Evolution of the optical properties of chromium spinels CdCr2O4, HgCr2S

Abstract

We report pressure-dependent reflection and transmission measurements on ZnCr2Se4, HgCr2S4, and CdCr2O4 single crystals at room temperature over a broad spectral range 200-24000 cm −1 . The pressure dependence of the phonon modes and the high-frequency electronic excitations indicates that all three compounds undergo a pressure-induced structural phase transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for CdCr2O4, HgCr2S4, and ZnCr2Se4, respectively. The eigenfrequencies of the electronic transitions are very close to the expected values for chromium crystal-field transitions. In the case of the chalcogenides pressure induces a red shift of the electronic excitation which indicates a strong hybridization of the Cr d-bands with the chalcogenide bands.

Structural transition in the magnetoelectricZnCr2Se4spinel under pressure

“…Our high-pressure Raman measurements revealed the substantial reduction of Raman signal above 12.5 GPa, thus hinting an insulator-metal transition. Our observation is consistent with a recent IR investigation 24 . Furthermore, our ab initio DFT study successfully reproduces this cubic-monoclinic structural transition.…”

“…Our high-pressure X-ray diffraction (XRD) study at room temperature shows that ZnCr 2 Se 4 undergoes a reversible structural transformation from the starting spinel phase towards a monoclinic structure initiating at 17 GPa. Our high-pressure Raman investigation evidences the substantial reduction of Raman intensity above 12.5 GPa, in agreement with the aforementioned IR study 24 . Finally, our ab initio density functional theory (DFT) studies successfully reproduce the cubic-monoclinic structural transition; in addition, the calculations indicate significant magnetic moment and band gap reduction accompanying the structural modification.…”

“…Finally, we would like to address the apparent inconsistency in the transition pressures recorded in our Raman (Fig. 2) and IR studies of Rabia et al 24 (insulatormetal transition in-between 11-14 GPa), and the cubic-monoclinic structural transition evidenced from our XRD investigation close to 17 GPa. It appears that the insulatormetal transition precedes the structural change, thus implying either (a) the pressureinduced metallization of the Fd3 m phase prior to the structural transformation, or (b) the appearance of an intermediate metallic phase prior to the adoption of the highpressure monoclinic structure.…”

“…Given that we have no evidence of structural disorder from our XRD study (Fig 3), we tend to interpret the vanishing of the transition between the Raman and IR investigations can be accounted for by the quasi-hydostatic sample environment of the latter study (CsI powder as PTM) 24 . The pressure-induced transition observed in our Raman study is reversible upon decompression (Fig.…”

Structural Behavior of ZnCr2S4 Spinel under Pressure

“…This electronic modification is evidenced by (a) an overall Raman intensity drop (Figure 7a) [26][27][28][29][30] , (b) mid-infrared reflectance 26,70 , and (c) theoretical predictions 71 . In Figure 6 we have marked the onset pressures for this electronic change P el for all of the relevant Cr-spinels, as evidenced by the overall Raman intensity reduction (we chose this method for determining P el , since it represents a complete data set for these compounds).…”