This work aims to synthetize superconducting Tl2Ba2CuO6 (Tl-2201)
single crystals at different Ar/O[2] partial pressures. The
experiments have been done at Chalmers (Göteborg) and at Unipress
(Warsaw) with the high pressure gas compressor attached to a pressure
vessel containing the furnace.Tl-2201 was chosen mainly for two
reasons:
i) Although Tl-2201 single crystals have been grown by various
methods, production of good quality crystals still remains a
challenge.
ii) Tc of Tl-2201 varies greatly (10-90 K) with e.g. oxygen and
thallium concentration in the sample. Since the equipment we use makes
it possible to have different gas mixtures as pressure transferring
media, we hope to succeed in growing a series of Tl-2201 crystals with
different Tc's correlating with varying Ar/O[2] partial pressures. The
first preliminary experiments presented here have, however been
performed in pure Ar.

The starting materials Tl[2]O[3], BaO or BaO2, and CuO were mixed in
the molar ratio Tl:Ba:Cu = 2:2:2, where excess CuO was added as a
flux, and pressed into pellets in a glove box under nitrogen
atmosphere. Crystallisations were done in BaZrO3 or Au crucibles,
loaded sample sizes being 5-10 g. Pressures of 0.8-1.1 GPa were used
and crystallisation was typically done by lowering the temperature
from 1000 C to 760 C at the cooling rates 10-24 C/h.

The existence of superconducting phase has been confirmed by
susceptibility data. At present we have preparedsamples with Tc = 88
K. However, the measurement was done on a few pieces of the sample
instead of a single crystal, the separation of good crystals from the
matrix without damage still being problematic. Powder X-ray
diffraction analysis, which also was done on a bulk sample, reveals
that in addition to Tl-2201 phase the samples also contain CuO and
Tl[2]O[3].

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