Giant pressure-induced volume collapse in the pyrite mineral MnS₂

Dramatic volume collapses under pressure are fundamental to geochemistry. In transition metal materials, collapses are usually driven by so-called 'spin state' transitions- the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show [1] that the classical S= 5/2 mineral Hauerite (MnS2) undergoes an unprecedented (ca. 22 %) volume collapse driven by a conceptually new magnetic mechanism. Using synchrotron x-ray diffraction, we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains is stable. This is confirmed as the thermodynamic ground state by in-situ laser heating. We show that magnetism is globally absent in the new phase, as the low-spin quantum S=1/2 moments are quenched by dimerisation. Our results show how the emergence of metal-metal bonding can stabilise giant spin-lattice coupling in Earth's minerals.

Size-dependent non-space filling atomic packing in metallic nanoparticles

Ni-Pd nanoparticles synthesized for CO catalysis are characterized by transmission electron microscopy and total X-ray scattering. The sizes of these nanoparticles can be tuned to size with great control over the monodispersity of the samples. The pair distribution functions of the reveal a local ordering within the highly disordered atomic structure within the nanoparticles. The PDFs show a size-dependent deviation from typical bulk face centered cubic (fcc) structure for these materials. The long-range isotropic disorder within these non-fcc nanoparticles can be fitted using an exponentially damped single-mode sine wave. Below a diameter of 5 nm, the Ni-Pd nanoparticles exhibit local ordering of atoms as found in typical icosahedral clusters. The transition from fcc to non-space filling atomic packing of icosahedral clusters in a nanoparticle is modeled to show the structural origin of the observed PDFs. Understanding this type of disorder can give insight into structure-property relations for applications in heterogeneous catalysis.

Investigation of metal supported MCF catalysts using PDF technique

Functionalisation of silica catalysts (e.g. MCF) supported by anchoring of organosilanes (e.g. aminopropylalkoxysilanes) may change the catalytic properties of the system and also can be used for the further grafting of noble metals [1-2]. In the case of metal loading, in the following calcination process thermal activation and thermal transformation take place leading to the sintering of metal particles, which results in creation of bigger metal crystals. The pair distribution function (PDF) technique is a powerful tool for studying amorphous, crystalline and partially crystalline materials [3]. It bases on model free Fourier transform of X-ray powder diffraction data, and gives the probability of finding any two atoms at a given interatomic distance. A long range order is not necessary and nanostructured materials may be investigated without constraints of conventional crystallographic methods. Structural changes of support (mesoporous cellular foams(MCF))and active sites of new series of metal catalysts (Au, Mo, Nb, Zr) were investigated using PDF technique. All data were collected at ID15 beamline at ESRF: a)catalysts at different steps of synthesis, before and after reactions - measurements in static conditions; b) calcinations processes and in-situ oxidation reactions - measurements at different temperature and with gas flow at ambient pressure. Acknowledgements: National Science Centre in Poland (Project NCN-reg.no.2013/10/ST5/00642) is acknowledged for the partial financial support.