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-The α-polymorph of the simple binary FeSe phase has recently emerged as a superconductor with an ambient P T c of ∼8-13 K. 1,2 Its structure comprises stacks of edgesharing FeSe 4 tetrahedra with a packing motif essentially identical to that of the FeAs layers in the families of the FeAs-based high-T c superconductors 3,4,5,6,7 but lacking any interleaved ion spacers or insulating layers. The structural analogy is reinforced by the observation that below 70 K the high-temperature crystal structure becomes metrically orthorhombic (space group Cmma), 8 displaying an identical distortion of the FeSe layers to that observed in the iron oxyarsenide family. 9,10 Theoretical calculations also find a very similar 2D electronic structure to that of the FeAs-based superconductors with cylindrical electron sections at the zone corner and cylindrical hole surface sections. 11 Moreover, superconductivity in FeSe is very sensitive to defects and disorder and occurs over a limited range of FeSe 1−δ nonstoichiometry. 12The effect of applied pressure on T c provides crucial information in differentiating between competing models of superconductivity and in the FeSe binary, T c is initially extremely sensitive to P and rises rapidly to 27 K at 1.48 GPa. 2 At the same time, antiferromagnetic spin fluctuations present above T c are strongly enhanced by pressure. 13 In the FeAs-based superconductors, the response of T c to pressurization is complex and sensitively depends on the composition of the materials and their doping level. Both positive and negative initial pressure coefficients, dT c /dP have been measured. Typically for the REFeAsO 1−x F x families, dT c /dP is positive at low doping levels and switches over to a negative value as x increases. 14,15,16,17,18 Moreover, for systems where the initial dT c /dP is positive, there is a critical value of P above which the trend is reversed and T c then decreases...

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High-resolution powder neutron diffraction has been used to study the crystal structure of the fullerene Cso in the temperature range 5 K to 320 K. Solid C , adopts a cubic structure at all temperatures. The experimental data provide clear evidence of a continuous phase transition at ca. 90 K and confirm the existence of a first-order phase transition at 260 K. In the hightemperature face-centred-cubic phase (T > 260 K), the Cs0 molecules are completely orientationally disordered, undergoing continuous reorientation. Below 260 K, interpretation of the diffraction data is consistent with uniaxial jump reorientation principally about a single (111) direction.In the lowest-temperature phase (T < 90 K), rotational motion is frozen although a small amount of static disorder still persists. 1. Introduction. c 6 0 buckminsterfullerene [l-31 is the most stable member of the whole family of closed carbon cage molecules-the fullerenes [4,5]. Its extraction and purification from arc-processed carbon [2,31 have not only enabled the original structural proposal [ll to be confirmed [2,3,61 but have also led to numerous experiments that are revealing many novel physical and chemical properties [7]. At room temperature, crystalline c& adopts a face-centred cubic crystal structure in which each of the c 6 0 molecules is orientationally disordered [8,9]. The structure may be regarded as cubic-closed-packed in which rotations and orientations of individual c 6 0 molecules are uncorrelated with their neighbours [9]. 13C NMR [lo-121 and quasi-elastic neutron scattering [13] measurements confirm this rapid isotropic reorientation at room temperature that results in time-averaging of the truncated icosahedron to spherical symmetry. Perhaps surprisingly in view of the almost spherical symmetry, this orientational disorder does not persist to low temperatures. Differential scanning calorimetry (DSC) [14] and X-ray diffraction [9] measurements established the existence of a first-order phase transition near 250 K. More recent work [9,15,16] has confirmed an ordered simple cubic crystal structure for Cw at low temperatures. The reason for the orientational order has been discussed in terms of van der Waals bonding and electrostatic repulsion that results in the facing of the

The newly discovered superconductor FeSe(1-x) (x approximately 0.08, T(c)(onset) approximately 13.5 K at ambient pressure rising to 27 K at 1.48 GPa) exhibits a structural phase transition from tetragonal to orthorhombic below 70 K at ambient pressure-the crystal structure in the superconducting state shows remarkable similarities to that of the REFeAsO(1-x)F(x) (RE = rare earth) superconductors.

The crystal structure of a solid controls the interactions between the electronically active units and thus its electronic properties. In the high-temperature superconducting copper oxides, only one spatial arrangement of the electronically active Cu(2+) units-a two-dimensional square lattice-is available to study the competition between the cooperative electronic states of magnetic order and superconductivity. Crystals of the spherical molecular C(60)(3-) anion support both superconductivity and magnetism but can consist of fundamentally distinct three-dimensional arrangements of the anions. Superconductivity in the A(3)C(60) (A = alkali metal) fullerides has been exclusively associated with face-centred cubic (f.c.c.) packing of C(60)(3-) (refs 2, 3), but recently the most expanded (and thus having the highest superconducting transition temperature, T(c); ref. 4) composition Cs(3)C(60) has been isolated as a body-centred cubic (b.c.c.) packing, which supports both superconductivity and magnetic order. Here we isolate the f.c.c. polymorph of Cs(3)C(60) to show how the spatial arrangement of the electronically active units controls the competing superconducting and magnetic electronic ground states. Unlike all the other f.c.c. A(3)C(60) fullerides, f.c.c. Cs(3)C(60) is not a superconductor but a magnetic insulator at ambient pressure, and becomes superconducting under pressure. The magnetic ordering occurs at an order of magnitude lower temperature in the geometrically frustrated f.c.c. polymorph (Néel temperature T(N) = 2.2 K) than in the b.c.c.-based packing (T(N) = 46 K). The different lattice packings of C(60)(3-) change T(c) from 38 K in b.c.c. Cs(3)C(60) to 35 K in f.c.c. Cs(3)C(60) (the highest found in the f.c.c. A(3)C(60) family). The existence of two superconducting packings of the same electronically active unit reveals that T(c) scales universally in a structure-independent dome-like relationship with proximity to the Mott metal-insulator transition, which is governed by the role of electron correlations characteristic of high-temperature superconducting materials other than fullerides.