Low-temperature structural phase transitions in ${\mathrm{NH}}_4$L(${\mathrm{SO}}_4$${)}_2$⋅${4\mathrm{H}}_2$O (L=La–Dy): Specific-heat measurements

From the specific-heat (${\mathit{C}}_{\mathit{p}}$) measurements on the isostructural ${\mathrm{NH}}_4$L(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O (L=La, Ce, Pr, Nd, Sm, Eu, Gd, and Tb) Tb) crystals at 100 K <T<300 K, it is shown that these crystals undergo two structural phase transitions, ${\mathit{T}}_1$ and ${\mathit{T}}_2$, in the aforesaid temperature range. The observed shapes of the anomalies in the specific heat and their associated entropies are such that ${\mathrm{NH}}_4$La(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O crystals have a low-temperature phase-transition behavior that is different from that observed in ${\mathrm{NH}}_4$L(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O (L=Ce, Pr, Nd, Sm, Eu, Gd, and Tb). Also, our room-temperature infrared results and unit-cell parameters of ${\mathrm{NH}}_4$Dy(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O crystals indicate that the lattice is isostructural with the rest of the series but no transitions occur at 100 K <T<300 K. Unlike other isostructural lanthanide series, the phase-transition temperatures for ${\mathrm{NH}}_4$L(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O show no straightforward correlation either with the unit-cell parameters or with the host-lanthanide-ion’s ionic radius. Further, the lattice-stability model, which successfully maps the trend of phase-transition temperatures for isostructural lanthanide orthoaluminates or pentaphosphates, is inadequate for predicting the observed behavior of ${\mathrm{NH}}_4$L(${\mathrm{SO}}_4$ ${)}_2$⋅${4\mathrm{H}}_2$O lattices.