In situ x-ray diffraction measurements of MgSiO 3 were performed at high pressure and temperature similar to the conditions at Earth's core-mantle boundary. Results demonstrate that MgSiO 3 perovskite transforms to a new high-pressure form with stacked SiO 6 -octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D″ seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D″ discontinuity.

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...