Pressure dependence of the Raman spectra of the IV-VI layer compounds GeS and GeSe

The effect of hydrostatic pressure on the Raman-active Ag and B3g modes in the IV-VI orthorhombic layer compounds GeS and GeSe is reported. The pressure coefficients and corresponding mode Gr\"uneisen parameters of the rigid-layer modes are found to be an order of magnitude larger than those of the rocksalttype modes. This behavior is consistent with the weaker interlayer forces of these materials as compared to intralayer forces.