Strong atomic displacements around nitrogen in tantalum determined by energy-dispersive x-ray diffraction

Abstract

The nitrogen induced attenuation of Bragg reflections (static Debye-Waller Factor) of tantalum single crystals containing nitrogen has been measured by energy-dispersive x-ray diffraction. N has been dissolved in Ta up to a concentration ofc=3.3% N/Ta. From a detailed study of the concentration andK dependence (K=scattering vector) of the static Debye-Waller Factor exponent 2L we conclude:i) 2L increases linearly with the nitrogen content forc≦1.8% N/Ta,ii) The displacements of the atomic shells due to N on the octahedral sites areu ₁=0.45 (5) Å,u ₂=(−)0.11 (2) Å.u ₃=0.25 (2) Å,iii) The thermal vibrations of those atoms close to N are changed, due to the strong static displacements that they experience. The unusual strong and extended displacements around N in Ta are in contradiction to calculations where two-force models have been used. We propose arguments that displacements are responsible for the specific site chosen by hydrogen atoms when they are trapped by nitrogen in Ta.