La2 –xSrxCuO4 –δ: structural, magnetic and transport measurements on antiferromagnets, insulators and superconductors

Abstract

The structural, magnetic and conducting properties of the La_{2 –x}Sr_xCuO_{4 –δ} system (0≤x≤ 0.13, 0.01 ≤δ≤ 0.04) are examined as functions of temperature, and combined with assignment of the formal Cu oxidation state. High-resolution powder neutron structural results confirm that the correct space group is Abma, ruling out charge-density wave behaviour as well as other Fermi-surface instability mechanisms, as being responsible for the tetragonal-to-orthorhombic phase transition. The reduced magnitude and eventual disappearance of the orthorhombic distortion as the formal Cu oxidation state increases is well described by the better matching of the (La,Sr)—O and Cu—O bond distances in the two layers, (Ln₂O₂) and (CuO₂), resulting in the tolerance factor increasing into the stability range of the K₂NiF₄ structure. The structural data reveal that there is a smooth variation of all structural parameters as the formal Cu oxidation state increases, except in the bond-length ratio (d_ax–d_eq)/(d_ax+d_eq), where d_ax and d_eq are the axial and equatorial Cu—O bond distances of the severely elongated CuO₆ octahedra. This ratio peaks at the formal Cu oxidation state nearest to + 2 in our series (i.e.+ 2.01) to a value of 0.1207(3), decreasing at both higher and lower oxidation states. An electronic Jahn–Teller mechanism is found superior to a superexchange mechanism in explaining the orbital ordering, indicating that the holes formed on Sr²⁺ doping have σ* character. Analysis of the magnetic Bragg scattering resulting from antiferromagnetic ordering of the copper spins leads to the evaluation of the staggered Cu moment which is reduced on doping and disappears at a formal Cu oxidation state of between +2.01 and +2.04. The data support localised models for the Cu spin system and provide no evidence for spin-density wave behaviour. Analysis of resistivity data at low temperatures shows the occurrence of variable-range hopping even after the long-range magnetic order has disappeared, demonstrating that the electronic states at the Fermi level are localised. Doping by oxygen vacancies and/or Sr²⁺ cations leads to the introduction of impurity states into the Mott–Hubbard gap. Each carrier extends over two or three copper sites, with the localisation length ξ increasing to beyond the mean interdopant separation on approaching the metal–insulator transition. As the temperature increases, there is a crossover to a transport mechanism based on excitation to the mobility edge in the overlapping upper-Hubbard impurity and valence bands with the metal–insulator transition being of the Anderson type.