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Ferroelastic Pb3(PO)4)2 crystal exhibits two types of domain walls. We constructed the geometrical lattice structure in the vicinity of the domain walls and observed the tilted angle of the W'-walls geometrically. Using the polarization microscope we investigated the temperature dependence of the domain wall and the change of the conoscope image at the phase transition temperature. Near Tc. the narrow domains between two W'-walls were widened and new needle shape W-walls appeared. Above Tc we observed the separation of the Isogyre in the conoscope image by the rotation of the crystal plate, which was inferred due to the infinitesimal lattice mismatch between the cleavage planes above Tc.

Nuclear magnetic resonance (NMR) of 51V in the ferroelastic BiVO4 single crystals has been investigated at a fixed frequency of 6 MHz up to 533 K just above its Curie temperature, Tc = 528 K. The seven resonance lines of 51V(I = 7/2) due to the quadrupole interaction were measured in the three mutually perpendicular crystal planes. While the quadrupole coupling constant (e2qQ/h) is found to be staying nearly the same, whereas the asymmetry parameter (η) is decreased with the increasing temperature. Above Tc, e2qQ/h = 5.031 MHz, and η = 0 were obtained at T = 533 K and the principal Z axis of the electric field gradient tensor is along the crystallographic b axis, same as in the ferroelastic phase. These results are consistent with the phase transition from the monoclinic ferroelastic phase below Tc to the tetragonal paraelastic phase above Tc.

Investigations have been made on the sintering process of PbTiO3 from the mixed PbO-TiO2 system by means of Raman scattering measurements. It has been observed that the Raman spectrum changes from the initial state PbO-TiO2 mixed system to the crystalline state of PbTiO3. The Raman spectrum from the initial state shows a superposed pattern of PbO and TiO2 modes. An almost similar spectral pattern to that from the perfect crystalline powder of PbTiO3 is first obtained in the sample fired around 700°C. As the firing temperature increases, the spectral pattern changes gradually to that of PbTiO3. The behavior of PbO and TiO2 modes in this process is discussed.