Neutron-diffraction study of ND4Cl in the tricritical region

Abstract

A detailed neutron scattering investigation has been made of the order-disorder transition in a crystal of N${\left({\mathrm{D}}_{0.93}{\mathrm{H}}_{0.07}\right)}_4$Cl as a function of both temperature and pressure, with emphasis on the region close to the tricritical point (TCP). At atmospheric pressure, the temperature dependence of the order parameter $\Psi$ was obtained by means of a detailed crystal structural analysis, which revealed that the transition was first order. It was also found that $\Psi$ could be determined with suitable accuracy simply by measuring the intensity of the (221) reflection, which is negligibly small in the disordered region. Intensity data for this reflection were obtained at elevated pressures up to a maximum of 6 kbar both as a function of pressure and temperature, and the phase line was accurately located. This was approximately linear, with an average slope of about 8 °K/kbar. From the behavior of the critical scattering, which showed quite abrupt changes in the disordered region close to the TCP, the latter was determined to lie at 128 ± 10 bar and (250.2 ± 0.1) °K. The critical exponents at the TCP were determined for scans made at constant pressure and temperature, and were found to have significantly different values, $2{\beta }_t=0.36\mathrm{}\pm 0.01 \mathrm{and} 0.28\pm 0.01$, respectively. In the second-order region a similar tendency was observed for scans made through a given transition point in these directions. At 280 °K, $2\beta$ for constant pressure and temperature scans, respectively, had values 0.61 ± 0.04 and 0.50 ± 0.04, the former being close to the three-dimensional Ising value.