Temperature dependence of the quadratic electrogyration coefficient ${\pi }_{13}$ of a nonenantimorphic crystal, ferroelectric K${\mathrm{H}}_2$P${\mathrm{O}}_4$, was successfully measured by using our general method. ${\pi }_{13}$ exhibits an unusual temperature dependence near the transition point. This behavior is primarily similar to that of the electrostrictive coefficient $Q_{13}$ of K${\mathrm{H}}_2$P${\mathrm{O}}_4$. But it reveals the electronic contribution which must be of special interest for elucidating the transition mechanism of the crystal.

• Received 21 November 1978

DOI:https://doi.org/10.1103/PhysRevLett.42.1427