Temperature dependence of spontaneous strain in ferroelastic thallous nitrate

TlNO₃ is a ferroelastic crystal belonging to the orthorhombic Aizu species m3mFmmm(ss) at room temperature. Lattice-parameter measurements by X-ray powder diffraction at various temperatures are reported for this crystal. Spontaneous-strain components are calculated from the lattice parameters. The spontaneous strain has a large magnitude of 7.80 x 10^-2 at room temperature. Its two components are found to be rather insensitive to change of temperature. This may be due in part to the improper nature of this ferroelastic. Anomalies indicating either new isomorphous phase transitions or crossover behaviour are observed at 309, 329 and 399 K. Between 352 and 416 K the crystal belongs to the rhombohedral Aizu species m3mF3. The critical exponent of the spontaneous strain for the rhombohedral phase is found to change from 0.72 (3) to 0.60 (5) at 399 K.