Pressure dependence of the impurity-trapped exciton emission in BaF2:Eu and BaxSr1-xF2:Eu

The normal and anomalous emission of Eu2+ in SrF2, BaF2, and BaxSr1-xF2 are studied as a function of x and as a function of pressure for x=0, 0.3, and 1. The anomalous emission converts to the normal emission at pressures of about 40 kbar and 30 kbar, for x=0.3 and x=1, respectively. The anomalous emission becomes redshifted as a function of x. The wavelength dependence of the emission lifetime as a function of x provides evidence for the inhomogeneous nature of the emission. It is suggested that each distinct site, with its distribution of Ba and Sr in the 12 nearest-neighbor cation positions, has its unique emission spectrum and that this becomes redshifted with increasing numbers of Ba. The observed anomalous emission then results from the sum of emission spectra from the distinct sites weighted by their statistical probability. The nature of the emission can be understood with a configurational coordinate model assuming that the localized 4f(6)5d(1) states and the impurity-trapped exciton states are in thermal equilibrium. For pure BaF2, a reversal in the relative energies of the two states occurs at the phases transition. For the mixed crystal, the relative energies depend on the number of Ba neighbors so that some sites exhibit anomalous emission while those with one or no Ba show the normal emission.