  We show that the transition in AMnO3 from the orthorhombic perovskite phase
to the hexagonal phase is promoted by inducing disorder on the A-site. The gap
between the orthorhombic and the hexagonal phase is widened for disordered,
mixed yttrium-gadolinium manganite samples. At the cost of the orthorhombic
phase a two phase region emerges. The phase separation exhibits very unusual
thermodynamical behaviour. We also show that high pressure synthesis favours
the orthorhombic phase. YMnO3 is formed in the orthorhombic phase at 15 kbar.
