The influence of changes in the nominal composition and preparation co
nditions on the formation of Tl(1223) under a high isostatic gas press
ure was studied. Optimization of the cation substitution allowed to in
crease the phase purity, whereas substitution of small amounts of oxyg
en by fluorine decreased the formation temperature of the Tl(1223) pha
se and extended the temperature range favorable for grain growth. A tw
o-step reaction involving substantial melting produced well shaped, pl
ate-like grains.