The X-ray storage phosphor materials BaFCl 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: ¦ C Q ¦ = 1.7 MHz at T = 296 K, C Q −1( ∂C Q /∂T) p = 1.7 × 10 −3 K −1. The coupling in BaFBr is ¦ C Q ¦= 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.