Strong 3D and 1D magnetism in hexagonal Fe-chalcogenides FeS and FeSe vs. weak magnetism in hexagonal FeTe

Pressure-Modulated Structural and Magnetic Phase Transitions in Two-Dimensional FeTe: Tetragonal and Hexagonal Polymorphs

Two-dimensional (2D) Fe chalcogenides with their rich structures and properties are highly desirable for revealing the torturous transition mechanism of Fe chalcogenides and exploring their potential applications in spintronics and nanoelectronics. Hydrostatic pressure can effectively stimulate phase transitions between various ordered states, allowing one to successfully plot a phase diagram for a given material. Herein, the structural evolution and transport characteristics of 2D FeTe were systematically investigated under extreme conditions by comparing two distinct symmetries, i.e., tetragonal (t) and hexagonal (h) FeTe. We found that t-FeTe presented a pressure-induced transition from an antiferromagnetic state to a ferromagnetic state at ∼3 GPa, corresponding to the tetragonal collapse of the layered structure. Contrarily, the ferromagnetic order of h-FeTe was retained up to 15 GPa, which was evidently confirmed by electrical transport and Raman measurements. Furthermore, T-P phase diagrams for t-FeTe and h-FeTe were mapped under delicate critical conditions. Our results can provide a unique platform to elaborate the extraordinary properties of Fe chalcogenides and further develop their applications.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Vacancy-plane-mediated exfoliation of sub-monolayer 2D pyrrhotite

Conventional exfoliation exploits the anisotropy in bonding or compositional character to delaminate 2D materials with large lateral size and atomic thickness. This approach, however, limits the choice to layered host crystals with a specific composition. Here, we demonstrate the exfoliation of a crystal along planes of ordered vacancies as a novel route toward previously unattainable 2D crystal structures. Pyrrhotite, a non-stoichiometric iron sulfide, was utilized as a prototype system due to its complex vacancy superstructure. Bulk pyrrhotite crystals were synthesized by gas-assisted bulk conversion, and their diffraction pattern revealed a 4C superstructure with 3 vacancy interfaces within the unit cell. Electrochemical intercalation and subsequent delamination yield ultrathin 2D flakes with a large lateral extent. Atomic force microscopy confirms that exfoliation occurs at all three supercell interfaces, resulting in the isolation of 2D structures with sub-unit cell thicknesses of 1/2 and 1/4 monolayers. The impact of controlling the morphology of 2D materials below the monolayer limit on 2D magnetic properties was investigated. Bulk pyrrhotite was shown to exhibit ferrimagnetic ordering that agrees with theoretical predictions and that is retained after exfoliation. A complex magnetic domain structure and an enhanced impact of vacancy planes on magnetization emphasize the potential of our synthesis approach as a powerful platform for modulating magnetic properties in future electronics and spintronics.

The Controllable Synthesis of High-Quality Two-Dimensional Iron Sulfide with Specific Phases

2D Fe-chalcogenides have drawn significant attention due to their unique structural phases and distinct properties in exploring magnetism and superconductivity. However, it remains a significant challenge to synthesize 2D Fe-chalcogenides with specific phases in a controllable manner since Fe-chalcogenides have multiple phases. Herein, a molecular sieve-assisted strategy is reported for synthesizing ultrathin 2D iron sulfide on substrates via the chemical vapor deposition method. Using a molecular sieve and tuning growth temperatures to control the partial pressures of precursor concentrations, hexagonal FeS, tetragonal FeS, and non-stoichiometric Fe₇ S₈ nanoflakes can be precisely synthesized. The 2D h-FeS, t-FeS, and Fe₇ S₈ have high conductivities of 5.4 × 10⁵ S m^-1 , 5.8 × 10⁵ S m^-1 , and 1.9 × 10⁶ S m^-1 . 2D tetragonal FeS shows a superconducting transition at 4 K. The spin reorientation at ≈30 K on the non-stoichiometric Fe₇ S₈ nanoflakes with ferrimagnetism up to room temperature has also been observed. The controllable synthesis of various phases of 2D iron sulfide may provide a route for synthesizing other 2D compounds with various phases.

DFT-aided machine learning-based discovery of magnetism in Fe-based bimetallic chalcogenides

With the technological advancement in recent years and the widespread use of magnetism in every sector of the current technology, a search for a low-cost magnetic material has been more important than ever. The discovery of magnetism in alternate materials such as metal chalcogenides with abundant atomic constituents would be a milestone in such a scenario. However, considering the multitude of possible chalcogenide configurations, predictive computational modeling or experimental synthesis is an open challenge. Here, we recourse to a stacked generalization machine learning model to predict magnetic moment (µB) in hexagonal Fe-based bimetallic chalcogenides, Fe_xA_yB; A represents Ni, Co, Cr, or Mn, and B represents S, Se, or Te, and x and y represent the concentration of respective atoms. The stacked generalization model is trained on the dataset obtained using first-principles density functional theory. The model achieves MSE, MAE, and R² values of 1.655 (µB)², 0.546 (µB), and 0.922 respectively on an independent test set, indicating that our model predicts the compositional dependent magnetism in bimetallic chalcogenides with a high degree of accuracy. A generalized algorithm is also developed to test the universality of our proposed model for any concentration of Ni, Co, Cr, or Mn up to 62.5% in bimetallic chalcogenides.

Phase-controllable growth of ultrathin 2D magnetic FeTe crystals

Two-dimensional (2D) magnets with intrinsic ferromagnetic/antiferromagnetic (FM/AFM) ordering are highly desirable for future spintronic devices. However, the direct growth of their crystals is in its infancy. Here we report a chemical vapor deposition approach to controllably grow layered tetragonal and non-layered hexagonal FeTe nanoplates with their thicknesses down to 3.6 and 2.8 nm, respectively. Moreover, transport measurements reveal these obtained FeTe nanoflakes show a thickness-dependent magnetic transition. Antiferromagnetic tetragonal FeTe with the Néel temperature (TN) gradually decreases from 70 to 45 K as the thickness declines from 32 to 5 nm. And ferromagnetic hexagonal FeTe is accompanied by a drop of the Curie temperature (TC) from 220 K (30 nm) to 170 K (4 nm). Theoretical calculations indicate that the ferromagnetic order in hexagonal FeTe is originated from its concomitant lattice distortion and Stoner instability. This study highlights its potential applications in future spintronic devices.

The Synthesis of a Quasi-One-Dimensional Iron-Based Telluride with Antiferromagnetic Chains and a Spin Glass State

The report on the superconductivity of the two-legged spin ladders BaFe₂S₃ and BaFe₂Se₃ has established 123-type iron chalcogenides as a novel subgroup in the iron-based superconductor family and has stimulated the continuous exploration of other iron-based materials with new structures and potentially novel properties. In this paper, we report the systematic study of a new quasi-one-dimensional (1D) iron-based compound, Ba₉Fe₃Te₁₅, including its synthesis and magnetic properties. The high-pressure synthesized Ba₉Fe₃Te₁₅ crystallized in a hexagonal structure that mainly consisted of face-sharing FeTe₆ octahedral chains running along the c axis, with a lattice constant of a = 10.23668 Å; this led to weak interchain coupling and an enhanced one-dimensionality. The systematic static and dynamic magnetic properties were comprehensively studied experimentally. The dc magnetic susceptibility showed typical 1D antiferromagnetic characteristics, with a T_max at 190 K followed by a spin glass (SG) state with freezing at T_f ≈ 6.0 K, which were also unambiguously proved by ac susceptibility measurements. Additionally, X-ray magnetic circular dichroism (XMCD) experiments revealed an unexpected orbital moment for Fe²⁺, i.e., 0.84 μ_B per Fe in Ba₉Fe₃Te₁₅. The transport property is electrically insulating, with a thermal activation gap of 0.32 eV. These features mark Ba₉Fe₃Te₁₅ as an alternative type of iron-based compound, providing a diverse candidate for high-pressure studies in order to pursue some emerging physics.