Intra- and interlayer contributions to the lattice vibrations in MoO3

We report new experimental results for the Raman- and infrared-active modes in the layered compound MoO3. The observed frequencies are explained in terms of a valence-force-field model, including interlayer interactions. Most of the 45 optical modes are dominated by either intralayer or interlayer contributions. However, in spite of the weak interlayer coupling, some of the intermediate-frequency modes are strongly perturbed by the interlayer interactions, which explains the large Davydov splittings observed. Moreover, the two lowest-frequency shear modes cannot be described within the rigid-layer approximation. These facts are a direct consequence of the specific bond geometry in MoO3.