Lattice Dynamics of β-SnWO4: Experimental and Ab Initio Calculations

The vibrational properties of beta-SnWO4 at center of the Brillouin zone are investigated by Raman at low temperature and infrared spectroscopies in connexion with ab initio calculations. In this cubic structure archetype of new materials with ionic conductivity properties, all 29 optical modes predicted by group theory are Raman-active. All lines have been unambiguously identified on the basis of depolarization ratio measurements. The phonon spectrum is calculated at the center of the Brillouin zone by GGA pseudopotentials within the PWSCF/QE suite. A remarkable agreement is obtained with the experimental frequencies, and a fairly good one is also observed with the Raman scattering cross sections calculated using LDA pseudopotentials for the internal modes of WO42- tetrahedra and the lattice modes as well. All Raman lines are assigned for the first time, and the normal coordinates characteristics are described. The whole results give a large credit to the calculation of the phonon spectra in this structural arrangement with such ions. The phonon density of states is deduced, and the characteristics are explained on the basis of the phonon dispersions curves along the [xi 00] and [xi xi 0] high-symmetry directions of the Brillouin zone.