FeAs Compounds Research Articles

Role of covalent Fe-As bonding in the magnetic moment formation and exchange mechanisms in iron-pnictide superconductors

The electronic origin of the huge magnetostructural effect in layered Fe-As compounds is elucidated using LiFeAs as a prototype. The crucial feature of these materials is the strong covalent bonding between Fe and As, which tends to suppress the exchange splitting. The bonding-antibonding splitting is very sensitive to the distance between Fe and As nuclei. We argue that the fragile interplay between bonding and magnetism is universal for this family of compounds. The exchange interaction is analyzed in real space, along with its correlation with covalency and doping. The range of interaction and itinerancy increase as the Fe-As distance is decreased. Superexchange makes a large antiferromagnetic contribution to the nearest-neighbor coupling, which develops large anisotropy when the local moment is not too small. This anisotropy is very sensitive to doping.

Pressure-induced shift ofTcinKxSr1−xFe2As2(x=0.2,0.4,0.7): Analogy to the high-Tccuprate superconductors

The systematic pressure shifts of ${T}_{c}$ were investigated in the whole phase diagram of the FeAs-based superconducting compound ${\text{K}}_{x}{\text{Sr}}_{1\ensuremath{-}x}{\text{Fe}}_{2}{\text{As}}_{2}$. Different regions, arising from corresponding responses of ${T}_{c}$ to pressure ($d{T}_{c}/dp>0$, $\ensuremath{\simeq}0$, or $<0$), can be clearly distinguished. This reveals an interesting similarity of the FeAs superconductors and the high-${T}_{c}$ cuprates. This behavior is a manifestation of the layered structure of the FeAs compounds and the pressure-induced charge transfer between the $({\text{Fe}}_{2}{\text{As}}_{2})$ and (K/Sr) layers. The coexistence of superconductivity and spin-density wave behavior were also observed, and the pressure effects on the latter is explored.

LiFeAs: An intrinsic FeAs-based superconductor withTc=18 K

The synthesis and properties of LiFeAs, a high-${T}_{c}$ Fe-based superconducting stoichiometric compound, are reported. Single crystal x-ray studies reveal that it crystallizes in the tetragonal PbFCl type (P4/nmm) with $a=3.7914(7)\text{ }\text{\AA{}}$ and $c=6.364(2)\text{ }\text{\AA{}}$. Unlike the known isoelectronic undoped intrinsic FeAs compounds, LiFeAs does not show any spin-density wave behavior but exhibits superconductivity at ambient pressures without chemical doping. It exhibits a respectable transition temperature of ${T}_{c}=18\text{ }\text{K}$ with electronlike carriers and a very high critical field, ${\text{H}}_{c2}(0)>80\text{ }\text{T}$. LiFeAs appears to be the chemical equivalent of the infinite layered compound of the high-${T}_{c}$ cuprates.

Solid state phase equilibria in the Fe-Ga-As system

Solid state phase equilibria in the ternary Fe-Ga-As diagram were determined at 600 °C using powder X-ray diffraction as experimental techniques. Very limited solid solubilities were measured in the binary constituent Fe-Ga and Fe-As compounds. In the Fe-rich part of the diagram, a ternary phase was evidenced which corresponds in fact to a solid solution into which Ga and As substitute one another on the same sublattice. This phase, which can be expressed by the general formula Fe 3 Ga 2− x As x (0.20 ≤ x ≤ 1.125), crystallizes in hexagonal symmetry; it is structurally derived from the NiAs type-structure (B8 1 ) and can be considered, owing to its c/a ratio (1.23 ≤ c / a ≤ 1.28) close to , as hexagonal-pseudocubic. The original feature of this experimental diagram is the occurrence of a tie-line between this ternary phase and the semiconductor GaAs which differs significantly from the theoretical diagram, estimated from simplified calculations proposed by Schmid-Fetzer. In particular, it is the first time that such a ternary phase M x (Ga, As) y (M = transition metal) is found to be in thermodynamic equilibrium with GaAs. This fact suggests that the solid state interdiffusions which occur during the annealing of a Fe/GaAs heterostructure could lead to a ferromagnetic, epitaxial and stable Fe 3 GaAs/GaAs contact.

Weak ferromagnetism in Cr 1− xFe xAs compound system

Magnetic measurements reveal that the compounds Cr 1− x Fe x As with 0.1 < x < 0.9 are weak itinerant electron ferromagnets although CrAs and FeAs compounds have a similar double spiral spin ordering.