Structural relaxation around Cr3+ in YAlO3–YCrO3 perovskites from electron absorption spectra

The structural relaxation around Cr3+ in YAl1-xCrxO3 perovskites (Pnma space group) was investigated and compared with analogous Cr-Al joins (corundum, spinel, garnet). Eight compositions (xCr3+ = 0, 0.035, 0.075, 0.135, 0.25, 0.5, 0.75, 1.0) were prepared by sol-gel combustion and analyzed by a combined X-ray diffraction (XRD) and electron absorption spectroscopy (EAS) approach. At variance to what observed in the series of (REEs,Y)AlO3 perovskites, the unit cell parameters and the XRD averaged octahedral (Cr,Al)-O and [VIII]Y-O bond distances scale linearly with the chromium fraction, in apparent agreement with the Vegard’s law.[1] In reality, the octahedral volume expansion due to Cr-Al substitution is not directly transferred to the whole unit cell, but it is partially compensated by tilting phenomena and through a significant change of the effective coordination number of Y in the A site below xCr3+ ~0.4. The optical parameters show an expected decrease of crystal field strength (10Dq) as well as an increase of covalency (B35) and polarizability (B55) towards YCrO3, but non linear trends outline some excess 10Dq, with constant B35 and B55, below xCr3+ ~0.4. The local Cr-O bond lengths, as calculated from EAS, indicate a compression from 1.98Å (xCr3+ =1.0) down to 1.95Å (xCr3+ =0.035), so that the relaxation coefficient is ε=0.54, implying a remarkably low structural relaxation in comparison with garnet (ε=0.74) [2], spinel (ε=0.68) [3] and corundum (ε=0.58) [4]. This ranking is likely due to a progressive decrease of polyhedral network flexibility, but the lowest relaxation degree of perovskite appears to be somewhat in contrast with its structural features. The enhanced covalent character of the Cr3+-O-Cr3+ bond in the one-dimensional arrangement of corner-sharing octahedra can be invoked as a factor limiting to some extent the perovskite network flexibility. The comparably lowest ε value of Y(Al,Cr)O3 perovskites found in this study can be also understood by considering an additional contribution to10Dq due to the electrostatic potential of the rest of lattice ions upon the localized electrons of the CrO6 octahedron.[5] Such an “excess” 10Dq increases when the point symmetry of the Cr site is low, as in perovskite, and would be non-linearly affected by the change of yttrium effective coordination number observed by XRD for xCr3+ below ~0.4. The above interpretation would justify the systematic underestimation of local Cr-O bond distances, as inferred from EAS, compared to what derived from XAS studies, implying a stronger degree of relaxation around Cr3+ of all the structures considered (ε~1 for garnet,[6] 0.83 for spinel,[7] 0.76 for corundum,[8] ~0.8 for perovskites). These results support the hypothesis that crystal field strength from EAS contains more information than previously retained, particularly an additional contribution from the next nearest neighbouring ions. Furthermore this insight into the interdependence of colour on crystal structure is a key to understand in depth the colour mechanism and design new and more efficient pigments, exploiting the different chromatic spectra from the chromium end-member green) to the Cr-doped aluminium end-member (red).