Observation of orthorhombic-tetragonal phase equilibria in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_{3-x}{\mathrm{Fe}}_x{\mathrm{O}}_{7-\delta }$

Recent orthorhombic-tetragonal ($O-T$) phase equilibria predicted by Khachaturyan and Morris have been observed experimentally in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_{3-x}{\mathrm{Fe}}_x{\mathrm{O}}_7$ ($0.01\le x\le 0.2$) by Mössbauer techniques. The relative proportions of $O$ and $T$ phases are found to be functions of Fe concentration. The $T$ phase grows from, the coexists with, the $O$ phase as $x$ increases. A nonzero fraction of $T$ phase persists down to the smallest values of $x$. A model is presented to interpret the structural aspects of the $O-T$ transition.