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Superconductivity in Hydrides Doped with Main Group Elements Under Pressure

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Superconductivity in Hydrides Doped with Main Group Elements Under Pressure Nov. Supercond. Mater. 2017; 3:14–22 Review Article Open Access Andrew Shamp and Eva Zurek* Superconductivity in Hydrides Doped with Main Group Elements Under Pressure DOI 10.1515/nsm-2017-0003 mospheric conditions, such as SiH4 [3–10], GeH4 [11–14], Received November 5, 2016; accepted January 18, 2017 and AlH3 [15–17]. He also pointed out that a second com- ponent, which can be seen as an impurity, may potentially Abstract: A priori crystal structure prediction techniques reduce the pressure required to metallize hydrogen [18]. have been used to explore the phase diagrams of hydrides Pressure can affect which combinations of elements of main group elements under pressure. A number of novel form stable phases, and the chemical compositions of phases with the chemical formulas MHn, n > 1 and M = these phases. Therefore, building on Ashcroft’s proposals, Li, Na, K, Rb, Cs; MHn, n > 2 and M= Mg, Ca, Sr, Ba; HnI density functional theory (DFT) calculations were carried with n > 1 and PH, PH2, PH3 have been predicted to be out to predict the structures of hydrides with stoichiome- stable at pressures achievable in diamond anvil cells. The tries that are not stable at atmospheric conditions, in the hydrogenic lattices within these phases display a num- δ− − − hopes of uncovering hitherto unknown superconductors ber of structural motifs including H2 ,H ,H3, as well as [19]. Since then, numerous theoretical studies have been one-dimensional and three-dimensional extended struc- performed that investigate the phase diagrams of hydro- tures. A wide range of superconducting critical tempera- gen doped with the electropositive alkali metals or al- tures, Tcs, are predicted for these hydrides. The mecha- kaline earth metals [20–34], as well as the electronega- nism of metallization and the propensity for superconduc- tive element iodine [35, 36] under pressure. Experimen- tivity are dependent upon the structural motifs present in tal work on the hydrides of lithium and sodium confirmed these phases, and in particular on their hydrogenic sub- that species with unique stoichiometries such as NaH and lattices. Phases that are thermodynamically unstable, but 3 NaH can be synthesized in diamond anvil cells, but the dynamically stable, are accessible experimentally. The ob- 7 phases that have been reported to date do not exhibit su- served trends provide insight on how to design hydrides perconductivity [21, 37]. Nonetheless, as summarized in that are superconducting at high temperatures. this short review, the aforementioned theoretical explo- Keywords: superconductivity, hydrides, high pressure, rations have: revealed that rich structural variety may be density functional theory, materials prediction found in the hydrogenic sublattices, suggested a number of ways that hydrogen may be metalized via doping, and 1 Introduction shown that the propensity for superconductivity is linked to the nature of the hydrogenic motifs. In a seminal work Ashcroft proposed that because the hy- Recent experimental studies carried out by Drozdov, p drogen is “chemically precompressed” in solids composed Eremets, and Troyan on hydrides containing a -block ele- of the group 14 hydrides, less physical pressure would be ment have yielded exciting results. It was shown that when T required to metallize them as compared to pure hydro- hydrides of sulfur were prepared at ≥ 300 K they be- T gen [1, 2]. Moreover, he hypothesized that these phases come superconducting with a critical temperature, c, of have the potential to be superconducting at high temper- 203 K at 150 GPa [38], and at 207 GPa compressed phos- T atures. Ashcroft’s predictions led to a plethora of theoreti- phine yielded a c of 103 K [39]. Theoretical studies, which cal, and a few experimental, studies that searched for su- have shown that the superconductivity observed in Ref. perconductivity in hydrides that are known to exist at at- [38] arises primarily from an H3S phase [40–55] whose structure has been recently confirmed experimentally [56], will not be described within this short review. We will, however, discuss the results of first principles calculations Andrew Shamp: Department of Chemistry, State University of New carried out for the hydrides of phosphorus [57–59]. Impor- York at Buffalo, Buffalo, NY 14260-3000, USA *Corresponding Author: Eva Zurek: Department of Chemistry, tantly, these studies revealed that the phases made in ex- State University of New York at Buffalo, Buffalo, NY 14260-3000, periment are not necessarily the thermodynamic minima, USA, E-mail: [email protected] © 2017 Andrew Shamp and Eva Zurek, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Superconductivity in Hydrides Doped with Main Group Elements Under Pressure Ë 15 * and that a mixture of phases could yield the observed su- the H2 σu-orbitals, which become partially filled [19]. As perconductivity. a consequence, these systems are already good metals at These studies highlight that new hydrogen-rich ma- 1 atm (but they are not stable). This mechanism of metal- terials, some of which may have a Tc higher than those lization yields phases with a high density of states (DOS) previously thought possible for Bardeen-Cooper-Schrieffer at the Fermi level, EF. (BCS)-type superconductors, can be attained under pres- 2. At 1 atm the volume of the R3¯m-LiH6 unit cell was sure, and they have reinvigorated the search for high- calculated to be a factor of two smaller than an optimized + temperature superconductivity in high-pressure hydrides. H6 lattice because ionic attraction between Li and three −1/3 (H2) molecules leads to “Madelung precompression”. 2 How to Metallize with an However, this method of precompression was only effec- tive at low pressures because the presence of the Li+ core Electropositive Element yielded a structure with a volume larger than that of ele- mental H2 at high pressures. Therefore, this metallization A number of chemical trends have been observed for com- mechanism is likely only effective for ionic compounds pressed alkali metal and alkaline earth polyhydrides [33]. that undergo pressure-induced band gap closure at rela- The pressure at which the polyhydrides were calculated tively low pressures. to become stable with respect to decomposition into the 3. The valence electrons of the electropositive element “classic” hydride (of MH or MH2 stoichiometry) and H2 de- can be transferred to a single hydrogen atom yielding a hy- − creases with decreasing ionization potential of the dopant dridic hydrogen, H , within a lattice of H2 molecules, as metal. For example, whereas LiHn phases are thermody- in the Pm-NaH9 structure illustrated in Fig. 1(b) [26]. Be- − namically stable above 100 GPa, CsHn structures are ac- cause an H impurity donor band forms between the H2 * cessible above 2 GPa. The stoichiometries of the thermo- σg- and σu-levels, the band gap of this system is smaller at dynamically stable polyhydrides, eg. those which com- 1 atm than in elemental H2. Therefore, phases possessing prised the convex hull, possessed an odd number of hy- H− should undergo metallization due to band broadening drogen atoms if the dopant atom was from Group I, and an and overlap at lower pressures than those required to met- even number it was from Group II. The only exception to allize pure hydrogen. However, calculations have shown this trend was lithium, wherein LiH2, LiH6, and LiH8 were that the DOS at EF in the metalized systems is generally found to be thermodynamically stable. In general, past a quite low. − δ− threshold pressure the hydrogen content in the structure It has been proposed that the formation of H vs. H2 with the most negative enthalpy of formation, ∆HF, de- molecules depends upon the number of formal “effectively creased with increasing pressure for the alkali metal poly- added electrons” donated from the electropositive element hydrides, whereas for the alkaline earth polyhydrides it in- to the hydrogenic sublattice [22]. Because these electrons * creased. Thus, it is likely that at very high pressures the fill the antibonding σu-orbitals in H2, the H–H bond elon- mole % ratio of hydrogen found in the structure that corre- gates. If the filling is large enough (for example because sponds to the lowest point on the convex hull will be larger the H:M ratio is low) this bond is expected to break, form- in the alkaline earth polyhydrides than in the alkali metal ing hydridic hydrogens. − polyhydrides. 4. Because H is a soft base it can interact with H2, − Whereas “chemical precompression” can be em- an extremely weak acid, to form H3, the simplest example ployed to metallize covalent hydrides at pressures lower of a three–center four–electron (3c-4e) bond. Calculations than those required for elemental hydrogen, different ways carried out with a variant of the configuration interaction ionic − to metallize become important for hydrides. We have (CI) method have shown that H3 is asymmetric with H–H found five mechanisms that hasten band gap closure, and distances measuring 0.75 and 2.84 Å, and the barrier be- four of these are explicitly illustrated schematically in Fig. tween the two possible minima is symmetric, with H–H 1. Which route is taken depends upon the nature of the hy- distances measuring ∼1.06 Å [60]. Symmetric and asym- − drogenic motifs present in the lattice, and this in turn is metric H3 motifs have been observed in polyhydrides con- governed by the nature of the dopant element. In the para- taining Na [21], K [28], Rb [27], Cs [29], Sr [23, 32] and Ba graphs that follow these five approaches are described by [31]: one example is the RbH5 phase illustrated in Fig. 1(c) way of example. [27]. In these species metallization occurs via the pressure 1.
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