Incommensurate modulation of calcium barium niobate (CBN28 and Ce:CBN28).
Incommensurate modulation of calcium barium niobate (CBN28 and Ce:CBN28).
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铌酸钙钡（CBN28 和 Ce:CBN28）的不相称调节。
DOI：
10.1107/s0108768111054863
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发表时间：
2012
期刊：
影响因子：
--
通讯作者：
M. Mühlberg
中科院分区：
文献类型：
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作者：
H. Graetsch;C. S. Pandey;J. Schreuer;M. Burianek;M. Mühlberg
在3 + 2维超空间群P4bm（aa½，-aa½）的超中心环境X4bm（AA0,-AA0）中，细化了Ca(0.28)Ba(0.72)Nb(2)O(6) （CBN28）和Ce(0.02)Ca(0.25)Ba(0.72)Nb(2)O(6) （Ce:CBN28）的不相关调制晶体结构。这两种化合物均具有四方钨青铜型结构。CBN28的调制包括Ba和Ca原子的波状分布以及未完全占据的Me2位点上具有15倍氧配位的空位。职业调制与原子位移参数的调制和Me2的位置参数的非常弱的调制耦合在一起。由于刚性NbO(6)八面体的协同倾斜，周围的O原子表现出强烈的位移调制，振幅高达约0.2 Å。具有12倍配位的Me1位和Nb原子几乎不受调制的影响。只观测到一阶卫星，调制用一阶谐波描述。在Ce:CBN28中，铈似乎位于Me2和Me1位点上。当每单位钙的分子式中加入0.02个铈时，波长和结构调制只会发生微弱的改变。
The incommensurately modulated crystal structures of Ca(0.28)Ba(0.72)Nb(2)O(6) (CBN28) and Ce(0.02)Ca(0.25)Ba(0.72)Nb(2)O(6) (Ce:CBN28) were refined in the supercentred setting X4bm(AA0,-AA0) of the 3 + 2-dimensional superspace group P4bm(aa½,-aa½). Both compounds are isostructural with a tetragonal tungsten bronze-type structure. The modulation of CBN28 consists of a wavy distribution of Ba and Ca atoms as well as vacancies on the incompletely occupied Me2 site with 15-fold oxygen coordination. The occupational modulation is coupled with a modulation of the atomic displacement parameters and a very weak modulation of the positional parameters of Me2. The surrounding O atoms show strong displacive modulations with amplitudes up to ca 0.2 Å owing to the cooperative tilting of the rigid NbO(6) octahedra. The Me1 site with 12-fold coordination and Nb atoms are hardly affected by the modulations. Only first-order satellites were observed and the modulations are described by first-order harmonics. In Ce:CBN28 cerium appears to be located on both the Me2 and Me1 sites. Wavevectors and structural modulations are only weakly modified upon substitutional incorporation of 0.02 cerium per formula unit of calcium.