  The PZT material near the morphotropic phase boundary where a monoclinic
phase was observed is studied. A theory of the tetragonal to monoclinic phase
transition was developed within the Landau free energy approach. The phase
transition from the tetragonal to the monoclinic phase is of the first order
near the second order. Dielectric response near the phase transition from the
tetragonal to the monoclinic phase was studied for the crystal and for the
ceramic and polycrystalline material taking into account the diagonal 33 and
the off diagonal 31 components of susceptibility. The later diverges at
temperature of stability boundary for the monoclinic phase, and it is zero in
the tetragonal phase. Such a behavior is not observed in the effective
dielectric static constant for ceramics and polycrystalline materials. We
explain absence of a large contribution of the off diagonal susceptibility by a
grain structure of the material. Above the percolation transition the off
diagonal contribution to the effective susceptibility is not present. Effective
susceptibility which is decreasing in tetragonal phase is in the monoclinic
phase also decreasing, however below phase transition temperature the decrease
is slower. A quantitative theoretical behavior of dielectric susceptibility for
both phases is found. Mechanical response of PZT material to a uniaxial stress
is studied theoreticaly for both phases. Observed temperature behavior of it, a
well around the transition temperature from the tetragonal to monoclinic phase
transition, is qualitatively explained.
