Magneto‐caloric materials offer the possibility to design environmentally friendlier thermal management devices compared to the widely used gas‐based systems. The challenges to develop this solid‐state based technology lie in the difficulty of finding materials presenting a large magneto‐caloric effect over a broad temperature span together with suitable secondary application parameters such as low heat capacity and high thermal conductivity. A series of compounds derived from the PbFCl structure is investigated using a combination of computational and experimental methods focusing on the change of cell volume in magnetic and non‐magnetic ground states. Scaling analysis of the magnetic properties determines that they are second order phase transition ferromagnets and that the magnetic entropy change is driven by the coupling of magneto‐elastic strain in the square‐net through the magnetic transition determined from neutron and synchrotron X‐ray diffraction. The primary and secondary application related properties are measured experimentally, and the c/a parameter is identified as an accurate proxy to control the magnetic transition. Chemical substitution on the square‐net affords tuning of the Curie temperature over a broad temperature span between 252 and 322 K. A predictive machine learning model for the c/a parameter is developed to guide future exploratory synthesis.

In the lacunar spinels, with the formula AB 4 X 8 , transition-metal ions form tightly bound B 4 clusters resulting in exotic physical properties such as the stabilization of Néel-type skyrmion lattices, which hold great promise for energy-efficient switching devices. These properties are governed by the symmetry of these compounds with distortion of the parent noncentrosymmetric F 4̅3 m space group to the polar R 3 m , with recent observation of a coexisting Imm 2 low-temperature phase. In this study, through powder neutron diffraction, we further confirm that a metastable Imm 2 coexists with the R 3 m phase in GaMo 4 Se 8 and we present its structure. By applying the mode crystallography approach to the distortions together with anisotropic microstrain broadening analysis, we postulate that the formation origin of the minority Imm 2 phase stems from the high compressive stress observed in the R 3 m phase. Bond valence sum analysis also suggests a change in electronic configuration in the transition to Imm 2 which could have implications on the electrical properties of the compound. We further establish the nature of the magnetic phase transition using critical exponent analysis obtained from single-crystal magnetization measurements which shows a mixture of tricritical mean-field and 3D Heisenberg behavior [β = 0.22(4), γ = 1.19(1), and δ = 6.42(1)]. Magnetoentropic mapping performed on a single crystal reveals the signature of a positive entropy region near the magnetic phase transition which corresponds to the skyrmion phase field observed in a polycrystalline sample.

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