Coupled neutron – γ-ray diffraction: a method for accurate measurement of strains. Application to the study of the ferroelectric phase transition of KH2PO4

Abstract

A method that employs simultaneous neutron and γ-ray diffraction techniques is proposed for accurate measurements of strains with or without an applied electric field (0–100 kV m⁻¹). Instead of an absolute determination of the position of the neutron Bragg peak, a relative measurement of the neutron and γ peak positions is used and allows a determination of the lattice-parameter changes with an accuracy of within 2 × 10⁻⁶. This technique avoids systematic errors originating in sample rotations by taking advantage of a reference directly related to the crystal lattice. The method appears to be particularly important for crystals that exhibit structural phase transitions, as demonstrated by the measurement of the strain U_xx in KH₂PO₄ crystals as a function of temperature and applied electric field. In this case the errors would amount to ±20%. The results demonstrate the need to re-examine the theory previously proposed by Uesu, Tanaka & Kobayashi [Ferroelectrics (1974), 7, 247–249] to interpret the electromechanical properties of KH₂PO₄.