From Semiconducting to Metallic: Jahn–Teller-Induced Phase Transformation in Skyrmion Host GaV₄S₈

Abstract

Lacunar spinels, GaM4X8 (M = V, Nb, Mo, Ta, W; X = S, Se, Te), constitute a rare class of compounds with multiferroic properties. Recently, one member of this family, GaV4S8, received global attention due to the Néel-type skyrmions discovered in this material. Previous investigations strongly indicate the important role of the structure behind the multiferroicity, for example, the strong impact of ferroelectric transition on the exchange interactions at ∼40 K. Inspired by the delicate entanglement of lattice, spin, and charge degrees of freedom, in the present work, we aimed to use pressure to alter the structure and thus change the material’s properties to establish the inter-relation between the structural, optical, and electrical properties for a better understanding of this skyrmion host material. Upon this objective, in situ high-pressure measurements of single crystal/powder X-ray diffraction, electrical conductivity, and Raman spectroscopy were carried out by using a diamond anvil cell. These studies revealed the pressure-induced structural transformation from cubic to orthorhombic, along with a transition from semiconductor to metallic state in GaV4S8. The phase changes coincide with the variation in the optical property in this material explored by Raman spectra. We also determined the bulk modulus of the two phases of GaV4S8 by fitting the data set of unit cell volumes against pressure with the second-order birth-Murnaghan equation of state, and explained the mechanisms of phase transitions by means of the Jahn–Teller effect and the anisotropic changes in bonding lengths during compression.

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

“…The Rietveld refinement confirms the presence of two coexisting phases with rhombohedral and orthorhombic crystal structures at low temperature (<51 K), which is unique to GaMo 4 Se 8 among the lacunar spinel family. We showed that the strain modes play an important role in the possible stabilization of the metastable orthorhombic phase and the recent observation of a metal–insulator transition in the pressure-induced orthorhombic phase of GaV 4 S 8 motivates further electrical characterization of the mixed phase observed in this Mo compound. In addition to that, magnetic measurements were carried out on a single-crystal sample.…”

Magnetism in the Néel skyrmion host GaV$_4$S$_8$ under pressure

“…Taking the change in cell volume from Ref. [8], and assuming the muon sites remain unchanged (a reasonable assumption given the minimal change to the lattice parameters), we can see that the spectrum is almost unchanged. Reducing the cycloidal period to λ C = 8 nm (equivalent to approximately doubling the relative strength of the DMI) changes the form, but not the position of the features (i.e.…”

“…On increasing temperature, C ordering then takes place in the region 5 < ∼ T < T c 13 K. Under the application of an applied field, in polycrystalline samples of GaV 4 S 8 the SkL phase is stabilized at 10 < ∼ T < ∼ 13 K, 40 < ∼ µ 0 H ext < ∼ 100 mT [7]. The application of hydrostatic pressure on GaV 4 S 8 has been studied previously at room temeprature [8]. At lower pressures there is a gradual reduction in the lattice parameters of the cubic phase with increasing pressure.…”

Strain driven conducting domain walls in a Mott insulator

“…Below T JT , the material undergoes a magnetic ordering at ∼13 K, with several competing magnetic phases, including a Néel-type skyrmion lattice phase. [50] It is also well documented that the electronic structure of GaV 4 S 8 is very sensitive to inter-atomic spacing, and can be modified with either hydrostatic pressure, [51,52] or chemical pressure. [53] 3…”