LAURIONITE-TYPE M(OH)X (M=BA, PB, X=CL,BR,I) AND SR(OH)I - AN IR AND RAMAN-SPECTROSCOPIC STUDY

IR, Raman and single-crystal Raman spectra of laurionite-type Sr(OH)I, Ba(OH)Cl, Ba(OH)Br, Ba(OH)I, Pb(OH)Cl, Pb(OH)Br and Pb(OH)I, and of d euterated specimens and Ba(OH)Cl1-xBrx solid solutions recorded at 90- 520 K are presented and assigned to OH (OD) stretching, librational an d translational modes. The main results obtained are (i) unusually lar ge structural HID isotope effects of Ba(OH)Cl due to Landau-type isost ructural reorientation of OH- ions and (ii) the very different nature and strengths of the hydrogen bonds formed. Thus, the strengths of the partly linear (Ba(OH)Cl), partly multifurcated hydrogen bonds range a s Pb(OH)I (nu(OH): 3497 cm(-1))>Pb(OH)Br>Pb(OH)Cl>Sr(OH)I(3591 cm(-1)) , as shown by the red-shift of the OH stretches and the blue-shift of the OH librations. In the cases of Ba(OH)Br (3605 cm(-1)) and Ba(OH)I (3590cm(-1)), hydrogen bonding. is negligible. The stronger hydrogen b onds formed in the lead compounds are caused by the greater synergetic effect of Pb2+ ions compared to those of Sr2+ and Ba2+.