Disorder-induced TA Raman lines in mixed Cu-halide crystals

The Raman spectra of $\mathrm{Cu}{\mathrm{Cl}}_x{\mathrm{Br}}_{1-x}$ and $\mathrm{Cu}{\mathrm{Br}}_x{\mathrm{I}}_{1-x}$ solid solutions are shown to contain extremely intense disorder-induced first-order TA phonons. The intensity of these lines is a function of the concentration $x$, and is highest in Cu${\mathrm{Cl}}_{0.60}$ ${\mathrm{Br}}_{0.40}$. The TA spectrum, well resolved at low temperature, fits well the density of states (DOS) calculated from inelastic-neutron-scattering data. Assuming a disorder model in which ${\mathrm{Cu}}^{+}$ populates the central site or off-center sites, the high-intensity of this line and its temperature and pressure dependence are explained. The Grüneisen parameters of the TA lines were measured in various mixed crystals, and fit well other related thermodynamic parameters. The low-temperature 2TA lines of both pure and mixed crystals fit the calculated two-phonon DOS. The spectral analysis at high temperature is discussed, and it is found that ${\omega }_{2\mathrm{TA}}\mathrm{}\left(T\right)\mathrm{}$ follows nicely $2{\omega }_{\mathrm{TA}}\mathrm{}\left(T\right)\mathrm{}$.