POLARIZED OPTICAL-ABSORPTION SPECTROSCOPY OF THE TL2MNF5-CENTER-DOT-H2O 1D MANGANESE(III) SINGLE-CRYSTAL

The polarized optical absorption spectra of the ID manganese (III) flu oride Tl2MnF5 . H2O are investigated in the 9.5 -300 K temperature ran ge. Throughout the work, special emphasis is placed on the correlation between the different spectroscopic parameters and the structural and magnetic properties of the title compound. Three prominent, strongly polarized broadbands are observed at 11,700, 17,500, and 20,800 cm-1, which are assigned to the spin-allowed crystal field bands within the d4 electronic configuration of the Jahn-Teller elongated MnF63- comple x (D4h). Both the polarization and the temperature dependence of the o scillator strengths indicate that these bands are electric-dipole-assi sted by odd parity vibrations, whereas the spin-forbidden transitions are induced by a pairwise exchange mechanism. A salient feature of the present work is the observation of vibronic progressions to a1g modes of 525, 505, and 375 cm-1 in the low temperature spectra of the 5B1g --> 5A1g and 5B2g bands. The nature of these modes and the experimenta l Huang-Rhys factors are analyzed in terms of the linear electron-phon on coupling coefficients, which are derived by correlating the optical spectra of several fluorides with the local geometry around the Mn(II I). The presence of an exciton magnon peak in the low temperature spec tra is also noteworthy. The temperature dependence of the intensity of this peak as well as of the spin-forbidden transitions provides evide nce of magnetic ordering below T(N) = 28 K and allows us to estimate a n intrachain exchange constant of J = 11 cm-1. (C) 1994 Academic Press , Inc.