Symmetry progression and possible polar metallicity in NiPS3 under pressure

Abstract

Abstractvan der Waals solids are ideal platforms for the discovery of new states of matter and emergent properties under external stimuli. Under pressure, complex chalcogenides like MPS3 (M = Mn, Ni, Co, V) host sliding and structural transitions, insulator-to-metal transitions, the possibility of an orbitally-selective Mott state, piezochromism, and superconductivity. In this work, we bring together diamond anvil cell techniques, infrared and Raman scattering spectroscopies, and X-ray diffraction with a detailed symmetry analysis and first-principles calculations to uncover a series of high-pressure phases in NiPS3. Remarkably, we find five different states of matter between ambient conditions and 39 GPa—quite different than in the other MPS3 materials. Even more strikingly, infrared spectroscopy and X-ray diffraction combined with a symmetry analysis reveal both metallicity and loss of the inversion center above ~23 GPa suggesting that NiPS3 may be a polar metal with a P3m1 space group under these conditions and P1 symmetry under maximum compression. In addition to identifying a candidate polar metal ripe for further inquiry, we suggest that pressure may tune other complex chalcogenides into this elusive state.

Metal Site Substitution and Role of the Dimer on Symmetry Breaking in FePS3 and CrPS4 under Pressure

“…NiPS 3 under pressure is different. Our recent work reveals that this system hosts an elusive polar metal state under compression . Depending upon the nature of any magnetic state, the high-pressure phase of NiPS 3 could potentially be a metallic multiferroic.…”

“…Obviously, infrared and Raman scattering spectroscopies are complementary techniques. Both are needed for a full symmetry analysis of phonons in this material. , Infrared also reveals electronic properties like metallicity. Typical signatures include the development of a Drude and screening of phonons.…”

“…This unexpected pathway eventually creates a metallic antiferromagnetic state with a P2/m space group�a finding is similar to the unexpected complexity of the symmetry progression in NiPS 3 , although in the Ni system, it results from a smaller van der Waals gap. 6,13,23,50 We attribute the difference in CrPS 4 to the absence of the structurally stabilizing P−P dimer, layer corrugation, the decreased size of the van der Waals gap, and changes in orbital occupation resulting from the metal center selection. This work deepens the understanding of competing states of matter in complex chalcogenides and helps to develop the potential of these interesting materials for optoelectronics, sensors, and low-power memory applications.…”

“…This is exciting because many of these phases have novel properties that are highly sought-after and cannot be realized via thermal pathways. − Pressure, for instance, provides the means to modify typical structural features such as lattice parameters, bond lengths and angles, and the interlayer van der Waals gap, − all of which are strongly coupled to electronic and magnetic properties in the M PS 3 series. Examples of the different areas of phase space and transitions under compression include, but are not limited to, insulator-to-metal transitions, unique symmetry evolutions, piezochromism, orbitally selective Mott states, polar metals, and superconductivity. − Pressure- and strain-induced structural, electronic, and magnetic transitions, if achieved in a reversible manner, may give rise to revolutionary advances in low-power, high-speed electronic devices. − Specifically, nonlinear responses under external stimuli, which arise due to strong electron correlations within transition metal ions, can stabilize multiple coexisting phases that can be harnessed for applications. − Pressure-induced phase transitions and the structure–property relations that can be unraveled in these systems are therefore of great contemporary interest.…”

An ab initio study of electron-hole pairs in a correlated van der Waals antiferromagnet: NiPS$_3$

“…In particular, NiPS 3 has gained attention for being a chargetransfer insulator with a zig-zag antiferromagntic ground state following the highly anisotropic XXZ Heisenberg model [6]. The ground state can be further tuned by thinning to yield strong magnetic fluctuations [7] and a possible Mott metal-insulator transition under pressure [8,9]. These interesting properties resemble those of the high-T c cuprates, thus suggesting NiPS 3 may host unconventional superconductivity [10,11].…”