Twisting two-dimensional iron sulfide layers into coincident site superlattices via intercalation chemistry

Layered van der Waals (vdW) materials are susceptible not only to various stacking polymorphs through translations but also twisted structures due to rotations between layers. Here, we study the influence of such layer-to-layer twisting through the intercalation of ethylenediamine (EDA) molecules into tetragonal iron sulfide (Mackinawite FeS). Selected area electron diffraction patterns of intercalated FeS display reflections corresponding to multiple square lattices with a fixed angle between them, contrary to a single square lattice seen in the unintercalated phase. The observed twist angles of 49.13° and 22.98° result from a superstructure formation well described by the coincident site lattice (CSL) theory. According to the CSL theory, these measured twist angles lead to the formation of larger coincident site supercells. We build these CSL models for FeS using crystallographic group-subgroup transformations and find simulated electron diffraction patterns from the model to agree with the experimentally measured data.

Sr(Ag1-x Lix )2 Se2 and [Sr3 Se2 ][(Ag1-x Lix )2 Se2 ] Tunable Direct Band Gap Semiconductors

Synthesizing solids in molten fluxes enables the rapid diffusion of soluble species at temperatures lower than in solid-state reactions, leading to crystal formation of kinetically stable compounds. In this study, we demonstrate the effectiveness of mixed hydroxide and halide fluxes in synthesizing complex Sr/Ag/Se in mixed LiOH/LiCl. We have accessed a series of two-dimensional Sr(Ag_1-x Li_x )₂ Se₂ layered phases. With increased LiOH/LiCl ratio or reaction temperature, Li partially substituted Ag to form solid solutions of Sr(Ag_1-x Li_x )₂ Se₂ with x up to 0.45. In addition, a new type of intergrowth compound [Sr₃ Se₂ ][(Ag_1-x Li_x )₂ Se₂ ] was synthesized upon further reaction of Sr(Ag_1-x Li_x )₂ Se₂ with SrSe. Both Sr(Ag_1-x Li_x )₂ Se₂ and [Sr₃ Se₂ ][(Ag_1-x Li_x )₂ Se₂ ] exhibit a direct band gap, which increases with increasing Li substitution (x). Therefore, the band gap of Sr(Ag_1-x Li_x )₂ Se₂ can be precisely tuned via fine-tuning x that is controlled by only the flux ratio and temperature.

Topologically driven linear magnetoresistance in helimagnetic FeP

The helimagnet FeP is part of a family of binary pnictide materials with the MnP-type structure, which share a nonsymmorphic crystal symmetry that preserves generic band structure characteristics through changes in elemental composition. It shows many similarities, including in its magnetic order, to isostructural CrAs and MnP, two compounds that are driven to superconductivity under applied pressure. Here we present a series of high magnetic field experiments on high-quality single crystals of FeP, showing that the resistance not only increases without saturation by up to several hundred times its zero-field value by 35 T, but that it also exhibits an anomalously linear field dependence over the entire range when the field is aligned precisely along the crystallographic c-axis. A close comparison of quantum oscillation frequencies to electronic structure calculations links this orientation to a semi-Dirac point in the band structure, which disperses linearly in a single direction in the plane perpendicular to field, a symmetry-protected feature of this entire material family. We show that the two striking features of magnetoresistance-large amplitude and linear field dependence-arise separately in this system, with the latter likely due to a combination of ordered magnetism and topological band structure.

Long-range magnetic order in hydroxide-layer-doped (Li1-x-y Fe x Mn y OD)FeSe

The (Li1-x Fe x OH)FeSe superconductor has been suspected of exhibiting long-range magnetic ordering due to Fe substitution in the LiOH layer. However, no direct observation such as magnetic reflection from neutron diffraction has been reported. Here, we use a chemical design strategy to manipulate the doping level of transition metals in the LiOH layer to tune the magnetic properties of the (Li1-x-y Fe x Mn y OD)FeSe system. We find Mn doping exclusively replaces Li in the hydroxide layer resulting in enhanced magnetization in the (Li0.876Fe0.062Mn0.062OD)FeSe superconductor without significantly altering the superconducting behavior as resolved by magnetic susceptibility and electrical/thermal transport measurements. As a result, long-range magnetic ordering was observed below 12 K with neutron diffraction measurements. This work has implications for the design of magnetic superconductors for the fundamental understanding of superconductivity and magnetism in the iron chalcogenide system as well as exploitation as functional materials for next-generation devices.

Proton and ammonia intercalation into layered iron chalcogenides

Structurally related to the iron-based superconductors, two new intercalated iron chalcogenides (H0.5NH3)Fe2Ch2 where Ch = S, Se have been prepared. By topochemical conversion, the protons were exchanged by lithium to form (Li0.5NH3)Fe2Ch2. Hydrogen bonding plays a significant role in the guest-host interactions of these intercalated phases.

Superconductivity and magnetism in iron sulfides intercalated by metal hydroxides

Inspired by naturally occurring sulfide minerals, we present a new family of iron-based superconductors. A metastable form of FeS known as the mineral mackinawite forms two-dimensional sheets that can be readily intercalated by various cationic guest species. Under hydrothermal conditions using alkali metal hydroxides, we prepare three different cation and metal hydroxide-intercalated FeS phases including (Li1-x Fe x OH)FeS, [(Na1-x Fe x )(OH)2]FeS, and K x Fe2-y S2. Upon successful intercalation of the FeS layer, the superconducting critical temperature Tc of mackinawite is enhanced from 5 K to 8 K for the (Li1-x Fe x OH) δ+ intercalate. Layered heterostructures of [(Na1-x Fe x )(OH)2]FeS resemble the natural mineral tochilinite, which contains an iron square lattice interleaved with a hexagonal hydroxide lattice. Whilst heterostructured [(Na1-x Fe x )(OH)2]FeS displays long-range magnetic ordering near 15 K, K x Fe2-y S2 displays short range antiferromagnetism.