The magnetic phase diagram of the spin-chain system Ca_2+xY_2-xCu₅O_10-δ: Oxygen hole-doping corrections

Recently, the antiferromagnetic transition temperature in the coupled spin-chain system Ca_2+xY_2-xCu₅O_10-δ was found to decrease with increasing x and disappear around x = 1.4 [K. Kubo, S. Kurogi, Y. Koike, T. Nishizaki, N. Kobayashi, Phys. Rev. B 71 (2005) 104413]. We report the results of X-ray diffraction, magnetic susceptibility, heat capacity, and iodometric titration measurements, and find that such behavior corresponds to oxygen-deficient specimens. We propose that the correct phase diagram is a function of both cation doping and oxygen content, with hole concentration p = (x - 2δ) per formula unit. We find that the true phase transition occurs just below p = 1.0, corresponding to a hole concentration of 0.20 per copper. For example, for x = 1.00, the single crystals of Kubo et al. (2005) correspond to our polycrystalline specimens with δ = 0.37 (p = 0.26), a very large oxygen-defect effect. Such a composition has only a slightly suppressed Néel temperature, while for our fully oxygenated x = 1.00 samples, the antiferromagnetic transition is completely suppressed.