Phase diagrams of as-grown and O2-annealed FeTe1-xSex determined from magnetic susceptibility measurement were obtained. For as-grown samples, the antiferromagnetic order was fully suppressed in the range region x \geq 0.15, and weak superconductivity appeared when x \geq 0.1. Beginning at x = 0.5, weak superconductivity was found to evolve into bulk superconductivity. Interestingly, for O2-annealed samples, complete suppression of magnetic order and the occurrence of bulk superconductivity were observed when x \geq 0.1. We found that O2-annealing induces bulk superconductivity for FeTe1-xSex. Oxygen probably plays a key role in the suppression of the magnetic order and the appearance of bulk superconductivity.
We have systematically investigated the annealing effect on the superconductivity of iron chalcogenide Fe1+y(Te1-xSex). The atmospheres used for annealing include O2, N2, I2 vapor, air and vacuum. We observed that annealing in O2, I2 and air could enhance superconductivity for the underdoped samples, consistent with the results reported in literatures. Interestingly, we found that annealing in N2 also leads to superconductivity enhancement, similar to the annealing effects of O2, I2 and air. However, vacuum annealing does not enhance superconductivity, which indicates that the enhanced superco…
We observed bulk superconductivity in the FeTe1-xSex crystals with a low Se concentration after the excess Fe atoms existing unavoidably in crystals as-grown are partially removed by means of annealing in the air. A revised magnetism and superconductivity phase diagram is obtained via resistivity and magnetic susceptibility measurements for FeTe1-xSex system. It is found that bulk superconductivity coexists with antiferromagnetic order in the crystals with 0.05<x<0.20. The phase diagram is very similar to the case of the K-doping or Co-doping BaFe2As2, as well as of SmFeAsO1-xFx iron-pnictide …
The iron chalcogenide Fe1+y(Te1-xSex) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe1+yTe exhibits antiferromagnetic order with in-plane magnetic wave-vector (pi, 0). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave-vector (pi, pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe-superconductors exhibit superconducting spin resonances around (pi, pi), suggesting a common symm…
Ordering of Interstitial Iron Atoms and Local Structural Distortion Induced by Iron Polycomplex in Fe1+yTe1-xSex as Seen via Transmission Electron Microscopy
[(Li0.8Fe0.2)OH]FeS and the series [(Li0.8Fe0.2)OH]Fe(S1-xSex) (0<x<1) were synthesized by hydrothermal methods and characterized by X-ray single crystal and powder diffraction, EDX and chemical analysis. Selenium-rich compounds show the coexistence of magnetic ordering with superconductivity known from the pure selenium compound. Sulphur doping decreases the critical temperature through chemical pressure until superconductivity is completely absent in [(Li0.8Fe0.2)OH]FeS, while the ferromagnetism in the [(Li0.8Fe0.2)OH] layers persists. The Li:Fe ratio in the hydroxide layer, and thus the cha…
Among iron based superconductors, the layered iron chalcogenide Fe(Te1-xSex) is structurally the simplest and has attracted considerable attentions. It has been speculated from bulk studies that nanoscale inhomogeneous superconductivity may inherently exist in this system. However, this has not been directly observed from nanoscale transport measurements. In this work, through simple micromechanical exfoliation and high precision low-energy ion milling thinning, we prepared Fe(Te0.5Se0.5) nano-flake with various thickness and systematically studied the correlation between the thickness and sup…
We report a distinct experimental approach to point-contact Andreev reflection spectroscopy with diagnostic capability via a unique design of nano-scale normal metal/superconductor devices with excellent thermo-mechanical stability, and have employed this method to unveil the existence of two superconducting energy gaps in iron chalcogenide Fe1+yTe1-xSex which is crucial for understanding its pairing mechanism. This work opens up new opportunities to study gap structures in superconductors and elemental excitations in solids.
We have performed systematic angle-resolved photoemission spectroscopy (ARPES) of iron-chalcogenide superconductor FeTe1-xSex to elucidate the electronic states relevant to the superconductivity. While the Fermi-surface shape is nearly independent of x, we found that the ARPES spectral line shape shows prominent x dependence. A broad ARPES spectrum characterized by a small quasiparticle weight at x = 0, indicative of incoherent electronic states, becomes progressively sharper with increasing x, and a well-defined quasiparticle peak appears around x = 0.45 where bulk superconductivity is realiz…
We report synthesis of under doped non-superconducting parent compound of iron chalcogenide, i.e FeTe1-xSex (x=0.0, 0.01, 0.03, 0.05) single crystals by flux free method. These FeTe1-xSex (x=0.0, 0.01, 0.03, 0.05) single crystals are crystallized in the tetragonal structure with space group P4/nmm at room temperature. Detailed scanning electron microscopy (SEM) results showed that the crystals are formed in slab-like morphology structure. Powder X-Ray diffraction (XRD) results show that (0 0 l) peaks are shifted towards higher angel with increasing Se concentration. Coupled magnetic and struct…
We discovered a novel annealing method for Fe-chalcogenide superconductors. It was found that sulfur annealing deintercalated excess Fe via formation of FeS2. Due to its specifics, sulfur annealing is applicable when preparing Fe-chalcogenide-based wires or cables.
Superconducting single crystals of pure FeTe1 xSex and FeTe0.65Se0.35 doped with Co, Ni, Cu, Mn, Zn, Mo, Cd, In, Pb, Hg, V, Ga, Mg, Al, Ti, Cr, Sr or Nd into Fe ions site have been grown applying Bridgman's method. It has been found that the sharpness of transition to the superconducting state in FeTe1 xSex is evidently inversely correlated with crystallographic quality of the crystals. Among all of the studied dopants only Co, Ni and Cu substitute Fe ions in FeTe0.65Se0.35 crystals. The remaining examined ions do not incorporate into the crystal structure. Nevertheless, they form inclusions t…
Superconductivity in anti-PbO-type iron chalcogenides Fe1-xTe1-ySey (x = 0, 0.1, y = 0.1 0.4) depends on the amount (x) of interstitial iron atoms located between the FeTe1-ySey layers. Non-superconducting samples of nominal Fe1.1Te1-ySey convert to superconductors with critical temperatures up to 14 K after annealing at 300°C in an oxygen atmosphere. The process is irreversible upon subsequent hydrogen annealing. Magnetic measurements are consistent with the formation of iron oxides suggesting that oxygen annealing preferably extracts interstitial iron from Fe1-xTe1-ySey which interfere with …
The iron-based superconductors had a significant impact on condensed matter physics. They have a common structural motif of a two-dimensional square iron lattice and exhibit fruitful physical properties as a strongly correlated electron system. During the extensive investigations, quasi-one-dimensional iron-based spin-ladder compounds attracted much attention as a platform for studying the interplay between magnetic and orbital ordering. In these compounds, BaFe2S3 and BaFe2Se3 were found to exhibit superconductivity under high pressure, having a different crystal and magnetic structure at low…
It has been clarified that bulk superconductivity in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ can be induced by annealing in an appropriate atmosphere to remove the harmful effects of excess iron. In order to clarify the details of the annealing process, we studied the changes in the physical properties and reaction products of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ annealed in pnictogen (P, As, Sb) atmospheres. Crystals annealed in a pnictogen atmosphere show bulk superconductivity and the values of $T_{c}$ and $J_{c}$ are about $14~$K and 2-4 $\times 10{^5}~$A/cm$^2$ ($2~$K, self-field), respectively. It is a…
Iron chalcogenide Fe(Te,Se) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors. By intercalating spacer layers, superconducting transition temperature has been raised over 40 K. On the other hand, the presence of excess Fe is almost unavoidable in Fe(Te,Se) single crystals, which hinders the appearance of bulk superconductivity and causes strong controversies over its fundamental properties. Here we report a syst…
Thermal conductivity measurements in magnetic fields have been carried out for FeSe$_{0.3}$Te$_{0.7}$ single crystals as-grown and annealed at 400 {\degree}C for 100 h in vacuum ($\sim$ 10$^{-4}$ Pa). It has been found that the thermal conductivity in the {\it ab}-plane, {\kab}, of the annealed crystal shows an enhancement at low temperatures just below the superconducting transition temperature, {\tc}, owing to the thermal conductivity due to quasiparticles, while {\kab} of the as-grown crystal does not. This suggests that FeSe$_{1-x}$Te$_{x}$ is a strongly correlated electron system. It has …
The search for Majorana bound state (MBS) has recently emerged as one of the most active research areas in condensed matter physics, fueled by the prospect of using its non-Abelian statistics for robust quantum computation. A highly sought-after platform for MBS is two-dimensional topological superconductors, where MBS is predicted to exist as a zero-energy mode in the core of a vortex. A clear observation of MBS, however, is often hindered by the presence of additional low-lying bound states inside the vortex core. By using scanning tunneling microscope on the newly discovered superconducting…
The momentum distribution of the energy gap opening at the Fermi level of superconductors is a direct fingerprint of the pairing mechanism. While the phase diagram of the iron-based superconductors promotes antiferromagnetic fluctuations as a natural candidate for electron pairing, the precise origin of the interaction is highly debated. We used angle-resolved photoemission spectroscopy to reveal directly the momentum distribution of the superconducting gap in FeTe1-xSex, which has the simplest structure of all iron-based superconductors. We found isotropic superconducting gaps on all Fermi su…
Co-doped BaFe2As2 has been previously shown to have an unusually significant improvement of Tc (up to 2 K, or almost 10%) with annealing 1-2 weeks at 700 or 800 C, where such annealing conditions are insufficient to allow significant atomic diffusion. While confirming similar behavior in optimally Co-doped SrFe2As2 samples, the influence on Tc of strain induced by grinding to ~50 micron sized particles, followed by pressing the powder into a pellet using 10 kbar pressure, was found to increase the annealed transition width of 1.5 K by approximately a factor of ten. Also, the bulk discontinuity…
We have grown single crystals of the iron-based ladder material BaFe_2S_3, which is superconductive under high pressure, adopting different conditions. By comparing the behaviors of these samples, it is found that the in situ annealing process can affect the crystal structure and the electrical transport, enhance the antiferromagnetic transition temperature, and reduce the extrinsic ferromagnetic component of the system. An in-depth analysis indicates that the crystal quality is improved by the in situ annealing in terms of reducing both the Fe deficiency and the Fe impurity in the samples. Th…
Since the discovery of superconductivity at 26 K in oxy-pnictide LaFeAsO1-xFx, enormous interests have been stimulated in the field of condensed matter physics and material sciences. Among the many kind of structures in the iron pnictide superconductors, FeSe with the PbO structure has received special attention since there is not poisonous pnictogen element in chemical composition and its structure is the simplest one. However, the superconducting transition temperature (Tc) in iron chalcogenide compounds is not enhanced as high as other iron pnictide superconductors under ambient pressure un…
The difference in thermal stability of F- and H- in 1111-type iron based superconductors, allows selective hydrogen elimination from non-superconductive CaFeAsF0.8H0.2 by thermal annealing. The analyzed chemical composition of the resulting samples indicates that incorporated hydrogen was selectively eliminated by thermal annealing at 553 K for 72 hours. The resulting hydrogen-eliminated sample shows bulk superconductivity with Tc = 29 K. This technique may be used for indirect electron doping for AeFeAeF (Ae; alkali-earth metal) iron based superconductor described by CaFeAsF1-xHx => CaFeAsF1-…
Effects of iodine annealing to induce bulk superconductivity in Fe1+yTe0.6Se0.4 have been systematically studied by changing the molar ratio of iodine to the sample and annealing temperature. The optimal condition to induce bulk superconductivity with Tc ~14.5 K and self-field Jc(2 K) ~ 5x10^5 A/cm2 is found to be a molar ratio of iodine of 5-7 % at the annealing temperature of 400 C. Furthermore, the fact that no compounds containing iodine are detected in the crystal and a significant amount of FeTe2 is produced after the iodine annealing strongly indicate that the excess iron is consumed to…
Topological superconductors, whose edge hosts Majorana bound states or Majorana fermions that obey non-Abelian statistics, can be used for low-decoherence quantum computations. Most of the proposed topological superconductors are realized with spin-helical states through proximity effect to BCS superconductors. However, such approaches are difficult for further studies and applications because of the low transition temperatures and complicated hetero-structures. Here by using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we discover that the iron-based supercondu…
We have systematically investigated and compared different methods to induce superconductivity in iron chalcogenide Fe1+yTe0.6Se0.4 including annealing in vacuum, N2, O2, I2 atmosphere, and immersing samples into acid and alcoholic beverages. Vacuum and N2 annealing are proved to be ineffective to induce superconductivity in Fe1+yTe0.6Se0.4 single crystal. O2 and I2 annealing, acid and alcoholic beverages can induce superconductivity by oxidizing the excess Fe in the sample. Superconductivity in O2 annealed sample is in bulk nature, while I2, acid and alcoholic beverages can only induce superc…
In this paper, we address a number of outstanding issues concerning the nature and the role of magnetic inhomogenities in the iron chalcogenide system FeTe1-xSex and their correlation with superconductivity in this system. We report morphology of superconducting single crystals of FeTe0.65Se0.35 studied with transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy and their magnetic and superconducting properties characterized with magnetization, specific heat and magnetic resonance spectroscopy. Our data demonstrate a presence of nanometre scal…
The effect of vacuum annealing on superconductivity is investigated in Fe(Se,Te) single crystals. It is found that superconductivity is not enhanced by annealing under high vacuum (~ 10^(-3) Pa) or by annealing in a sealed evacuated quartz tube. In a moderate vacuum atmosphere (~ 1 Pa), iron oxide layers are found to show up on sample surfaces, which would draw excess Fe out of the crystal. Thus, it is suggested that remanent oxygen effectively works to remove excess Fe from the matrix of Fe(Se,Te) crystals, resulting improvement of superconducting transition temperature. Our transport measure…
New iron-based intercalation superconductors Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) with Tc = 45 K have successfully been synthesized via intercalation of dissolved alkaline metal in ethylenediamine. The c-axis lengths of Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) are 20.74(7) Å and 21.9(1) Å, respectively, and are about 50 % larger than that of KxFe2Se2, indicating that not only alkaline metal but also ethylenediamine is intercalated between the Se-Se layers of FeSe. It seems that the high-Tc of Ax(C2H8N2)yFe2-zSe2 (A = Li, Na) is caused by the possible two-dimensional electronic structure due to the large c…
Following the discovery of superconductivity in epsilon-iron, subsequent experiments hinted at non-Fermi liquid behaviour of the normal phase and sensitive dependence of the superconducting state on disorder, both signatures of unconventional pairing. We report further resistive measurements under pressure of samples of iron from multiple sources. The normal state resistivity of epsilon-iron varied as rho_0+AT^{5/3} at low temperature over the entire superconducting pressure domain. The superconductivity could be destroyed by mechanical work, and was restored by annealing, demonstrating sensit…
The parent compound of an unconventional superconductor must contain unusual correlated electronic and magnetic properties of its own. In the high-Tc potassium intercalated FeSe, there has been significant debate regarding what the exact parent compound is. Our studies unambiguously show that the Fe-vacancy ordered K2Fe4Se5 is the magnetic, Mott insulating parent compound of the superconducting state. Non-superconducting K2Fe4Se5 becomes a superconductor after high temperature annealing, and the overall picture indicates that superconductivity in K2-xFe4+ySe5 originates from the Fe-vacancy ord…
By direct quenching or post-annealing followed by quenching, we have successfully obtained a series of KxFe2-ySe2 samples with different properties. It is found that the samples directly quenched in the cooling process of growth show superconductivity and the one cooled with furnace is insulating even though their stoichiometries are similar. The sample cooled with furnace can be tuned from insulating to superconducting by post-annealing and then quenching. Based on the two points mentioned above, we conclude that the superconducting state in KxFe2-ySe2 is metastable, and quenching is the key …
Two iron-chalcogenide superconductors Li(x)[C5H5N](y)Fe(2-z)Se2 and Cs(x)Fe(2-z)Se2 in the as-prepared and annealed state have been investigated by means of the Moessbauer spectroscopy versus temperature. Multi-component spectra are obtained. One can see a non-magnetic component due to iron located in the unperturbed Fe-Se sheets responsible for superconductivity. Remaining components are magnetically ordered even at room temperature. There is some magnetically ordered iron in Fe-Se sheets perturbed by presence of the iron vacancies. Additionally, one can see iron dispersed between sheets in t…
We have studied local magnetic moment and electronic phase separation in superconducting K$_{x}$Fe$_{2-y}$Se$_2$ by x-ray emission and absorption spectroscopy. Detailed temperature dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properties. By cooling down, the local magnetic moment of Fe shows a sharp drop across the superconducting transition temperature (T$_c$) and the coexisting phases exchange spectral weights with the low spin state gaining intensity at the expense of the higher spin state. After annealing the sa…
We report on a new iron selenide superconductor with a TC onset of 45K and the nominal composition Lix(C5H5N)yFe2-zSe2, synthesized via intercalation of dissolved alkaline metal in anhydrous pyridine at room temperature. This superconductor exhibits a broad transition, reaching zero resistance at 10K. Magnetization measurements reveal a superconducting shielding fraction of approximately 30%. Analogous phases intercalated with Na, K and Rb were also synthesized and characterized. The superconducting transition temperature of Lix(C5H5N)yFe2-zSe2 is clearly enhanced in comparison to the known su…
We show that the interference between scattering by impurities and by critical spin fluctuations gives rise to anisotropic transport in the Ising-nematic state of the iron pnictides. The effect is closely related to the non-Fermi liquid behavior of the resistivity near an antiferromagnetic quantum critical point. Our theory not only explains the observed sign of the resistivity anisotropy $Δρ$ in electron doped systems, but also predicts a sign change of $Δρ$ upon sufficient hole doping. Furthermore, our model naturally addresses the changes in $Δρ$ upon sample annealing and alkaline-earth sub…
In the layered iron-based superconductor YFe2Ge2, a high Sommerfeld ratio of ~100 mJ/molK^2 and a T^(3/2) temperature dependence of the electrical resistivity at low temperature T indicate strong electronic correlations and point towards an unconventional pairing state. We have investigated the role of composition and annealing conditions in optimizing the growth of high-quality YFe2Ge2. Our findings confirm that bulk superconductivity is observed in samples with disorder scattering rates less than 2 k_B T_c/hbar. Fe deficiency on the Fe site is identified as the dominant source of disorder, w…
The c-axis textured Sr1-xKxFe2As2 tapes produced by cold rolling and post-annealing, could carry a high super-current over 2*104 A/cm2. However, the magnitude is far from its maximum, because of the current obstacles associated with various defects in the material. To predict the maximal transport critical current density, we modeled the current paths in a c-axis textured polycrystal as a three-dimensional flow network, and calculated the maximum flow with the Ford-Fulkerson algorithm. It indicates that a much higher super-current of about 2*105 A/cm2 could be achieved in an ideal c-axis textu…
The appearance of superconductivity at ambient pressures in the undoped iron pnictide parent compound SrFe2As2 is studied experimentally using several techniques and approaches to aid in understanding the nature of this phase. Low temperature magnetization measurements of single crystals of SrFe2As2 reveal diamagnetic screening due to the onset of superconductivity below 21 K, with volume fraction estimates varying between 0 and 15 percent. The effects of heat treatment and cold-working via severe mechanical deformation on the superconducting phase found are studied, showing that superconducti…
We study the relationship between precipitate morphology and superconductivity in KxFe1.6+ySe2 single crystals grown by self-flux method. Scanning electron microscopy (SEM) measurements revealed that superconducting phase forms a network in the samples quenched above iron vacancy order-disorder transition temperature Ts. For the samples obtained by natural cooling down to room temperature in the furnace, referred to as furnace cooling, superconducting phase aggregates into micrometer-sized rectangular bars and aligns as disconnected chains. Accompanying this change in morphology the supercondu…
We report on temperature-dependent soft X-ray absorption spectroscopy (XAS) measurements utilizing linearly polarized synchrotron radiation to probe magnetic phase transitions in iron-rich Fe1+yTe. X-ray magnetic linear dichroism (XMLD) signals, which sense magnetic ordering processes at surfaces, start to increase monotonically below the Néel temperature TN = 57 K. This increase is due to a progressive bicollinear antiferromagnetic (AFM) alignment of Fe spins of the monoclinic Fe1+yTe parent phase. This AFM alignment was achieved by a [100]-oriented biasing field favoring a single-domain stat…
Here, we report the synthesis and characterization of sulphur-substituted iron telluride i.e. FeTe1-xSx; (x = 0-30 %) system and study the impact of low temperature oxygen (O2) annealing as well. Rietveld analysis of room temperature x-ray diffraction (XRD) patterns shows that all the compounds are crystallized in a tetragonal structure (space group P4/nmm) and no secondary phases are observed. Lattice constants are decreased with increasing S concentration. The parent compound of the system i.e. FeTe does not exhibit superconductivity but shows an anomaly in the resistivity measurement at aro…
London penetration depth, $λ(T)$, was measured in single crystals of SrFe$_2$(As$_{1-x}$P$_x$)$_2$ ($x=$0.35) iron - based superconductor. The influence of disorder on the transition temperature, $T_c$, and on $λ(T)$ was investigated. The effects of scattering controlled by the annealing of as-grown crystals was compared with the effects of artificial disorder introduced by 2.5~MeV electron irradiation. The low temperature behavior of $λ(T)$ can be described by a power-law function, $Δλ(T)=AT^n$, with the exponent $n$ close to one in pristine annealed samples, as expected for superconducting g…
Temperature-dependent London penetration depth, $λ(T)$, was measured in optimally - doped, $x=$0.35, as-grown ($T_c \approx$25 K, RRR=$ρ(300K)/ρ(T_c)$=4.5) and annealed ($T_c \approx$35 K, RRR=6.4) single crystals of SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ iron - based superconductor. Annealing increases the RRR and decreases the absolute value of the London penetration depth from $λ(0) = 300 \pm 10$ nm in as-grown sample to $λ(0) = 275 \pm 10$ nm. At low temperatures, $λ(T) \sim T$ indicating superconducting gap with line nodes. Analysis of the full - temperature range superfluid density is consisten…
We study the effects of Cu substitution in Fe1.1Te, the non-superconducting parent compound of the iron-based superconductor, Fe1+yTe1-xSex, utilizing neutron scattering techniques. It is found that the structural and magnetic transitions, which occur at \sim 60 K without Cu, are monotonically depressed with increasing Cu content. By 10% Cu for Fe, the structural transition is hardly detectable, and the system becomes a spin glass below 22 K, with a slightly incommensurate ordering wave vector of (0.5-d, 0, 0.5) with d being the incommensurability of 0.02, and correlation length of 12 angstrom…
We report systematic 31P-NMR study on iron (Fe)-based superconductors SrFe2(As_{1-x}P_{x})2 (Sr122AsP), in which a superconducting (SC) transition temperature Tc at x=0.35 increases from Tc=26 K up to 33 K by annealing an as-grown mono-crystalline sample. The present NMR study has unraveled that Tc reaches a highest value of 33 K at x=0.35 around a quantum critical point at which antiferromagnetic (AFM) order disappears. When noting that the SC transition disappears at x=0.6 where the AFM spin fluctuations (SFs) are no longer present, we remark that the onset and increase of Tc are apparently …
We investigate the magnetic pair-breaking due to Mn impurities in the optimally electron doped Sr(Fe$_{0.88} $Co$_{0.12} $)$_{2} $As$_2 $ superconductor to deduce the symmetry of the superconducting order parameter. Experiments on the as-grown crystals reveal a T$_c $ suppression rate of $ \sim $30 mK/$μΩcm$, which is in close agreement with similarly slower values of T$_c $ suppression rates reported previously for various transition metal impurities, both, magnetic and non-magnetic, in several structurally analogous iron-based superconductors. However, careful annealing of these crystals at …
The crystal and magnetic structures of the superconducting iron based chalcogenides RbyFe2-xSe2 have been studied by means of single crystal synchrotron x-ray and high resolution neutron powder diffraction in the temperature range 2-570 K. The ground state of the crystal is an intrinsically phase separated state with two distinct by symmetry phases. The main phase has the iron vacancy ordered \surd5x\surd5 superstructure (I4/m space group) with AFM ordered Fe spins. The minority phase does not have \surd5x\surd5-type of ordering and has smaller in plane lattice constant a and larger tetragonal…
