DETERMINATION OF THE STRUCTURES OF BA(OH)CL, BA(OD)CL AND BA(OD)BR BY SINGLE-CRYSTAL X-RAY-DIFFRACTION AND NEUTRON POWDER DIFFRACTION

The crystal structures of Ba(OD)CI and Ba(OD)Br have been determined by neutron powder diffraction at 295 K and 1.5 K and Rietveld refinement methods {R(I) = 9.3, 8.2 [Ba(OD)Cl], 5.7, and 4.9% [Ba(OD)Br]), that of Ba(OH)Cl by single-crystal X-ray diffraction at 295 K (R = 4.1 % for 1260 reflections). They are isostructural to laurionite Pb(OH)CI {space group Pnma, Z = 4, a = 738.97(3), b = 443.69(3), and c = 913.90(5) pm [Ba(OH)Cl], a = 738.07(4), b = 447.62(2) and c = 897.93(5) pm [Ba(OD)Cl] and a = 759.65(2), b = 438.77(1), and c = 1030.22(2) pm [Ba(OD)Br] at 295 K}. Whereas hydrogen bonds are not present in the bromide, in the case of Ba(OH)CI weak hydrogen bonds to adjacent Cl- ions are formed which are strengthened on going down to lower temperatures due to reorientation of the OH- ions from trifurcated H-bonds to nearly linear ones. The hydrogen bonds of Ba(OD)Cl have been established to be stronger than those of Ba(OH)CI. This becomes apparent by both the smaller cell volume [296.65(4) pm3 instead of 299.6(1) pm3] and the different transition temperature (170 K - 300 K instead of 100 K - 180 K).