INTERNAL STRAIN AND THE FERROELECTRIC TRANSITION IN FERROELASTIC NH4HSO4 CRYSTALS AS STUDIED BY EPR OF VO2+ IMPURITY PROBES

The ferroelectric phase transition and its relation to the spontaneous strain in ferroelastic NH4HSO4 crystals were investigated using VO2+ ions as an EPR probe. The impurity ions were found to be interstitially trapped at sites surrounded by crystallographically inequivalent NH4+ and SO42- ions. The polar VO2+ axes exhibited temperature-dependent displacements in two distinct directions with different energies. The differential properties of VO2+ ions in NH4HSO4 crystals were used to verify the presence of internal stress in the ferroelastic phases, and the corresponding strain was studied in the range between -120 and + 100°C. The results indicate that the ferroelectric phase transition occurs as a consequence of lattice instability caused by the internal strain. At the second-order structural transition a dipolar lattice emerges in the crystal and the spontaneous polarization appears as a result of internal entropy transfer to the strained lattice. © 1979.