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Energy-dispersive synchrotron x-ray-diffraction studies were performed on powder samples of Hg1212 and Hg1223 in a diamond-anvil cell at pressure up to 300 kbar. High compressibility was found. A reversible structural transition was observed, which is suggested to correspond to a pressure-induced ordering along the c axis. The accommodation of the Ba ion at high pressure is studied, and the effect it might have on the electronic structure of these materials is discussed by employing results from cluster calculations.

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A sol-gel-based preparation route of precursors to be used in the synthesis of the high-T, superconductor HgBa,CaCu,O, + (Hg-1212) has been investigated. The samples were synthesized by reacting the precursor ' Ba,CaCu,O, made from acetate-tartrate solutions containing appropriate amounts of Ba, Ca and Cu ions, and HgO in closed ampoules at 630-730 "C and yielded the 121 2 phase as the major phase together with the starting precursor and Hg(1). The advantages of this method are that the impurity CaHgO, did not form and that the finegrained powders reacted only slightly with the silica tubes. The onset superconducting transition temperatures were 11 1 and 127 K, for the as-synthesized and oxygen-treated samples, respectively.

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