Jahn-Teller driven electronic instability in thermoelectric tetrahedrite

Tetrahedrite, Cu12Sb4S13, is an abundant mineral with excellent thermoelectric properties owing to its low thermal conductivity. The electronic and structural origin of the intriguing physical properties of tetrahedrite, including its metal-to-semiconductor transition, remains largely unknown. This work presents the first determination of the low-temperature structure of tetrahedrite that accounts for its unique properties. Contrary to prior conjectures, our results show that the trigonal-planar copper cations remain in planar coordination below the metal-to-semiconductor transition. The atomic displacement parameters of the trigonal-planar copper cations, which have been linked to low thermal conductivity, increase by 200% above the metal-to-semiconductor transition. The phase transition is consequence of the orbital degeneracy of the highest occupied 3d cluster orbitals of the copper clusters found inside the sodalite cages in the cubic phase. This study reveals that a Jahn-Teller electronic instability leads to the formation of “molecular-like” Cu57+ clusters and suppresses copper rattling vibrations due to the strengthening of direct copper-copper interactions. Our first-principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes. The present results provide insights on the interplay between phonon transport, electronic properties and crystal structure in mixed-valence compounds

STRIPPING FOIL SIMULATIONS FOR ISIS INJECTION UPGRADES

Abstract ISIS, the pulsed neutron and muon spallation source located at the Rutherford Appleton Laboratory (UK), currently delivers a mean beam power of 0.2 MW to target. A 70 MeV H − linear accelerator feeds into a 50 Hz, 800 MeV proton synchrotron (through a 0.3 µm aluminium oxide stripping foil), accelerating up to 3×10 13 protons per pulse. Potential injection scheme upgrades, aiming to raise average beam power towards 0.5 MW with a new 180 MeV linear accelerator, are being studied. Detailed consideration of the injection stripping foil forms a key element of this study: scattering, stripping efficiency and foil lifetime are significant factors in determining loss levels, which consequently limit operational intensity. This paper describes the identification of a suitable stripping foil specification for successful 180 MeV H − charge exchange injection into the ISIS synchrotron. A simulation code was developed to investigate electron stripping, scattering events and temperature rises, in order to witness their subsequent effect on foil lifetime. ANSYS models were also used to investigate the heat transfer and temperature distribution within thin foils

Phase Transitions in the Hexagonal Tungsten Bronze RbNbW2O9 (dataset)

Cifs related to the refined structures as determined from powder neutron data. Raw electrical dat