Vibrational Raman Spectra of CaMoO4 and CaWO4 at High Pressures

Raman vibrational spectra at pressures as high as 40 kbar are reported for the scheelite and previously unknown high‐pressure phases of CaMoO₄ and CaWO₄. The pressure derivatives of the vibrational frequencies, $\mathrm{d\nu \hspace{0.17em}/\hspace{0.17em}dP}$ , are all between 0.0 and 1.0 cm⁻¹·kbar⁻¹ which is normal for ionic crystals; however, the observed pressure dependences of the frequencies of the internal modes are qualitatively different from those predicted according to Scott's interpretation of the splittings of the scheelite spectra. The appearance of the high‐pressure phases is indicated by splittings of $E_g$ modes at 145 and 797 cm⁻¹ in CaMoO₄ and 117 and 797 cm⁻¹ in CaWO₄. The spectra of the high‐pressure phases and the dependences of the apparent transition pressures upon orientation of the crystallographic axes relative to the small stress anisotropy in the high‐pressure cell suggest that the high‐pressure phases have structures similar to that of NiWO₄.