Neutron diffractometry investigation of the tricritical point of KH2 PO4

Abstract

The spontaneous shear strain and the variation of the lattice-constant C in KH2PO4 (K.D.P.) have been studied as functions of the temperature and hydrostatic pressure using a high-resolution neutron diffractometer. The measurements were performed in a range of ± 5 K around the transition temperature and for pressure up to 4 kbar. All the data are consistently fitted, using a Landau-type free energy expansion with the quadratic and quartic term coefficients varying linearly with the temperature and pressure. Both these coefficients vanish simultaneously at the tricritical point (P* = 2.8 ± 0.3 kbar, T* = 109 ± 1 K). This mean-field behaviour is in agreement with recent theories of critical phenomena: Fits with power-laws are also discussed