Observation of a continuous phase transition in a shape-memory alloy

TL;DR

This study investigates the nature of the phase transition in AuZn and Au_{0.52}Zn_{0.48} alloys, revealing a continuous transition in AuZn consistent with mean-field theory and identifying a critical endpoint in the phase diagram.

Contribution

It provides the first evidence of a continuous phase transition in a shape-memory alloy and characterizes the critical endpoint using neutron scattering, specific heat, and pressure measurements.

Findings

AuZn exhibits a continuous phase transition.

The transition in AuZn aligns with mean-field theory predictions.

A critical endpoint is identified at T_M*=2.7K and p*=3.1GPa.

Abstract

Elastic neutron-scattering, inelastic x-ray scattering, specific-heat, and pressure-dependent electrical transport measurements have been made on single crystals of AuZn and Au_{0.52}Zn_{0.48} above and below their martensitic transition temperatures (T_M=64K and 45K, respectively). In each composition, elastic neutron scattering detects new commensurate Bragg peaks (modulation) appearing at Q = (1.33,0.67,0) at temperatures corresponding to each sample's T_M. Although the new Bragg peaks appear in a discontinuous manner in the Au_{0.52}Zn_{0.48} sample, they appear in a continuous manner in AuZn. Surprising us, the temperature dependence of the AuZn Bragg peak intensity and the specific-heat jump near the transition temperature are in favorable accord with a mean-field approximation. A Landau-theory-based fit to the pressure dependence of the transition temperature suggests the presence of a critical endpoint in the AuZn phase diagram located at T_M*=2.7K and p*=3.1GPa, with a quantum saturation temperature \theta_s=48.3 +/- 3.7K.