Anomalous acoustic behavior of KH2PO4-type crystals at high pressure

The pressure dependences of the elastic constants of K${\mathrm{H}}_2$P${\mathrm{O}}_4$, K${\mathrm{D}}_2$P${\mathrm{O}}_4$, Rb${\mathrm{H}}_2$P${\mathrm{O}}_4$, and N${\mathrm{H}}_4$ ${\mathrm{H}}_2$P${\mathrm{O}}_4$ have been studied for pressures up to 20 kbar at 23°C. For N${\mathrm{H}}_4$ ${\mathrm{H}}_2$P${\mathrm{O}}_4$ a complete set of six constants was measured, while for the other three materials only $C_{11}$, $C_{33}$, $C_{44}$, and $C_{66}$ were measured. For all four materials the $C_{44}$ and $C_{66}$ constants exhibit anomalous pressure dependences, while the other measured constants appear to have normal behavior. The anomalous constants all have striking nonlinear pressure dependences and all ultimately soften with increasing pressure. For K${\mathrm{H}}_2$P${\mathrm{O}}_4$ and K${\mathrm{D}}_2$P${\mathrm{O}}_4$ both the $C_{66}^E$ and $C_{66}^P$ constants were determined, and for these materials it is shown that the nonlinear pressure dependences are not due to piezoelectric coupling effects. The results of previous determinations of high-pressure phase transitions in the four crystals are reviewed, and the possible connection between the anomalous shear modes and pressure-induced phase transitions is discussed.