Hydrogen-induced modification of the electronic structure and magnetic states in Fe, Co, and Ni monohydrides

Hydrogen-induced modification of electronic structures, magnetic states, and crystal structures of transition metal ($\text{TM}=\text{Fe}$, Co, and Ni) monohydrides was investigated using TM $K$-edge x-ray magnetic circular dichroism (XMCD), x-ray diffraction, and first-principles calculations. The TM hydrides undergo narrowing of TM $3d$ density of states (DOS) and significant shifts in the Fermi energy $E_{\text{F}}$ due to hydrogenation, which is responsible for the magnetic properties of the TM hydrides: ferromagnetic FeH, ferromagnetic CoH, and paramagnetic NiH. The reconstruction of the electronic structure is mainly attributed to the appearance of bonding and antibonding states together with hydrogen-induced volume expansion. We demonstrate that the characteristic XMCD profile of the TM hydrides near the absorption edge probed the reconstruction of the electronic structure above $E_{\text{F}}$. The pressure dependence of XMCD revealed that the ferromagnetic state of FeH is less stable than that of CoH under pressure. The different hydrogenation processes between CoH and NiH at room temperature are reported by means of x-ray diffraction.

Figure 1

Crystal structures of (a) dhcp-FeH ($P{6}_3/mmc$), (b) hcp-CoH ($P{6}_3/mmc$), (c) fcc-CoH and NiH ($Fm\overline{3}m$). Each atomic position is indicated by the Wyckoff position.

Figure 2

Molar volume $V$ and $c/a$ ratio as a function of hydrogen pressure $p_{{\text{H}}_2}$: (a) $V$ of Co and CoH, (b) $c/a$ ratio in hcp-CoH${}_x$, and (c) $V$ of Ni and NiH. Solid lines denote curves fitted by the Birch-Murnaghan equation of states. The crosses in panel (a) denote $V$ of the fcc-Co minor phase. The inset of panel (c) illu-strates an enlarged plot of $V$ at the lower pressure region of NiH.

Figure 3

X-ray diffraction patterns of (a) Co and (b) Ni and their hydrides at selected pressures. Solid gray lines denote fitted curves of x-ray diffraction patterns by using RIETAN 2000 program.[34] Indices shown at bottom and top of the panels correspond to $hkl$ reflection from TM and TM hydride, respectively. The dots in panel (a) indicate 200 and 220 reflections from the fcc-Co minor phase. Asterisks indicate diffraction from ruby chips.

Figure 4

Total DOS (thick lines) of (a) dhcp-FeH, (b) fcc-CoH, and (c) fcc-NiH together with DOS of pure metals with the same crystal structure (thin lines). For dhcp-FeH, the Wyckoff position of hydrogen atom is set to $4f(1/3,1/3,z)$ and $z=0.880$. The vertical line indicates the Fermi energy $E_{\text{F}}$. The insets in panel (a) and (b) represent the total DOS of bcc-Fe and hcp-Co, respectively.

Figure 5

Partial DOS of Ni-$s$, $p$, $d$, and H-$s$ orbitals with up-spins in Ni (thin lines) and NiH (thick lines). The panel (d) shows an enlarged figure of the Ni-$d$ orbital.

Figure 6

XANES and XMCD spectra of TM and their hydrides: (a) Fe, (b) Co, (c) Ni. The vertical broken lines indicate the absorption edge $E_0$.

Figure 7

XANES and XMCD spectra of Co and CoH as a function of reciprocal space $k$. The broken lines represent normalized XANES and XMCD profiles of CoH with assumption of the same unit cell volume of Co.

Figure 8

Calculated XANES (upper part) and XMCD spectra (lower part) of TMs (thin lines) and TM hydrides (thick lines): (a) Fe, (b) Co, (c) Ni. The average profiles of the two spectra originating from the Fe(2a) and Fe(2c) sites is shown for FeH.

Figure 9

Integrated intensity $I_{\text{mcd}}$ of XMCD for FeH (closed diamonds), CoH (closed circles), and NiH (closed triangles) as a function of $p_{{\text{H}}_2}$. $I_{\text{mcd}}$ for pure metals is represented by open symbols. For comparison, the pressure dependence of Co metal and Ni metal using standard pressure transmitting medium is shown by the broken lines.[24, 43] $p_{\text{c}}$ represents an estimated critical pressure of ferromagnetic FeH.