Session: P1O

Till recently it was considered that velocity of piezoactive acoustic waves propagating in piezoelectric materials decreases at total or partial switching off of piezoeffect. However, recently there appear publications showing that there are such materials and crystallographic orientations for which the velocity of bulk as well as surface acoustic waves in presence of piezoeffect (vpz) may be less than the velocity of the same waves at total or partial switching off piezoeffect (v). It is also well known that measure of piezoactivity of acoustic waves is the square of electromechanical coupling coefficient that is most frequently defined as K2 = (v2 pz − v2)/v2 pz or as K2 ≈ 2(vpz − v)/vpz if vpz − v << vpz. But for that situations when vpz < v the aforementioned expression loses sense because it shows the possibility of the negative value of K2. Such cases lead to necessity of more precise definition of this coefficient and the most correct way is energy consideration. But there are no unambiguous approaches to definition of electromechanical coupling coefficient from this consideration. At present there exist at least two generally used definitions: K2 1 = Wel/(Wel + Wmech) and K2 2 = W 2 elmech/(Wel ×Wmech), where Welmech, Wel, and Wmech are densities of electromechanical, electrical, and mechanical energies of acoustic waves. In this paper the coefficients K1, K2, and K have been calculated for all types of bulk acoustic waves propagating in crystals of quartz, lithium tantalate, lithium niobate, and potassium niobate for various propagation directions in the main crystallographic cuts. It has been shown that for weak piezoelectric materials all three definitions give practically the same values for all propagation directions. As for strong piezoelectric materials, in general case the values of these coefficients are different. At that the values of coefficients K1 and K are most close each other and they coincide for a number of crystallographic orientations. It has been also shown that for every type of bulk acoustic wave for arbitrary crystallographic orientation the density of electromechanical energy is identical with density of electric energy. This work was supported by the grant from RFBR #01-02-16266a.