Intrinsic Polarity of Crystals: Elementary Models, Heat Capacity and Transfer, Negative Thermal Expansion

In all dielectrics, electric polarization is induced by an external electric field, but polar crystals also exhibit intrinsic polarity, forming non-centrosymmetric polar and polar-neutral structures of pyroelectrics and piezoelectrics. It is postulated that intrinsic polarity in crystal structures arising due to the different electronegativities of neighboring ions. The emergence of intrinsic polarity is facilitated by high ionic charge, large Lorentz factor and high anharmonicity of ions vibrations that are the characteristic of mixed covalent-ionic bonds. Polar crystal structures are characterized by reduced symmetry and the ability to electrically respond to mechanical, thermal, optical, and other nonelectrical stimuli. The paper examines the causes of polarity and antipolarity in crystals, assuming that the dynamics of polar cluster formation corresponds to the concept of configurational entropy. Experimental data and explanations for many manifestations of intrinsic polarity in crystals are presented.