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.

供暖与制冷环节的碳排放占英国二氧化碳总排放量的20%以上，具备大热卡效应（Caloric Effects）的材料有望在该领域的脱碳进程中发挥关键作用。压卡效应（Barocaloric）材料近来展现出取代传统烃类制冷剂的巨大潜力。研究人员近期在几何阻挫反钙钛矿Mn3GaN的一级反铁磁转变（奈尔温度TN）处观测到了巨压卡效应（BCE），该效应因有序相内被抑制的自旋涨落而得到增强。我们近期针对结构相近的Mn3NiN开展的研究结果显示，其熵变更大；我们将这一现象归因于Ni基与Ga基化合物中Mn原子参与的电子杂化性质存在差异。为阐明压卡效应的内在作用机制，我们近期针对多种Mn3AN开展了非弹性中子散射与对分布函数（Pair Distribution Function, PDF）实验，这是锰基反钙钛矿材料领域内首次开展此类实验。我们发现在奈尔温度TN附近，声子的温度依赖性呈现反常特征且与A位原子种类相关，这与我们针对驱动该一级相变的不同机制所提出的预测一致。为完善本研究，我们提议在Mn3(A,B)N体系中对上述特征进行测量：该体系不仅具备大压卡效应，其相变的热滞后现象（对循环压卡效应性能极为有害）也比Mn3AN小一个数量级。