Size-dependent mechanical properties of Mg nanoparticles used for hydrogen storage
(1) High resolution TEM analysis on the crystallographic of the nanoparticles.
The Beam direction BD=[2-1-10]. The normal of the edges of the particle were along
[0002], [01-10] and [01-11], respectively. Therefore the particle surfaces should be {0001} basal, {01-10} prismatic and {01-11} pyramidal, as shown in figure s1a and b. There were also pre-existing defects in the as-received nanoparticles. Figure s1c shows the <a>-type dislocations in an as-received particle.
Figure S1. HRTEM images of the Mg nanoparticles
(2) Dislocation analysis on Mg nanoparticles before and after compression.
Before compression:

Figure S2. G dot b analysis on the dislocations. The diffraction pattern and the different dark field TEM images by using different g vectors are shown.
BD=[11-23]. The relative long dislocation line was visible under g=[01-1-1], [10-1-1] and
[1-100]. According to g dot analysis, this dislocation was determined as <a+c>-type. The shorter dislocation segment was visible under g= [10-1-1] and [1-100], but invisible under g=[01-1-1]. Therefore the corresponding burgers vector was determined as [-2110].
After compression:
Figure S3. Dark field TEM images of a nanoparticle after compression with different g vectors.
BD=[11-23]. The relative long dislocation line was visible under g=[01-1-1], [10-1-1] and
[1-100]. According to g dot analysis, this dislocation was determined as <a+c>-type.
(3) Molecular dynamics simulations
Molecular dynamics (MD) simulations were performed by LAMMPS to investigate the deformation mechanism in compression tests of Mg nanoparticles. EAM interatomic potential for
Mg was applied 1 . All the compression tests were performed at temperature of 300 K and MD relaxation of 1 nano-second under 300K was performed before each tensile test. The strain rate was 2 × 10
7
s
-1
and the maximum strain is 0.2. Mg nanoparticles without any pre-exited defects inside were created as Fig. 4 (a), with the dimension D (the minimum diameter for the spherical to contain such nanoparticle) decrease from 24 nm to 8 nm with step size of 4 nm. The contact stress and strain was calculated in the same ways as those described in experimental parts in the main text.

2
(4) Density functional theory calculations of ideal shear stress
We performed density functional theory (DFT) calculations by using the Vienna ab initio simulation package (VASP) in non-spin-polarized conditions to calculate the ideal shear stress for
Mg 2 . The primitive supercell for hcp lattice that contains 2 Mg atoms was used. We used Mg pseudopotential generated by projector augmented wave (PAW) method within Perdew-Burke-
Ernzerhof (PBE) exchange-correlation functional
3
. Brillouin zone integrations were performed on a grid of 23 × 23 × 13 using kinetic cutoff energy of 400 eV and first-order Methfessel-Paxton smearing of 0.4 eV
4
. A fixed shear strain was applied to the hcp primitive supercell along certain slip direction on a slip plane (basal (0001), prismatic (
(1010)
), pyramidal (
(1011)
and
(1211)
).
In the Cartesian coordination defined by the slip direction and slip plane normal direction, the supercell was relaxed under the fixed shear strain so that all the other stress tensor components are less than 0.05 GPa, except the shear stress corresponding to the applied shear strain. The applied shear strain increases from 0 to 0.30 with a step size of 0.02.
(5) Movies
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