Accurate determination of strain tensors from small shifts of reflections measured on a four-circle diffractometer

Formulas are given for the derivation of the changes in Bragg angles and the corresponding full asymmetric strain tensor from the shifts of reflections measured on a four-circle diffractometer. The effects of using either an ω- or a 2θ-scan mode are discussed. A least-squares program was written on the basis of the derived formulas and applied to data measured on the piezoelectric crystals of AgGaS₂ under the influence of an external electric field. The obtained piezoelectric constants are in good agreement with values determined by conventional methods.