Nonlinear pressure dependence of the direct band gap in adamantine ordered-vacancy compounds

Abstract

A strong nonlinear pressure dependence of the optical absorption edge has been measured in defect chalcopyrites CdGa 2 Se 4 and HgGa 2 Se 4 . The behavior is due to the nonlinear pressure dependence of the direct band-gap energy in these compounds as confirmed by ab initio calculations. Our calculations for CdGa 2 Se 4 , HgGa 2 Se 4 and monoclinic ␤-Ga 2 Se 3 provide evidence that the nonlinear pressure dependence of the direct band-gap energy is a general feature of adamantine ordered-vacancy compounds irrespective of their composition and crystalline structure. The nonlinear behavior is due to a conduction band anticrossing at the ⌫ point of the Brillouin zone caused by the presence of ordered vacancies in the unit cell of these tetrahedrally coordinated compounds.

High-pressure optical and vibrational properties of CdGa2Se4: Order-disorder processes in adamantine compounds

“…11). In our previous work, 16 we suggested that the strong nonlinear pressure dependence of the direct bandgap is a general feature common to all OVCs due to a conduction band anticrossing at the C point of the Brillouin zone caused by two facts: (i) the presence of ordered vacancies in adamantine OVCs and (ii) the doubling of the unit cell along the c axis with respect to the zinc blende structure.…”

“…It can be observed that the edge shifts to higher energies below 8 GPa, and it shifts to lower energies from 8 GPa till 16 GPa. In our previous work, 16 we explained that the optical absorption edge of DCCdGa 2 Se 4 and DC-HgGa 2 Se 4 was of direct character and that the direct bandgap shows a strong non-linear pressure dependence at low pressures. Figure 11 shows the pressure dependence of the experimentally estimated direct bandgap energy (circles) in DC-CdGa 2 Se 4 , as obtained by extrapolating the linear fit of the high-energy part of the (ah ) 2 versus h plot to zero absorption.…”

“…16. Lattice dynamics calculations of phonon modes were performed in the defect chalcopyrite structure at the zone center (C point) of the Brillouin zone.…”

“…CdGa 2 Se 4 is one of the most studied adamantine ternary compounds, and the behavior of DC-CdGa 2 Se 4 under pressure has been subject of recent studies. [11][12][13][14][15][16] In particular, two Raman scattering measurements under pressure 11,12 found a phase transition near 20 GPa to the disordered rock salt (DR) structure [see Fig. 1(c)].…”

“…As regards high-pressure studies in CdGa 2 Se 4 , recent optical absorption measurements in DCCdGa 2 Se 4 and DC-HgGa 2 Se 4 were conducted and provided evidence that all OVCs are expected to have a strong nonlinear pressure dependence of the direct bandgap energy at low pressures. 16 In summary, previous high-pressure Raman scattering studies in DC-CdGa 2 Se 4 were far from being conclusive because (i) there is some controversy regarding the assignment of the symmetries of the Raman-active modes, (ii) there is controversy regarding the presence of the two stages of disorder in this compound at high pressure; and (iii) there is controversy regarding the reversibility of the orderdisorder phase transition leading from the DC to the DR structure taking place near 20 GPa in CdGa 2 Se 4 . Furthermore, the mechanisms of the order-disorder phase transitions are far from being completely understood 11 these stages of disorder are difficult to be evidenced by x-ray diffraction measurements.…”