Abstract
samples with compositions between and were investigated by magic-angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR), powder x-ray diffraction (XRD), neutron vibrational spectroscopy (NVS), and neutron powder diffraction (NPD). The rigid-lattice MAS-NMR spectrum for a sample in the triclinic phase shows a single phase with two well-resolved resonances at and , indicating that two inequivalent sites are occupied, as was observed previously in . For , the ratio of spectral intensities of the two lines is 1:0.76, indicating that the site corresponding to the line has the lower site energy and is fully occupied. Similarly, the neutron vibrational spectra for clearly confirm that at least two sites are occupied. For , XRD indicates that of the metal atoms are in the phase, corresponding to an upper composition for the phase of , consistent with the MAS-NMR and neutron vibrational spectra indicating that is single phase. The MAS-NMR and NVS of indicate a mixed-phase sample and clearly show that the two inequivalent sites observed at cannot be attributed to the sites normally occupied in the phase. For , NPD results indicate a -phase boundary of at , increasing to at and below, in general agreement with the phase boundary estimated from the NVS and MAS-NMR spectra of . Rigid-lattice MAS-NMR spectra of and show a ratio of spectral intensities of and , respectively, indicating complete occupancy of the lower-energy site, consistent with the NPD results. For each composition, the correlation time for deuterium hopping was determined at the temperature where resolved peaks in the MAS-NMR spectrum coalesce due to motion between inequivalent sites. The measured correlation times are consistent with previously determined motional parameters for .
- Received 7 June 2006
DOI:https://doi.org/10.1103/PhysRevB.74.184109
©2006 American Physical Society