Next neighbors effect along the Ca–Sr–Ba-åkermanite join: Long-range vs. short-range structural features

Abstract

Abstract An original approach has been developed herein to explore the correlations between short- and long-range structural properties of solid solutions. X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) data were combined on a (Ca,Sr,Ba) 2 (Mg 0.7 Co 0.3 )Si 2 O 7 join to determine average and local distances, respectively. Instead of varying the EAS-active ion concentration along the join, as has commonly been performed in previous studies, the constant replacement of Mg 2+ by a minimal fraction of a similar size cation (Co 2+ ) has been used to assess the effects of varying second-nearest neighbor cations (Ca, Sr, Ba) on the local distances of the first shell. A comparison between doped and un-doped series has shown that, although the overall symmetry of the Co-centered T1 -site was retained, greater relaxation occurs at the CoO 4 tetrahedra which become increasingly large and more distorted than the MgO 4 tetrahedra. This is indicated by an increase in both the quadratic elongation ( λ T1 ) and the bond angle variance ( σ 2 T1 ) distortion indices, as the whole structure expands due to an increase in size in the second-nearest neighbors. This behavior highlights the effect of the different electronic configurations of Co 2+ (3 d 7 ) and Mg 2+ (2 p 6 ) in spite of their very similar ionic size. Furthermore, although the overall symmetry of the Co-centered T1-site is retained, relatively limited ( 2+ –O occur along the solid solution series and large changes are found in molar absorption coefficients showing that EAS Co 2+ -bands are highly sensitive to change in the local structure.