Pressure-driven formation and stabilization of superconductive chromium hydrides

Abstract

Chromium hydride is a prototype stoichiometric transition metal hydride. The phase diagram of Cr-H system at high pressures remains largely unexplored due to the challenges in dealing with the high activation barriers and complications in handing hydrogen under pressure. We have performed an extensive structural study on Cr-H system at pressure range 0 ∼ 300 GPa using an unbiased structure prediction method based on evolutionary algorithm. Upon compression, a number of hydrides are predicted to become stable in the excess hydrogen environment and these have compositions of Cr2Hn (n = 2–4, 6, 8, 16). Cr2H3, CrH2 and Cr2H5 structures are versions of the perfect anti-NiAs-type CrH with ordered tetrahedral interstitial sites filled by H atoms. CrH3 and CrH4 exhibit host-guest structural characteristics. In CrH8, H2 units are also identified. Our study unravels that CrH is a superconductor at atmospheric pressure with an estimated transition temperature (T c) of 10.6 K, and superconductivity in CrH3 is enhanced by the metallic hydrogen sublattice with T c of 37.1 K at 81 GPa, very similar to the extensively studied MgB2.

Synthesis ofNi2H3at high temperatures and pressures

“…It is not clear whether this is an artifact of DFT preferring nonmagnetic solutions or if metal hydrides are inherently nonmagnetic. Note that recent structure predictions in a similar system, Cr-H, also exclusively find nonmagnetic hydrides [42]. Either way, this should not affect the stability of Ni 2 H 3 against γ 2 -NiH and H 2 .…”

On Distribution of Superconductivity in Metal Hydrides

“…Among all predicted stable phases a -ZrH 16 is the most promising from superconducting point of view (see Table 5). This hydride is structurally close to the titanium hydride P2 1 /c-TiH 14. We calculated the EPC parameters for predicted ZrH 16 which are found to be λ = 1.19, ω log =852.3 K, T C (Allen-Dynes) = 88.4 K. Calculated values are significantly higher than predicted values for ZrH (T C = 10 K 25 ). Taking into account new data on ZrH 16 , a new function maxT C (N d ) with monotonic behavior was reconstructed (see Figure 3).…”

“…First of all it is necessary to note the unique layered structure of erbium hydride -ErH 15 including enneagon (9-membered) rings of hydrogen atoms. The closest analogue of such structures are -XH 16 , for instance, -AcH 16 12 , however, enneagon H-rings do not occur in any of the previously studied superhydrides. Higher hydrides of lutetium (LuH 12 , LuH 13 ) already have lower symmetry and their properties are close to polyhydrides C2/m-HfH 14 , -ZrH 16 , P21/c-TiH 14 .…”

The Search for Superconductivity in High Pressure Hydrides

“…Recently, CSP has been employed to predict the most stable structures of compounds with the Cr x H y stoichiometry up to pressures of 300 GPa. 133 Whereas at 1 atm CrH was the only thermodynamically stable phase found, pressure promoted the stabilization of hydrogen rich phases. When ZPE effects were included Cr 2 H 3 , CrH 2 , CrH 3 , CrH 4 , and CrH 8 were found to lie on the convex hull at some pressure.…”