Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum 695 019, India.
Inorg Chem. 2012 Feb 20;51(4):2409-19. doi: 10.1021/ic202383f. Epub 2012 Jan 26.
Quaternary pyrochlore-type solid solutions, CaGdZrNb(1-x)Ta(x)O(7) (x = 0, 0.2, 0.4, 0.6, 0.8, 1), were prepared by a high-temperature ceramic route. The pyrochlore phases of the compounds were confirmed by powder X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy. The crystallographic parameters of the pyrochlore compounds were accurately determined by Rietveld analysis of the powder XRD data. The isovalent substitution of Ta in place of Nb at the B site can reveal the effect of chemical bonding on lattice thermal expansion and oxide ion conductivity because both Nb and Ta have the same ionic radius (0.64 Å). Lattice thermal expansion coefficients of the samples were calculated from high-temperature XRD measurements, and it was found that the thermal expansion coefficient decreases with substitution of Ta. Oxide ion conductivity measured by a two-probe method also shows the same trend with substitution of Ta, and this can be attributed to the high bond strength of the Ta-O bond compared to that of the Nb-O bond. Microstructural characterization using scanning electron microscopy proves that the size of the grains has a small effect on the oxide ion conductivity. Our studies established the role of chemical bonding in deciding the conductivity of pyrochlore oxides and confirmed that the 48f-48f mechanism of oxide ion conduction is dominant in pyrochlore oxides.
四方烧绿石型固溶体 CaGdZrNb(1-x)Ta(x)O(7) (x = 0, 0.2, 0.4, 0.6, 0.8, 1) 是通过高温陶瓷路线制备的。粉末 X 射线衍射 (XRD)、拉曼光谱和透射电子显微镜证实了化合物中的烧绿石相。通过对粉末 XRD 数据的 Rietveld 分析，准确确定了烧绿石化合物的晶体学参数。B 位上 Ta 对 Nb 的同价取代可以揭示化学键对晶格热膨胀和氧离子电导率的影响，因为 Nb 和 Ta 具有相同的离子半径 (0.64 Å)。从高温 XRD 测量计算了样品的晶格热膨胀系数，发现随着 Ta 的取代，热膨胀系数减小。通过两探针法测量的氧离子电导率也表现出相同的趋势，这归因于 Ta-O 键的高键强度与 Nb-O 键相比。使用扫描电子显微镜的微观结构表征证明，晶粒尺寸对氧离子电导率的影响很小。我们的研究确立了化学键在决定烧绿石氧化物电导率方面的作用，并证实了氧离子传导的 48f-48f 机制在烧绿石氧化物中占主导地位。
