Temperature dependence of the crystal-optics characteristics of [N(CH3)4]2ZnCl4:Ni2+ crystals with correlated motion of tetrahedral groups

The optical birefringence, optical indicatrix rotation, and residual intensity have been experimentally investigated in the parent and incommensurate phases of [N(CH₃)₄]₂ZnCl₄ crystals doped with Ni²⁺. The temperature dependences obtained are nonlinear in a wide temperature range (T _i − 360 K). It is shown that the nature of this nonlinearity is related to the presence of local spatial regions of the correlated motion of tetrahedral groups. It is established that the deformation of tetrahedral groups increases the temperature range of existence of these regions.