Anomalous temperature dependence of third-order elastic constants in NH 4Cl and NH 4Br near the λ point

Abstract

Second order elastic constants of NH 4Cl and NH 4Br crystals were measured under hydrostatic and uniaxial pressures near the λ-type phase transitions. Third order elastic constants were calculated from the pressure derivatives of the second order elastic constants. Results show that the temperature dependences of the third order elastic constants of NH 4Cl differ from those of NH 4Br dramatically. In addition, it is found that the coefficients 1 54 (C 111 + 6C 112 + 2C 123) and ( 1 24 √3)(C 111 − 3C 112 + 2C 123) of strains in the free energy of NH 4Cl show quite different temperature dependences from those of NH 4Br.