Mode Crystallography Analysis through the Structural Phase Transition and Magnetic Critical Behavior of the Lacunar Spinel GaMo 4 Se 8 .

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.
Competing Interests: The authors declare no competing financial interest.
(© 2021 The Authors. Published by American Chemical Society.)