ELECTRIC QUADRUPOLE INTERACTION OF BA-137 IN BARIUM FLUOROHALIDES

The X-ray storage phosphor materials BaFCl and BaFBr differ greatly in their Ba-137 nuclear electric quadrupole interaction, despite a commo n crystal structure. The quadrupole coupling constant in BaFCl is rela tively small and increases rapidly with temperature, in an unusual man ner: \C-Q\ = 1.7 MHz at T = 296 K, C-Q(-1)(partial derivative C-Q/part ial derivative T)(p) = 1.7 x 10(-3) K-1. The coupling in BaFBr is \C-Q \ = 17.0 MHz at 296 K. The difference in quadrupole coupling at room t emperature is explained using an electrostatic point-charge model of t he crystals.