Electronic state and local surrounding of 119Sn in calcium-substituted holmium orthochromites

Abstract

The influence of Ca²⁺ doped into the holmium sublattice on the magnetically active surrounding of Sn⁴⁺ ions located in the chromium sublattice of Ho_1–xCa_xCr_0.997Sn_0.003O₃ (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 Ho_0.9Ca_0.1Cr_0.997Sn_0.003O₃, 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.