N.M.R. study of changes in bonding and the metal-non-metal transition in liquid caesium-antimony alloys

Abstract

¹³³Cs N.M.R. results for the resonance shift and nuclear relaxation rate in liquid Cs-Sb alloys are presented for the concentration range 0≤x _Sb≤0·62. On the Cs-rich side of the compound composition Cs₃Sb, the correlation between Knight shift and magnetic susceptibility or electrical conductivity strongly indicates that a pseudogap forms in this liquid as the metal-non-metal transition is approached. Before the onset of the non-metallic state near x _Sb = 024 the electron correlation time τ reaches a value of 4 × 10⁻¹⁴ s. At the composition Cs₃Sb, both the shift and the relaxation rate are very sensitive to small deviations from stoichiometry; the minimum observed value of the shift is 0·16%, that of the relaxation rate T ⁻¹ ₁ is 2 × 10³ s⁻¹. The behaviour close to stoichiometry is not inconsistent with non-degenerate localized states within the gap. On the Sb-rich side of Cs₃-Sb only a small variation in shift is found and T ⁻¹ ₁ decreases to 500 s⁻¹ at the compound composition CsSb. In this concentration range a strong reduction in the s-electron density at the Cs nucleus is inferred from the change in the hyperfine field. This result and, in particular, the long correlation time τ = 10⁻¹² s near CsSb, together with the estimated quadrupolar correlation time > 10⁻¹¹ s show that long-lived molecular groups, probably with partial covalent bonding character, are probably present in non-metallic liquid Cs-Sb alloys.