Optical absorption of Co 2+ in MgIn 2 S 4 , CdIn 2 S 4 , and HgIn 2 S 4 spinel crystals
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The optical absorption spectra of cobalt-doped MgIn2S4, CdIn2S4, and HgIn2S4 single crystals grown by a chemical transport reaction were investigated at 7 K. The values of Dq and B calculated from the observed spectra show that the d electrons are more delocalized in the order of MgIn2S4, CdIn2S4, and HgIn2S4. Because this order corresponds to the decreasing order of the electronegativity differences between the metal and sulfur atoms of the host crystals, we conclude that as the metal–sulfur bonds in the host crystals are more covalent, the d electrons of the cobalt impurities are more spread out and the cobalt–sulfur bonds are also more covalent.
I. INTRODUCTION II
III 2
VI 4
Ternary compounds of the A B X type exhibit a broad transparency region, high nonlinear susceptibility, and high photosensitivity. These properties make AIIBIII2XVI4 compounds promising optoelectronic materials. Among these compounds, MgIn2S4, CdIn2S4, and HgIn2S4 are interesting because their structures are different from the defect chalcopyrite structure that the majority of A II B III 2 X VI 4 compounds have. MgIn 2 S 4 , CdIn2S4, and HgIn2S4 have the cubic spinel structure with the space symmetry group Fd3m.1 As in all normal spinels, AII ions occupy eight tetrahedral holes and BIII ions fill 16 octahedral holes in CdIn2S4 and HgIn2S4 unit cells. But the structure of MgIn2S4 is partially inverse spinel in which Mg and In atoms occupy both tetrahedral and octahedral sites.2 Several authors have studied the 3d transition metal impurities in these crystals. The electron paramagneticresonance and the optical absorption spectra of cobaltdoped CdIn2S4 have been studied,3,4 and Fiorani et al. have investigated the electron paramagnetic-resonance powder spectra of Co2+ and Cr3+ in MgIn2S4.5 But in the case of HgIn2S4, only the photoconductivity and the photoluminescence properties of pure crystals have been reported,6,7 and there has been no study on 3d transition metal impurities in this compound. Studies on the cobalt energy-level splitting induced by the localized crystal field are useful tools to understand the crystal structure and the crystal-field strength of the host lattice.8,9 Therefore, the comparison of energy levels of cobalt impurities in these spinel crystals gives important information about the effect of the substituted Mg, Cd, and Hg atoms on the localized crystal field. Such a comparison, however, has not been J. Mater. Res., Vol. 18, No. 3, Mar 2003
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made yet, except for the comparison of the hyperfine constants found for Co2+ in MgIn2S4 and CdIn2S4 by Fiorani et al.5 Therefore, in this work, the optical absorption spectra of cobalt-doped MgIn2S4, CdIn2S4, and HgIn2S4 single crystals measured at 7 K are reported in the near-infrared and visible regions. We calculate the crystal field parameter Dq and the Racah parameter B from the observed spectra and compare these values to investigate the effect of Mg, Cd, and Hg atoms on the localized crystal field. The distortion
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