It is demonstrated that doping of NaCl with CuCl increases the elastic constants of NaCl and modifies their dependence on temperature. The changes are significantly larger than estimated for a statistical distribution of the doping material. The crystallinity of embedded CuCl is considered to be responsible for additional elastic effects. This conclusion is verified by studying different kinds of annealing effects. The elastic interaction between guest and host crystals is interpreted as a relaxation of the NaCl lattice which closes the spatial gap caused by the lattice misfit. This results in a spontaneous strain which modifies the elastic constants due to the contribution of nonlinear terms.

No abstract

K 2 Ge 8 O 17 shows a structural phase transition near T c ≈ 269 K. The temperature dependence of the optical birefringence n, of the elastic stiffness constants c ij and of the effective electro-optical coefficients r (eff ) is determined. At T c the derivative ∂ n/∂T changes its sign. Elastic properties are studied by recording the acoustic resonances of a rectangular parallelepiped. We discuss those features of the method which are important in the investigation of phase transitions. No signature of elastic softening near T c is observed in K 2 Ge 8 O 17 . Below T c , c 44 and c 66 increase continuously. The elastic hardening is accompanied by the onset of a small electro-optical effect. Its existence demonstrates the loss of the centre of symmetry, its anisotropy is consistent with a triclinic symmetry, and its magnitude suggests a spontaneous polarization of about 6 × 10 −4 C m −2 in the low-temperature phase. Although K 2 Ge 8 O 17 is reminiscent of a weak ferroelectric material there are some observations which are uncommon to a paraelectric-ferroelectric phase transition.