We present comprehensive measurements of the structural, magnetic and electronic properties of layered van-der-Waals ferromagnet VI3 down to low temperatures. Despite belonging to a wellstudied family of transition metal trihalides, this material has received very little attention. We outline, from high-resolution powder x-ray diffraction measurements, a corrected room-temperature crystal structure to that previously proposed and uncover a structural transition at 79 K, also seen in the heat capacity. Magnetization measurements confirm VI3 to be a hard ferromagnet (9.1 kOe coercive field at 2 K) with a high degree of anisotropy, and the pressure dependence of the magnetic properties provide evidence for the two-dimensional nature of the magnetic order. Optical and electrical transport measurements show this material to be an insulator with an optical band gap of 0.67 eV -the previous theoretical predictions of d-band metallicity then lead us to believe VI3 to be a correlated Mott insulator. Our latest band structure calculations support this picture and show good agreement with the experimental data. We suggest VI3 to host great potential in the thriving field of low-dimensional magnetism and functional materials, together with opportunities to study and make use of low-dimensional Mott physics.Two-dimensional van-der-Waals (vdW) magnetic materials have in recent years become the subject of a wide range of intense research 1 . While a large portion of research into two-dimensional materials has centered on graphene, the addition of magnetism into such a system leads to many interesting fundamental questions and opportunities for device applications 2-6 . Particularly for future spintronics applications, semiconducting or metallic materials which exhibit ferromagnetism down to monolayer thickness are an essential ingredient. This has led to a large volume of recent publications on two-dimensional honeycomb ferromagnet CrI 3 7-12 . CrI 3 and VI 3 belong to a wider family of MX 3 transition metal trihalides, with X = Cl, Br, I, which were synthesized in the 60s 13,14 but have since seen little interest until recently 15 . VI 3 is an insulating two-dimensional ferromagnet with a Curie Temperature, T c , given as 55 K and reported to have the layered crystal structure of BiI 3 with space group R-3 [16][17][18] . As shown in a recent review 15 , there is very little available information on VI 3 other than the structure and the expected S = 1 from the 3d 2 configuration of the vanadium sites. Calculations using density functional theory, which additionally yield the exchange constants, have suggested VI 3 to not only remain ferromagnetic down to a single crystalline layer, but to also exhibit Dirac half-metallicity, of interest for spintronic applications 19 .In these vdW materials, hydrostatic pressure forms an extremely powerful tuning parameter. Given the weak mechanical forces between the crystal planes, the application of pressure will dominantly have the effect of pressing the ab planes together, and gradually an...

Two-dimensional materials have proven to be a prolific breeding ground of new and unstudied forms of magnetism and unusual metallic states, particularly when tuned between their insulating and metallic phases. In this paper we present work on a new metal to insulator transition system FePS3 . This compound is a two-dimensional van-der-Waals antiferromagnetic Mott insulator. Here we report the discovery of an insulator-metal transition in FePS3, as evidenced by x-ray diffraction and electrical transport measurements, using high pressure as a tuning parameter. Two structural phase transitions are observed in the x-ray diffraction data as a function of pressure and resistivity measurements show evidence of the onset of a metallic state at high pressures. We propose models for the two new structures that can successfully explain the x-ray diffraction patterns.

We report Raman scattering measurements of BaC6, SrC6, YbC6, and CaC6, which permit a systematic study of the phonons and the electron-phonon interaction within the doped graphene layers of these compounds. The out-of-plane carbon phonon softens as the spacing of the graphene layers is reduced in the series BaC6, SrC6, YbC6, and CaC6. This is due to increasing charge in the π * electronic band. Electron-phonon interaction effects between the in-plane carbon modes at ≈ 1500 cm −1 and the π * electrons cause a strong non-adiabatic renormalization. As charge is transferred into the π * band these non-adiabatic effects are found to increase concurrent with a reduction in the phonon lifetime.