Infrared Optical Properties of ( C U G~~P~)~-, , (6 Ge), Solid SolutionsInfrared reflectivity spectra of (CuGe,P,)I -,(6 Ge), mixed crystals with compositions in the range z = 0.07-0.33 are measured at room temperature in the wavenumber range froni 180 to 4000 cni-l. An analysis of the spectra reveals six vibrational modes. The composition dependence of the mode frequencies and of t h e free carrier concentration and mobility is given and compared with infrared optical data for CuGe,P,.