Stretchingpotentialandequilibriumlength oftheOHbondinsolids ViktorSzalay a, * ,L aszl  oKov acs a ,ManfredW€ ohlecke b ,EugenLibowitzky c a Crystal Physics Laboratory, Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary b Department of Physics, University of Osnabr€ uck, D-49069 Osnabr€ uck, Germany c Institute for Mineralogy and Crystallography, University of Vienna, Vienna, Austria Received3May2001;infinalform30November2001 Abstract Correlationhasbeenobservedbetweentheexperimentallydeterminedharmonicfrequencies, x,andanharmonicity coefficients, X,ofOHbondstretchingvibrationsinalkalihalide,oxide,andhydratecrystals.Ithasbeenshownthat simplesecond-orderperturbationapproximationformulasof x and X canexplainthe x–X correlation,andtheycanbe used to obtain the shape of the stretching potential and an estimate of the equilibrium length of an OH bond in crystallineenvironment. Ó 2002ElsevierScienceB.V.Allrightsreserved. The presence of an OH bond and the related stretching vibration has been observed by vibra- tional spectroscopy in many crystals of very dif- ferent electronic structure and chemical bonding [1–3]. One may sort these crystals into three groups. In crystals of the first group (I) the OH bondisfoundinimpuritymolecules,e.g.,takethe alkalihalideshavingOH  defects.Oxidecrystals, e.g., simple oxides and perovskites, with the pro- ton as impurity form the second group (II). Crystals containing the OH bond as part of a stoichiometric constituent rather than that of an impurity species, e.g., hydrates and hydroxides, belongtothethirdgroup(III).Althoughstudiesof theOHstretchoritsisotopereplicainsolidsare generally restricted to the fundamental and the lowest overtone transitions, a large collection of dataandsomeempiricalcorrelationsamongthem are available. Correlations are very important, becausetheycanbeusedtopredicttheresultsof experiments,torefineandadvanceourmodelsand understanding, and to draw general conclusions relatedtogroupsofcrystalsratherthantojustone crystal. The correlation between hydrogen-bond distances and stretching frequencies is a widely studied example [4–6]. Other examples are the correlationbetweendeuteronquadrupolecoupling constants and OD stretching frequencies [7], the correlationbetweenOHstretchingfrequenciesand outofplanebendingfrequencies[6],andthefre- quency versus anharmonicity correlation in hy- drates [1]. In a recent survey of oxide crystals an 5March2002 ChemicalPhysicsLetters354(2002)56–61 www.elsevier.com/locate/cplett * Correspondingauthor.Fax:+36-1-392-2223. E-mail address: viktor@power.szfki.kfki.hu(V.Szalay). 0009-2614/02/$-seefrontmatter Ó 2002ElsevierScienceB.V.Allrightsreserved. PII:S0009-2614(02)00099-4