Frustrated Under Pressure—Multi-Component Manganese Nitride Antiperovskites for Future Solid-State Refrigeration

With the heating and cooling sector accounting for a fifth of the world’s electricity consumption, and 7-10% of the world’s greenhouse gas emissions, more environmentally friendly and energy efficient alternatives to gas refrigerants have become a key area of research.¹ One such solution is barocalorics (BC), a type of caloric that heats and cools under the application and release of pressure with virtually no global warming potential, and none of the associated risks with regards to flammability and containment that gas refrigerants pose.²

Manganese nitride (Mn−N) antiperovskites (APs) (Mn₃AN, space group: Pm-3m, No. 221) are a family of BCs that exhibit magnetovolume effects when they undergo a first-order phase transition.³ These Mn-N APs typically exhibit non-collinear antiferromagnetic (AFM) ordering, usually forming the Γ^4g and Γ^5g magnetic structures, which arise from the frustrated Mn-Mn spin interactions.⁴ Furthermore, they exhibit a negative thermal expansion (NTE), seeing a volume increase when transitioning on cooling, and a volume decrease on heating. It is this coupling of the magnetism to the lattice volume that gives rise to the BC effect (BCE) and has led to a growing interest in researching Mn−N APs for future refrigeration devices, as giant-BCEs (ΔS > 10 Jkg⁻¹K⁻¹) have been observed in both Mn₃GaN and Mn₃NiN.^4,5

Currently, the biggest drawback with Mn-N APs as BCs is that they require large pressures (p > 100 MPa) to drive the transition, hence recent research has focused on decreasing the thermal hysteresis so lower operating pressures are required.⁶ A method of doing this is through doping the metal A-site to form multi-component Mn-N APs, (e.g. Mn₃(A,B,C...)N), as preliminary results have shown that A-site doping in Mn-N APs can minimise the transition hysteresis.^6,7 The chemical flexibility of Mn-N APs and the ability to easily dope the A-site offers a vast number of possible AP combinations, and possibly more efficient Mn-N BC materials to synthesise and characterise.

Using solid-state synthesis methods, we have successfully synthesised and characterised 16 ternary (Mn₃(A,B,C)N) and 1 quaternary Mn-N APs (Mn₃(A,B,C,D)N) with high purity, confirmed using x-ray diffraction and Rietveld refinement. Through A-site doping, we see a drastic improvement in the thermal hysteresis, NTE and transitional entropy. A-site doping also facilitates the manipulation of the transition temperature, enabling the design of Mn-N APs with suitable operating temperatures for cooling and heating applications. We also observe the evolution of unique magnetic behaviour that differs from the undoped counterparts. Neutron studies revealed unique frustrated magnetic structures in our doped samples, even when their undoped counterparts exhibit no first-order transition. We understand that this behaviour arises from the sharing of A-site valence electrons between dopants, with the outer electrons (s- and p-electrons) playing a key role in maximising the NTE, and consequently, the BCE. In addition, interesting phonon behaviour arises from the A-site across the transition that is unique for each Mn-N AP, although how this correlates to the lattice dynamics of the materials is still unknown and requires further investigation. With this research, we hope to shed light on the unique and vast possibilities that doping the A-site of Mn-N APs proposes, not just for calorics, but the broader applications as well.

1 Strahan, D. Clean cold and the Global Goals - Birmingham.ac.uk.
2 Aprea, C., Greco, A., Maiorino, A. & Masselli, C. International Journal of Heat and Technology 36, 1155–1162 (2018).
3 Takenaka, K. et al. Science and Technology of Advanced Materials 15, 015009 (2014).
4 Boldrin, D. et al. Physical Review X 8, (2018).
5 Samathrakis, I. & Zhang, H. Physical Review B 101, (2020).
6 Rendell-Bhatti, F. et al. Journal of Physics: Energy 5, 024018 (2023).
7 Boldrin, D. Applied Physics Letters 118, 170502 (2021).