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

The structural relaxation around Cr3+ in YAl1-xCrxO3 perovskites was investigated and compared with analogous Cr-Al joins (corundum, spinel, garnet). Eight compositions (xCr3+ from 0 to 1) were prepared by sol-gel combustion and were analyzed by a combined X-ray diffraction (XRD) and electron absorption spectroscopy (EAS) approach. The unit cell parameters and the XRD averaged octahedral (Cr,Al)-O and [VIII]Y-O bond distances scale linearly with the chromium fraction. The optical parameters show an expected decrease of crystal field strength (10Dq) and an increase of covalency (B35) and polarizability (B55) toward YCrO3, but a nonlinear trend outlines some excess 10Dq below xCr3+ ~ 0.4. The local Cr-O bond lengths, as calculated from EAS, indicate a compression from 1.98 Å (xCr3+ ) 1.0) down to 1.95 Å (xCr3+ ) 0.035) so that the relaxation coefficient of perovskite (? ) 0.54) is the lowest in comparison with garnet (? ) 0.74), spinel (?) 0.68), and corundum (? ) 0.58) 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 the perovskite polyhedral network flexibility. The increased probability of Cr-O-Cr clusters for xCr3+ greater than ~0.4 is associated to diverging trends of nonequivalent interoctahedral angles. The relatively low relaxation degree of Y(Al,Cr)O3 can be also understood by considering an additional contribution to 10Dq because of the electrostatic potential of the rest of the lattice ions upon the localized electrons of the CrO6 octahedron. Such an "excess" of 10Dq increases when the point symmetry of the Cr site is low, as in perovskite, and would be affected by the change of yttrium effective coordination number observed by XRD for xCr3+ greater than ~0.4. This would justify the systematic underestimation of local Cr-O bond distances, as inferred from EAS, compared to what is derived from X-ray absorption (XAS) studies implying a stronger degree of relaxation around Cr3+ of all the structures considered and supporting the hypothesis that 10Dq from EAS contains more information than previously retained particularly an additional contribution from the next nearest neighboring ions.