Luminescence decay curves have been measured for the Sm^{3+} ^{4}G_{5/2} → ^{6}H_{7/2} emission in the cubic crystal Cs_{2}NaSm_{x}Eu_{y}Y_{1-x-y}Cl_{6} (x = 0.005 to x = 1, y = 0 to y = 0.99) over the temperature range 10 K to 300 K using pulsed laser excitation into the ^{4}G_{5/2} state of Sm^{3+}. The luminescence from the ^{4}G_{5/2} state of Sm^{3+} is strongly quenched by both, cross relaxation to nearest-neighbour Sm^{3+} ions and energy transfer to the ^{5}D_{0} state of Eu^{3+}. We interpret these processes in terms of a recently developed discrete shell model. The dependence of energy transfer from the Sm^{3+} donor ion to Eu^{3+} acceptor ions on y is readily studied and modelled. The temperature dependence shows that the cross relaxation occurs mainly by electric dipole vibronic-electric dipole vibronic interaction while in the energy-transfer process magnetic dipole allowed electronic contributions are also involved.
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