X-Ray Absorption Spectroscopy of Strontium(II) Coordination: I. Static and Thermal Disorder in Crystalline, Hydrated, and Precipitated Solids and in Aqueous Solution

Abstract

Detailed analyses of crystalline, hydrated, and precipitated strontium compounds and an aqueous strontium solution by synchrotron extended X-ray absorption fine structure (EXAFS) were used to quantify local thermal and static disorder and to characterize strontium coordination in a variety of oxygen-ligated bonding environments. Analysis of anharmonic vibrational disorder (i.e., significant contribution from a third cumulant term (C3) in the EXAFS phase-shift function) in compounds with low and high static disorder around strontium showed that first-shell anharmonic contributions were generally not significant above experimental error in the EXAFS fits (R±0.02 Å with and without C3). The only case in which a significant apparent decrease in Sr–O distance was observed with increasing temperature, and for which a third cumulant term was significant, was for dilute strontium in aqueous solution. Empirical parameterization of Debye–Waller factor (σ2) for strontium compounds as a function of backscatterer atomic number (Z), interatomic Sr–Z distance, and temperature of spectral data collection showed systematic increases in σ2 as a function of increasing temperature and Sr–Z bond length. At values of σ2 greater than ≈0.025 Å2 (for N<12 and RSr-Z>3 Å), backscattering was generally not significant above noise levels in spectra of compounds of known crystal structure. Comparison of the EXAFS spectra of freshly precipitated SrCO3 (spectra collected wet) to that of dry, powdered strontianite (SrCO3(s)) indicated no significant differences in the local atomic structure around strontium. Analysis of partially hydrated strontium in natural Ca-zeolite (heulandite) showed that strontium is substituted only in the calcium (Ca2) site. Backscattering from aluminum and silicon atoms in the zeolite framework were apparent in the EXAFS spectra at low and room temperature at distances from central strontium of <4.2 Å. Comparison of strontium structural coordination determined in this and previous studies suggests that previous EXAFS determinations of hydrated strontium may have underestimated first-shell interatomic distances and coordination numbers because minor contributions to the EXAFS phase-shift and amplitude functions were not accounted for, either theoretically or empirically.