuations, while LaTiO₂N from LTO-4/3 presents a larger effective crystallite size of 50-52 nm with some lattice spacing fluctuations. UV-vis diffuse reflectance analysis reveals that, unlike LaTiO₂N from LTO-1, the spectra of LaTiO₂N from LTO-4/3 show a quite low absorption background above the wavelength of the optical absorption edge (∼580 nm), suggesting good crystallinity with a very low surface defect density. Both oxynitride NPs appear to offer utility as inorganic pigments with different colours, while LaTiO₂N NPs from LTO-4/3 have advantages for various applications, including potential as a visible-light-driven water splitting photocatalyst.

Solving the puzzle of the dielectric nature of tantalum oxynitride perovskites

The dielectric properties of tantalum oxynitride perovskites are of interest because of the unique optical characteristics of these compounds based on their visible light absorption. Unfortunately, such perovskites are thermally metastable, which makes it challenging to prepare high-quality bulk samples for the study of dielectric properties. Recently, studies of small single crystals of BaTaO2N showed the compound to be ferroelectric, and this can be well explained based on the non-centrosymmetric crystal structure proposed previously using first-principles and molecular dynamic investigations.

High-Temperature Synthesis and Dielectric Properties of LaTaON2

The development of new synthetic methodologies of perovskite oxynitrides is challenging but necessary for the search of new compounds and the investigation of new properties. Here, we report a new method of preparation of the perovskite LaTaON₂ that has been investigated as a pigment and photocatalyst for water splitting. The synthesis proceeds through the solid-state reactions under N₂ at 1500 °C between La₂O₃, LaN, and Ta₃N₅ or between LaN and TaON, which are completed after 3 h and lead to sintered, highly crystalline samples with particle sizes up to 1 μm. Nitrogen-deficient samples LaTaO_1+xN_2-x with x ≤ 0.35 are prepared by changing the N/O ratio in the mixture of reactants. Electron diffraction, synchrotron diffraction, and neutron diffraction studies on stoichiometric and nitrogen-deficient compounds indicate that they crystallize in the monoclinic space group I2/m with lattice parameters for LaTaON₂ of a = 5.71458(7), b = 8.05987(10), c = 5.74772(6) Å, and β = 89.982(3)°. The three anion sites of the I2/m structure are partially occupied by oxygen and nitrogen, with a preference of nitride for two positions with occupancies of 77 and 88%. This anion distribution is different from that reported in previous studies of samples prepared by ammonolysis at lower temperature, suggesting that the synthesis conditions affect the anion order of this perovskite. Optical measurements indicate a band gap of about 1.9 eV, which is close to that observed in samples prepared by other methods. The determined dielectric permittivity for LaTaON₂ ε_r ≈ 200, reported for the first time for a highly nitrided pseudocubic perovskite, is similar to that observed in perovskites with one nitrogen per formula.

Unveiling the mechanisms behind the ferroelectric response in the Sr(Nb,Ta)O2N oxynitrides

Oxynitride perovskites of the type ABO₂N have attracted considerable attention thanks to their potential ferroelectric behavior and tunable bandgap energy, making them ideal candidates for photocatalysis processes. Therefore, in order to shed light on the origin of their ferroelectric response, here we report a complete analysis of the structural and vibrational properties of SrNbO₂N and SrTaO₂N oxynitrides. By employing first-principles calculations, we analyzed the symmetry in-equivalent structures considering the experimentally reported parent I4/mcm space group (with a phase a⁰a⁰c^- in Glazer's notation). Based on the I4/mcm reference within the 20-atoms unit-cell, we found and studied the ensemble of structures where different octahedral anionic orderings are allowed by symmetry. Thus, by exploring the vibrational landscape of the cis- and trans-type configuration structures and supported by the ionic eigendisplacements and the Born effective charges, we explained the mechanism responsible for the appearance of stable ferroelectric phases in both anionic orderings. The latter goes from covalent-driven in the trans-type ordering to the geometrically-driven in the cis-type configuration. Finally, we found in both cases that the biaxial xy epitaxial strain considerably enhances such ferroelectric response.

Low temperature synthesis of barium oxynitridosilicates using BaCN2 and SiO2

Barium oxynitridosilicates, Ba3Si6O12N2 and Ba3Si6O9N4, were obtained from a mixture of BaCN2 and SiO2 at 800 °C, which is several hundred degrees lower than the temperature required in solid state reactions using BaCO3, SiO2 and Si3N4. The low-temperature formation mechanism was investigated by thermogravimetry analysis in conjunction with gas chromatography and mass spectroscopy. The phase ratio between the oxynitridosilicates was controlled by tuning the reaction temperature, duration, and atmosphere. Almost single-phase Ba3Si6O12N2 was obtained by reaction at 800 °C for 15 h under a N2 atmosphere, but the product changed to Ba3Si6O9N4 after 50 h at 800 °C or by heating at 950 °C for 15 h. The photoluminescence properties of Eu-doped products obtained at 800 °C using a mixture of BaCN2 : Eu and SiO2 were investigated.

Environmentally Benign Synthesis and Color Tuning of Strontium-Tantalum Perovskite Oxynitride and Its Solid Solutions

A facile method was successfully developed to prepare strontium-tantalum perovskite oxynitride, SrTaO₂N, and its solid solutions. Urea was employed as a solid nitriding agent to eliminate the use of toxic NH₃ gas. In addition, utilization of sol-gel-derived Ta₂O₅ gel as a Ta precursor allowed for completion of nitridation within a shorter period and at a lower calcination temperature compared with the conventional ammonolysis process. Optimization of the reaction conditions, such as the urea content, allowed for the production of solid solutions of SrTaO₂N and Sr_1.4Ta_0.6O_2.9. The products exhibited optical absorption and chromatic colors because of the narrower band gaps of oxynitrides compared with those of oxides. The O/N ratios of the solid solutions were easily adjusted by varying the amount of urea in the mixture of precursors. As a result, the colors of the products ranged from yellow to brown. The nitridation process and products developed in this study are interesting environmentally benign alternatives to conventional inorganic pigments.

Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON2

Non-centrosymmetric polar compounds have important technological properties. Reported perovskite oxynitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. Reported here is the first hexagonal perovskite oxynitride BaWON₂ , which shows a polar 6H polytype. Synchrotron X-ray and neutron powder diffraction, and annular bright-field in scanning transmission electron microscopy indicate that it crystalizes in the non-centrosymmetric space group P6₃ mc, with a total order of nitride and oxide at two distinct coordination environments in cubic and hexagonal packed BaX₃ layers. A synergetic second-order Jahn-Teller effect, supported by first principle calculations, anion order, and electrostatic repulsions between W⁶⁺ cations, induce large distortions at two inequivalent face-sharing octahedra that lead to long-range ordered dipoles and spontaneous polarization along the c axis. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity.

High-Quality Heteroepitaxial Growth of Thin Films of the Perovskite Oxynitride CaTaO2N: Importance of Interfacial Symmetry Matching between Films and Substrates

Perovskite oxynitrides have been studied with regard to their visible light-driven photocatalytic activity and novel electronic functionalities. The assessment of the intrinsic physical and/or electrochemical properties of oxynitrides requires the epitaxial growth of single-crystalline films. However, the heteroepitaxy of perovskite oxynitrides has not yet matured compared to the progress realized in work with perovskite oxides. Herein, we report the heteroepitaxial growth of CaTaO₂N thin films with (100)_pc, (110)_pc, and (111)_pc crystallographic surface orientations (where the subscript pc denotes a pseudocubic cell) on SrTiO₃ substrates using reactive radio frequency magnetron sputtering, along with investigations of crystallinity and surface morphology. Irrespective of surface orientation, stoichiometric CaTaO₂N epitaxial thin films were grown coherently on SrTiO₃ substrates and showed clear step and terrace surfaces in the case of low values of film thickness of approximately 20 nm. A (110)_pc-oriented film was also more highly crystalline than (100)_pc- and (111)_pc-oriented specimens. This relationship between crystallinity and surface orientation is ascribed to the number of inequivalent in-plane rotational domains, which stems from the symmetry mismatch between the orthorhombic CaTaO₂N and cubic SrTiO₃. A CaTaO₂N thin film grown on a lattice- and symmetry-matched orthorhombic DyScO₃ substrate exhibited a significant crystallinity and a clear step and terrace surface even though the film was thick (∼190 nm). These results are expected to assist in developing the heteroepitaxial growth of high-quality perovskite oxynitride thin films.