The synthesis conditions of HgBa2CuO4 + δ were studied. It was observed that the best phase purity was achieved at an apparent cation ratio of Hg:Ba:Cu = 0.7:2:1 that was obtained by either Hg deficient initial compositions or extra Hg absorbents, e.g. Au foil. No significant impurities or site mixing can be identified by X-ray/neutron powder diffraction investigations in these non-stoichiometric samples. However, their large paramagnetic background suggests the existence of (Ba, Cu) oxides. Our data also show that this apparent Hg deficiency is likely to be synthesis-reaction related.
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