1 Proc.1995 IEEE Frequency Control Symposium, San Francisco, CA, 31 May -2 June, 1995, pp.671-678 THE ANOMALOUS ELASTIC ANISOTROPY OF Li 2 B 4 O 7 AND ITS INFLUENCE ON SAW PROPERTIES NATALYA F.NAUMENKO Crystallography Dept., Moscow Steel & Alloys Institute, 117936, Leninski prosp.,4. Moscow, Russia Abstract The anomalous elastic anisotropy of Li 2 B 4 O 7 caused by the "incorrect" relation between two elastic modula, c 33 and c 44 , is proved to be responsible for the existence of earlier found undamped longitudinal horizontally polarized (LH) type leaky waves. A simple provement is presented to show that “ray-polarized” quasilongitudinal bulk wave propagating in the plane of reflectional symmetry satisfies the stress-free conditions when the boundary plane is parallel to polarization vector and orthogonal to the plane of reflectional symmetry. The close relation between the particular directions, such as acoustic axes, and the existence of “non-leaky” waves is analyzed using the “exceptional wave line” method. LH type leaky waves exist in crystal cuts with Euler angles (,,90), where  is arbitrary when =40...46. If =45 the permitted interval of  angle is the largest: =35-68. 1.Introduction The acoustic anisotropy of any crystal which means the anisotropy of bulk acoustic wave (BAW) velocities in infinite medium plays a very important role in the investigation of surface acoustic waves (SAW). The geometrical image of acoustic anisotropy is the refraction (or slowness) surface. While analyzing this characteristic surface one can predict the behavior of SAW in selected crystal cuts, for example, the splitting of general SAW solution into two separate waves with different velocities and mechanical displacement (polarization) vectors or the existence of SAW with “supersonic” velocity in relation to the slowest bulk mode. The analysis of acoustic anisotropy should include the search for particular directions [1]: longitudinal and transverse normals and acoustic axes. While propagating along two first particular directions one of three bulk modes is pure longitudinal or pure transverse respectively. If propagation direction is parallel to one of acoustic axes, at least two bulk modes have the same velocity and thus degenerate. The last type of particular directions in crystals of middle symmetry systems - trigonal, tetragonal and hexagonal - is usually localized in the planes of reflectional symmetry or the planes orthogonal to even-fold symmetry axes. The bulk waves, propagating along 3-,4- and 6-fold symmetry axes also degenerate. However in some crystals the acoustic axes of “general position” can exist. The example is quartz, a crystal with very strong acoustic anisotropy. An infinite number of bulk modes with different polarization vectors can propagate parallel to the acoustic axis and if the last one is of conical type [2], the Poynting vector strikes a cone while propagation direction rotates around the axis. It is obvious that all acoustic properties change rapidly in the vicinity of this particular direction. The refraction surface is characterized by the negative curvature in this special area. Thus if the acoustic axis occurs to be in the sagittal plane for the specified SAW orientation it leads to the appearance of additional “limiting bulk waves”[3], propagating parallel to the surface. The corresponding surface skimming bulk waves (SSBW) can be observed experimentally. Usually only slowest and middle bulk modes degenerate but some exclusions are known, so called “longitudinal acoustic axes”. For example, in parathellurite TeO 2 (symmetry 422) in the plane (100) the velocities of the fast and the middle bulk waves coinside. As a result there is a strong deviation of power flow vector from the propagation direction for quasilongitudinal wave. In (001) plane the angle between these two vectors exceeds 73. The other important result is the existence of crystallographic orientations in TeO 2 , where the quasilongitudinal bulk wave satisfies the stress-free mechanical boundary conditions. This phenomenon was discovered first in [4]. The other example of crystal with anomalous elastic anisotropy is Li 2 B 4 O 7 . 2.Elastic anisotropy of Li 2 B 4 O 7 and ray-polarized waves The crystal of lithium tetraborate Li 2 B 4 O 7 (symmetry 4mm) is one of the most promising new piezoelectric materials for SAW devices due to the high piezoelectric