Vibrational Spectroscopy 71 (2014) 37–40 Contents lists available at ScienceDirect Vibrational Spectroscopy jou r n al hom ep age: www.elsevier.com/locate/vibspec Anomalous temperature dependence of the OH - absorption band in stoichiometric LiNbO 3 crystals K. Lengyel, I. Kiss, L. Kovács ∗ , V. Szalay, G. Corradi Wigner Research Centre for Physics, Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Konkoly Thege M. út 29-33, H-1121 Budapest, Hungary a r t i c l e i n f o Article history: Received 18 October 2013 Received in revised form 18 December 2013 Accepted 9 January 2014 Available online 22 January 2014 Keywords: Hydroxyl ion Stretching vibration LiNbO3 a b s t r a c t Infrared absorption spectra of the stretching vibration of OH - and OD - ions in stoichiometric LiNbO 3 crys- tal have been measured in the temperature range 10–310 K. The band parameters, halfwidth and position, have been determined with high accuracy by assuming quasi-Voigt line shapes. Anomalous behaviour of the OH - band position has been observed and interpreted by phonons coupled to the stretching vibration with coupling constants of alternate signs. © 2014 Elsevier B.V. All rights reserved. 1. Introduction From time to time interesting papers appear on the anomalous temperature dependence of various physical properties of LiNbO 3 crystals. A number of them present anomalous behaviour above room temperature, mainly between 50 and 120 ◦ C (see e.g. [1] and references therein). Glass and Lines [2] measured the spon- taneous polarization of LiNbO 3 crystals of different compositions at low temperatures and observed a maximum at about 30 K in the ratio of the pyroelectric coefficient and the thermal capacity. Simi- larly, Bravina et al. [3] also presented some peculiarities in the low temperature pyroelectric behaviour and dielectric permittivity of lithium niobate. Phonon anomalies in the IR and Raman spectra were observed between 100 and 200 K by Golubovic et al. [4]. In addition, Fernández-Ruiz et al. [5] detected a structural anomaly using neutron diffraction at low temperature on a nearly congru- ent LiNbO 3 crystal. Two minima in the unit cell volume at 55 and 100 K have been found together with a shortening of the distance between the Li and O layers in the unit cell. Recently, Yao et al. [6] performed low temperature (4 ≤ T ≤ 90 K) neutron powder diffrac- tion measurements on both a congruent LiNbO 3 crystal grown using 7 Li isotopes and on a near-stoichiometric Mg-doped crys- tal. However, contrary to [5] they found no anomalous structural behaviour in either sample. If there were an anomalous behaviour in the lattice parameters and atomic distances of the structure of ∗ Corresponding author. Tel.: +36 13922588; fax: +36 13922223. E-mail address: kovacs.laszlo@wigner.mta.hu (L. Kovács). LiNbO 3 it should show up in the temperature dependence of the vibrational properties of the built-in hydroxyl ions (OH - ) which are very sensitive to the changes of their local environment [7]. LiNbO 3 crystals grown in air always contain protons form- ing OH - molecules with the oxygen ions of the lattice. The OH - molecules fixed in the lattice possess 3 different (1 stretching and 2 bending) infrared active vibrational modes [8]. The stretch- ing mode can be observed in the absorption spectrum, while the bands corresponding to the bending modes are mostly screened by phonon absorption and can be detected through combination tran- sitions [9]. The stretching vibrational spectrum strongly depends on the composition of the LiNbO 3 crystals [7]. A typical stretch mode spectrum of the incorporated OH - ions in undoped congruent LiNbO 3 crystals contains a broad absorption band consisting of sev- eral overlapping components in the 3450–3500 cm -1 wavenumber range, while that of the stoichiometric crystals shows only a single narrow band at about 3465 cm -1 with a bandwidth of about 3 cm -1 at 300 K [10,11]. The OH - vibrational frequency determined by the potential formed by the neighbouring ions changes with tempera- ture as observed in vapour phase equilibrated nearly stoichiometric LiNbO 3 crystals between 1.5 and 600 K [12]. Anomalous changes in the lattice parameters and atomic distances around the hydroxyl ions should result in anomalous temperature dependence of the vibrational frequency. However, no such behaviour was observed in [12]; that might be due to inaccuracies of the decomposition of overlapping band components and/or the coarse temperature grid (50 K steps) employed. The aim of the present work is a systematic, high accuracy mea- surement of the absorption spectrum of the incorporated OH - ions 0924-2031/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.vibspec.2014.01.003