Using anin situx-ray diffraction (XRD) technique, the electric-field induced change in thed-spacings&Dgr;d/dof the individual microcrystals or “crystallographic” strain is measured on lead magnesium niobate—lead titanate—barium titanate electrostrictive ceramics of composition 0.970(0.900PMN−0.100PT)−0.030BT from zero field to saturation. This crystallographic strain is then compared to the bulk longitudinal strain as measured by&Dgr;l/lusing a photonic sensor. The crystallographic electric-field induced strain behavior for virgin samples could be fit to a constitutive model, where the induced strain is proportional to the square of the polarization with an electrostrictive coefficientQdetermined to be 1.4(×106cm4/C2).Attempts to fit the bulk strain behavior on the same sample required lower values of the electrostrictive coefficientQ∼1, and generally yielded worse fits where the model predicted greater than the observed strain values at low electric fields. Subsequentin situXRD measurements on previously electric-field cycled samples yielded much lower crystallographic strain values with a significantly different electric field dependence. ©1997 American Institute of Physics.