Interstitial Hydrogen in $\mathrm{Fe}/\mathrm{V}$ Superstructures: Lattice Site Location and Thermal Vibration

We report real space location of hydrogen in single crystalline $\mathrm{Fe}/\mathrm{V}$ superstructures. Anisotropic strain is quantified versus hydrogen concentration by using the yield of backscattered primary 2 MeV ${}^4\mathrm{H}\mathrm{e}$ ions for incidence in different crystallographic directions. From a comparison of ion channeling in combination with ${}^1\mathrm{H}({}^{15}\mathrm{N},\alpha \gamma ){}^{12}\mathrm{C}$ nuclear reaction analysis and Monte Carlo simulations we show that hydrogen is located in octahedral $z$ sites and quantify its vibrational amplitude of 0.2 Å.

Figure 1

(a) HAADF-STEM image showing the sample cross section of the superlattice along $<110>$, respectively, $<100>$ of MgO, with $Z$-number contrast where lighter elements appear darker. The panel to the right sketches the schematic sample layout: $\text{MgO}/[\mathrm{V}14\mathrm{ML}/\mathrm{Fe}2\mathrm{ML}]\times 23/\mathrm{V}14\mathrm{ML}/\mathrm{Pd}7\mathrm{nm}$. (b) Image of the single crystalline superlattice with atomic resolution. The right panel shows the tetragonal distorted bct vanadium unit cell with interstitial hydrogen occupying ${\mathrm{T}}_z$ or ${\mathrm{O}}_z$ sites [33].

Figure 2

(a) Schematic of the RBS geometry and angular map of primary 2 MeV ${}^4\mathrm{He}$ ions backscattered from the hydrogenated $\mathrm{Fe}/\mathrm{V}$ superlattice around [012] orientation revealing the inclined [012] axis relative to bcc (dashed line). (b) Anisotropic expansion of the $\mathrm{Fe}/\mathrm{V}$ superlattice versus average hydrogen concentration. (c) Minimum axial channeling yield normalized by integrated Pd backscattering yield for different average hydrogen loads; (White filled circle), [001]; (Black filled circle), [012].

Figure 3

Angular line scan of backscattering primary 6.55 MeV ${}^{15}\mathrm{N}$ ions and comparison to simulation of average ${}^{15}\mathrm{N}$-NEP assuming different ion divergences in the lattice represented by the solid lines. (inset) Angular map of simulated average ${}^{15}\mathrm{N}$-NEP to $\mathrm{Fe}/\mathrm{V}$ atoms. The dotted line indicates the experimental angular line scan.

Figure 4

(Upper panels) Simulated NEP for 6.55 MeV primary ${}^{15}\mathrm{N}$ ions steered through the $\mathrm{Fe}/\mathrm{V}$ superlattice to hydrogen atoms in ${\mathrm{O}}_z$ (left), between ${\mathrm{O}}_z$ and ${\mathrm{T}}_z$ sites (middle), and ${\mathrm{T}}_z$ sites (right). Incident beam angles are varied around the [001] channel. (insets) The location of hydrogen atoms (red) in the V unit cell (yellow) for a projection along [001] [33]. (Lower panels) Normalized $\gamma$ yield (red) from the angular line scan (dashed black line in upper viewgraphs) is compared to the NEP for different thermal vibrational amplitudes of hydrogen (continuous lines).