EFFECT OF PRESSURE AND TEMPERATURE ON 4F-4F LUMINESCENCE PROPERTIES OF SM2+ IONS IN MFCL CRYSTALS (M=BA, SR, AND CA)

We present effects of pressure and temperature on 4f-4f luminescence p roperties of Sm2+ ions in MFCl crystals (M=Ba, Sr, and Ca). Temperatur e experiments indicated that a two-step nonradiative process (D-5(J) - ->4f(5)5d thermal crossover followed by 4f(5)5d-->(5)(DJ'<J) downward feeding) strongly influenced the luminescence intensity distribution f rom the D-5(J) (J= 0,1,2) states of Sm2+. We quantitatively analyzed t he temperature dependence of the intensities and lifetimes of D-5(J) ( J=0,1,2) luminescence using a single configurational coordinate model and luminescence rate equations. Within the context of the model, we o btained the upward thermal crossover rates of the D-5(J)-->4f(5)5d pro cess and the downward feeding fractions of the 4f(5)5d-->D-5(J) proces s. Pressure experiments showed a strong shift of the 4f(5)5d state to lower energy. This shift led to an increase in the degree of 4f-5d ele ctronic admixture, a decrease in the thermal crossover barrier between the D-5(J) levels and the 4f(5)5d state, and decreases in the lifetim es of the D-5(0) and D-5(1) states. We analyzed the change in lifetime of the D-5(0) State with pressure in the framework of the conventiona l Judd-Ofelt second-order electric dipole and extended third-order ele ctric dipole mechanisms and obtained good agreement with the experimen tal result. We found that the third order electric dipole mechanism, w hich becomes activated through 3f-5d spin-orbit coupling, leads to a r elaxation of the second order electric dipole forbiddenness of the D-5 (0)<->F-7(0) transition and to significant contributions to the radiat ive D-5(J)-->F-7(J') transitions of Sm2+. [S0163-1829(98)04742-0].