Ferroelastic behavior and the monoclinic-to-orthorhombic phase transition in $M{\mathrm{P}}_5{\mathrm{O}}_{14}$ ($M=\mathrm{L}\mathrm{a}-\mathrm{T}\mathrm{b}$)

The crystallographic transitions of the pseudo-orthorhombic monoclinic I rare-earth pentaphosphates are examined. The atom positions allow a smooth transition into strict orthorhombic symmetry and a second-order monoclinic-to-orthorhombic phase charge is observed for La${\mathrm{P}}_5$ ${\mathrm{O}}_{14}$, Pr${\mathrm{P}}_5$ ${\mathrm{O}}_{14}$, Nd${\mathrm{P}}_5$ ${\mathrm{O}}_{14}$, Tb${\mathrm{P}}_5$ ${\mathrm{O}}_{14}$, and ${\mathrm{Nd}}_x{\mathrm{La}}_{1-x}{\mathrm{P}}_5{\mathrm{O}}_{14}$. The monoclinic distortion angle $\delta$ ($\delta =\beta -90\mathrm{}°\approx 0.5°$) is determined using a He-Ne laser to measure the divergence between the two reflections obtained from the top surface of a twinned crystal. The transitions are monitored by observing the change in $\delta$ with temperature. The transition temperatures increase linearly with the decrease in the ionic radius of the rare-earth ion from 118°C for La${\mathrm{P}}_5$ ${\mathrm{O}}_{14}$ to 174°C for Tb${\mathrm{P}}_5$ ${\mathrm{O}}_{14}$. The two types of twin boundary possible in the monoclinic I crystals are observed by several microscopic techniques and both are found to be very mobile via ferroelastic reorganization. A shear stress of only 14 ± 3 kN/${\mathrm{m}}^2$ is required to induce motion of the twin boundary perpendicular to [001]. This stress is two orders of magnitude lower than values that have been reported to induce ferroelastic reorganization in other crystals. The twin boundaries can be readily introduced mechanically into, and removed from, the crystals using very small forces.