On the nature of the phase transitions of aluminosilicate perrhenate sodalite

Abstract

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 P 4 ¯ 3 n $P\overline{4}3n$ (> 218.6(1 K) to a statically disordered (<218.6(1) K) SOD template in P 4 ¯ 3 n $P\overline{4}3n$ . 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 P 4 ¯ 3 n $P\overline{4}3n$ to P m 3 ¯ n $Pm\overline{3}n$ , the structure with the sodalites full framework expansion. Noteworthy is the high decomposition temperature of 1320(10) K.