SHG-INVESTIGATIONS OF INCOMMENSURATE A2BX4-CRYSTALS

The complete tensor of nonlinear optical susceptibilities of Rb2ZnC14 including the relative signs of its components has been measured in the ferroelectric commensurate phases between IS K and the lock-in temperature using second harmonic generation (SHG). In contrast to published data the susceptibilities turn out to be not proportional to the spontaneous polarization below 140 K. Due to the change to monoclinic symmetry new coefficients appear in the ferroelectric low-temperature phase, which are two orders of magnitude smaller than those of the orthorhombic phase. The decomposition of the tensor of nonlinear susceptibilities into irreducible parts reveals that the proportionality holds between spontaneous polarization and the vector invariants. The temperature dependence of the septor parts of the tensor resembles that of the coefficients. © 1990, Taylor & Francis Group, LLC. All rights reserved.