Strain-coupling and relaxation dynamics in multicaloric ammonium sulphate (NH4)2SO4

The properties of materials that lay the groundwork for greenhouse gas-free solid-state cooling are intricately tied to their proximity to a phase transition---a phenomenon that unfolds reversibly in response to an externally applied field. Enter ammonium sulfate, a material that holds the promise of effectiveness and affordability within this remarkable realm. Its allure stems from a structural phase transition occurring at 225 K, which can be driven by changing hydrostatic pressure and electric fields. However, because of a large change in volume, cycling crystals through the transition typically results in mechanical failure. In this work, resonant ultrasound spectroscopy reveals the strain relaxation mechanisms responsible for this failure and a thermal pathway by which it can be avoided.