Preparations in Air or Lidded Containers

In another procedure (49), the reactants (n2Os, Ba(NOs)2, CaO, and CuO) were heated in two steps, the first in a lidded Pt crucible (1000 C, 15-25 min), and the second (as pellets wrapped in gold foil) in a sealed silica tube (865-905°C, 3-10 h) that had been flushed with oxygen. Products were multiphase, but a 

All of these procedures involve heating T1203 in non-sealed systems, and all are typified by superconducting product stoichiometries far different from the starting compositions. Again, in addition to safety concerns, little control of superconducting-phase composition and reproducibility of synthetic conditions is afforded by use of non-hermetically sealed reaction containers. The problem appears to be more complex than simply thallium reactant loss factors related to reaction kinetics are most likely quite important for the preparation of these metastable phases. 

Other researchers attempted improved control of product stoichiometry by wrapping reactant-mixture pellets (Tl2Os, BaO, CaO, and CuO) in gold foil and then sealing them in silica ampoules that had been flushed with oxygen (8)(51). Multiphase samples, however, were still obtained. Nominal starting metal compositions of 1 1 3 3 and 2 2 2 3 (Tl Ba Ca Cu) resulted in the largest amounts of 2223 and 2212 phases, respectively the samples 

In numerous initial experiments with various proportions of reactants heated at 850-915°C for 15 min to 3 h, most of the products were mixtures containing the 2212 and 2223 phases with superconductivity onset temperatures ranging from 100 to 127 K (6). 

Essentially single-phase 2223 (Tc = 125 K with zero resistance at 122 K) was obtained by heating stoichiometric quantities of the reactants at 890°C for 1 h (6X56) if this mixture was heated for more than 2 h or at temperatures above 900°C, a mixture resulted with 2212 as the major phase and 2223 as a minor component (56). The authors note that the temperature range in which the thallium-based superconductors can be formed without decomposition to barium cuprate phases is very narrow (6). 

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