Phase Transition adn Crystal Structure Analysis Using Rietveld Method in the <TEX>$(Na_{0.3}Sr_{0.7})(Ti_{0.7}M_{0.3})O_3 (M=Ta, Nb)$</TEX> System

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J. Korean Ceram. Soc. > Volume 32(5); 1995 > Article

Journal of the Korean Ceramic Society 1995;32(5): 582.

Rietveld 법을 이용한 $(Na_{0.3}Sr_{0.7})(Ti_{0.7}M_{0.3})O_3 (M=Ta, Nb)$ 계에서의 결정구조 해석과 상전이 특성

정훈택, 김호기¹

동신대학교 무기재료공학과
¹한국과학기술원 무기재료공학과

Phase Transition adn Crystal Structure Analysis Using Rietveld Method in the $(Na_{0.3}Sr_{0.7})(Ti_{0.7}M_{0.3})O_3 (M=Ta, Nb)$ System

ABSTRACT

The crystal structures of (Na0.3Sr0.7)(Ti0.7M0.3)O3 (M=Ta, Nb) compounds were determined using the Rietveld method. Due to the tilting of a oxygen octahedron, (Na0.3Sr0.7)(Ti0.7Nb0.3)O3 had a superlattice of doubled a, b and c of simple perovskite. The crystal structure of (Na0.3Sr0.7)(Ti0.7M0.3)O3 was tetragonal with a space group 14/mmm. The crystal structure of (Na0.3Sr0.7)(Ti0.7M0.3)O3 was a cubic with space group Pm3m, in which no tilting of oxygen octahedron was observed. The difference in the oxygen tilting of these two materials was due to the larger covalency of Nb-O bond than that of Ta-O bond, which induced a strong $pi$Nb0 bonding in (Na0.3Sr0.7)(Ti0.7M0.3)O3. Therefore, the higher transition temperature of (Na0.3Sr0.7)(Ti0.7M0.3)O3 could be related to the larger tilting of oxygen octahedron.

Key words: $(Na_{0.3}Sr_{0.7})(Ti_{0.7}M_{0.3})O_3 (M=Ta, Nb)$ system, X-ray rietveld analysis, Oxygen octahedron tilting and transition temperature