Oxygen Interstitials in Superconducting La₂CuO₄:  Their Valence State and Role

Abstract

The nature of the much debated valence state of an interstitial oxygen atom in oxygen-doped La2CuO4 is the subject of this paper. In model cluster calculations, we studied the position, charge, and spin state of the interstitial oxygen atoms in this superconductor. The models considered allow the interstitial oxygen to move off a symmetrical position, to have varying spin and charge, and to be surrounded by various magnetic environments. UB3LYP calculations show that a model having an interstitial oxygen atom with a total spin of 1 is lowest in energy; the interstitial oxygen atoms here act as stable radicals with a net charge of -1. These results agree with experimental evidence for the paramagnetic behavior for interstitial atoms. The energy associated with a spin flip at a Cu site in our models is lower if interstitial oxygen has a local electron spin density, compared to the case when it does not. We provide a possible explanation for the increase of the doping concentrations of interstitial oxygen with a decrease of the Néel temperature of this system. The relative stability of the models we consider depends on their spin states, accompanied by structural changes; this explains indirectly the experimental change of the slope (from 2 to 1.3) of the linear relationship between the hole concentration and the oxygen content. Our results support a stripe phase in high temperature superconductivity; in our calculations, hole doping to the copper oxide layer comes only through the formation of an oxygen interstitial pair, not from any change of the local structural environment and magnetic field around the single interstitial.

Study of Defect Chemistry in the System La2–xSrxNiO4+δ by 17O Solid-State NMR Spectroscopy and Ni K-Edge XANES

“…Precedent exists in the literature for unusually charged oxygen species in these and related systems. Lee and Hoffmann discussed the possibility of interstitials with peroxide- or subperoxide-like character in the isostructural cuprate La 2 CuO 4 , and Demourgues et al suggested for highly hyperstoichiometric samples of La 2 NiO 4+δ (δ ≈ 0.25) the association of interstitial defects with axial oxygen sites, forming O 3 5– polyoxide clusters . On the other hand, the 17 O NMR shift of the O i resonance does not appreciably change between x = 0 and in any of the Sr-substituted samples, whereas any change in the peroxide-like character of O i would be expected to alter the nuclear shielding and thus the observed NMR shift. , Therefore, covalency changes involving the interstitial sites would seem unlikely.…”

High-T$$_c$$ Cuprates: a Story of Two Electronic Subsystems

“…Two key predictions of this mechanism have been confirmed, one by direct measurement [ 74 ], which shows that substitution of Cu by Zn reverses the orbital transition locally, and the other by calculations [ 58 , 86 ], which show that the dopant charge remains localized in its vicinity, i.e., does not reach the planes physically—the charge redistribution around the dopant is found to be neutral overall within a single spacer layer. At first sight, the above metallization of the Cu–O ionic bond would give rise to a mixture of Cu and Cu configurations in the plane, bathed by a Fermi liquid of oxygen hole carriers.…”

Mechanism of metallization and superconductivity suppression in YBa2(Cu0.97Zn0.03)3O6.92revealed by67Zn NQR

“…The metallic alloying scenario, wherein the planes are doped with delocalized dopand holes, is contradicted by ab initio calculations showing that the Sr hole in La 2−x Sr x CuO 4 remains localized [14]. It is similarly found that the interstitial oxygen in La 2 CuO 4+δ preferentially creates an inert peroxide, O 2− 2 , with the apical oxygen [15]. A more remote possibility, the impurity-band scenario, is excluded experimentally [16].…”