Intrinsic point defects in niobium: Trapping at¹⁰⁰Pd and¹¹¹In impurities observed by PAC

Abstract

Radiation-induced lattice defects in high-purity niobium have been investigated in the temperature range of 30K to 540 K by means of γ-γ perturbed angular correlation (PAC) measurements using the radioactive probes¹⁰⁰Pd/¹⁰⁰Rh and¹¹¹In/¹¹¹Cd. Both probes were produced within the niobium samples by means of heavy-ion nuclear reactions. At the Pd impurities trapping of defects occurred during heavy-ion irradiation at about 30 K in two defined configurations: defect 1(Pd) withv _Q1=e ² qQ/h=42(±2) MHz, η₁=0 and defect 2 (Pd) withv _Q2=(±2) MHz, η₂=1. Two defects were observed at the In impurities in annealing stage III (around 250 K) after heavy-ion as well as electron irradiations: defect 1(In) withv _Q1=87(±1) MHz, η₁=0 and defect 2(In) withv _Q2=105(±2) MHz, η₂=0.65(±0.02). A third defect (defect 3(In):v _Q3=177(±2) MHz, η₃≲0.2) appeared above 260 K after heavy-ion irradiation only. The data are interpreted in terms of interstitial trapping at the Pd impurities and vacancy trapping at the In impurities. Information on the microscopic structure of defect 1(In) and 2(In) is obtained from a PAC-single-crystal experiment. For defect 1(In) axial 〈111〉-symmetry is found, which leads us to identify this defect with a monovacancy as nearest neighbor with respect to the In probe. Defect 2(In) is the trapped divacancy for which an orientation is found that is consistent with both vacancies being nearest neighbor to the probe but second nearest neighbors to each other.