Analysis of divalent Co, Zn, Pb, and Ba XAFS spectra of synthetic and natural calcite samples containing trace concentrations of these heavy metals confirms their substitution in the unique Ca site in octahedral coordination with varying degrees of local distortion. The existence of each trace metal at the single site in the bulk crystal is significant in view of previous studies showing that these trace elements are incorporated differentially at multiple, structurally distinct surface sites occurring in nonequivalent growth steps on calcite surfaces. The octahedral coordination for Ba is particularly noteworthy because of its large size (35% larger than host Ca) and the fact that it rarely exists as a major constituent with such a low coordination number. Analysis of the local distortion and relaxation around the impurities shows a nearly complete contraction of the structure around Co (super 2+) and Zn (super 2+) . The Co-O and Zn-O first-shell distances are only slightly longer than in CoCO ₃ and ZnCO ₃ , with a site compliance of approximately 80-90%. Displacements of higher shells relative to those in calcite decrease rapidly, but irregularly, over a short distance, and the relaxation may be largely confined within 6-7 Aa of the impurity. The dilation around the large Pb (super 2+) and Ba (super 2+) ions in calcite also shows a high degree of site compliance (85-90%). Relaxation around Pb and Ba also appears to be restricted, but extending further for Ba than for Pb. The limited observations suggest that compliance of the octahedral site in calcite is larger than for the cation site in the rocksalt structure. The high compliance for the metal site in calcite may reflect the corner-sharing topology of the structure and is also probably the reason that calcite shows such a wide range of isovalent substitutions in nature. The findings also provide a direct indication of the local strain associated with a dilute substitutional solid solution.

Phosphorus K-edge X-ray absorption near-edge structure (XANES) spectroscopy was performed on phosphate mineral specimens including (a) twelve specimens from the apatite group covering a range of compositional variation and crystallinity; (b) six non-apatite calcium-rich phosphate minerals; (c) 15 aluminium-rich phosphate minerals; (d) ten phosphate minerals rich in either reduced iron or manganese; (e) four phosphate minerals rich in either oxidized iron or manganese; (f) eight phosphate minerals rich in either magnesium, copper, lead, zinc or rare-earth elements; and (g) four uranium phosphate minerals. The identity of all minerals examined in this study was independently confirmed using X-ray powder diffraction. Minerals were distinguished using XANES spectra with a combination of pre-edge features, edge position, peak shapes and post-edge features. Shared spectral features were observed in minerals with compositions dominated by the same specific cation. Analyses of apatite-group minerals indicate that XANES spectral patterns are not strongly affected by variations in composition and crystallinity typical of natural mineral specimens.

The use of anion redox reactions is gaining interest for increasing rechargeable capacities in alkaline ion batteries. Although anion redox coupling of S²⁻ and (S₂)²⁻ through dimerization of S–S in sulfides have been studied and reported, an anion redox process through electron hole formation has not been investigated to the best of our knowledge. Here, we report an O3-NaCr_2/3Ti_1/3S₂ cathode that delivers a high reversible capacity of ~186 mAh g⁻¹ (0.95 Na) based on the cation and anion redox process. Various charge compensation mechanisms of the sulfur anionic redox process in layered NaCr_2/3Ti_1/3S₂, which occur through the formation of disulfide-like species, the precipitation of elemental sulfur, S–S dimerization, and especially through the formation of electron holes, are investigated. Direct structural evidence for formation of electron holes and (S₂)ⁿ⁻ species with shortened S–S distances is obtained. These results provide valuable information for the development of materials based on the anionic redox reaction.