No abstract

X-ray diffraction experiments under high pressure have been carried out on BaFCl, BaFBr, and BaFI that belong to the family of layered ionic crystals with the matlockite PbFCl structure. Experiments up to 30 GPa with different pressure transmitting media and different powder proportions were undertaken to determine the anisotropic stress component contribution. The bulk and linear moduli were determined for each compound and structural phase transitions have been observed near 21 and 27 GPa for BaFCl and BaFBr, respectively. The difference in bond strength due to the layered properties of these compounds results in clear differences in compressibilities along the two principal directions, perpendicular and parallel to the C4 axis of the tetragonal matlockite structure. The anisotropic behavior of the unit-cell parameters observed under compression is attributed to the anisotropic coordination of the highly polarizable halogen anion. This phenomenon created a large static dipole at the halogen and involves an energy contribution that seems to stabilize the layered structure.

Experimental results given in Part I and II are interpreted with the help of a band scheme including five energy levels: β (1.91 eV above the VB, due to exciton—neutral acceptor complexes creation with 0.1 eV binding energy), A (0.55 eV above the VB, copper vacancies V Cu−), B (0.76 eV above A, [V Cu−—V O+] vacancy associations), C (0.97 eV above A, [VCu—VCu] vacancy associations) and D (0.38 eV below the CB, due to oxygen vacancies V+O). The strongest absorption bands, as well as the minima in the photoconductivity spectrum are explained by optical transitions from A to B and C levels. The defect density responsible for the level C is about 1017 cm−3. This scheme is consistent with results obtained by other authors on the electrical properties and the luminescence of Cu2O.

The morphotropic phase transition in the Pb(Zr x Ti 1−x )O 3 system has been studied by far-infrared reflectance spectroscopy. The reflectance measurements are performed on compositions with x = 0.0, 0.10, 0.20, 0.30, 0.35, 0.40 and 0.525 at room temperature in the frequency range 40-600 cm −1 . The spectrum is fitted with a model based on the factorized form of the dielectric function. The dependences of the mode frequencies, damping and oscillator strength on x are reported. The static dielectric constant is calculated from the frequencies obtained from the fitting. The observed experimental data show incomplete softening of the lowest E(1TO) mode towards the morphotropic phase boundary. The oscillator strength of the lowest mode carries nearly all the contribution to the lattice part of the dielectric constant.