Modeling the Superlattice Phase Diagram of Transition Metal Intercalation in Bilayer 2H-TaS2

Van-der-Waals hosts intercalated with transition-metal (TM) ions exhibit a range of magnetic properties strongly influenced by the structural order of the intercalants. However, predictive computational models for the intercalant ordering phase diagram are lacking, complicating experimental pursuits to target key structural phases. Here we use density functional theory (DFT) to construct a pairwise lattice model and Monte Carlo to determine its associated thermodynamic phase diagram. To circumvent the complexities of modeling magnetic effects, we use the diamagnetic ions Zn2+ and Sc3+ as computationally accessible proxies for divalent and trivalent species of interest (Fe2+, Co2+, V3+ and Cr3+), which provide insights into the thermodynamic phase diagram well above the paramagnetic transition temperature. We find that electrostatic coupling between intercalants is almost entirely screened, so the pairwise lattice model represents a coarse-grained charge density reorganization. The resulting phase diagram reveals that entropically favored √3 × √3 ordering and coexisting locally ordered √3 × √3 and 2 × 2 domains persist across a range of temperatures and stoichiometries. This occurs even at quarter-filling of interstitial sites (corresponding to bulk stoichiometries of M0.25TaS2; M = intercalant) where there are experimental reports of 2 × 2 and √3 × √3 coexistence but a thermodynamic preference for long-range 2 × 2 order is often assumed. Accompanying Raman spectra revealing quenching-induced √3 × √3 order in 2H-V0.25NbS2 supports that deficient √3 × √3 domains may dominate 2 × 2 at high temperatures in a range of TM-intercalated transition metal dichalcogenides.

Crystal Chemistry and Design Principles of Altermagnets

Altermagnetism was very recently identified as a new type of magnetic phase beyond the conventional dichotomy of ferromagnetism (FM) and antiferromagnetism (AFM). Its globally compensated magnetization and directional spin polarization promise new properties such as spin-polarized conductivity, spin-transfer torque, anomalous Hall effect, tunneling, and giant magnetoresistance that are highly useful for the next-generation memory devices, magnetic detectors, and energy conversion. Though this area has been historically led by the thin-film community, the identification of altermagnetism ultimately relies on precise magnetic structure determination, which can be most efficiently done in bulk materials. Our review, written from a materials chemistry perspective, intends to encourage materials and solid-state chemists to make contributions to this emerging topic through new materials discovery by leveraging neutron diffraction to determine the magnetic structures as well as bulk crystal growth for exploring exotic properties. We first review the symmetric classification for the identification of altermagnets with a summary of chemical principles and design rules, followed by a discussion of the unique physical properties in relation to crystal and magnetic structural symmetry. Several major families of compounds in which altermagnets have been identified are then reviewed. We conclude by giving an outlook for future directions.

Fe Site Order and Magnetic Properties of Fe1/4NbS2

Transition-metal dichalcogenides (TMDs) have long been attractive to researchers for their diverse properties and high degree of tunability. Most recently, interest in magnetically intercalated TMDs has resurged due to their potential applications in spintronic devices. While certain compositions featuring the absence of inversion symmetry such as Fe1/3NbS2 and Cr1/3NbS2 have garnered the most attention, the diverse compositional space afforded through the host matrix composition as well as intercalant identity and concentration is large and remains relatively underexplored. Here, we report the magnetic ground state of Fe1/4NbS2 that was determined from low-temperature neutron powder diffraction as an A-type antiferromagnet. Despite the presence of overall inversion symmetry, the pristine compound manifests spin polarization induced by the antiferromagnetic order at generic k points, based on density functional theory band-structure calculations. Furthermore, by combining synchrotron diffraction, pair distribution function, and magnetic susceptibility measurements, we find that the magnetic properties of Fe1/4NbS2 are sensitive to the Fe site order, which can be tuned via electrochemical lithiation and thermal history.

Discovery of Charge Order in the Transition Metal Dichalcogenide Fe_{x}NbS_{2}

The Fe intercalated transition metal dichalcogenide (TMD), Fe_{1/3}NbS_{2}, exhibits remarkable resistance switching properties and highly tunable spin ordering phases due to magnetic defects. We conduct synchrotron x-ray scattering measurements on both underintercalated (x=0.32) and overintercalated (x=0.35) samples. We discover a new charge order phase in the overintercalated sample, where the excess Fe atoms lead to a zigzag antiferromagnetic order. The agreement between the charge and magnetic ordering temperatures, as well as their intensity relationship, suggests a strong magnetoelastic coupling as the mechanism for the charge ordering. Our results reveal the first example of a charge order phase among the intercalated TMD family and demonstrate the ability to stabilize charge modulation by introducing electronic correlations, where the charge order is absent in bulk 2H-NbS_{2} compared to other pristine TMDs.

Pressure-Driven Sequential Lattice Collapse and Magnetic Collapse in Transition-Metal-Intercalated Compounds FexNbS2

Volume collapse under high pressure is an intriguing phenomenon involving subtle interplay between lattice, spin, and charge. The two most important causes of volume collapse are lattice collapse (low-density to high-density) and magnetic collapse (high-spin to low-spin). Herein we report the pressure-driven sequential volume collapses in partially intercalated Fe_xNbS₂ (x = 1/4, 1/3, 1/2, 2/3). Because of the distinct interlayer atomic occupancy, the low-iron-content samples exhibit both lattice and magnetic collapses under compression, whereas the high-iron-content samples exhibit only one magnetic collapse. Theoretical calculations indicate that the low-pressure volume collapses for x = 1/4 and x = 1/3 are lattice collapses, and the high-pressure volume collapses for all four samples are magnetic collapses. The magnetic collapse involving the high-spin to low-spin crossover of Fe²⁺ has also been verified by in situ X-ray emission measurements. Integrating two distinct volume collapses into one material provides a rare playground of lattice, spin, and charge.