Crystal structure of the new FeSe1-x superconductor (original) (raw)
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EPL (Europhysics Letters), 2012
PACS 62.50.-p-High-pressure effects in solids and liquids PACS 74.62.Fj-Effects of pressure on superconducting transition temperature PACS 61.50.Ks-Crystallographic aspects of phase transformations; pressure effects Abstract-Simultaneous high pressure x-ray diffraction and electrical resistance measurements have been carried out on a PbO type α-FeSe 0.92 compound to a pressure of 44 GPa and temperatures down to 4 K using designer diamond anvils at synchrotron source. At ambient temperature, a structural phase transition from a tetragonal (P4/nmm) phase to an orthorhombic (Pbnm) phase is observed at 11 GPa and the Pbnm phase persists up to 74 GPa. The superconducting transition temperature (T C) increases rapidly with pressure reaching a maximum of ~28 K at ~ 6 GPa and decreases at higher pressures, disappearing completely at 14.6 GPa. Simultaneous pressure-dependent x-ray diffraction and resistance measurements at low temperatures show superconductivity only in a low pressure orthorhombic (Cmma) phase of the α-FeSe 0.92. Upon increasing pressure at 10 K near T C , crystalline phases change from a mixture of orthorhombic (Cmma) and hexagonal (P63/mmc) to a high pressure orthorhombic (Pbnm) phase near 6.4 GPa where T C is maximum. Introduction.-The pressure variable has always played a pivotal role in the discovery and optimization of novel superconducting materials. Discovery of high temperature superconductivity in a new class of iron-based layered compounds has received extensive attention recently [1-6]. Undoped iron-based layered compounds like REOFeAs (RE = trivalent rare earth metal), and AFe 2 As 2 (A = divalent alkaline earth metal) are non-superconducting at ambient pressure and are known to exhibit tetragonal to orthorhombic structural transition and antiferromagnetic (AFM) ordering on cooling. The AFM ordering and structural transition is suppressed under high pressure or chemical doping and superconductivity is induced [1-4]. However, the critical relationships between structure, magnetism, and superconductivity still remain unresolved. More recently, superconductivity was reported at 8 K in α-FeSe 1-δ samples with PbO-type tetragonal structure [5]. At ambient conditions, α-FeSe 1-δ has a structure composed of stacks of edge-sharing FeSe 4tetrahedral layers stacked along c-axis [5-7] while, the structure of FeAs-based superconductors consists of edge sharing FeAs 4tetrahedra stacked layer by layer with separating elements like REO in REOFeAs or A in AFe 2 As 2 between the FeAs 4 layers [1, 2]. The tetragonal α-FeSe undergoes a structural phase transition to an orthorhombic (Cmma) below 70 K upon cooling [7]. One remarkable aspect of superconductivity in binary FeSe-system is the strong relationship between the superconducting state and pressure. Recently, the T C onset was shown to increase at huge rate of dTc/dP = 9.1 K/GPa (the largest for any of the known FeAs-compounds), dramatically reaching an onset of 27 K at 1.5 GPa [6]. This sensitivity of T C to pressure convincingly indicates that there is a strong correlation between the superconducting properties and changes in the crystal structure of FeSe-system under pressure. A number of pressure-dependent structural and resistance measurements of the layered Fe-based systems have been reported that are aimed at understanding the critical relationship between compression behavior of crystal structure and superconductivity [6-11]. However, none of the previous works reported simultaneous high pressure and low temperature resistivity and x-ray diffraction measurements on the same sample in the same experiment. Here, we report simultaneous high pressure resistance and x-ray diffraction experiments using a designer-DAC to precisely elucidate the effect of pressure on the observed superconducting properties and the local crystallographic modulations of FeSe 1-δ. In addition, there have been some disagreements on structural phase modulations in FeSe 1-δ system under pressure. While some report a tetragonal to a hexagonal phase (P63/mmc) transition above 12 GPa [8, 9], a phase transition from the
Physical Review B, 2009
FeSe with the PbO structure is a key member of the family of new high-Tc iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted FeSe4 tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that application of hydrostatic pressure first rapidly increases Tc which attains a broad maximum of 37 K at ∼7 GPa (this is one of the highest Tc ever reported for a binary solid) before decreasing to 6 K upon further compression to ∼14 GPa. Complementary synchrotron X-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of α-FeSe, revealing an extremely soft solid with a bulk modulus, K0 = 30.7(1.1) GPa and strong bonding anisotropy between inter-and intra-layer directions that transforms to the more densely packed β-polymorph above ∼9 GPa. The non-monotonic Tc(P ) behavior of FeSe coincides with drastic anomalies in the pressure evolution of the interlayer spacing, pointing to the key role of this structural feature in modulating the electronic properties. PACS numbers: 74.70.Dd, 74.25.Ha, 61.05.C-
Structural and superconductivity study on α-FeSex
Journal of Physics and Chemistry of Solids, 2010
We have successfully synthesized the a-FeSe x binary tetragonal superconductors with nominal composition of FeSe x (x= 0.6-1.0) via conventional solid state reactions between Fe and Se sealed in quartz tubes. Fe and b-FeSe are the most commonly seen impurities in this binary system. A lowtemperature annealing at 400 1C is found to be crucial to remove b-FeSe, which is the thermodynamic stable phase with hexagonal symmetry. For all the samples of FeSe x , superconductivity is confirmed by magnetic measurements as well as resistivity measurements with their T c at around 8 K. We noticed that their T c does not vary with the different nominal Se amount. High-resolution synchrotron X-ray diffraction analysis revealed that the unit cell parameters of all these samples do not change within the error range, and their structure only tolerate the same very small amount of Se deficiency. Based on this study, we concluded that the a-FeSe x superconductor only exist in a very narrow deficiency range.
FAST TRACK COMMUNICATION: Superconducting and normal phases of FeSe single crystals at high pressure
Journal of Physics-condensed Matter, 2009
We report on the synthesis of superconducting single crystals of FeSe, and their characterization by X-ray diffraction, magnetization and resistivity. We have performed ac susceptibility measurements under high pressure in a hydrostatic liquid argon medium up to 14 GPa and we find that TC increases up to 33-36 K in all samples, but with slightly different pressure dependences on different samples. Above 12 GPa no traces of superconductivity are found in any sample. We have also performed a room temperature high pressure X-ray diffraction study up to 12 GPa on a powder sample, and we find that between 8.5 GPa and 12 GPa, the tetragonal PbO structure undergoes a structural transition to a hexagonal structure. This transition results in a volume decrease of about 16%, and is accompanied by the appearance of an intermediate, probably orthorhombic phase.
Tetragonal-to-Orthorhombic Structural Phase Transition at 90 K in the Superconductor Fe1.01Se
Physical Review Letters, 2009
In this letter we show that superconducting Fe 1.01 Se undergoes a structural transition at 90 K from a tetragonal to an orthorhombic phase but that non-superconducting Fe 1.03 Se does not. Further, high resolution electron microscopy study at low temperatures reveals an unexpected additional modulation of the crystal structure of the superconducting phase involving displacements of the Fe atoms, and that the non-superconducting material shows a distinct, complex nanometerscale structural modulation. Finally, we show that magnetism is not the driving force for the phase transition in the superconducting phase.
2010
Syntheses of superconducting iron chalcogenides FeSe1-x (x = 0-0.15) and FeTe1-ySey (y = 0.3-0.55) were performed. Superconducting phase of iron selenide was obtained by the solid-state reaction and from liquid phase. The highest values of critical temperature (T-c = 8.2-8.7 K) exhibit FeSe1-x obtained by the crystallization from a melt with excess of iron less than 1 mol\%. The samples from a melt contain up to 78\% of tetragonal phase, as estimated by the X-ray diffraction. Lattice parameters and unit cell volume for the samples exhibiting highest T-c and sharpest transition to superconducting state are limited to narrow range, with c/a ratio close to 1.469. The samples with excess of selenium contain higher amount of hexagonal phase than stoichiometric one. Superconducting single-crystalline samples of FeTe1-ySey (up to 100\% of tetragonal phase) were obtained using Bridgman's method. When y value increases, the volume of unit cell decreases. The critical temperature T-c cha...
Pressure-induced effects on the structure of the FeSe superconductor
Solid State Communications, 2009
A polycrystalline sample of FeSe, which adopts the tetragonal PbO-type structure (P4/nmm) at room temperature, has been prepared using solid state reaction. We have investigated pressure-induced structural changes in tetragonal FeSe at varying hydrostatic pressures up to 0.6 GPa in the orthorhombic (T=50 K) and tetragonal (T =190 K) phases using high resolution neutron powder diffraction. We report that the structure is quite compressible with a Bulk modulus ≈31 GPa to 33 GPa and that the pressure response is anisotropic with a larger compressibility along the cc-axis. Key bond angles of the SeFe4 pyramids and FeSe4 tetrahedra are also determined as a function of pressure.
Synthesis and Physical Properties of FeSe1/2Te1/2 Superconductor
2009
One of the most important properties of very recently reported FeSe based superconductors is the robustness of their superconductivity under applied magnetic field. The synthesis and control of superconductivity in FeSe based compounds is rather a difficult task. Synthesis and physical property characterization for optimized superconductivity of FeSe1/2Te1/2 at 13 K is reported here. The compound crystallized in a tetragonal structure with lattice parameters a = 3.8008(10) and c = 6.0187 (15) A. Magnetization measurements indicated bulk superconductivity with lower critical field (Hc1) of around 180 Oe. By applying Ginzburg Landau (GL) theory, the Hc2(0) value is estimated to be = 1840 kOe for the 90% of resistive transition. A heat capacity measurement revealed bulk superconductivity by a hump at Tc near 13 K, and an expected decrease was observed under an applied magnetic field.