Crystal structure of and low-temperature phase transitions in isostructural RbSm(SO4)2⋅4H2O andNH4Sm(SO4)2⋅4H2O

The crystal structure of RbSm(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O was determined at 296 K using the x-ray-crystallographic-diffraction technique. The unit cell is monoclinic with P$2_1$/c space group. The structure can be envisioned as consisting of the following constituents: a nine-coordinated Sm polyhedron, two crystallographically independent sulfate tetrahedra, an interlayer water, and a caged Rb cation. Both inequivalent sulfates bridge the nine-coordinated Sm polyhedra, forming crisscrossing networks of chains along the [100] and [001] directions, which result in rippled layers in the structure parallel to the (010) plane. It is argued that the monovalent cation Rb is caged by 11 sulfate oxygens and 2 water molecules, and the ${\mathrm{Rb}}^{+}$ ion rattles in a cavitylike geometry. The crystal structure of RbSm(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O has been compared with the published structure of the ${\mathrm{NH}}_4$Sm(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O lattice, and the two structures are isostructural in the conventional sense. However, from our proposed hydrogen-bonding scheme for water molecules and from a comparative analysis of the two structures, it is suggested that there are significant differences in the monovalent cation coordination in the two lattices. Also, the hydrogen-bond strengths, which are crucial to the cohesion of the lattice, show significant differences in these two isostructural lattices. Infrared and thermal dehydration probing further support our contention that there are three distinct types of water in lanthanide double sulfates. From the specific-heat measurements at 100T${\mathrm{SO}}_4$${)}_2$⋅${4\mathrm{H}}_2$O lattice undergoes a single λ transition at 232±1 K, unlike ${\mathrm{NH}}_4$Sm(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O, which has two structural transitions in the above temperature range. The λ anomaly reflects logarithmic dependence near the transition temperature, thus suggesting the transition is order-disorder in origin.