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 abstractDoped 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
Highlights
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
The unit cell of CaCuO2 optimized in its metallic state shows the CuO bond length of 1.913 Å, not far from the experimental value of 1.928 Å (Table 1)
The last method is very elegant and it works well for many other families of superconductors, such as bismuthates (Ba(II) ➔ K(I)), plumbates, etc. This type of doping may rather be reproduced using theoretical calculations for periodic systems, and we have chosen to mimic this particular method in our computational modelling approach
Summary
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. 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- (: hole) or La- (: electron) doped CaCu(II)O2 precursor. While hope never dies and many researchers show considerable optimism (https://www. houstonchronicle.com/local/history/medical-science/article/ Houston-scientist-Dr-Paul-Chu-upends-the-physics-8406495. php), are there any rational reasons why the situation might be considerably improved in the near future?
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