Ferroelectrics, 447:63–68, 2013 Copyright © Taylor & Francis Group, LLC ISSN: 0015-0193 print / 1563-5112 online DOI: 10.1080/00150193.2013.821893 Vibrational Fingerprints of LiNbO 3 -LiTaO 3 Mixed Crystals S. SANNA, 1 A. RIEFER, 1 S. NEUFELD, 1 W. G. SCHMIDT, 1 G. BERTH, 2 M. R ¨ USING, 2 A. WIDHALM, 2 AND A. ZRENNER 1 1 Lehrstuhl f¨ ur Theoretische Physik, Universit¨ at Paderborn, Warburgestraße 100, 33098 Paderborn, Germany 2 Center for Optoelectronics and Photonics (CeOPP) and Department Physik, Universit¨ at Paderborn, Warburgestraße 100, 33098 Paderborn, Germany Atomistic simulations in the framework of the density functional theory have been used to model morphologic and vibrational properties of lithium niobate–lithium tantalate mixed crystals as a function of the [Nb]/[Ta] ratio. Structural parameters such as the crystal volume and the lattice parameters a and c vary roughly linearly from LiTaO 3 to LiNbO 3 , showing only minor deviations from the Vegard behavior. Our ab initio calcu- lations demonstrate that the TO 1 , TO 2 and TO 4 vibrational modes become harder with increasing Nb concentration. TO 3 becomes softer with increasing Nb content, instead. Furthermore, the investigated zone center A 1 -TO phonon modes are characterized by a pronounced stoichiometry dependence. Frequency shifts as large as 30 cm −1 are ex- pected as the [Nb]/[Ta] ratio grows from 0 to 1. Therefore, spectroscopic techniques sensitive to the A 1 modes (such as Raman spectroscopy), can be employed for a direct and non-destructive determination of the crystal composition. Keywords Ferroelectrics; vibrational properties; LiNbO 3 ; LiTaO 3 ; mixed crystals I. Introduction Both lithium niobate (LiNbO 3 , LN) and lithium tantalate (LiTaO 3 , LT) are optical ma- terials widely used for a variety of applications ranging from optical modulators over photo-refractive and acoustic devices to frequency doubling. Thereby mainly the piezo- and ferroelectric properties of the materials are exploited. In particular, LN is probably the most important electro-optic material, while LT is mainly employed as a LN replacement for shorter wavelength applications [1]. Both materials crystallize within the R3c space group and share several common physical and chemical properties. Commercially available crystals are usually Czochralsky grown and lithium deficient ([Li] /[Nb], [Ta] < 1), which results in a highly defective structure of congruent composition. Above the Curie tempera- ture (607 ◦ C for LT and 1142 ◦ C for LN) a phase transition to a high-symmetry paraelectric phase (space group R ¯ 3c) occurs. Mixed crystals have recently attracted the attention of the scientific community, as they offer the possibility to tune the physical properties by varying the composition. Lithium niobate-tantalate (LNT, LiNb 1-x Ta x O 3 ) is one of the simplest ferroelectric mixed crystals, Received September 19, 2012; in final form March 13, 2013. ∗ Corresponding author. E-mail: simone.sanna@uni-paderborn.de [191]/63 Downloaded by [Universitatsbibliothek Paderborn] at 23:58 22 September 2013