We have investigated the effects of isotopic composition on the band gap of CuCl on a series of samples made out of the stable isotopes 63 Cu, 65 Cu, 35 Cl, and 37 Cl. Besides specimens containing elements with the natural abundances, we have measured samples with monoisotopic sublattices as well as artificial mixtures of isotopes. With nonlinear ͑two-photon absorption, second-harmonic generation͒ and linear ͑luminescence͒ optical spectroscopy we find that the fundamental gap of CuCl increases by 364͑18͒ eV/amu when increasing the Cl mass. However, it decreases by 76͑5͒ eV/amu when increasing the Cu mass. Using a two-oscillator model for the lattice dynamics of CuCl we show that these rates are consistent with the anomalous increase of the band gap with increasing temperature. These effects can be traced back to the strong p-d mixing in the copper halides. From the temperature dependence of the band gap of CuBr we also estimate the changes of its gap with isotopic composition.

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The transverse-optic ͑TO͒ phonons of zinc-blende-type CuCl (␥-CuCl͒ and the longitudinal-optic ͑LO͒ phonons of ␥-CuBr exhibit striking anharmonic self-energy anomalies at the center of the Brillouin zone. We investigate here by Raman spectroscopy the dependence of the LO and TO phonons of ␥-CuI on pressure, temperature, and isotopic mass of copper, in the search for related effects. We find that the pressure dependence of the TO phonon linewidth is qualitatively different at low temperature and close to room temperature. A model is developed to interpret these differences based on anharmonic decay into two and three phonons ͑two different channels͒. In order to make semiquantitative predictions we used the experimental pressure dependence of the TO phonons and that of phonons at the edge of the Brillouin zone that we obtained by linearresponse ab initio local-density approximation calculations. We have also calculated the pressure dependence of the zone-edge phonons of ␥-CuCl and ␥-CuBr and compared it with the few existing experimental results. In this process, interesting anomalies concerning the transverse-acoustic phonons at the zone edge have been found.

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