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Piezoelectric and dielectric measurements were performed on different batches of polarized NaNbO3 ceramics. It has been established that, as a result of polarization, in these ceramics there appears a metastable ferroelectric phase existing for a long time. The presence of this phase manifests itself as a piezoelectric effect accompanied by a piezoresonance dispersion of the dielectric permittivity. The magnitudes and temperature dependencies of the piezoelectric coefficients d33 of the NaNbO3 ceramics were measured for the first time.

Neutron diffraction studies of antiferromagnetic MnO confined to MCM-41 type matrices with channel diameters 24÷87Å demonstrate a continuous magnetic phase transition in contrast to a discontinuous first order transition in the bulk. The character of the magnetic transition transforms with decreasing channel diameter, showing the decreasing critical exponent and transition temperature, however the latter turns out to be above the Néel temperature for the bulk. This enhancement is explained within the framework of Landau theory taking into consideration the ternary interaction of the magnetic and associated structural order parameters.

The phenomenological theory of phase transitions in multiferroic MnWO₄ is suggested. The theoretical model uses the assumption that the magnetic order is driven by the instability in the (1/4;1/2;1/2) point of the Brillouin zone, which is justified by the symmetry of the low-temperature magnetic phase. It is shown that the experimentally observed incommensurate magnetic order is due to the Lifshitz invariants allowed for the corresponding order parameters. Invariants responsible for the magnetoelectric interaction are found and a schematic phase diagram is calculated. The influence of the magnetic field on the phase transition sequence is also analyzed. It is suggested that the description of the phase transitions in MnWO₄ starting from the orthorhombic praphase significantly simplifies the approach and allows us to draw important conclusions.

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