Lattice Defects in Lithium Tantalate

Lithium tantalate single crystals are used for piezoelectric devices. The lattice defects of this structure and their possible role on piezoelectric performances are investigated. Synthetic crystals are grown by a Czochralski process. To get homogeneous material it is necessary to start from a non-equimolar mixture of Li20 and Ta2O5 powders leading to a congruent melt. The resulting crystals arc non-stoichiometric with an atomic ratio [Li]/[Li + Ta] ≈ 48%, and this induces a first kind of lattice defects: the point defects associated to this non-stoichiometry. When cooled down from high temperature, LiTa03 suffers a second-order phase transition from a paraelectric phase R3c to a ferroelectric phase R3c which is the stable phase at room temperature. A second kind of lattice defects (ferroelectric domains) is generally nucleated at the transition. These defects constitute a poison for piezoelectric applications because the polarization vector c is reversed. One can in principle prevent their occurrence by a poling process (cooling under a static electric field). Dislocations and twins are other as-grown lattice defects; they can also be introduced by the usual machining processes (sawing, grinding…). Furthermore because of the very high values of the piezoelectric constants, the stress field of the dislocations can indtice ferroelectric domains around them, even at room temperature, and such domains cannot be removed by poling. The experimental techniques used are infrared spectroscopy and differential scanning calorimetry for the characterization of point defects and non-stoichiometry; chemical etching and transmission electron microscopy for the characterization of dislocations and twins. As-grown defects are studied and the ones introduced by machining; these latter ones are simulated by scratching and by plastic deformation under confining pressure. A few constant strain rate tests are also performed in the temperature range 20 to 700 °C. The subsequent TEM investigations allow the activated glide systems to be characterized. Les monocristaux de tantalate de lithium sont utilises pour la fabrication de composants piezoelectriques. On s'interesse dans ce travail aux defauts de resea u de cette structure et a leurs consequences possibles sur les performances de ces composants. Les cristaux sont synthetises par tirage Czochralski. II est necessaire pour obtenir un materiau homogene de partir d'un melange Li2O et Ta205 correspondant a la fusion congruentc. Les cristaux ainsi obtenus ne sont pas stoechio- metriques et presentent un rapport atomique [Li]/[Li + Ta] as 48%. Ceci entraine Ic premier type de defaut: les defauts ponctuels associes a la non-stoechiometrie. Quand on le rcfroidit a partir d'une temperature elevee, le tantalate de lithium subit une transition de phase du second ordre de la phase paraelectrique R3c a la phase ferroelectrique R3c qui est la phase stable a tem perature ambiante. Un second type de defaut (domaines ferroelectriques) est generalement miclee a cette transition. Ces defauts sont un poison pour les applications piezoelectriques puisque le vecteur de polarisation est renverse. On peut en principe eviter leur apparition par polage (refroidissement sous champ electrique). Les dislocations et les macles sont d'autres defauts possibles qui peuvent egalement etre introduits lors de Pusinage (sciage, polissage, …). De plus, du fait desfortes valeurs des constantcs piezoelectriques, le champ dc contrainte des dislocations peutnucleer des domaines ferroelectriques. meme a temperature ambiante, et ces domaines ne sont paselimines lors du polage. Les techniques experirnentales utilisees sont la spectroscopie infrarouge et la calorimetrie pour la caracterisation des defauts ponctuels et de la non-stoechiometrie; 1'attaqtie chimique et la microscopie electronique en transmission pour la caracterisation des dislocations