X-ray absorption spectroscopy of strontium(II) coordination - I. Static and thermal disorder in crystalline, hydrated, and precipitated solids and inaqueous solution

Detailed analyses of crystalline, hydrated, and precipitated strontium comp ounds and an aqueous strontium solution by synchrotron extended X-ray absor ption 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 (C-3) in the EXA FS phase-shift function) in compounds with low and high static disorder aro und strontium showed that first-shell anharmonic contributions were general ly not significant above experimental error in the EXAFS fits (R +/- 0.02 A ngstrom with and without C-3) The only case in which a significant apparent decrease in Sr-O distance was observed with increasing temperature, and fo r which a third cumulant term was significant, was for dilute strontium in aqueous solution. Empirical parameterization of Debye-Waller factor (sigma( 2)) for strontium compounds as a function of backscatterer atomic number (Z ), interatomic Sr-Z distance, and temperature of spectral data collection s howed systematic increases in sigma(2) as a function of increasing temperat ure and Sr-Z bond length. At values of sigma(2) greater than approximate to 0.025 Angstrom(2) (for N < 12 and RSr-Z > 3 Angstrom), backscattering was generally not significant above noise levels in spectra of compounds of kno wn crystal structure. Comparison of the EXAFS spectra of freshly precipitat ed SrCO3 (spectra collected wet) to that of dry, powdered strontianite (SrC O3(s)) indicated no significant differences in the local atomic structure a round strontium. Analysis of partially hydrated strontium in natural Ca-zeo lite (heulandite) showed that strontium is substituted only in the calcium (Ca2) site. Backscattering from aluminum and silicon atoms in the zeolite f ramework were apparent in the EXAFS spectra at low and room temperature at distances from central strontium of <4.2 Angstrom. Comparison of strontium structural coordination determined in this and previous studies suggests th at previous EXAFS determinations of hydrated strontium may have underestima ted first-shell interatomic distances and coordination numbers because mino r contributions to the EXAFS phase-shift and amplitude functions were not a ccounted for, either theoretically or empirically. (C) 2000 Academic Press.