Electric quadrupole interaction of 137Ba in barium fluorohalides

The X-ray storage phosphor materials BaFC1 and BaFBr differ greatly in their 137Ba nuclear electric quadrupole interaction, despite a common crystal structure. The quadrupole coupling constant in BaFCl is relatively small and increases rapidly with temperature, in an unusual manner: [CQ[ = 1.7 MHz at T = 296 K, CQ-1(delta CQ/delta T)p = 1.7 x 10(-3) K-1. The coupling BaFBr is [CQ[ = 17.0 MHz at 296 K. The difference in quadrupole coupling at room temperature is explained using an electrostatic point-charge model of the crystals.