Composition‐dependent behavior of Co(d⁷) optical transitions in Cd_1–xCo_xGa₂S₄ mixed crystals

Abstract

The optical transitions due to the cobalt transition metal impurity centers with composition x in C d1-k Co x Ga 2 S 4 mixed crystals were investigated by optical absorption spectroscopy. The optical absorption spectra showed three crystal-field transitions between the ground state 4 A 2 ( 4 F) and the excited states 4 T 2 ( 4 F), 4 T 1 ( 4 F), and 4 T 1 ( 4 P) of substitutional Co 2+ ions that were similar for all the investigated compositions. The crystal-field parameter (Dq) and the Racah parameter (B) obtained from the observed absorption bands showed a composition dependence such that the Dq parameter increases; on the other hand, the B parameter decreases with increasing composition x, which implies that the Racah parameter (B) depends strongly on the host crystal. Also, the total splitting between the highest and the lowest states of the 4 T 1 ( 4 P) state split due to spin -orbit coupling showed a composition dependence varying from 1576 cm -1 to 1232 cm -1 with increasing x. The spin -orbit coupling parameter λ varied from 394 cm -1 to 308 cm -1 with composition x.

Colloidal Nanocrystals of Wurtzite Zn1−xCoxO (0 ≤ x ≤ 1): Models of Spinodal Decomposition in an Oxide Diluted Magnetic Semiconductor

“…From eq 2, a reduction of B from 760 cm -1 for dilute Zn 1-x Co x O (ref 43) to 736 cm -1 for w-CoO, while increasing Dq from 400 to 403.5 cm -1 , reproduces both the signs and the magnitudes of the experimental shifts very well: ∆E( 4 A 2 f 4 T 1 (P)) ) -300 cm -1 , ∆E( 4 A 2 f 4 T 1 (F)) ) +47 cm -1 ). Comparable changes in Dq and B have been seen in Co 2+ -doped CdTe crystals under hydrostatic pressure, 60 in Cd 1-x Co x Ga 2 S 4 with increasing x, 61 and in Co 2+ -doped Cd 1-x Zn x Se with increasing x. 62 Although the absolute values of Dq and B obtained here for w-CoO are subject to the same uncertainties as those determined for Zn 1-x Co x O (ref 43), the trends are reliable.…”