Pressure dependence of the isomer shift of Sn¹¹⁹

Abstract

Using recoilless gamma resonance absorption of the 23.8 keV transition of Sn¹¹⁹ we have measured the isomer shift as a function of hydrostatic pressure up to 100 kbar. The tin isotope was introduced into the substances SnMg₂, metallic tin, Pd(12 wgt.% Sn), Sn (SO₄)₂·2H₂O, and SnAu. Except for SnMg₂, all compounds exhibit a linear relation between isomer shift and pressure, which reflects a decrease of ¦ ψ(0)¦², the probability density of the electrons at the nuclear site, with increasing pressure. This decrease is interpreted by a volume-dependent screening of the 5s electrons by the 5p electrons. The non-linear dependence of the isomer shift upon pressure in the case of SnMg₂ is interpreted in terms of the superposition of two counteracting effects, with band repopulation and electronic shielding effects prevailing in the low and high pressure branches, respectively. Measurements of the pressure dependence of both the electrical resistivity and the lattice structure of SnMg₂ lead to a picture, which resulted in a calculation of the pressure dependence of the isomer shift, which is in agreement with the experimental observations.