Rubidium Polyhydrides Under Pressure: Emergence of the Linear H₃⁻ Species

Abstract

The structures of compressed rubidium polyhydrides, RbH(n) with n>1, and their evolution under pressure are studied using density functional theory calculations. These phases, which start to stabilize at only P = 2 GPa, consist of Rb(+) cations and one or more of the following species: H(-) anions, H(2) molecules, and H(3)(-) molecules. The latter motif, the simplest example of a three-center four-electron bond, is found in the most stable structures, RbH(5) and RbH(3) , which metallize above 200 GPa. At the highest pressures studied, our evolutionary searches find an RbH(6) phase which contains polymeric (H(3)(-))(∞) chains that show signs of one-dimensional liquid-like behavior.

Theoretical study of the ground-state structures and properties of niobium hydrides under pressure

“…The reference level is pure niobium and pure hydrogen, each in its most stable form at the pressure specified. As can be seen, at 1 atm, the lower hydrides Nb 4 (H 2 ) x phases with respect to their separated counterparts; hydrogen molar content (x = 0 corresponds to pure niobium; x = 1 to pure hydrogen) for the ground state and P = 1 atm, 10, 50, 100, and 400 GPa. The stoichiometric index n (in NbH n ) is indicated at the bottom.…”

“…57 Returning to NbH 4 at 300 GPa, the H-H separation in the top and bottom faces is 1.58Å, while the H-H separation between atoms in top or bottom and the sides is 1.59Å. These H-H separations are also in excess of those for pure solid hydrogen and in the high pressure structures of GeH 4 and SnH 4 , so that all in all, we arrive at the conclusion that there are no semimolecular paired hydrogen units in the high-pressure structure of NbH 4 .…”

“…10. NbH 6 has also a smaller ZPE of 0.074 eV/NbH 6 compared with the sum of ZPEs for NbH 4 and H 2 , so NbH 6 is also expected to become more stable at a lower pressure of ∼354 GPa when ZPE is included [see Fig. 15(b)].…”

“…17) indicates that the electronic DOS at the Fermi level originates, as expected, mostly from the Nb 4d orbitals. However, in the higher hydrides, given the polarity Nb δ+ H δ− , one also expects and obtains contributions to the DOS near the Fermi level from the hydrogens as well, especially for NbH 4 and NbH 6 . The hydrides studied have reasonably high electronic densities of states at the Fermi level, as one can judge through comparison with the DOS obtained in a free electron approximation (the red dot in each DOS).…”

“…These metastable molecules have been formed in near isolation and characterized in a neon matrix by Wang and Andrews; 15 4 by reacting laser ablated niobium with molecular hydrogen in excess neon, and then characterized them spectroscopically. 15 While stable against atomization, all these molecular niobium hydrides have large positive heats of formation relative to the separated elements and small barriers to oligomerization; thus they do not survive any warming up of the matrix in which they are being observed.…”