High-pressure phase transition in ZnSe

A high-pressure Mössbauer spectrometer for the 93.3-keV resonance in⁶⁷Zn and an X-ray diffractometer of the Guinier type are used to investigate⁶⁷ZnSe powder at high external pressures. At 13 GPa, a phase transition from the zinc blende to the fcc (NaCl) structure is observed. The transition is accompanied by a 15.2% decrease of the volume of the unit cell. The Lamb-Mössbauer factor (LMF) increases fromf=0.53% at ambient pressure tof=1.07% at 6.1 GPa. It thendecreases tof=0.91% as the pressure is further increased to 8.2 GPa. This behavior is investigated by lattice-dynamical calculations using an 11-parameter rigid-ion model. The decrease of the LMF at high pressures is caused by the softening of the TA phonon modes, which occurs already far below the phase transition. The s-electron density (0) at the⁶⁷Zn nucleus increases with reduced volume. The change of (0) as well as theoretical Hartree-Fock cluster calculations show that covalency of bonding 4s/4p states of Zn and Se plays an essential role and determines the isomer shift.