The new solid solutions have been investigated by different experimental methods in the temperature range 10 K - 300 K. X-ray and calorimetric measurements show that the incommensurate phase characteristic of is also observed for . Raman spectra were obtained for and . Two low-frequency, temperature-dependent bands were observed over the whole temperature range investigated. These results are discussed in connection with structural disorder and the presence of precursor effects of an incommensurate phase transition.