Ferroelastic behavior of NdxLa1−xP5O14 crystals

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Abstract
Ferroelastic behavior has been observed in NdxLa1−xP5O14 single crystals. In this paper experimental emphasis is on the x=0.75 composition. In these crystals, which belong to the monoclinic space group P21/c‐C52h at room temperature, twinning is present and it gives rise to surface deformations. These have been studied by an optical method. The domains have also been studied by observation between crossed polarizers and by x‐ray diffraction techniques. The ferroelastic nature of the twinning is demonstrated by the ability to alter the domain pattern by a small shear stress of suitable orientation. The domain structure vanishes at 141±2 °C. Above this temperature the crystals are most likely orthorhombic with space group Pncm‐D72h (conventional setting Pmna). The characteristic ferroelastic parameter, the spontaneous strain, is a pure shear in the (010) plane, amounting to 4×10−3 at 20 °C. The temperature dependence of this strain indicates a second‐order transition, in agreement with differential thermal analysis results. Besides the NdxLal−x ultraphosphates in the range x=0 to x=1, many other crystals such as the rare earth (La through Tb) and Bi ultraphosphates can be expected to behave in a similar manner.

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