ELSEVIER Physica C 282-287 (1997) 65-68 PHYSICA High Pressure Synthesis and Structure of the Superconducting Mercury Cuprates (Hgl-xMx)Ba2Can-lCunO2+2n+3 with M= C, S. P.Bordet 1, S.Loureiro 1, S.LeFloch 1, J.J.Capponi 1, C.Chaillout I, J.Chenavas 1, j.L.Tholence 2, E.V.Antipov 3, P.G.Radaelli 4, M.Marezio 1. 1 Laboratoire de Cristallographie, BP 166, 38042 Grenoble Cedex 09, France 2 LEPES, CNRS, BP 166, 38042 Grenoble Cedex 09, France 3 Inorganic Chemistry Department, Moscow State University, 119899, Moscow, Russian Federation 4 ILL, BP 156, 38042 Grenoble Cedex 09, France. In mercury superconducting cuprates synthesized at high pressure, partial carbon substitution on the mercury site occurs when the precursors are not absolutely carbon free. Hg-12(n-1)n samples made from carbonate- containing precursors show lower Tcs and smaller c parameter. A model of the structural arrangement of the CO3 -2 groups is proposed from neutron diffraction data and crystal-chemistry considerations. The third oxygen of the oxyanion is situated in the (Hg,C)Ob layer and hinders the incorporation of extra-oxygen atoms. The (Hgl_xCx)Ba2CuO4+ ~ phase diagram shows three different phases, namely HgBa2CuO4+8, Hg0.5C0.5Ba2CuO4+ 8 and CBa2CuO4+O. The latter two phases are not superconducting due to the insufficient hole concentration on the superconducting layer. Sulphur can also partially replace mercury in Hgl-xSxBa2CuO4+O and forms a solid solution up to x=0.15. Neutron diffraction shows that the sulphur atoms arrange as SO4 -2 oxyanions. The larger S-O distances induce steric limitations to the presence of other sulphate anions in the same mesh. 1. Introduction In the structures of the mercury cuprates of general formula HgBa2Can-lCunO2n+2+3, oxygen- deficient perovskite blocks alternate with oxygen- deficient NaCI blocks. Tc first increases with n up to n=3 where it reaches the record value of 135 K, and then progressively decreases. Bulk samples of compounds with n<6 have been synthetised, and their structures have been determined, mainly by using neutron powder diffraction. The specific structural features of these compounds can be summarized as follows : i) The Cu-O apical distances are very large (=2.8 A) compared to other high Tc cuprates, ii) The Hg cations adopt a dumbbell-type oxygen coordination along the c axis with two apical oxygens, iii) The oxygen site O 2 in the Hg layer is located at the center of the mesh and is only partially occupied. This occupancy ~ controls the doping of the superconducting layers and the maximum occupancy depends on n. The in-plane Hg-O distance is very long (= 2.75 ]k), and the oxygen anion is practically only bonded to two Ba cations along the c axis. Due to the linear coordination of Hg cations, these compounds show a great ability for the partial replacement of Hg cations by various chemical species, such as other cations or oxyanions. Numerous studies have been devoted to the effects of such substitutions on the structural and physical properties of mercury cuprates. However, the bad consequence of this ability, is the difficulty to prepare substitution-free samples which are needed for precise investigations of the physical and structural properties. It also complicates the comparison between results obtained on samples prepared with different synthesis techniques. The possibility of Hg substitution was first suspected due to the low occupancy of high thermal parameter values obtained for Hg in diffraction studies. For example, Wagner et al. [1] first reported the subtitution of Cu for Hg in Hg-1201 prepared by the sealed tube technique. Alexandre et al. [2], working with samples prepared by the high pressure-high temperature (HP-HT) technique, indicated that such substitution either did not take place, or would only exist in a very narrow range. Based on x-ray diffraction and iodometric titration results, they concluded that replacement of Hg by carbon was likely to occur in such samples. Kopnine et al. [3] also reported on the detrimental effect on Tc of the substitution of C for Hg in HP-HT samples. 0921-4534/97/$17.00 © ElsevierScienceB.V. All rights reserved. PII S0921-4534(97)00211-6