Lattice Perturbations in CaWO4 and CaMoO4

Frequency assignments for the vibrational modes of species $B_u$ in CaWO₄ and CaMoO₄ are made. $B_u$ modes are both infrared inactive and Raman inactive in unstrained material, but become infrared active and exhibit $E_u$ characteristics under $\mathrm{xz}$ or $\mathrm{yz}$ shear strain and $A_u$ characteristics under $\mathrm{xy}$ shear strain. In the present paper previously published infrared data are interpreted in terms of strain‐induced mode mixing, and the nature of the crystalline strain is discussed. The assignment of the three $B_u$ modes in CaWO₄ completes the task of identifying the 18 internal modes of the WO₄⁼ ions.