High-Temperature Polymorphism in Metastable BiMnO₃

Abstract

The multiferroic perovskite BiMnO3, synthesized under high-pressure conditions, decomposes if heated at room-pressure in the temperature range of 500−650 °C. Comparative studies by high-temperature X-ray diffraction, electron diffraction, thermal analysis, and magnetic investigation revealed the existence of a complex pathway to decomposition, depending on the heating rate, pressure, and atmosphere that involves different metastable phases. In particular the as-prepared monoclinic phase (I) transforms to a second monoclinic form (II) at 210 °C and then to an orthorhombic phase (III) at 490 °C. These phase transitions, fast and reversible, occur on heating with a drop in volume and are moved at higher temperatures when pressure is decreased. The transition from II to III, typically observed in inert atmosphere, can be detected also in air when the heating rate is kept sufficiently high. When III is heated in an oxygen-containing atmosphere a slow irreversible transition to variants IV and then V takes place with kinetics depending on temperature, heating rate, and oxygen partial pressure. Both IV and V are oxidized ferromagnetic phases containing Mn4+ characterized by a modulated structure based on fundamental triclinic perovskite cells. Their magnetic behavior shows a strong analogy with thin films of BiMnO3, suggesting for the latter an oxidized nature and for the former a possible multiferroic behavior.

Perovskite solid solutions with multiferroic morphotropic phase boundaries and property enhancement

“…Details of the space group determination can be found elsewhere. 38 This is the same crystal structure of the high temperature phase of BiMnO 3 , which transforms into the monoclinic one at 770 K. 41,42 The monoclinic to orthorhombic transformation can be also induced by high pressures, 43 and by oxygen hyperstoichiometry, 44 as well as by the simultaneous substitution of Pb 2þ for Bi 3þ and Ti 4þ for Mn 3þ as shown here. This might be an example of chemical pressure effects, like those described in the Pb(Zr,Ti)O 3 system that have been proposed to be responsible of the appearance of the MPB.…”

Orbital ordering and magnetic interactions in BiMnO3

“…In an analogy with [40,41], we will call it 'double exchange interaction', although, strictly speaking, it is not a regular double exchange sinceĜ ↑ RR also includesV ↑ R , which takes into account the effects of the orbital incorporates the effects of the second order with respect to {t R R } and is inversely proportional to ex . Therefore, in the following it will be called 'superexchange interaction' 11 . We will also consider two approximations for J D RR and J S RR .…”

“…We will also consider two approximations for J D RR and J S RR . In the first one,ĥ R R will be regular Hartree-Fock Hamiltonian for the majority-spin states and 11 The division of (6) into the 'double exchange' and 'superexchange' parts was first introduced in [41] for the halfmetallic cubic compounds where, similar to BiMnO 3 (figure 7), the bandwidth W is smaller than ex . The idea is based on the 1/ ex expansion for the Green functionĜ ↓ R R .…”

“…The first one, at 474 K, takes place without changing the monoclinic symmetry [11]. The phase transition at 770 K is monoclinic to orthorhombic [11] and was believed likely to be ferroelectric-paraelectric.…”

“…The phase transition at 770 K is monoclinic to orthorhombic [11] and was believed likely to be ferroelectric-paraelectric. Nevertheless, it is worth noting that there is also another point of view according to which the onset of the ferroelectric behavior is expected only around 450 K, i.e.…”