High-pressure $hP3$ yttrium allotrope with $\mathrm{Ca}{\mathrm{Hg}}_2$-type structure as a prototype of the $hP3$ rare-earth hydride series

A high-pressure (HP) yttrium allotrope, $hP3$-Y (space group $P6/mmm$), was synthesized in a multi-anvil press at 20 GPa and 2000 K which is recoverable to ambient conditions. Its relative stability and electronic properties were investigated using density functional theory calculations. A $hP3$-Y derivative hydride, $hP3-\mathrm{YH}x$, with a variable hydrogen content ($x=2.8$, 3, 2.4), was synthesized in diamond anvil cells by the direct reaction of yttrium with paraffin oil, hydrogen gas, and ammonia borane upon laser heating to ∼3000 K at 51, 45 and 38 GPa, respectively. Room-temperature decompression leads to gradual reduction and eventually the complete loss of hydrogen at ambient conditions. Isostructural $hP3-\mathrm{NdH}x$ and $hP3-\mathrm{GdH}x$ hydrides were synthesized from Nd and Gd metals and paraffin oil, suggesting that the $hP3$-Y structure type may be common for rare-earth elements. Our results expand the list of allotropes of trivalent lanthanides and their hydrides and suggest that they should be considered in the context of studies of HP behavior and properties of this broad class of materials.

Figure 1

Crystal structure of the $hP3$ yttrium allotrope. (a) Unit cell, (b) graphenelike nets of Y2 atoms alternating with nets of Y1 atoms along the $c$ direction, and (c) a view along the [110] direction showing the stacking of nets of yttrium atoms (oriented perpendicular to the page) along the $c$ direction. Yttrium atoms, Y1 and Y2, are shown in dark green and light green, respectively.

Figure 2

Results of density functional theory (DFT) calculations. (a) Phonon dispersion curves for $hP3$-Y ($P6/mmm$) calculated at 1 bar along high-symmetry directions in the Brillouin zone and resulting phonon density of states. (b) Calculated enthalpy difference between known yttrium allotropes (hcp, α-Sm type, and dhcp) and $hP3$-Y: $\mathrm{\Delta }H=H_i-H_{hP3-\mathrm{Y}}$.

Figure 3

Pressure dependence of the volume per yttrium atom for $hP3$-Y and yttrium hydrides, $hP3-\mathrm{Y}{\mathrm{H}}_x$ and $cF4-\mathrm{Y}{\mathrm{H}}_x$, determined in this paper. The literature data for previously known yttrium allotropes, fitted by a single Birch-Murnaghan (BM3) equation of state (EoS; black line), are given for comparison. All blue symbols represent experimental values for $cF4-\mathrm{Y}{\mathrm{H}}_x$, green symbols for $hP3-\mathrm{Y}{\mathrm{H}}_x$. Open and filled symbols represent data obtained at high-pressure–high-temperature (HP-HT) conditions and on decompression, respectively. The density functional theory (DFT)-calculated pressures for given volume for $hP3$-Y are shown by red triangles and solid line.

Figure 4

Electronic density of states (eDOS) calculations at 1 bar. (a) Total eDOS for $hP2$-Y and $hP3$-Y. Partial eDOS for (b) $hP2$-Y and (c) $hP3$-Y. The inset in (c) shows relative contributions to the total eDOS of the $s$ and $d$ states for $hP3$-Y (dashed brown line = $s$ states, dashed blue line = $d$ states) and for $hP2$-Y (solid brown line = $s$ states, solid blue line = $d$ states). The vertical dashed line indicates the Fermi energy.

Figure 5

Electronic band structure along high-symmetry directions in the hexagonal Brillouin zones of (a) $hP2$-Y and (b) $hP3$-Y at 1 bar. Brillouin zone and density functional theory (DFT)-calculated Fermi surfaces of (c) $hP2$-Y and (d) $hP3$-Y at 1 bar.