No abstract

No abstract

No abstract

A phenomenological theory of the martensitic fcc-hcp transformation is proposed and applied to the illustrative example of cobalt. The fcc and hcp structures are shown to result from different ordering mechanisms from a disordered polytypic structure and to be intrinsically faulted. The three, fcc, hcp, and disordered polytype, structures are inserted in the framework of the segregation process which leads to the formation of close-packed structures from the melt. The essential features reported for the fcc-hcp transformation in cobalt are explained within the preceding model, namely, the asymmetry of the interphase region, the phonon spectrum, the ␦-shape of its specific heat anomaly, and the existence of an intermediate modulated structure. The property of the transformation enthalpy to be different on heating and cooling is related to the different degree of order of the hcp and fcc structures. The partial dislocation mechanism currently assumed for the transformation is deduced from the secondary shear strains involved at the transformation.

No abstract

No abstract

At variance with structural ferroic phase transitions which give rise to macroscopic tensors coupled to macroscopic fields, criteria defining antiferroelectric (AFE) phase transitions are still under discussion due to the absence of specific symmetry properties characterizing their existence. They are recognized by the proximity of a ferroelectric (FE) phase induced under applied electric field, with a double hysteresis loop relating the induced polarization to the electric field and a typical anomaly of the dielectric permittivity. Here, we show that there exist indeed symmetry criteria defining AFE transitions. They relate the local symmetry of the polar crystallographic sites emerging at an AFE phase transition with the macroscopic symmetry of the AFE phase. The dielectric properties of AFE transitions are deduced from a Landau theoretical model in which ferroelectric and ferrielectric phases are shown to stabilize as the result of specific symmetry-allowed couplings of the AFE order- parameter with the field-induced polarization.Comment: 7 pages, 5 figures, 1 tabl

No abstract