2P054
Ab-initio investigations on theoretically predicted perovskite andperovskite-like oxynitrides: novel materials for photocatalysis(University of Tokyo) Kubo Ayako, Giorge Giacomo, Yamashita Koichi[Introduction] Visible light-responsive photo catalysts have recently received wide attention
since their high potentiality as water splitters. Among all the suggested materials, metaloxynitrides MTaO2N (M= Ca, Sr, Ba) in the perovskite polymorph are reported to beextremely performing in H2 evolution [1]. Noticeable theoretical results predicting therelevant performances of some of these perovskite and perovskite-like oxynitride structures
have been already obtained [2], still necessitating of experimental support.
Here, we investigate the structural, electronic, and optical properties of MgTaO2N, atheoretically predicted performing photo catalyst not yet experimentally synthesized. We also
investigate those of CaTaO2N to validate our calculation by comparing the results to theexperimentally known properties.
[Experiment] First-principles calculations were performed within the framework of DFT
using the VASP, applying PAW potentials. Effects of interelectronic exchange and correlation
to the total energies were treated in the GGA-PBE. All results rely on well-converged
structures with respect to the energy-cutoff (500 eV) for the pseudo potential and k point
sampling. In all the calculations, cell parameters was allowed to fully relax.
Due to the pure theoretical nature of the material, at first(a)(b)

we have optimized perovskite structure of MgTiO3 and CaTiO3.
We have also optimized the geikielite polymorph of MgTiO3,
which is the most stable structure of MgTiO3. Next, we havesubstituted Ti with Ta and O with N and finally compared
the properties of the two so obtained structures of MgTaO2N.

Figure 1 the structures ofMgTiO3(a) Perovskite (b) Geikielite[Result and Discussion]
I. Favored anion orderings in the perovskite polymorph of MTaO2N (M = Mg, Ca)We first investigated the stability of trans-MTaO2N (Fig.2 (a)) and cis-MTaO2N (Figure 2).
Four spatial configurations of cis-TaO4N2 octahedra sharing a common axial N are possible:
namely, the “0-deg”, “90-deg”, “180-deg”, and “270-deg” configurations (Fig.2 (b) ~ (e))

Table 1 shows the energy of MTaO2N in five structural models. The cis-type structures arestable compared to the trans-type ones by at least 0.75 eV / FU in CaTaO2N and 0.27 eV / FUin MgTaO2N. On the other hand, the energy differences among four configurations of cis-typestructures are within 0.01 eV / FU in Table 1 The stabilities of perovskite structures [eV / FU]
CaTaO2N and 0.06 eV / FU intransMgTaO2N. From these results, we canguess

thatthereexists

Mvarying

cis-type structures in the MTaO2Ncrystals.
(a)

(b)

cis0-deg

90-deg180-deg

270-deg

Ca-43.88-44.12-44.13-44.12-44.13Mg-42.06-42.27-42.33-42.32-42.27

(c)

(e)

(d)

Figure 2 structures of perovskite MgTaO2N. (a) trans (b) ”0-deg” (c) “90-deg” (d) “180-deg”(e) “270-deg” [Orange: Mg atoms; red: O; white: N; Ta atoms @ the octahedral center]

II. Favored anion orderings in the geikielite structure of MgTaO2N
Second, we investigated the stability of three spatial configurations of geikielite structuresof MgTaO2N, namely “trans”, “parallel”, and “cis” (Fig.3 (a) ~ (c)). In “trans” structure, there
are some N anions which coordinate a Ta ion in trans configuration, while all N anions
coordinate in cis configuration in “cis” and “parallel” structures. N anions are put on parallel
planes in “parallel”, while two N anions

(a)

(b)

(c)

line up vertically to the planes in “cis”. The
most stable one is cis-type consistently
with the case of perovskite MTaO2N.
Table 2 The stabilities ofgeikielite structures [eV / FU]Mtransparallelcis

Mg-42.53-42.53-42.60Figure 3 structures of geikielite MgTaO2N(a) trans (b) parallel (c) cis [same notation as Fig.2]

We will investigate the electronic and optical properties such as band structures using
these optimized structures. The results will be shown on the session.
[1] D. Yamasita et al., Solid State Ionics, 2004, 172, 591[2] I. E. Castelli et al., Energy and Environmental Science, 2012, 5, 5814