Structural and calorimetric studies of order-disorder in CdMg(CO3)2
Structural and calorimetric studies of order-disorder in CdMg(CO3)2
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CdMg(CO3)2 有序无序的结构和量热研究
DOI：
10.1016/0022-4596(87)90161-7
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发表时间：
1987
期刊：
影响因子：
--
通讯作者：
A. Navrotsky
中科院分区：
文献类型：
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作者：
C. Capobianco;B. Burton;P. Davidson;A. Navrotsky
用粉末X射线衍射和溶液量热法研究了CdMg（CO 3）2在室温附近的无序化。长程有序参数在此范围内从1变为0，无序焓为13.7 ± 0.8kJ/mole。由CdCO 3和MgCO 3生成有序CdMg（CO 3）2的焓为−5.6 ± 0.8 kJ/mole;无序相的焓为+8.1 ± 0.8 kJ/mole。这些数据支持充满活力的模型，假设积极的相互作用，镉和镁的阳离子层和层之间的负相互作用（导致订购）。一个合理的拟合所观察到的相位关系，实现使用点近似（PA）的广义布拉格-威廉姆斯模型或四面体近似（TA）的集团变分法（CVM）。然而，这些模型并没有给出一个定量的拟合的焓和长程序参量随温度的变化。特别是，所观察到的有序-无序转变发生在一个较小的温度范围比预测的更急剧，也许是因为更强烈的合作行为，其中碳酸酯基团以及二价阳离子发挥作用。
The disordering of CdMg(CO3)2was studied near room temperature by powder X-ray diffraction and solution calorimetry using samples quenched from 600 to 850°C. The long-range order parameter changes from unity to zero in this range and the enthalpy of disordering is 13.7 ± 0.8 kJ/mole. The enthalpy of formation of ordered CdMg(CO3)2from CdCO3and MgCO3is −5.6 ± 0.8 kJ/mole; that of the disordered phase is +8.1 ± 0.8 kJ/mole. These data support energetic models which assume positive interactions of Cd and Mg within cation layers and negative interactions (leading to ordering) between layers. A reasonable fit to the observed phase relations is achieved using either the point approximation (PA) of the generalized Bragg-Williams model or the tetrahedron approximation (TA) of the cluster variation method (CVM). These models however, do not give a quantitative fit to the variation of enthalpy and long-range order parameter with temperature. In particular, the observed order-disorder transition occurs more sharply over a smaller temperature range than predicted, perhaps because of more strongly cooperative behavior in which the carbonate groups as well as the divalent cations play a role.