The superconducting mercury cuprates of general formula HgBa2Can­1CunO2n+2+ (n = 1 to 6) exhibit the highest critical temperature (133 K for n = 3) of all high-Tc superconductors. HP-HT synthesis is used to prevent HgO decomposition and allows the synthesis to proceed by solid state reaction of oxides. In previous experiments [1,2], we had demonstrated the power of in situ synchrotron diffraction for the investigation of the reaction mechanism involved in the HP-HT synthesis. We present here the results of new experiments devoted to the related Hg2Ba2YCu2O8­ (Hg-2212) compound, and to the study of single crystal growth using the flux technique for HgBa2CuO4+(Hg-1201).

The experiments were carried out at ID30, using a large-volume press. The samples were contained in gold capsules to reproduce the laboratory conditions used previously. High-quality diffraction images were recorded every 1 min using a CCD/image intensifier and 77 keV radiation. The reactions were studied on increasing temperatures, and for different pressure values up to 6 GPa.

The in situ study of the Hg-2212 synthesis was motivated by the observation of possible cation substitution depending on applied pressure and starting material. In fact, experiments at 4 GPa indicated a different chemical path depending on the choice of the initial mixture. Starting from a mixture of oxides and metallic copper, the compound is formed at 900 °C through a complex reaction involving an intermediate high-pressure phase. Whereas, starting from a pre-reacted "Ba2YCu2Oy" precursor mixed with HgO, the Hg-2212 phase is formed directly at 850 °C.

Figure 101
Fig. 101: Diffractograms recorded during the Hg-1201 crystal growth in a BaO 2 rich flux at 6 GPa. 

Due to its high Tc of 97 K and simple structure, Hg-1201 may constitute a model compound to investigate the structural and physical effects at the origin of high Tc. Key experiments for these studies, such as neutron diffraction or NMR, require the use of large, high-quality, single crystals. A possible way to grow such crystals would be to use the flux technique at high pressure. However, no information about the melting points and phase equilibria in the system Hg-Ba-Cu-O, crucial to control the crystal growth, was available until now. The in situ diffraction technique developed at ID30 is very well suited for such investigations, since both melting and crystallisation processes can be detected on diffraction images. Figures 101 and 102 illustrate the results of a typical in situ Hg-1201 crystal growth experiment at 6 GPa, using a BaO2 rich flux to prevent substitution by cations from the flux. On Figure 101, the flux melting is detected by the concomitant disappearance of crystalline phases and growth of a large diffuse bump typical of a liquid. The remaining Bragg peaks detected at higher temperatures belong to Hg-1201 and CuO (and gold from the container). As seen in Figure 102, apart from the broad diffuse signal from the melt, these phases appear as strong localised Bragg spots, indicating the growth of a small number of crystalline grains. These results are now being used for the preparation of mm-size crystals in a large volume high-pressure apparatus in our laboratory.

Figure 102
Fig. 102: Diffraction image of Hg-1201 growing in a BaO 2 rich flux at 990 °C and 6 GPa.

References
[1] ESRF Highlights, 66 (1997/1998).
[2] S. Le Floch, M. Mezouar, B. Antérion, P. Toulemonde, A. Prat, C. Bougerol-Chaillout and P. Bordet, AIRAPT-17 Proceedings, Science and Technology of High Pressure, M.H. Manghnani, W.J. Nellis and M.F. Nicol (Eds.), 2, 726-729 (2000).

Principal Publication and Authors
P. Toulemonde (a), S. Le Floch (a), P. Bordet (a), J.-J. Capponi(a), M. Mezouar(b) and P. Odier (a), Supercond. Sci. Technol., 13, 1129-1134 (2000).
(a) Laboratoire de Cristallographie, CNRS, Grenoble (France)
(b) ESRF