Luminescence Study of Na6MnCl8 and Na6MnBr8 Microcrystals in NaCl and NaBr Lattices

Abstract

AbstractA detailed study of the luminescence and excitation spectra as well as of the lifetime (τ) on Na6MnX8 (X = Cl, Br) precipitates in the 300 to 2 K temperature range is made. In spite of the large distance between nearest Mn2+ ions (0.797 nm for Na6MnCl8), it is demonstrated that even at 2 K luminescence takes place at excitation traps after an exchange induced transfer between close Mn2+ ions. At variance to compounds like MnF2, only one trap (with depth of 350 and 480 cm−1 for X = Cl and Br, respectively) together with killers is mainly responsible for the observed temperature dependence of the luminescence. Aside from the inhomogeneous broadening of the emission band, the extrinsic character of the luminescence is underlined by the experimental Stokes shift observed at 10 K. The analysis of the bandwidth and the vibrational progressions on the 4T1g(G) band is perfectly explained in terms of coupling to the a1g mode. The increase of the Huang‐Rhys factor on passing from Na6MnCl8, S(a1g) = 5, to Na6MnBr8, S(a1g) = 8, is reasonably explained through a microscopic model. It is demonstrated that the lifetime‐temperature dependence is mainly governed by the overlap between the absorption and emission bands. This overlap is estimated subtracting the odd‐phonon contribution to the Stokes shift since phonon assistance is not required for excitation transfer through superexchange. The existence of Mn2+ −Mn2+ transfer down to 2 K is related to the perfect cubic symmetry of these compounds, and to a small coupling to the Jahn‐Teller eg mode. The different thermal shifts experienced by the luminescence and the excitation 4T1g(G) band are also discussed.