Abstract
Analytic formula for the shear stiffness c55 of A2BX4-type crystals is derived in the domain-like regime near the phase transition temperature Tc from the incommensurate into commensurate ferroelectric phase. The anomalous part of c55 is shown to be proportional to the domain wall density. A comparison with the dielectric susceptibility χ3 is made. Theoretical and experimental temperature dependences of c55 in K3SeO4 are discussed.