Optical absorption spectrum of Cr 3+ in uvarovite garnet

Abstract

The absorption spectrum of Cr 3+ in uvarovite garnet has been studied both at room and liquid-air temperatures. The absorption bands of uvarovite garnet have been attributed to a Cr 3+ ion on the basis of the observed features. Of the four bands observed, three have been recorded as sharp lines and one as a broad band. At liquid-air temperature this broad band has been found to split into two. Two of the closely lying sharp lines observed in the red in uvarovite were assigned to the same transition 4 A 2 → 2 E while the third sharp line has been assigned to the 4 A 2 → 2 T 1 transition. The broad band on the violet end has been ascribed to the 4 A 2 → 4 T 2 transition. The two sharp lines assigned to the 4 A 2 → 2 E transition could be successfully interpreted as due to the splitting in the 2E state on account of a trigonal distortion coupled with spin-orbit coupling. The calculated splitting in 2E has been found to be in excellent agreement with the experimental value. Thus the observed bands of the uvarovite garnet have been established for the first time to belong to Cr 3+ in octahedral symmetry with a trigonal distortion. The crystal-field parameters which give a good fit to the observed band positions are B = 718 cm −1, C = 4.5 B, Dq = 1700 cm −1, v = −2044 cm −1 and λ = 59 cm −1.