Crystal structure, magnetism and phase transformation in perovskites A2CrNbO6(A = Ca, Sr, Ba)
Crystal structure, magnetism and phase transformation in perovskites A2CrNbO6(A = Ca, Sr, Ba)
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DOI：
10.1039/ft9969201051
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发表时间：
1996
期刊：
影响因子：
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通讯作者：
J. Choy;Seung‐Tae Hong;Kyoung-Shin Choi
中科院分区：
文献类型：
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作者：
J. Choy;Seung‐Tae Hong;Kyoung-Shin Choi
根据Rietveld精修，Ca_2CrNbO_6和Sr_2CrNbO_6分别为单斜晶系，a= 5.4195（1）A，B= 5.4907（1）A，c= 7.7073（2）A和立方晶系，a= 7.8732（1）A（Fmtext-装饰：overline 3 m，No.225）。在大气压下，Ba 2CrNbO 6结晶为8H六方结构，a= 5.7404（1）A，c= 18.7744（5）A（P63/mmc，No. 196），但在高压（80 kbar）和高温（930 °C）下，它经历结构相变，成为立方钙钛矿，a= 8.0550（4）A（Fmtext装饰：overline 3 m）。磁化率测量表明，立方对称的Ca_2CrNbO_6、Sr_2CrNbO_6和Ba_2CrNbO_6的磁行为符合居里-外斯定律，而六方对称的Ba_2CrNbO_6的磁行为与其它三种材料有很大的不同。基于几种模型的磁性计算表明存在强（铬-铬）共价键，通过面对共享的八面体在六方相。这些化合物的晶体结构和磁性的变化可以根据阳离子的大小和金属与氧离子之间的共价键的性质来合理化。
According to the Rietveld refinement, Ca2CrNbO6 and Sr2CrNbO6 have been found to be monoclinic with a= 5.4195 (1)A, b= 5.4907 (1)A, c= 7.7073 (2)A, and cubic with a= 7.8732 (1)A(Fmtext-decoration:overline3m, No. 225), respectively. Under atmospheric pressure, Ba2CrNbO6 crystallizes into an 8H hexagonal structure with a= 5.7404 (1)A and c= 18.7744 (5)A(P63/mmc, No. 196), but at high pressure (80 kbar) and high temperature (930 °C), it undergoes a structural phase transformation to the cubic perovskite with a= 8.0550 (4)A(Fmtext-decoration:overline3m). Magnetic susceptibility measurements show that Ca2CrNbO6, Sr2CrNbO6 and Ba2CrNbO6 with cubic symmetry follow the Curie–Weiss law, while the magnetic behaviour of Ba2CrNbO6 with hexagonal symmetry is significantly different from the others. Magnetic calculations based upon several models indicate the presence of strong (Cr—Cr) covalent bonding through face sharing of the octahedra in the hexagonal phase. The variation of crystal structures and magnetic properties in these compounds can be rationalized in terms of cationic size and the nature of the covalent bonding between metal and oxygen ions.