Xiaofang Lai

Observation of Superconductivity in Tetragonal FeS

The possibility of superconductivity in tetragonal FeS has attracted considerable interest because of its similarities to the FeSe superconductor. However, all efforts made to pursue superconductivity in tetragonal FeS have failed so far, and it remains controversial whether tetragonal FeS is metallic or semiconducting. Here we report the observation of superconductivity at 5 K in tetragonal FeS that is synthesized by the hydrothermal reaction of iron powder with sulfide solution. The obtained samples are highly crystalline and less air-sensitive, in contrast to those reported in the literature, which are meta-stable and air-sensitive. Magnetic and electrical properties measurements show that the samples behave as a paramagnetic metal in the normal state and exhibit superconductivity below 5 K. The high crystallinity and the stoichiometry of the samples play important roles in the observation of superconductivity. The present results demonstrate that tetragonal FeS is a promising new platform to realize high-temperature superconductors.

Enhanced Superconductivity in Restacked TaS2 Nanosheets

Since interface superconductivity was discovered at the interface between two insulating layers LaAlO3 and SrTiO3, such interface-induced superconducting systems have been a research hotspot in superconductivity. Here, we report homogeneous interfaces formed by stacking chemically exfoliated monolayer TaS2 nanosheets randomly. Enhanced superconductivity of Tc = 3 K is observed, compared with 0.8 K of parent 2H-TaS2. The measurement of heat capacity shows the increase of electronic specific-heat coefficient γ of restacked TaS2 nanosheets compared to parent 2H-TaS2 crystals. Density functional theory calculations indicate that increase and delocalization of electron states near the Fermi surface due to the homogeneous interfaces effects could account for the enhanced superconductivity.

Observation of superconductivity in 1T′-MoS2 nanosheets

Studies on the preparation and physical properties of phase-pure 1T′-MoS2 are still scarce although a 1T′ phase MX2 (M = Mo and W; X = Se and Te) has recently been reported to be a Weyl semimetal, a quantum spin Hall insulator, and a superconductor. Herein, we report a perfect single-layer 1T′-MoS2 structure based on edge-sharing octahedra. Pure 1T′-MoS2 nanosheets were successfully prepared using LiMoS2 single crystals. The arrangement of zig-zag Mo–Mo chains in the 1T′-MoS2 layer evolved from that of parent Mo–Mo diamond-like chains in LiMoS2 crystals after the removal of all lithium ions. A well-defined 1T′-type lattice structure was observed and characterized by scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Surprisingly, superconductivity with an onset transition temperature (Tc) of 4.6 K has been observed in the lithium-free 1T′-MoS2 nanosheets, which is not observed in semiconducting bulk or single-layer 2H-MoS2.

Synthesis, crystal structure and physical properties of [Li0.85Fe0.15OH][FeS]

The layered mixed anion compound with the formula [Li0.85Fe0.15OH][FeS] was synthesized via a facile hydrothermal method. [Li0.85Fe0.15OH][FeS], which is determined by single crystal X-ray diffraction and refined by the SHELXTL program, crystallizes in the tetragonal space group of P4/nmm (a = b = 3.6886(3) A, c = 8.915(1) A, V = 121.29(2) A3, Z = 2). The structure features alternatively packed anti-PbO-like [Li0.85Fe0.15OH] and anti-PbO [FeS] layers. The sample was characterized by Field Emission Scanning Electron Microscopy (FESEM) and High-Resolution Transmission Electron Microscopy (HRTEM). Powder X-ray diffraction results confirm the phase purity of the as-synthesized crystals. Temperature-dependent measurements of magnetic susceptibility reveal the presence of a paramagnetic-to-ferromagnetic phase transition around 50 K, accompanied by the metal-to-semiconductor phase transition in the temperature-dependent resistance of the [Li0.85Fe0.15OH][FeS] single crystal.

Tailoring the photocatalytic activity of layered perovskites by opening the interlayer vacancy via ion-exchange reactions

Layer-structured materials have shown great promise in photocatalytic applications. The criteria for effective design and selection of superior layered photocatalysts according to their crystal structures are extremely important. Herein, a series of layered perovskites MLa2Ti3O10 (M = Ca, Sr, Ba) and K2xCa1−xLa2Ti3O10 (x = 0.05, 0.11, 0.25) were prepared by an ion-exchange approach from K2La2Ti3O10. Their photocatalytic properties were evaluated by the degradation of methyl orange (MO) and phenol and by photocatalytic hydrogen evolution. Their catalytic activities were significantly improved compared to K2La2Ti3O10, in the order Ca > Sr > Ba > K for both organic pollutant degradation and H2 evolution. The optimized composition of K2xCa1−xLa2Ti3O10 with x = 0.11 shows an increase of four times in photocatalytic efficiency in comparison to pristine K2La2Ti3O10. The underlying mechanism of the improved performance is discussed in detail in terms of the packing factor model, which demonstrates that a more open structure with a lower packing factor possesses higher photocatalytic activity in the layered perovskites.

Ammonia and iron cointercalated iron sulfide (NH3)Fe0.25Fe2S2: hydrothermal synthesis, crystal structure, weak ferromagnetism and crossover from a negative to positive magnetoresistance

The discovery of superconductivity in anti-PbO-type FeS has aroused a renewed interest in the intercalation compounds of FeS. Here we report a novel intercalation compound of FeS with the chemical composition of (NH3)Fe0.25Fe2S2, which is synthesized via a new hydrothermal reaction. This material crystallizes in the tetragonal space group I4/mmm, preserving the FeS tetrahedral layers with ammonia and excess iron forming planes in between. The microstructure and thermal stability of the sample were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric analyses (TGA). These results suggest that (NH3)Fe0.25Fe2S2 is not sensitive to electron beam irradiation and is more thermally stable than the other ammonia intercalated iron selenide superconductors. Physical property measurements show that it is a ferromagnetic semiconductor. By using first-principles calculations we assess that the low-temperature ferromagnetism originates from the interlayer rather than the intralayer iron. The transport properties at low temperatures are dominated by electron-like carriers and the sign reversal and strong temperature dependence of the Hall coefficient may be caused by a multi-band effect. Most importantly, an unusual crossover from negative to positive magnetoresistance with increasing temperature was identified, which reveals relatively strong coupling between carriers and magnetic moments as well as disorder.

Suppression of superconductivity and structural phase transitions under pressure in tetragonal FeS

Pressure is a powerful tool to study iron-based superconductors. Here, we report systematic high-pressure transport and structural characterizations of the newly discovered superconductor FeS. It is found that superconductor FeS (tetragonal) partly transforms to a hexagonal structure at 0.4 GPa, and then completely transforms to an orthorhombic phase at 7.4 GPa and finally to a monoclinic phase above 9.0 GPa. The superconducting transition temperature of tetragonal FeS was gradually depressed by pressure, different from the case in tetragonal FeSe. With pressure increasing, the S-Fe-S angles only slightly change but the anion height deviates farther from 1.38 Å. This change of anion height, together with the structural instability under pressure, should be closely related to the suppression of superconductivity. We also observed an anomalous metal-semiconductor transition at 6.0 GPa and an unusual increased resistance with further compression above 9.6 GPa. The former can be ascribed to the tetragonal-orthorhombic structural phase transition, and the latter to the electronic structure changes of the high-pressure monoclinic phase. Finally, a phase diagram of tetragonal FeS as functions of pressure and temperature was mapped out for the first time, which will shed new light on understanding of the structure and physics of the superconducting FeS.

Synthesis, structure, magnetic and photoelectric properties of Ln3M0.5M′Se7 (Ln = La, Ce, Sm; M = Fe, Mn; M′ = Si, Ge) and La3MnGaSe7

Six new isostructural compounds, with the formulas La3Fe0.5GeSe7, La3MnGaSe7, Ce3Fe0.5SiSe7, Ce3Mn0.5SiSe7, Sm3Fe0.5SiSe7 and Sm3Mn0.5GeSe7, have been successfully synthesized via a molten salt method. Their structures are determined by single crystal X-ray diffraction and they crystallize in the Ce6Al3.33S14 structure type (space group: P63, Pearson symbol: hP24). Pure phases of the Ce3Fe0.5SiSe7, Ce3Mn0.5SiSe7, Sm3Fe0.5SiSe7 and Sm3Mn0.5GeSe7 compounds were obtained by solid state reaction and were characterized by powder X-ray diffraction (PXRD), scanning electron microscope (SEM), ultraviolet-visible-infrared (UV-vis-IR) absorbance spectroscopy, and magnetization measurements. The Ce3Fe0.5SiSe7 and Ce3Mn0.5SiSe7 compounds show paramagnetic domination accompanied by antiferromagnetic contributions, while the Sm3Mn0.5GeSe7 and Sm3Fe0.5SiSe7 compounds show anti-ferromagnetic phase transitions with Neel temperatures of 13 K and 24 K, respectively. Optical measurements reveal that all of the four compounds can absorb most of visible light. These four compounds also show photoelectric properties with the photocurrent densities of 81, 1.3, 1.8 and 0.8 μA cm−2, respectively.

Effects of Iron Doping on the Physical Properties of Quaternary Ferromagnetic Sulfide: Ba2Fe0.6V1.4S6

The mixed-metal sulfide compound with the formula Ba2Fe0.6V1.4S6 was successfully synthesized via solid-state reaction. Ba2Fe0.6V1.4S6 has a quasi-one-dimensional structure and crystallizes in the hexagonal space group P63/mmc. The structure is composed of face-sharing anion octahedron [MS6]8- (M = V or Fe) units to construct infinite chains along the c axis, in which the Fe atoms randomly occupy the V sites. The Ba2+ ions reside between adjacent chains. Magnetic susceptibility measurements reveal a transition between paramagnetism and ferromagnetism around 25 K. The small polaron hopping (SPH) conduction behavior has been observed in the higher temperature region (75-300 K), while in the lower temperature region (25-74 K), the resistivity features a variable range hopping mechanism (VRH). The analysis of density of states indicates that Fe-3dz2 and S-3p states mainly dominate the valence band maximum, while Fe-3dz2 states contribute significantly to the magnetic susceptibility.