Raman-scattering study of the high-pressure phase transition inZn1−xMnxSe

Optical phonons of the mixed crystal ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Se (0≤x≤0.33) in the zinc-blende phase are investigated by Raman scattering. The optical phonons of ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Mn}}_{\mathrm{x}}$Se exhibit behavior intermediate between the two-mode and one-mode situations. The LO phonon of ZnSe continuously evolves into that of the ‘‘hypothetical’’ zinc-blende MnSe whereas the TO phonon and one component of the impurity mode merge into the gap mode of MnSe. The other component of the impurity mode transforms into the TO phonon of MnSe. Under hydrostatic pressure, in contrast to pure ZnSe, the mixed crystals undergo a high-pressure phase transition between 28 and 52 kbar depending on the composition. A new Raman peak appears in the high-pressure phase, which shows marked softening. When the pressure is reduced, the reverse transition occurs at a significantly lower pressure. At pressures below the reverse transition, the mixed crystal appears to be a mixture of the zinc-blende and another phase whose structure is closely related to that of the high-pressure phase. The energy of the ${\mathrm{Mn}}^{2+}$ internal transition shows a negative pressure coefficient.