THE INFLUENCE OF A 2ND-ORDER PHASE-TRANSITION ON IMPURITY LUMINESCENCE - A STUDY OF RBCDF3MN-2+

Precise photoluminescence measurements have been carried out on RbCdF3 :Mn2+ in the 9-300 K range to explore the structural phase transition of the host lattice at T(c) = 124 K using the first (M1) and second (M 2) moments of the emission band together with the lifetime r as probes . Aside from showing a sensitivity of M2(T) and r(T) to the phase tran sition, the present data reveal that the curve M1(T) experiences a sma ll but observable change of slope in the vicinity of T(c). It is demon strated that this jump, DELTA(t)(partial derivative M1/partial derivat ive T)p = 0.22 cm-1 K-1, is mainly associated with the increase experi enced by the local thermal expansion coefficient below T(c). The impor tance of both the implicit and explicit contributions to M1 (T) is ana lysed throughout this work. Neither the luminescence nor the excitatio n spectrum shows any evidence of tetragonal distortion in the MnF64- c omplex. This fact is explained through the R(ax) - R(eq) = 0.2 pm valu e (where R(ax) and R(eq) are the axial and equatorial Mn2+-F- distance s) at 30 K, and the coupling coefficient V(E) = 66 cm-1 pm-1 with the E(g) Jahn-Teller mode of MnF64-. Finally, we have found that this crys tal exhibits photochemical reactions under uv light irradiation, leadi ng to the formation of centres that transfer easily to Mn2+ ions. Thes e centres give rise to an intense absorption band at 310 nm, analogous ly to those previously formed under x-irradiation.