Thermodynamically nonequilibrium states in lanthanum manganite LaMnO$_3$ doped with 5 at% Ba

Mössbauer spectroscopy and X-ray diffraction have been used to study structural transitions in the LaMnO$_{3+\delta}$ compound doped with 5 at% Ba, annealed in vacuum, and cooled under different conditions: rapid cooling (quenching into liquid nitrogen), fast cooling, and slow cooling to room temperature. It has been shown experimentally that vacuum annealing and subsequent fast cooling result in the transition of the rhombohedral structure $(\delta> 0)$ of the synthesized compound to a mixture of three orthorhombic phases $Pnma$I, $Pnma$II, and $Pnma$II*, with stoichiometric composition in oxygen $(\delta = 0)$. Then, an unusual fluctuation change in the partial ratio of the phases is observed depending on the vacuum annealing time. The fluctuations disappear upon slow cooling and quenching of the sample.