Low temperature magneto-morphological characterisation of coronene and the resolution of previously observed unexplained phenomena

The polyaromatic hydrocarbon coronene has been the molecule of choice for understanding the physical properties of graphene for over a decade. The modelling of the latter by the former was considered to be valid, as since it was first synthesised in 1932, the physical behaviour of coronene has been determined extremely accurately. We recently discovered however, an unforeseen polymorph of coronene, which exists as an enantiotrope with the previously observed crystal structure. Using low-temperature magnetisation and crystallographic measurements, we show here for the first time that the electronic and magnetic properties of coronene depend directly on the temperature at which it is observed, with hysteretic behaviour exhibited between 300 K and 100 K. Furthermore we determine that this behaviour is a direct result of the appearance and disappearance of the newly-discovered polymorph during thermal cycling. Our results not only highlight the need for theoretical models of graphene to take into account this anomalous behaviour at low temperatures, but also explain puzzling experimental observations of coronene dating back over 40 years

Hydrogen-substituted β-tricalcium phosphate synthesized in organic media

cited By 1International audienceβ-Tricalcium phosphate (β-TCP) platelets synthesized in ethylene glycol offer interesting geometries for nano-structured composite bone substitutes but were never crystallographically analyzed. In this study, powder X-ray diffraction and Rietveld refinement revealed a discrepancy between the platelet structure and the known β-TCP crystal model. In contrast, a model featuring partial H for Ca substitution and the inversion of P1O4 tetrahedra, adopted from the whitlockite structure, allowed for a refinement with minimal misfits and was corroborated by HPO4 2- absorptions in Fourier-transform IR spectra. The Ca/P ratio converged to 1.443 ± 0.003 (n = 36), independently of synthesis conditions. As a quantitative verification, the platelets were thermally decomposed into hydrogen-free β-TCP and β-calcium pyrophosphate which resulted in a global Ca/P ratio in close agreement with the initial β-TCP Ca/P ratio (δCa/P = 0.003) and with the chemical composition measured by inductively coupled plasma (δCa/P = 0.003). These findings thus describe for the first time a hydrogensubstituted β-TCP structure, i.e. a Mg-free whitlockite, represented by the formula Ca21 - x(HPO4)2x(PO4)14 - 2x, where x = 0.80 ± 0.04, and may have implications for resorption properties of bone regenerative materials. © Christoph Stähli et al. 2016

Manganvesuvianite and tweddillite, two new Mn3+-silicate minerals from the Kalahari manganese fields, South Africa

AbstractThe new minerals manganvesuviante and tweddillite, both formed by hydrothermal alteration of primary manganese ores, are described from the Kalahari manganese fields (Republic of South Africa). In addition, single-crystal X-ray structure refinements of both new minerals are presented.Manganvesuvianite is a tetragonal vesuvianite mineral with the simplified formula Ca19Mn3+(Al,Mn3+,Fe3+)10(Mg,Mn2+)2Si18O69(OH)9, characterized by Mn3+ occupying the five-coordinated position (square pyramid). The crystals simple prismatic forms: {100}, {110} terminated by {101} and exhibit deep maroon red colour. With polarized light the crystals are strongly pleochroic, yellowish parallel to E and dark red to lilac parallel to O.Tweddillite is an epidote-group mineral (space group P21/m, a = 8.932(5), b = 5.698(4), c = 10.310(5) Å, β = 114.56(4), V = 477.3(8) Å3) with the simplified formula CaSr(Mn3+,Fe3+)2Al[Si3O12](OH), closely related to strontiopiemontite. The difference between strontiopiemontite and tweddillite is the concentration of octahedral Mn3+. Strontiopiemontite has Mn3+ mainly on the M3 site whereas tweddillite has Mn3+ with minor Fe3+ on M3 and M1. Tweddillite forms aggregates of very thin dark red {001} blades characterized by striking pleochroism. The crystals appear dark red parallel to b and orange-yellow parallel to a. Perpendicular to (001) the blades appear magenta to red.</jats:p