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) fluoride Tl2MnF5 . H2O are investigated in the 9.5 -300 K temperature range. 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- complex (D4h). Both the polarization and the temperature dependence of the oscillator strengths indicate that these bands are electric-dipole-assisted 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 experimental Huang-Rhys factors are analyzed in terms of the linear electron-phonon coupling coefficients, which are derived by correlating the optical spectra of several fluorides with the local geometry around the Mn(III). The presence of an exciton magnon peak in the low temperature spectra is also noteworthy. The temperature dependence of the intensity of this peak as well as of the spin-forbidden transitions provides evidence of magnetic ordering below T(N) = 28 K and allows us to estimate an intrachain exchange constant of J = 11 cm-1. (C) 1994 Academic Press, Inc.