Temperature dependence of orientationally disordered SO4 tetrahedra in mirabilite (Na2SO4·10H2O)

Mirabilite (Na 2 SO 4 ⋅10H 2 O) is one of the representative phase change materials. Temperature dependent structural behavior of mirabilite was investigated by using single-crystal X-ray diffraction (XRD) technique in the temperature range between 213 and 303 ​K. Thermal expansion of mirabilite was mainly ascribed to the expansion of two Na(H 2 O) 6 octahedra constituting the ribbon structure along the c -axis. Three O atoms out of four apices of SO 4 tetrahedron were split into two partially occupied O sites. The resulting disordered arrangement gave two orientations (major and minor) for the SO 4 tetrahedron. The SO 4 tetrahedron of the major orientation had the smaller volume than that of the minor orientation. The volume of the major SO 4 tetrahedron showed a decreasing trend as increasing temperature. This shrinkage of the major SO 4 tetrahedron would induce oxygen diffusion from the major SO 4 tetrahedron to the minor SO 4 tetrahedron. As a result, the O site occupancies belonging to the minor SO 4 tetrahedron increased from 0.17 to 0.27. Crystal structure of mirabilite consists of infinite ribbons of edge-sharing Na(H 2 O) 6 octahedra alternating with chains composed of SO 4 tetrahedra and two H 2 O molecules to form sheets parallel to the b - c plane. Since the fundamental structure of mirabilite is essentially the same as that of thenardite (Na 2 SO 4 ), phase change from mirabilite to thenardite immediately occurs after dehydration without formation of any intermediate phase. Temperature dependent structural behavior of mirabilite (Na 2 SO 4 ⋅10H 2 O) was investigated using single-crystal X-ray diffraction technique in the temperature range between 213 and 303 K. Thermal expansion of mirabilite was mainly ascribed to the expansion of Na(H 2 O) 6 octahedra constituting the ribbon structure along the c -axis. Three O atoms out of four apices of SO 4 tetrahedron were split into two partially occupied O sites. The resulting disordered arrangement gave two orientations for the SO 4 tetrahedron; minor orientation and major orientation. In the SO 4 tetrahedron belonging to the minor orientation, the O site occupancy factor and coordination volume showed increasing trends with temperature. On the contrary, those for the major SO 4 tetrahedron displayed decreasing trends. This contraction of the major SO 4 tetrahedron induces oxygen diffusion from the major SO 4 tetrahedron to the minor SO 4 tetrahedron. Since the fundamental structure of mirabilite is essentially the same as that of thenardite, phase change from mirabilite to thenardite immediately occurs without formation of any intermediate phase after dehydration. • Temperature dependence of crystal structure of mirabilite (Na 2 SO 4 ·10H 2 O) was measured using in-situ low-temperature single-crystal X-ray diffraction method. • Mirabilite maintains substantially its structure just below the dehydration temperature. • Anisotropic thermal expansion is ascribed to the continuous thermal expansion of Na(H 2 O) 6 octahedra constituting the ribbon structure along the c -axis. • The contraction of the major SO 4 tetrahedron induces oxygen diffusion from the major SO 4 tetrahedron to the minor SO 4 tetrahedron.