Constant-wavelength neutron diffraction study of cubic perovskites BaTaO2N and BaNbO2N

Abstract

Local structural disorder of perovskite oxynitrides BaTaO 2 N and BaNbO 2 N is investigated by constant-wavelength neutron powder diffraction measurements at 300 and 6 K. Temperature-dependent diffraction line-broadening analyses together with the Rietveld refinements confirm that both perovskites retain average structures of simple cubic symmetry down to 6 K. While the Ta and Nb atoms in the above oxynitrides have remarkably large displacement parameters, indicative of pronounced octahedral distortions, the rigidity of Ba sublattice seems to suppress the cooperative propagation of long-range octahedral tilting.©2011 The Ceramic Society of Japan. All rights reserved.Key-words : Oxynitrides, Perovskites, Atomic displacement parameters, Neutron diffraction, Rietveld refinement [Received January 24, 2011; Accepted March 6, 2011] The perovskite-type oxynitrides emerged recently as a promising material class for developing pigments, 1),2) photocatalysts,3)5) dielectrics, 6)8) colossal magnetoresistance, 9),10) etc. The functionalities relevant to above applications stem primarily from the coexistence of nitrogen and oxygen atoms within the anion sublattice. Nitrogen being more polarizable and less electronegative than oxygen, the oxynitride-type compounds generally have narrower band gaps and more covalent metal-anion bonds, compared with the analogous oxides. Such modifications in the electronic structure and the bonding nature have immediate impacts on the range of optical absorption and dielectric behaviors. 6)Besides the above described electronic effects, the mixed anion composition in oxynitrides presents intriguing structural subjects: anion ordering and local structural disorder. These two phenomena occur contrariwise to each other. That is, a complete O/N ordering in oxynitrides corresponds to an absence of local structural disorder, whereas a statistical O/N distribution means a substantial disruption of the lattice periodicity. Particularly the primitive cubic perovskites BaTaO 2 N and BaNbO 2 N are regarded to have purely random O/N arrangements in the anion sublattice, and hence to be typical examples in which the local and average structural descriptions largely deviate. In fact, while the conventional diffraction studies using X-ray 6) and neutron 11) radiations invariably indicated an average structure of Pm 3m symmetry for BaTaO 2 N and BaNbO 2 N, the zone-axis electron diffraction (ED), 12) extended X-ray absorption fine structure (EXAFS), 13) and pair-distribution-function (PDF) 14) analyses for BaTaO 2 N revealed evidences supporting short-range structural distortions around Ta atoms. On the one hand, above inconsistency reflects the sensitivity of the latter local probes to shortrange structural phenomena, but on the other hand, it raises a question: Would there be no local information retrievable from the bulk diffraction analysis of the above oxynitride perovskites? At this point it can be mentioned that the previous neutron powder diffraction (NPD) study of BaTaO 2 N and BaNbO 2 N, conducte...

Electronic Structures of Tetragonal ABX3: Role of the B-X Coulomb Repulsions for Ferroelectricity and Piezoelectricity

“…Due to the change of (Ti 1-3z Nb 3z )-(O 1-z N z ) Coulomb repulsions, the e 33 piezoelectric values are expected to be larger than that in tetragonal BaTiO 3 . Recent experimental paper reported that the most stable structure of BaNbO 2 N is cubic in room temperature [34]. Contrary to the experimental result, however, our calculations suggest that the tetragonal structure will be more stable than the cubic one, as shown in Fig.…”

NH3-Assisted Flux Growth of Cube-like BaTaO2N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity

“…9 Previously, X-ray absorption fine structure, 10 electron diffraction, 11 neutron diffraction, 12 and pair-distribution-function 13 analyses have shown that the real structure of BaTaO 2 N is based on irregularly deformed Ta(O,N) 6 …”

Unravelling the correlated electronic and optical properties of BaTaO2N with perovskite-type structure as a potential candidate for solar energy conversion

“…Moreover, the comparable size and charge between O and N give unusual bonding preference, 7 resulting in better performance of the oxynitrides over the pure metal-oxide or pure-nitride counterparts, making perovskite oxynitrides suitable candidates for solar energy conversion. 8 Barium tantalum oxynitride (BaTaO 2 N) has a perovskite-like structure, [9][10][11][12] with a band gap energy of 1.8 eV, making it a suitable choice as an active material in solar energy conversion devices. For solar water splitting, Higashi et al 13 were able to achieve an IPCE value of 10% (at 1.2 V RHE ) using BaTaO 2 N photoanodes co-loaded with CoO x and RhO x nanoparticles, which is the highest among photoanode materials with a band gap smaller than 2 eV.…”