Structural and magnetic properties in the powder form of Sn1−xCrxO2 solid solution

Structural and magnetic properties were studied in powder form of Sn_1−xCr_xO₂ with x=0.01, 0.02, 0.03, 0.04 and 0.05 in nominal composition. The structural parameters were obtained at room temperature by the Rietveld refinement of the x-ray powder diffraction profiles. Samples of x=0 to 0.04 are tetragonal phase with a space group P4₂/mnm. The lattice parameters indicate three-step changes with increasing Cr content. The distortion of the metal-oxygen octahedral unit occurs. The substitution of Cr ions on the Sn sites shortens the lattice parameters and the octahedral unit becomes elongated with a displacement of an apical oxygen from x=0 to x=0.02. The incorporation of Cr over x=0.02 leads to the recovery of the length of lattice parameters together with a relaxation of the octahedral unit. This result indicates a possible interstitial occupation of Cr ions from x=0.03 to x=0.04. The Cr doping reaches a saturation limit at x=0.05 with a trace of the excess Cr oxides in the x-ray study. A room temperature ferromagnetism appears in the sample with x=0.01 and becomes remarkable in one with x=0.02. The magnetization decreases with increasing the Cr doping with the amount x>0.02. Thus, the appearance of ferromagnetism highly correlated with the oxygen displacements at the apical position of the octahedral in the Sn_1−xCr_xO₂ system at room temperature. The critical oxygen displacement in the elongated octahedral at around x=0.02 may encourage the vacancy of the apical oxygen and eventually leads to appearance of a ferromagnetism based on an F-center exchange with a micro- and/or nano-structural transition. The observed ferromagnetism is highly correlated with the averaged structural change appeared in the x-ray powder diffraction.