We have studied as-synthesized, vacuum-annealed and high-pressure oxygen annealed HgBa2CuO4+δ(Hg-1201) using Raman scattering. The apical-oxygen vibrational frequencies showed a slight but systematic shift (590, 591 and 587 cm-1), in agreement with the slight change in the Hg-O bond length from neutron-diffraction results. This suggested that the valence of Hg did not change significantly with oxygen content. The intensity of the ∼ 570 cm-1 peak decreased significantly after vacuum anneal and increased after high-pressure oxygen anneal, confirming the early assignment of this mode to interstitial oxygen. The thermal stability of these samples was studied by increasing laser power. High power density resulted in the decomposition of Hg-1201, mainly to BaCuO2-δ, suggesting mercury loss upon local heating. It was found that the annealed samples decomposed more easily. In addition, one kind of crystallites exhibited a 326 cm-1 broad peak, which disappeared after high-power irradiation. We propose that this extra peak may come from HgO and/or the defect oxygen O (4).
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