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Variation of the optical absorption edge in AgGaS2 single crystals at high pressure

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In this paper the optical absorption edge of AgGaS 2 is measured as a function of pressure up to 26 GPa in order to verify the effect of the three phases transitions occurring in that pressure domain. The direct energy gap increases linearly with pressure at the rate of about 4.0 x 10 -2 eV GPa -1 up to 10.2 GPa. The absence of any discontinuity in the energy gap in the pressure range of 4.2-10.2 GPa confirms that the volume change, in the chalcopirite to monoclinic second-order transition, if it exists, is very small. When the pressure is raised above 10.2 GPa, the energy gap drops suddenly by about 1.1 eV and the spectral form of the absorption coefficient is typical of semiconductors with indirect-gap transitions. This is consistent with the transition to the orthorhombic-type structure and the pressure coefficient of the band gap is -7.5 x 10 -2 eV GPa -1 . In the pressure range 20-26 GPa (NaCl-type phase) the pressure coefficient is about -1.7 x 10 -2 eV GPa -1 . When we released the pressure to atmospheric values, we observe a red shift in the recovered sample of about 1 eV and after annealing to 24 hours at 500 K the band gap does not revert to its original value.