Sr(OH)Br, a metal hydroxide halide with unusual properties

The disorder of the hydroxide ions of the cubic polymorph of strontium hydr oxide bromide Sr(OH)Br(cP16) between the two OH- ion positions I and II has been studied by differential scanning calorimetry (DSC), impedance spectro scopy (electric conductivity), and temperature-dependent Raman spectroscopy (90-630 K). We found that the OH- position I, which is involved in weak tr ifurcated O-H ... Br hydrogen bonds (nu(OH) = 3543 cm(-1) at 298 K), is dep opulated with increasing temperature in favour of OH- position II, where th e OH- ions form stronger linear hydrogen bonds (nu(OH) = 3492 cm(-1)). The O ... Br distance of the latter bond decreases from 10 to 780 K by about 5 pm and, hence, the strength of the respective hydrogen bend increases. This is obviously the reason for the change of the population ratio OH- (II)/OH - (I) with increasing temperature, as revealed from both neutron diffractio n and Raman spectroscopic datal e.g., 0.08 at 10 K and 0.34 at 780 K (neutr on data). For the deuterated compound, these ratios are smaller because in the case of linear hydrogen bonds (OH- II) the strength of D bonds is somew hat greater than that of H bonds and vice versa in the case of trifurcated bonds (OH- I). This is caused by the different zero-point vibrational ampli tudes of the respective librational modes. At elevated temperatures, Sr(OH) Br displays large protonic conductivity a with an activation energy of cond uction of 60.5 kJ mol(-1) (500-670 K), exceeding sigma > 10(-2) Ohm(-1) cm( -1) above 700 K after a precipitous increase of sigma by 2 orders of magnit ude at about 673 K, As shown from the half-widths of the Raman bands the dy namics of local motions of the OH- ions is at least 1 order of magnitude fa ster than the macroscopic diffusion processes, (C) 2000 Academic Press.