The diminished effectiveness of the CuO flux above 1080◦C could be due to increased reduction of Cu2+ to Cu+ [24], and/or faster evaporation of the copper oxide at these temperatures. The failure of the CuO flux at these temperatures is demonstrated for example by experiments 14 and 15 (Table I) in which ZrO2 and TiO2 solid solutions reached equilibrium, whereas the ordered phase was present only as the seeds that were part of the starting mix, and had not grown. The complete equilibration of the samples with CuO at 1160◦C (runs 19 and 20, Table I) was possibly helped by the
fact that it was disordered (Zr,Ti)2O4 that crystallized, which is known to react more rapidly compared to the
ordered phase.