HIGH-PRESSURE X-RAY-DIFFRACTION AND NEUTRON-DIFFRACTION STUDIES OF BAF2 - AN EXAMPLE OF A COORDINATION-NUMBER OF 11 IN AX(2) COMPOUNDS
HIGH-PRESSURE X-RAY-DIFFRACTION AND NEUTRON-DIFFRACTION STUDIES OF BAF2 - AN EXAMPLE OF A COORDINATION-NUMBER OF 11 IN AX(2) COMPOUNDS
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DOI：
10.1103/physrevb.52.13247
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发表时间：
1995-11-01
影响因子：
3.7
通讯作者：
HULL, S
中科院分区：
文献类型：
--
作者：
LEGER, JM;HAINES, J;HULL, S
BaF2研究了角色散X射线和中子飞行时间粉末衍射作为压力的函数。具有萤石结构的常压相在约3GPa下转变为正交结构。原子的位置已被细化在原位为这一阶段的中子数据由Rietveld方法。这些精确的位置是在其他钴钨石型结构化合物中观察到的那些位置的典型。在12 GPa以上，发生第二次晶体学转变。的结构是Ni2In型，证实了Rietveld细化的X-射线衍射数据。在这些相变中，钡的配位数从8增加到9，然后增加到11，据我们所知，这是在AX（2）化合物中观察到的最高值。最后一个值表示要在其结构映射中添加新的结构类型。AX（2）化合物在正常条件下的摩尔体积与平均阳离子-阴离子距离的立方成线性关系。这些直线的斜率近似与配位数成反比。这表明填充效率的提高，并定量地解释了相变时压力对体积的影响。
BaF2 was studied by angle-dispersive x-ray and neutron time-of-flight powder diffraction as a function of pressure. The normal pressure phase which has the fluorite structure transforms at about 3 GPa to an orthorhombic structure. The atomic positions have been refined in situ for this phase from the neutron data by the Rietveld method. These refined positions are typical of those observed in other cotunnite-type structured compounds. Above 12 GPa, a second crystallographic transformation occurs. The structure is of the Ni2In type as confirmed by Rietveld refinements of the x-ray diffraction data. At these phase transformations, the coordination number of barium increases from 8 to 9 and then to 11, to our knowledge, the highest value observed in AX(2) compounds. This last value indicates a new structure type is to be added in their structure map. The molar volume of the AX(2) compounds under normal conditions varies linearly with the cube of the mean cation-anion distance. The slopes of these straight lines are approximately inversely proportional to the coordination number. This demonstrates the improving of the packing efficiency and explains quantitatively the effect of pressure on volume at phase transitions.