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  • 08월 28일 16시 이후 : 초록수정 불가능, 일정확인 및 검색만 가능

Simple and complex perovskite oxynitrides containing Ta+5

2008년 8월 11일 17시 16분 53초
금21B1심 이곳을 클릭하시면 발표코드에 대한 설명을 보실 수 있습니다.
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재료화학 - Multi-Functional Hybrid Materials II
저자 및
영남대학교 화학과, Korea
The syntheses, crystal structures, optical properties, and electrical properties of perovskite oxynitrides, AMO2N (A = Ba, Sr, Ca; M = Ta, Nb), LaTaON2, LaTiO2N, LaMg1/3Ta2/3O2N, LaMg1/2Ta1/2O5/2N1/2, and BaSc0.05Ta0.95O2.1N0.9, are described. The average crystal structure of BaTaO2N is a cubic perovskite, with a Ta–O/N distance of 2.056 Å. SrTaO2N and CaTaO2N are distorted by octahedral tilting, showing noticeably smaller Ta–O/N distances of approximately 2.02 Å. The Mg+2/Ta+5 and Sc+3/Ta+5 pairs have high preference for ordering in the oxides but in oxynitrides the mixed anion environment disturbs the long range order of cations. As determined from the diffuse reflection absorption spectra, the above perovskite oxynitrides have band gap energies of 1.8~2.4 eV. Impedance spectroscopy reveals that BaTaO2N and SrTaO2N have unexpectedly high bulk dielectric constants, k ~ 4900 and 2900, respectively at room temperature. The dielectric constants of both compounds are frequency dependent and show a relatively weak, linear dependence upon temperature with no sign of a phase transition over the temperature range 300-180 K. Thin film BaTaO2N was epitaxially grown on 100 cut SrTiO3 substrate with a conducting buffer layer of SrRuO3 by pulsed-laser-deposition. The dielectric measurements on BaTaO2N thin film roughly reproduced the bulk property. The extended X-ray absorption fine structure (EXAFS) spectroscopy at the Ta L3-edge, neutron pair distribution function (PDF) analysis, and zone axis electron diffraction, indicate that the Ta+5 coordination environment in BaTaO2N is severely distorted far from a regular octahedral picture, thus providing a clue to understand the high dielectric constant observed.