Measurement of the piezoelectric tensor of an organic crystal by the x-ray method: The nonlinear optical crystal 2-methyl 4-nitroaniline

The piezoelectric tensor of the organic nonlinear optical compound 2-methyl 4-nitroaniline (MNA) is refined from the measurements of the shift of the Bragg peaks on application of an external electric field. The ``three-step modulation'' method is used, in which the gating of the x-ray signal is synchronized with the variation of the electric field. The piezoelectric tensor is dominated by one coefficient, {}d₁₁{}=13.8(1)×{}10^-12 C/N, which is, to our knowledge, the largest ever observed in an organic molecular crystal and six times greater than the largest coefficient of {α} quartz. Here, the x axis is chosen along the polar axis of the crystal. The large linear electro-optical effect observed in MNA is reinterpreted in terms of the piezoelectric-photoelastic effect.