Crystal and Magnetic Structures in Layered, Transition Metal Dihalides and Trihalides

Abstract

Materials composed of two dimensional layers bonded to one another through weak van der Waals interactions often exhibit strongly anisotropic behaviors and can be cleaved into very thin specimens and sometimes into monolayer crystals. Interest in such materials is driven by the study of low dimensional physics and the design of functional heterostructures. Binary compounds with the compositions MX 2 and MX 3 where M is a metal cation and X is a halogen anion often form such structures. Magnetism can be incorporated by choosing a transition metal with a partially filled d-shell for M, enabling ferroic responses for enhanced functionality. Here a brief overview of binary transition metal dihalides and trihalides is given, summarizing their crystallographic properties and long-range-ordered magnetic structures, focusing on those materials with layered crystal structures and partially filled d-shells required for combining low dimensionality and cleavability with magnetism.

High-temperature ferromagnetic semiconductors: Janus monolayer vanadium trihalides

“…Different from Cr-based trihalides, two electrons are expected to occupy the t 2g states and two of three degenerate t 2g are half filled, leading to S = 1 and valence electronic configuration of 3d 2 with a magnetic moment of 2 µ B per V atom. 52,53 The band structure of VClI in Figure 2a shows the intrinsic semiconducting property with a 2.30 eV band gap which is a little less than 2.72 eV and 2.65 eV for VBrI and VClBr, respectively. In Figure 2b-e, it is found the indirect gap is formed between valance band maximum (VBM) at Γ point and conduction band minimum (CBM) at M point near the Fermi level.…”

Electrically tunable exchange splitting in bilayer graphene on monolayer Cr2X2Te6 with X = Ge, Si, and Sn

“…Several ferromagnetic insulators (FMI), like EuO and Yttrium-Iron-Garnet, have been considered in the context of graphene spintronics [22,[31][32][33][34]. However transition-metal trichalcogenides Cr 2 Si 2 Te 6 , Cr 2 Ge 2 Te 6 (CGT), and transition-metal trihalides CrI 3 , and their monolayers are now extensively discussed in literature as promising materials for low-dimensional spintronics because of their FMI ground state properties [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53]. These materials have attracted attention as substrates for topological insulators (TI) [49,50] and graphene [27], because ferromagnetic coupling is present.…”

Crystallography of Functional Materials

“…Two papers overviewed crystallographic structures and various related features to properties with an eye on being able to exploit the features and develop new materials in the future with improved properties. The paper by McGuire [7] presented an overview of the crystal structures metal di-and tri-halides, calling attention to the triangular and honeycomb transition metal cation nets and stacking sequences, along with their magnetic structures and properties. The paper by Cook et al [9] presented an overview of the crystallographic attributes of porphyrins with examples illustrating how the structural orientations of the porphyrin macrocycle and the inter-porphyrin covalent bonding present in multiporphyrins can influence the semiconducting properties.…”

“…The review paper by McGuire [7] examines the crystal structure and magnetic properties of MX 2 and MX 3 compounds where M is a transition metal with partially filled d-shells (M = Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Mo, Tc, Ru, Rh, and Ir) and X is either Cl, Br, or I. When M = Ti, V, Mn, Fe, Co, or Ni the MX 2 compound crystallizes in either the CdI 2 or CdCl 2 structure types.…”