  The existence of a two-partial Rayleigh wave coupled to an electrical field
in 2mm piezoelectric crystals is known but has rarely been investigated
analytically. It turns out that the Z-cut, X-propagation problem can be fully
solved, up to the derivation of the secular equation as a polynomial in the
squared wave speed. For the metallized (unmetallized) boundary condition, the
polynomial is of degree 10 (48). The relevant root is readily identified and
the full description of the mechanical and electrical fields follows. The
results are illustrated in the case of the superstrong piezoelectric crystal,
Potassium niobate, for which the effective piezoelectric coupling coefficient
is calculated to be about 0.1
