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Free keywords:
high-temperature in-situ X-ray diffraction; low-temperature in-situ X-ray diffraction; perrhenate sodalite; phase transitions
Abstract:
The temperature-dependent structure-property relationships of the aluminosilicate perrhenate sodalite |Na8(ReO4)2|[AlSiO4]6 (ReO4-SOD) were analysed via powder X-ray diffraction (PXRD), Raman spectroscopy and heat capacity measurements. ReO4-SOD shows two phase transitions in the investigated temperature range (13 K < T < 1480 K). The first one at 218.6(1) K is correlated to the transition of dynamically ordered P4̅ 3n (> 218.6(1 K) to a statically disordered (<218.6(1) K) SOD template in P4̅ 3n. The loss of the dynamics of the template anion during cooling causes an increase of disorder, indicated by an unusual intensity decrease of the 011-reflection and an increase of the Re-O2 bond length with decreasing temperature. Additionally, Raman spectroscopy shows a distortion of the ReO4 anion. Upon heating the thermal expansion of the sodalite cage originated in the tilt-mechanism causes the second phase transition at 442(1) K resulting in a symmetry-increase from P4̅ 3n to Pm3̄͞ n, the structure with the sodalites full framework expansion. Noteworthy is the high decomposition temperature of 1320(10) K.
