Mode Crystallography Analysis through the Structural Phase Transition and Magnetic Critical Behavior of the Lacunar Spinel GaMo4Se8.

Kieran Routledge,Jonathan Alaria,John B Claridge,Sheikh J Ahmed,Praveen Vir,Stanislav N Savvin,Nicholas Cook,Philip A E Murgatroyd

doi:10.1021/acs.chemmater.1c01448

Abstract

In the lacunar spinels, with the formula AB4X8, transition-metal ions form tightly bound B4 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 F4̅3m space group to the polar R3m, with recent observation of a coexisting Imm2 low-temperature phase. In this study, through powder neutron diffraction, we further confirm that a metastable Imm2 coexists with the R3m phase in GaMo4Se8 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 Imm2 phase stems from the high compressive stress observed in the R3m phase. Bond valence sum analysis also suggests a change in electronic configuration in the transition to Imm2 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.

Highlights

The class of compounds known as lacunar spinels, with the formula AB4X8 (A = Ga and Ge; B = V, Mo, Nb, and Ta; and X = S and Se), provides a diverse chemistry allowing the study of fundamental mechanisms and physical properties, which could be used in energy-efficient switching devices
The Rietveld refinement confirms the presence of two coexisting phases with rhombohedral and orthorhombic crystal structures at low temperature (
The critical exponent analysis shows that the compound primarily exhibits tricritical behavior as observed for other skyrmion hosting compounds and the magnetic phase diagram is complex, possibly due to the presence of various magnetic interactions

Summary

■ INTRODUCTION

Energy landscape calculations have shown that the R3m phase is the ground-state structure, but the large strain amplitude observed in the R3m phase could stabilize the metastable Imm[2] structure as observed in other spinel compounds[52] and is consistent with the fact that GaMo4Se8 presents the largest strain mode among the lacunar spinel family (Figure S11b) This effect might be incipient in other systems such as a GaMo4S8 where the diffraction pattern shows strong anisotropic broadening at low temperature.[53] In our diffraction pattern, the anisotropic microstrain tensor of the R3m phase from the 2 K refinement (Figure S12a) is largely directional along the c axis of the hexagonal cell (one of the [111] axes in the pseudo-cubic cell), whereas the Imm[2] microstrain (Figure S12b) is more isotropic. This reduced temperature stability range is predicted for the GaMo4S8 phases when the intensity of Hund coupling is increased as the single ion energy increases.[29]

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES