Understanding the giant barocaloric effects and thermal hysteresis in Mn3(A,B)N using inelastic neutron scattering

Materials with large caloric effects have the potential to play a major role in decarbonising our heating and cooling, which makes up over 20% of the UK’s CO2 emissions. Barocaloric materials have recently shown great promise in usurping conventional hydrocarbon refrigerants. Giant barocaloric effects (BCE) have recently been observed in the geometrically frustrated antiperovskite Mn3GaN at the 1st-order antiferromagnetic transition (TN), enhanced by suppressed spin fluctuations in the ordered phase. Our recent results on the closely related Mn3NiN have revealed yet larger entropy changes, which we interpret as due to differences in the nature of the electronic hybridisation with Mn between the Ni and Ga compounds. In order to understand the underlying mechanisms of the BCE, we have recently performed inelastic neutron scattering and pair distribution function (PDF) experiments on various Mn3AN, the first such experiments on Mn-based antiperovskites. We have found anomalous and A-site dependent phonon temperature dependencies close to TN, which is consistent with our prediction of the different mechanisms that drive this 1st-order transition. To complete our study, we propose to measure these features in Mn3(A,B)N, where not only are BCE large but thermal hysteresis of the transition (highly detrimental to cyclic BCE performance) is an order of magnitude smaller than Mn3AN.