Raman scattering intensities in alpha-quartz: A first-principles investigation - art. no. 094305

Using a first-principles density functional approach, we calculate the firs t-order Raman intensities of alpha -quartz. The dynamical charge tensors, v ibrational frequencies and eigenmodes, and polarizability tensors are obtai ned within a perturbational approach. We calculate Raman intensities by eva luating the variation of the polarizability tensors for finite displacement s of the atoms. Calculated intensities agree well with experimental data, s howing an average error of 13% for relative intensities. Using our first-pr inciples results as reference, we critically examine simple models for the Raman activity. We first consider a bond polarizability model, for which th e parameters are derived from our first-principles results for alpha -quart z. This model reproduces the first-principles intensities with an average e rror of 15%. In the attempt of reducing this error, we then introduce a mod el in which the symmetry of the first neighbor shell is accounted for in th e most general way. For alpha -quartz, this model extends the bond polariza bility model, which is recovered as a special case. The model, which fully accounts for the local symmetry, describes the first-principles results wit hin an average error of 12%, marginally improving upon the bond polarizabil ity model (15%). However, when these models with parameters derived for alp ha -quartz are applied to a cristobalite polymorph, only the bond polarizab ility model shows good transferability properties. These results support th e use of the bond polarizability model as a simple scheme for calculating R aman intensities in tetrahedrally bonded SiO2 systems.