Pressure dependence of elastic constants of orthorhombic Li2Ge7O15 above and below the ferroelectric transition

The pressure derivatives of the elastic constants c_ij of orthorhombic Li₂Ge₇O₁₅ have been determined at 293 K by the method of pressure-induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 0 and 1500 bar. Approaching the transition at ca. 630 bar, all P_ij = dc_ij/dp (i, j = 1, 2, 3; p pressure) develop strongly negative values. At higher pressures a similar behaviour with reversed sign is observed. The pressure derivatives of the pure “shear resistances” c₄₄, c₅₅, and c₆₆ depend only slightly on pressure even in the vicinity of the transition. The main interactions driving the transition are of the totally symmetric type. The values dK^?1/dp (K volume compressibility) deviate strongly from the quasi-invariant value of ca. 5 observed in almost all stable crystals (dK^?1/dp = ? 1750 at 620 bar and 1380 at 700 bar). The anomalous piezoelastic behaviour reflects the anomalous thermoelastic behaviour: negative P_ij in the low pressure (high temperature) phase correspond to positive T_ij = d log c_ij/dT (T temperature) and vice versa.