High-pressure Raman scattering and an anharmonicity study of multiferroic wolframite-type Mn0.97Fe0.03WO4.

A high-pressure Raman scattering study of wolframite-type Mn(0.97)Fe(0.03)WO(4) is presented up to 10.4 GPa. The phonon wavenumbers vary linearly with pressure. The mode Grüneisen parameters are larger for many bending and lattice modes when compared to the stretching modes due to the larger compressibility of Mn(Fe)O(6) octahedra when compared to WO(6) octahedra. Combining the pressure-dependent Raman data of this work with the temperature-dependent Raman data on this crystal previously reported by us has allowed estimation of the temperature-dependent pure lattice and intrinsic anharmonic contributions to the observed total Raman shifts as a function of temperature. It has been found that the observed unusual hardening of the 884, 698 and 674 cm(-1) stretching modes upon heating from 4 to about 150-200 K followed by the usual softening above 150-200 K is a result of a positive intrinsic anharmonic contribution and a negative pure lattice contribution; i.e., up to about 150-200 K the anharmonic contribution surpasses the lattice contribution and the total Raman shift is slightly positive whereas above 150-200 K the lattice contribution becomes dominant and the Raman bands exhibit the usual softening with increasing temperature.