Electronic state and local surrounding of 119Sn in calcium-substituted holmium orthochromites
Résumé
The influence of Ca2+ doped into the holmium sublattice on the magnetically active surrounding of Sn4+ ions located in the chromium sublattice of Ho1–xCaxCr0.997Sn0.003O3 (x = 0, 0.003, and 0.1) compounds was studied by 119Sn Mössbauer spectroscopy. At concentrations [Ca] = [Sn] = 0.3 mol %, an increase was observed in the spectral contribution of Sn4+ sites, having the full number of nearest-neighbor Cr3+cations (n = 6), where they perceived a magnetic field H(Sn)4.2 K = 82 kOe, compared to the contribution of the relevant sites in the undoped chromite (x = 0). This observation was interpreted as resulting from a reduced probability of appearance of Cr3+ vacancies in the nearest surrounding of heterovalent Sn4+ ions. For x = 0.1, on the contrary, the 119Sn spectrum revealed a reduced contribution from the Sn4+ sites with n = 6. This evolution is shown not to be due neither to the appearance of Cr4+ nor Cr6+ ions in the nearest neighborhood of Sn4+ in the chromium sublattice to balance the charge deficiency of the Ca2+ ions doped into the holmium sublattice. This allowed us to suggest that the observed effect was due to the onset of Sn4+ segregations in the structure of Ho0.9Ca0.1Cr0.997Sn0.003O3, which contained a far greater amount of Ca2+ ions whose charge deficiency was balanced mostly by Cr4+ formation. Studies of samples that were prepared under a hydrogen atmosphere revealed the reduction of Sn4+ to the oxidation state +2, with the concomitant stabilization of the formed Sn2+ ions on crystallite surfaces on sites having low coordination numbers.