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Disorder-order Transitions In Na_xcoo_2 (x～0.58)

机译：Na_xcoo_2（x〜0.58）的无序转变

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摘要

First-order structural transitions have been discovered for a γ-Na_xCoO_2 (x～0.58) sample at T_1=235 K and T_2=288 K. Based on the combined results of electron and neutron-diffraction experiments, the compound has been revealed to exhibit a 7~(1/2)a_0 × 7~(1/2)a_0-type (a_0 represents the a-axis length of the hexagonal basic structure) ordered superstructure with the space group of P6_3/m, being isostructural with K_4Co_7O_(14), due to the ordering of Na atoms in this temperature range. The Na ordering triggers a cooperative shift of Co-atom trimers parallel to the c axis, leading to an undulated arrangement of the CoO_2 conduction paths. Anomalous change in the magnetic susceptibility, electrical resistivity, and lattice parameters at T_1 and T_2 can be explained by the slight distortion of the CoO_2 layer. Above T_2, in contrast, the compound turns out to possess the γ-Na_xCoO_2-type structure, in which the Nal and Na2 atoms are randomly distributed.
机译：在T_1 = 235 K和T_2 = 288 K时，发现了γ-Na_xCoO_2（x〜0.58）样品的一阶结构转变。基于电子和中子衍射实验的组合结果，发现该化合物具有一个7〜（1/2）a_0×7〜（1/2）a_0型（a_0代表六边形基本结构的a轴长度）的有序上层建筑，其空间群为P6_3 / m，与K_4Co_7O_（ 14），因为在此温度范围内Na原子的排序。 Na顺序触发平行于c轴的Co原子三聚体的协同移动，从而导致CoO_2传导路径的波形排列。 T_1和T_2处磁化率，电阻率和晶格参数的异常变化可以通过CoO_2层的轻微变形来解释。相反，在T_2以上，该化合物具有γ-Na_xCoO_2型结构，其中Nal和Na2原子是随机分布的。

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