We report the high-pressure structural characterization of an organic polyiodide salt in which ap rogressive addition of iodine to triiodide groups occurs.C ompression leads to the initial formation of discrete heptaiodide units, followed by polymerization to a3Danionic network. Although the structural changes appear to be continuous,t he insulating salt becomes as emiconducting polymer above1 0GPa.T he features of the pre-reactive state and the polymerized state are revealed by analysis of the computed electron and energy densities.T he unusually high electrical conductivity can be explained with the formation of new bonds.Polyiodides (PIs) in crystal form were discovered about 200 years ago. [1] Their structural diversity,which is due to the bonding flexibility of iodine,r emains the subject of continuous interest. [2][3][4] Nowadays,v arious PIs are known, ranging from I 3 À to I 29 3À ,with the general formula I nÀ 2 m+n and n up to 4. Their building blocks consist of I 3 À as adonor and I 2 as an acceptor forming ac harge transfer (CT) complex. CT complexes can interact further to form extended structures with various topologies,u pt oc ubic networks.A st he aggregation of iodine units is somewhat unpredictable,t he design of PIs is not easy.Moreover,the classification of higher units is,thus far,pragmatically based on the distance between iodine atoms,but universal criteria are still lacking.Different theoretical approaches to study bonds in PIs exist, for example,calculations of the potential energy surface and bond order, [5] energy decomposition analysis (EDA), [6] analysis of the electron density and its Laplacian at bond critical points (BCP), [7] theelectron localization function, and the one-electron potential. [8] Furthermore,t he energy den-

High-pressure single-crystal to 20 GPa and powder diffraction measurements to 50 GPa, show that the structure of Pb2SnO4 strongly distorts on compression with an elongation of one axis. A structural phase transition occurs between 10 GPa and 12 GPa, with a change of space group from Pbam to Pnam. The resistivity decreases by more than six orders of magnitude when pressure is increased from ambient conditions to 50 GPa. This insulator-to-semiconductor transition is accompanied by a reversible appearance change from transparent to opaque. Density functional theory-based calculations show that at ambient conditions the channels in the structure host the stereochemically-active Pb 6s 2 lone electron pairs. On compression the lone electron pairs form bonds between Pb2+ ions. Also provided is an assignment of irreducible representations to the experimentally observed Raman bands.

Calcium carbonate is a relevant constituent of the Earth’s crust that is transferred into the deep Earth through the subduction process. Its chemical interaction with calcium-rich silicates at high temperatures give rise to the formation of mixed silicate-carbonate minerals, but the structural behavior of these phases under compression is not known. Here we report the existence of a dense polymorph of Ca 5 (Si 2 O 7 )(CO 3 ) 2 tilleyite above 8 GPa. We have structurally characterized the two phases at high pressures and temperatures, determined their equations of state and analyzed the evolution of the polyhedral units under compression. This has been possible thanks to the agreement between our powder and single-crystal XRD experiments, Raman spectroscopy measurements and ab-initio simulations. The presence of multiple cation sites, with variable volume and coordination number (6–9) and different polyhedral compressibilities, together with the observation of significant amounts of alumina in compositions of some natural tilleyite assemblages, suggests that post-tilleyite structure has the potential to accommodate cations with different sizes and valencies.

The low-temperature behavior of tetragonal copper sulfide, Cu 2 S, was investigated by powder and singlecrystal x-ray diffraction, calorimetry, electrical resistance measurements, and ambient temperature optical absorption spectroscopy. The experiments were complemented by density-functional-theory-based calculations. High-quality, polycrystalline samples and single crystals of tetragonal copper sulfide were synthesized at 5 GPa and 700 K in a large volume multianvil press. Tetragonal Cu 2 S undergoes a temperature-induced phase transition to an orthorhombic structure at around 202 K with a hysteresis of ±21 K, an enthalpy of reaction of 1.3(2) kJ mol −1 , and an entropy of reaction of 6.5(2) J mol −1 K −1. The temperature dependence of the heat capacity at the transition temperature indicates that the transition from the tetragonal to the low-temperature polymorph is not a single process. The structure of the low-temperature polymorph at 100 K was solved in space group P na2 1. The structure is based on a slightly distorted cubic close packing of sulfur with copper in threefold coordination similar to the structure of tetragonal copper sulfide. The electrical resistance changes several orders of magnitude at the transition following the temperature hysteresis. The activation energy of the conductivity for the tetragonal phase and the low-temperature polymorph are 0.15(2) and 0.22(1) eV, respectively. The direct band gap of the tetragonal polymorph is found to be 1.04(2) eV with the absorption spectrum following Urbach's law. The activation energies and the band gaps of both phases are discussed with respect to the results of the calculated electronic band structures.

The ferroelectric to paraelectric phase transition of multiferroic CaMnTi 2 O 6 has been investigated at high pressures and ambient temperature by second-harmonic generation (SHG), Raman spectroscopy, and powder and single-crystal x-ray diffraction. We have found that CaMnTi 2 O 6 undergoes a pressure-induced structural phase transition (P 4 2 mc → P 4 2 /nmc) at ∼7 GPa to the same paraelectric structure found at ambient pressure and T c = 630 K. The continuous linear decrease of the SHG intensity that disappears at 7 GPa and the existence of a Raman active mode at 244 cm −1 that first softens up to 7 GPa and then hardens with pressure are used to discuss the nature of the phase transition of CaMnTi 2 O 6 , for which a dT c /dP = −48 K/GPa has been found. Neither a volume contraction nor a change in the normalized pressure on the Eulerian strain is observed across the phase transition with all the unit-cell volume data following a second-order Birch-Murnaghan equation of state with a bulk modulus of B 0 = 182.95(2) GPa.

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A new iridium boride, β-Ir4B5, was synthesized under high-pressure/high-temperature conditions of 10.5 GPa and 1500 °C in a multianvil press with a Walker-type module. The new modification β-Ir4B5 crystallizes in a new structure type in the orthorhombic space group Pnma (no. 62) with the lattice parameters a = 10.772(2) Å, b = 2.844(1) Å, and c = 6.052(2) Å with R1 = 0.0286, wR2 = 0.0642 (all data), and Z = 2. The structure was determined by single-crystal X-ray and neutron powder diffraction on samples enriched in 11B. The compound is built up by an alternating stacking of boron and iridium layers with the sequence ABA′B′. Additionally, microcalorimetry, hardness, and compressibility measurements of the binary iridium borides α-Ir4B5, β-Ir4B5, Ir5B4, hexagonal Ir4B3–x and orthorhombic Ir4B3–x were carried out and theoretical investigations based on density function theory (DFT) were employed to complement a comprehensive evaluation of structure–property relations. The incorporation of boron into the structures does not enhance the compressibility but leads to a significant reduction of the bulk moduli and elastic constants in comparison to elemental iridium.

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