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. 2018 Aug 14;24(9):233.
doi: 10.1007/s00894-018-3777-6.

High-temperature superconductivity as viewed from the maximum hardness principle

Wojciech Grochala  1 , Mariana Derzsi  2   3
Affiliations

High-temperature superconductivity as viewed from the maximum hardness principle

Wojciech Grochala et al. J Mol Model. .

Abstract

The Maximum Hardness Principle - and its reformulation by Chattaraj as the Minimum Polarizability Principle - is an immensely useful concept which works in support of a chemical intuition. As we show here, it may also be used to rationalize the scarcity of high-temperature superconductors, which stems - inter alia - from rarity of high-density of state metals in Nature. It is suggested that the high-temperature oxocuprate superconductors as well as their iron analogues - are energetically metastable at T ➔ 0 K and p ➔ 0 atm conditions, and their tendency for disproportionation is hindered only by the substantial rigidity of the crystal lattice, while the phase separation and/or superstructure formation is frequently observed in these systems. This hypothesis is corroborated by hybrid density functional theory theoretical calculations for Na- (thus: hole) or La- (thus: electron) doped CaCu(II)O2 precursor. Non-equilibrium synthetic methods are suggested to be necessary for fabrication of high-temperature superconductors of any sort. Graphical abstract Doped oxocuprate superconductors are shown to be unstable with respect to phase separation (disproportionation) in accordance with the Maximum Hardness Principle; their metastability is mostly due to rigidity of [CuO2] sheets and preparation using high-temperature conditions.

Keywords: Band gap; Critical superconducting temperature; Density of states; Hardness; Metal; Superconductor.

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Figures

Graphical abstract
Graphical abstract
Doped oxocuprate superconductors are shown to be unstable with respect to phase separation (disproportionation) in accordance with the Maximum Hardness Principle; their metastability is mostly due to rigidity of [CuO2] sheets and preparation using high-temperature conditions
Fig. 1
Fig. 1
Crystal structures relevant to this work: infinite layer CaCu(II)O2 – top left [22], NaCu(III)O2 – bottom left [23], and LaCu(I)O2 – right [24]. The Cu–O bonds were drawn, while other bonds were omitted
Fig. 2
Fig. 2
(left) Magnetic model used in spin-polarized calculations of CaCuO2 and its doped variants. The small red balls are oxygen atoms, and light and dark blue balls represent Cu atoms with spin up and down, respectively (he biggest light-blue ball represents Cu center, on which a hole (or an extra electron) was imposed. The cationic layers (containing Ca, La and Na) are left out for clarity). (right) Optimized crystal structure of 1/8-doped CaCu(II)O2: dopant atoms sit in (0,0,0) and to allow for this, the original CaCuO2 cell contents is shifted by the (0.5,0.5,0.5) vector
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