Cubic ZnS under pressure: Optical-absorption edge, phase transition, and calculated equation of state

We have measured the effect of pressure on the energy of the direct-optical-absorption edge (E₀ gap, Γ₁₅ᵛ{→}Γ₁ᶜ) of cubic ZnS, covering the full stability range (0--15 GPa) of the tetrahedral phase. The E₀ gap exhibits a sublinear increase under pressure, with the corresponding (linear) gap deformation potential being -5.0(2) eV. X-ray diffraction shows the high-pressure phase of ZnS to have the NaCl-type structure for pressures up to at least 27 GPa. In contrast to earlier reports, this phase is not metallic but shows a broad optical-absorption edge with onset near 2 eV, which is characteristic of an indirect-gap material. Experimental results for the equation of state and band-gap deformation potential are compared with ab initio calculations based on local-density theory and the relativistic linear-muffin-tin-orbital method.