NONLINEAR ELASTIC COEFFICIENTS OF KDP

A new apparatus for the measurement of velocity changes due to applied mechanical uniaxial stress working from room temperature down to liqu id nitrogen temperatures is described. The experimental data. obtained with this apparatus allow to calculate nonlinear elastic stiffnesses. Results on Potassium Dihydrogen Phosphate (KDP) are presented. No ano maly of the nonlinear coefficient c(456) near the phase transition cou ld be found contrary to the Landau theory but in agreement with former measurements of the nonlinear piezomodul e(345).