The crystal structure and spectral properties of bulk MoS 2 were investigated at high pressures up to 51 GPa using a diamond anvil cell with synchrotron radiation in addition to high temperature X-ray diffraction and high pressure Raman spectroscopic analysis. While the crystal structure of MoS 2 is stable on increasing temperature, results of high pressure experiments show a pressure-induced isostructural hexagonal distortion to a 2H a -hexagonal P6 3 /mmc phase around 26 GPa as predicted by theoretical calculations reported earlier. The 2H a -hexagonal phase coexists with the ambient 2H c phase up to 51 GPa, the highest pressure achieved in our experiments. The Raman data obtained in our high pressure experiments show consistent changes in the vibrational modes. Furthermore, the diffraction data obtained for the shocked MoS 2 to pressures 8 GPa is found to be structurally resilient.

Transition–metal (TM) nitrides are a class of compounds with a wide range of properties and applications. Hard superconducting nitrides are of particular interest for electronic applications under working conditions such as coating and high stress (e.g., electromechanical systems). However, most of the known TM nitrides crystallize in the rock–salt structure, a structure that is unfavorable to resist shear strain, and they exhibit relatively low indentation hardness, typically in the range of 10–20 GPa. Here, we report high–pressure synthesis of hexagonal δ–MoN and cubic γ–MoN through an ion–exchange reaction at 3.5 GPa. The final products are in the bulk form with crystallite sizes of 50 – 80 μm. Based on indentation testing on single crystals, hexagonal δ–MoN exhibits excellent hardness of ~30 GPa, which is 30% higher than cubic γ–MoN (~23 GPa) and is so far the hardest among the known metal nitrides. The hardness enhancement in hexagonal phase is attributed to extended covalently bonded Mo–N network than that in cubic phase. The measured superconducting transition temperatures for δ–MoN and cubic γ–MoN are 13.8 and 5.5 K, respectively, in good agreement with previous measurements.

We present inelastic neutron scattering results on the geometrically frustrated pyrochlore Tb 2 Sn 2 O 7 . At high temperature T Ͼ 50 K, this system resembles the cooperative paramagnet Tb 2 Ti 2 O 7 , while at low temperature T ϳ 60 mK, it displays remarkably different behavior. Powder neutron scattering, susceptibility, and specific heat techniques have shown that below 0.87 K Tb 2 Sn 2 O 7 enters a partially ordered state that is characterized by two-sublattice ferrimagnetic long-range order which coexists with paramagnetic spin components. We show that ͑i͒ the low-temperature state produces a large internal field and collective excitations and ͑ii͒ the coexisting paramagnetic state persists down to 0.1 K, with spins fluctuating at a rate greater than 0.04 THz, resulting in a diffuse magnetic background to the diffraction patterns. A low-lying excitation at 1.2 meV partially softens as short-range correlations build up while cooling in the paramagnetic state.

No abstract

Uranium nitride (UN) is one of the most studied actinide materials as it is a promising fuel for the next generation of nuclear reactors. Despite large experimental and theoretical efforts, some of the fundamental questions such as degree of 5 f–electron localization/delocalization and its relationship to magneto-vibrational properties are not resolved yet. Here we show that the magnetostriction of UN measured in pulsed magnetic fields up to 65 T and below the Néel temperature is large and exhibits complex behavior with two transitions. While the high field anomaly is a field-induced metamagnetic-like transition and affects both magnetisation and magnetostriction, the low field anomaly does not contribute to the magnetic susceptibility. Our data suggest a change in the nature of the metamagnetic transition from first to second order-like at a tricritical point at T tri ∼ 24 K and H tri ∼ 52 T. The induced magnetic moment at 60 T might suggest that only one subset of magnetic moments has aligned along the field direction. Using the results obtained here we have constructed a magnetic phase diagram of UN. These studies demonstrate that dilatometry in high fields is an effective method to investigate the magneto-structural coupling in actinide materials.