Pressure Dependence of the Refractive Index of Amorphous Lone-Pair Semiconductors

The pressure dependence of the refractive index ($\frac{\mathrm{dn}}{\mathrm{dP}}$) of several amorphous chalcogenide semiconductors is reported. $\frac{\mathrm{dn}}{\mathrm{dP}}$ is positive for materials containing group-VI elements in twofold coordination (lone-pair semiconductors), whereas it is negative for tetrahedral semiconductors. Furthermore, $\frac{\mathrm{dn}}{\mathrm{dP}}$ is the same for the amorphous and crystalline phases of a given semiconductor. Although a one-oscillator (Penn) model adequately predicts $\frac{\mathrm{dn}}{\mathrm{dP}}$ for tetrahedral semiconductors, a similar approach does not predict the positive $\frac{\mathrm{dn}}{\mathrm{dP}}$ of lone-pair semiconductors. Local-field corrections appear to be the cause of the positive $\frac{\mathrm{dn}}{\mathrm{dP}}$. The Lorenz-Lorentz description of the local field predicts $\frac{\mathrm{dn}}{\mathrm{dP}}$ in agreement with experiment for most molecular lone-pair materials. It fails, however, for materials containing large concentrations of group-IV elements. The limitations of the local-field correction in describing $\frac{\mathrm{dn}}{\mathrm{dP}}$ of partially molecular semiconductors is discussed.