Raman microscopy/spectroscopy measurements are presented on high purity niobium (Nb) samples, including pieces from hot spot regions of a tested superconducting rf cavity that exhibit a high density of etch pits. Measured spectra are compared with density functional theory calculations of Raman-active, vibrational modes of possible surface Nb-O and Nb-H complexes. The Raman spectra inside particularly rough pits in all Nb samples show clear differences from surrounding areas, exhibiting enhanced intensity and sharp peaks. While some of the sharp peaks are consistent with calculated NbH and NbH 2 modes, there is better overall agreement with C-H modes in chain-type hydrocarbons. Other spectra reveal two broader peaks attributed to amorphous carbon. Niobium foils annealed to >2000 C in high vacuum develop identical Raman peaks when subjected to cold working. Regions with enhanced C and O have also been found by SEM/EDX spectroscopy in the hot spot samples and cold-worked foils, corroborating the Raman results. Such regions with high concentrations of impurities are expected to suppress the local superconductivity and this may explain the correlation between hot spots in superconducting rf (SRF) cavities and the observation of a high density of surface pits. The origin of localized high carbon and hydrocarbon regions is unclear at present but it is suggested that particular processing steps in SRF cavity fabrication may be responsible.

High purity niobium (Nb), subjected to the processing methods used in the fabrication of superconducting RF cavities, displays micron-sized surface patches containing excess carbon.High-resolution transmission electron microscopy and electron energy-loss spectroscopy measurements are presented which reveal the presence of nanoscale NbC coherent precipitates in such regions. Raman backscatter spectroscopy on similar surface regions exhibit spectra consistent with the literature results on bulk NbC but with significantly enhanced two-phonon scattering. The unprecedented strength and sharpness of the two-phonon signal has prompted a theoretical analysis, using density functional theory (DFT), of phonon modes in NbC for two different interface models of the coherent precipitate. One model leads to overall compressive strain and a comparison to ab-initio calculations of phonon dispersion curves under uniform compression of the NbC shows that the measured two-phonon peaks are linked directly to phonon anomalies arising from strong electron-phonon interaction. Another model of the extended interface between Nb and NbC, studied by DFT, gives insight into the frequency shifts of the acoustic and optical mode density of states measured by first order Raman. The exact origin of the stronger two-phonon response is not known at present but it suggests the possibility of enhanced electron-phonon coupling in transition metal carbides under strain found either in the bulk NbC inclusions or at their interfaces with Nb metal. Preliminary tunneling studies using a point contact method show some energy gaps larger than expected for bulk NbC.

High-pressure structure transition of nontoxic all-inorganic MHP CsSnBr3 was fully explored up to 15 GPa using an advanced structure search technique CALYPSO combined with first-principles calculations. Besides the known orthorhombic Pnma ground state phase, two high-pressure semiconducting Cmcm and P21/m phases of CsSnBr3 were firstly uncovered above 2.37 and 6.8 GPa, respectively. Both phase transitions of the Pnma → Cmcm at 2.37 GPa and Cmcm → P21/m at 6.8 GPa were characterized as first order with a volume reduction of 4.7% and 10.8%. The occurrences of high-pressure Cmcm and P21/m phases follow the enhanced distortions of Sn-Br polyhedrons and increased coordination of Sn atoms from 6 to 8 at elevated pressures. Compared to the direct band gap of the ambient-pressure Pnma phase, the Cmcm and P21/m phases exhibit a larger indirect band gap of 2.347 and 3.143 eV, respectively, originating from the movement away from the Fermi level of conduction bands driven by the twisting of Sn-Br polyhedrons under pressure. The light absorption performances of two high-pressure phases in comparison with the Pnma phase were studied by the calculated optical absorption coefficients.