Variation of the local properties in TmxSe (0.8 < x ≤ 1.0) : 169Tm Mössbauer and X-ray measurements

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Abstract
169Tm Mössbauer absorption measurements have been made on the system TmxSe in order to investigate the variation of the local properties with stoichiometry. Five samples spanning the whole range of composition where the overall symmetry remains cubic were examined from 1.4 to 295 K. For the samples with the highest lattice constants, a = 5.712 and 5.711 Å, there is no evidence of any significant departure from local cubic symmetry in the paramagnetic region, and in the magnetically saturated region we observe a distribution of hyperfine fields. For each of the samples with intermediate lattice constants (a = 5.685 and 5.672 Å) a distribution of quadrupole splitting is observed in the paramagnetic region probably related to a distribution of noncubic environments. Surprisingly, for the most nonstoichiometric sample (a = 5.626 Å) this distribution vanishes and there is only one dominant noncubic site present. A room temperature X-ray examination of this sample shows the presence of sharp superstructure reflections indicating a cationic vacancy segregation in every second {111} plane within a multi-domain structure. For the three smallest lattice parameters samples, in the paramagnetic region at low temperatures, the Mössbauer line shapes are influenced by magnetic relaxation. Adequate fits are obtained using a fluctuating hyperfine field model

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