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The hyperfine interactions of six different PAC (perturbed angular correlations) probes in the ferromagnetic spinel semiconductor HgCr 2 Se 4 have been investigated. Thereby the site occupation of the probes was determined and opposed to that in other comparable substances. All of the three lattice sites could be tested. Theoretically calculated hyperfine fields (WIEN97) were compared with the experimental values.

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Single crystalline, ferroelectric BaTiO 3 as material with the highest piezoelectric constants among the perovskites with ordered sublattices was implanted with 111 In( 111 Cd). The electric field gradient at the Ti position was measured with perturbed γγ-angular correlation spectroscopy (PAC) while the crystal was exposed to an external electric field. A quadratic dependence could be observed: ν Q (E) = (34.8(1) + 0.16(4) E/kV/mm + 0.080(2) E 2 /kV 2 /mm 2 ) MHz. Point charge model calculations reproduce the linear change of V zz , but not the quadratic term. The polarizability of the host ions of BaTiO 3 is known to be nonlinear with respect to an electric field. The resulting quadratic shift of the electron density is reflected in the strength of the EFG.

Electric field gradients have been measured at substitutional lattice sites in ternary semiconductors using Perturbed γ -γ Angular Correlation spectroscopy (PAC). The experimental results for AIBIIIC2 chalcopyrite structure compounds and • AIIB2IIIC4IV defect chalcopyrites are compared with ab nitio calculations. The latter were carried out with the WIEN code that uses the Full Potential Linearized Augmented Plane Wave method within a density functional theory. The agreement between experiment and theory is in most cases very good. Furthermore, the anion displacements in AgGaX2 -compounds (X: S, Se, Te) have been determined theoretically by determining the minimum of the total energy of the electrons in an elementary cell.