First-principles Studies of Phase Stability and the Neutral Atomic Vacancies in LiNbO 3 , NaNbO 3 and KNbO 3
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First-principles Studies of Phase Stability and the Neutral Atomic Vacancies in LiNbO3, NaNbO3 and KNbO3 Akio Shigemi and Takahiro Wada Department of Materials Chemistry, Ryukoku University, Seta, Otsu 520-2194, Japan. ABSTRACT We overall evaluated the enthalpies of formation and the formation energies of neutral vacancies in ANbO3 (A = Li, Na, K) using a plane-wave pseudopotential method within a density functional formalism. The LiNbO3 phase with the LiNbO3-type structure was confirmed to have lower enthalpy of formation than that with perovskite- or ilmenite-type structure. The NaNbO3 (R3c) and KNbO3 (Bmm2 and R3m) phases with the lowest symmetry were found to have the lowest enthalpy of formation. The formation energy of a A vacancy was found to be the lowest under an oxidizing atmosphere and that of an O vacancy was found to be the lowest under a reducing atmosphere. The formation energy of a Nb vacancy was the highest under both oxygen-rich and -poor conditions. These results are in agreement with the empirical rule that B site defects in perovskite-type oxide do not exist. INTRODUCTION Recently, lead-free piezoelectric ceramic materials have come under the spotlight from the viewpoint of environmental protection. ANbO3 (A = Li, Na and K) has been studied as a candidate for a lead-free piezoelectric material that can replace Pb(Zr,Ti)O3 (PZT). It is known that LiNbO3 and KNbO3 show piezoelectricity and NaNbO3 show antiferroelectricity at room temperature. Although LiNbO3 have the significantly distorted Perovskite (P)-type structure called LiNbO3 (LN)-type which is similar to Ilmenite (I)-type structure, LiNbO3 have the I-type structure [1,2] as a metastable phase. On the one hand, NaNbO3 and KNbO3 have normal P-type structure. In the stabilized phases of ANbO3 under atmosphere pressure, LiNbO3 shows the only structural phase transition between a paraelectric phase and a ferroelectric phase at 1210 ºC, but NaNbO3 and KNbO3 show a number of structural phase transitions between a paraelectric cubic phase and a ferroelectric rhombohedral phase [3]. In order to improve the piezoelectric property for ANbO3 ceramics, the solid solution partially replaced on A sites, e.g., (Na0.98Li0.02)NbO3 [4] and (Na0.5K0.5)NbO3 [5], was fabricated. However, it is difficult to fabricate high-density ANbO3 ceramics including the alkali metals on A sites by a conventional sintering process due to the volatility of the alkali metal component at sintering temperatures. Additionally, these ceramics sintered at high temperatures under a reducing atmosphere were found to include point defects. It is clear that these defects influence the dielectric and piezoelectric properties. As a first step in studying point defects in ANbO3, we focused our efforts on neutral vacancies using a supercell technique. In a previous study, we individually evaluated the enthalpies of formation of the various phases and the formation energies of neutral vacancies in LiNbO3 [6], NaNbO3 [7] and KNbO3 [8] using an ab initio calculation. In this r
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