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

• New method to assess short- vs. long-range properties of non-ideal solid solutions.
• Constant Co2+-for-Mg replacement turns the Ca–Sr–Ba-åkermanite join EAS-active.
• Co→Mg substitution does not produce any alteration in the average structure.
• Relaxation around Co-tetrahedra scales with second-nearest neighbor cation radius.
• Different electronic configurations of Co and Mg cause different local structures.

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(Mg0.7Co0.3)Si2O7 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 Mg2+ by a minimal fraction of a similar size cation (Co2+) 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 CoO4 tetrahedra which become increasingly large and more distorted than the MgO4 tetrahedra. This is indicated by an increase in both the quadratic elongation (λT1) and the bond angle variance (σ2T1) 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 Co2+ (3d7) and Mg2+ (2p6) in spite of their very similar ionic size. Furthermore, although the overall symmetry of the Co-centered T1-site is retained, relatively limited (<10 deg) angular variations in O–Co2+–O occur along the solid solution series and large changes are found in molar absorption coefficients showing that EAS Co2+-bands are highly sensitive to change in the local structure.

Average (Mg–O) vs. local (Co–Olocal) tetrahedral bond distances in function of the average X cation radius in the (Ca,Sr,Ba)2(Mg0.7Co0.3)Si2O7 join. Figure optionsDownload as PowerPoint slide