High-pressure phase transitions in semiconductor mixed crystals ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se (x≤0.29) are investigated using Raman scattering and photoluminescence (PL) up to a pressure of 150 kbar. The samples are characterized for composition and crystal structure. A Raman mode is observed between TO and LO frequencies for samples with x≤0.097 and is assigned to a disorder activated zone boundary phonon. Apart from the PL band at 2.1 eV due to the ${}_{}{}^4{}_{}{}^{}$${\mathit{T}}_1$${\to }^6$${\mathit{A}}_1$ transition between the crystal-field split levels of ${\mathrm{Mn}}^{2+}$, a PL band is observed at 1.94 eV in the samples with x=0.063 which is assigned to a native defect forming a complex with ${\mathrm{Mn}}^{2+}$. The intensity of the PL band reduces drastically with pressure and completely disappears at about 15 kbar. A posible model for this behavior is presented. The pressure dependence of the phonon frequencies and the behavior of PL suggest three transitions ${\mathit{P}}_1$, ${\mathit{P}}_2$, and ${\mathit{P}}_3$ for the mixed crystals with x=0.29, whereas those with x=0.063 and 0.097 show only ${\mathit{P}}_2$ and ${\mathit{P}}_3$ transitions. In the sample with x=0.29, TO mode splits into two components across ${\mathit{P}}_1$ and the low-frequency component exhibits softening. The crystals turn opaque across ${\mathit{P}}_2$ and the PL signal disappears suggesting that the band gap changes from direct to indirect. Intensities of Raman lines decrease and completely disappear across ${\mathit{P}}_3$. The difference in the behavior of the phase transitions for the low and the high values of x is understood on the basis of their different crystal structures.

• Received 18 May 1995

DOI:https://doi.org/10.1103/PhysRevB.52.11052