Second-order transitions in IR and NIR spectra of augelites Al2(OH)3(XO4) (X = P, As, V)

Caused by findings in mid-IR spectra of synthetic augelites Al-2(OH)(3)(XO4) (X = P, As, V) DRIFT-NIR spectra of these and of deuterated samples were measured. Based on symmetry considerations some weak mid-IR and the NIR bands are assigned to overlapping quasi-overtones (QOT's) and combinations of delta and gamma modes in the Al2OH and Al3OH groups of the augelite structure, and to QOT's of nu(OH/D) fundamentals and to nu + delta and nu + gamma combinations, respectively; even ternary combinations nu + 2delta and nu + delta + gamma are observed. From the nu(OH/D) fundamentals the anharmonicity parameters omega(e) and x(e)omega(e) were calculated. The variations of the x(e)omega(e) values are linked to the bonding differences in the two independent OH/D groups. (C) 2003 Elsevier Science B.V. All rights reserved.