Raman Spectroscopy of Tin Monoxide under High Pressure

Under ambient conditions, tin monoxide crystallizes in litharge structure which consists of tetragonal \textit{P4/nmm} symmetry. An orthorhombic distortion of this compound at high pressure is assumed to be driven by softening of the unobserved $B_{1g}$ phonon, which results in a spontaneous strain in the \textit{xy} plane of the tetragonal unit cell. In this case, a reduction of the symmetry into \textit{Pm2}$_{1}n$ occurs. The correlation between the tetragonal and orthorhombic symmetries shows a splitting of the degenerated $E_{g}$ phonon into a superposition of $A_{1}$ and $B_{1}$ phonons in the lower symmetry. This splitting was observed in our pressure dependent Raman scattering measurements. The changes in the pressure induced Raman spectrum of tin monoxide can be quantitatively obtained by first order perturbation theory. The frequency of this phonon under stress is obtained by diagonalizing the relevant matrix. This procedure gives a frequency shift and splitting of $E_{g}$ phonon as a function of pressure induced strain. By means of Landau's classical free energy theory this order parameter gives a critical pressure value of 1.03 GPa for the phase transition.