Structural relaxation around Cr3+ at the Na(Al1-xCrx)P2O7 octahedral site: an XRPD and EAS study

The isovalent substitution of chromium for aluminium at octahedral site along the Na(Al1-xCrx)P2O7 pyrophosphate solid solution has been investigated by means of the combined application of X-ray powder diffraction (XRPD) and electronic absorption spectroscopy (EAS). In agreement with the structural refinements, deconvolution of the optical spectra revealed a progressive decreasing of the crystal field strength parameter 10Dq moving toward the NaCrP2O7 end-member, meaning that the local chromium-oxygen bond distance increased along the join with the amount of chromium. The calculated structural relaxation coefficient around the substituent ion Cr3+ was epsilon=0.97 (i.e., very close to that predicted by the Hard Spheres model). A detailed comparison with compact crystal structures (i.e., perovskite, garnet, spinel, clinopyroxene, and corundum) as well as with other pyrophosphate compounds with II-NaMP2O7 structure-type (with M=Al, Cr, Fe, V, Ti, Mo and In) highlighted that compounds belonging to the Na(Al1-xCrx)P2O7 solid solution - characterized by a network connectivity which is basically a framework topology - undergo a structural relaxation that is confined within the first shell (i.e., occurs at the octahedral site), while P2O7 dimers act as rigid units.