A series of perovskite-type phases of alkaline-earth-based tantalum and niobium oxynitrides has been studied using both first-principles electronic-structure calculations and molecular-dynamics simulations, in particular by investigating different structural arrangements and anion distributions in terms of total-energy calculations. The structural properties are explained on the basis of COHP chemical bonding analyses and semiempirical molecular orbital calculations. We provide theoretical proof for the surprising result that the local site symmetries of these phases are lower than cubic because density-functional calculations clearly show that all crystallographic unit cells are better described as being orthorhombic with space group Pmc2(1) to optimize metal-nitrogen bonding; nonetheless, there is no contradiction with a macroscopic cubic description of the structures of BaTaO(2)N and BaNbO(2)N adopting space group Pm3m. Additionally, we find that the anionic sublattice is ordered in all compounds studied over a wide temperature range.

Recently, materials based on tantalum oxynitrides have become a focus of interest, for example, as nontoxic color pigments [1,2] or as photocatalysts. [3] The archetypal solid-state tantalum oxynitride TaON was first described by Brauer and Weidlein. [4] This polymorph, b-TaON, which is the only confirmed polymorph of TaON, crystallizes isotypically to ZrO 2 in the monoclinic baddeleyite structure type, in which the metal atoms are seven-coordinate. Powder neutron diffraction experiments later revealed that the oxygen and nitrogen atoms of b-TaON have an ordered arrangement.[5] bTaON is typically synthesized by treating the starting material b-Ta 2 O 5 with flowing ammonia gas at 800 8C. The existence of a hexagonal polymorph, a-TaON, which was proposed by Buslaev et al., [6] has been refuted on the basis of quantumchemical calculations. [7] In the Ta-O-N system, we have now prepared a new phase, as a light brown powder, through the ammonolysis of b-Ta 2 O 5 with dry ammonia gas flowing at a rate of 10 L h À1 . A maximum yield of 85 wt % was obtained at a reaction temperature of 850 8C and a reaction time of 5 h. More severe reaction conditions (higher flow rate, higher temperature) resulted in only the known phases b-TaON and Ta 3 N 5 . As the new phase could only be obtained as a powder, the structure determination was performed with powder X-ray diffraction (PXRD) data. Although the optimized reaction conditions yielded the maximum phase fraction of the new TaON polymorph, the product could not isolated as a pure phase: a number of competing phases such as b-TaON, Ta 3 N 5 , and a yet unidentified compound, possibly an oxynitride, were also produced. The large number of side products led to severe difficulties in the structure determination. Therefore, samples produced in reactions with shorter ammonolysis times, which contained a smaller fraction of the new phase, but only b-Ta 2 O 5 as a side phase, were used for the structure determination.Quantitative analysis of the oxygen and nitrogen contents of several samples (by hot-gas extraction), taking into account the phase fractions of the side products (determined by Rietveld refinement), gave values of 8.3 wt % oxygen and 6.0 wt % nitrogen. The resulting composition of TaO 1.1 N 0.9 is in good agreement with the formula TaON, within experimental error.In situ temperature-dependent PXRD experiments in a nitrogen atmosphere provided additional evidence for the stoichiometric composition. As can be seen in Figure 1, the new phase transforms into the known b-TaON phase at high temperature (ca. 900 8C). Simultaneous thermogravimetric (TG) and differential thermal analysis (DTA) revealed that no weight is lost during the phase transition. The phase fraction of b-Ta 2 O 5 is identical before and after the transition. We conclude that the new phase is, indeed, an unknown, metastable polymorph of TaON and propose that it be designated as g-TaON.The reflections of g-TaON were indexed to a C-centered monoclinic cell, and the space group C2/m (no. 12) was chosen. Rietvel...

Magnesium doped tantalum oxynitrides were prepared by ammonolysis of amorphous mixed oxides. An orange colored anatase-type phase with the composition Mg 0.05 Ta 0.95 O 1.15 N 0.85 was found. It is metastable and undergoes a phase transformation to a baddeleyitetype phase between 900 and 1000 1C. X-ray diffraction measurements indicate spacegroup I4 1 /amd with lattice parameters a ¼ 391:986ð6Þ pm and c ¼ 1011:19ð3Þ pm. A possible anion ordering was examined by theoretical methods and neutron diffraction experiments. In addition, anosovite-type (Ti 3 O 5 ) phases Mg x Ta 3Àx O 3x N 5À3x ; 0pxp0.3 were obtained. The electronic spectra of all phases were investigated by UV/vis spectroscopy. r

The geometric and electronic structure of the recently found new polymorph of tantalum oxynitride, gamma-TaON, and its structural stability were studied quantum-chemically at the density functional level. Results obtained by complementary quantum-chemical techniques with wavefunctions either expanded in atom-centered functions or in plane waves were compared, having employed pure density-functional functionals within the generalized gradient approximation as well as density-functional/Hartree-Fock hybrid methods. In particular, several plausible anion distributions were investigated and, in accordance with Pauling's second rule, it was found that the configuration in which nitrogen occupies crystallographic sites with highest coordination numbers is the most stable one. Theoretically generated local structural parameters were used to improve the accuracy of the experimentally derived information. The bonding situation in the most stable configuration was investigated by an analysis of the density of states.