Complex Structural Behavior of BiMn₇O₁₂ Quadruple Perovskite

Abstract

Structural properties of a quadruple perovskite BiMnO were investigated by laboratory and synchrotron X-ray powder diffraction between 10 and 650 K, single-crystal X-ray diffraction at room temperature, differential scanning calorimetry (DSC), second-harmonic generation, and first-principles calculations. Three structural transitions were found. Above T = 608 K, BiMnO crystallizes in a parent cubic structure with space group Im3̅. Between 460 and 608 K, BiMnO adopts a monoclinic symmetry with pseudo-orthorhombic metrics (denoted as I2/m(o)), and orbital order appears below T. Below T = 460 K, BiMnO is likely to exhibit a transition to space group Im. Finally, below about T = 290 K, a triclinic distortion takes place to space group P1. Structural analyses of BiMnO are very challenging because of severe twinning in single crystals and anisotropic broadening and diffuse scattering in powder. First-principles calculations confirm that noncentrosymmetric structures are more stable than centrosymmetric ones. The energy difference between the Im and P1 models is very small, and this fact can explain why the Im to P1 transition is very gradual, and there are no DSC anomalies associated with this transition. The structural behavior of BiMnO is in striking contrast with that of LaMnO and could be caused by effects of the Bi lone electron pair.

High-Pressure Synthesis, Structures, and Properties of Trivalent A-Site-Ordered Quadruple Perovskites RMn7O12 (R = Sm, Eu, Gd, and Tb)

“…(TOO = 650 K) 10 and BiMn7O12 (TOO = 608 K). 21 The evolution of HT laboratory XRPD patterns and temperature dependence of the lattice parameters of GdMn7O12 are shown on parameters.] The crystal structure parameters of (Tb0.88Mn0.12)Mn7O12 are summarized in Table 2, and Figure 4 shows the fitting results.…”

Rise of A-site columnar-ordered A2A′A′′B4O12 quadruple perovskites with intrinsic triple order

“…There are a number of temperature-driven distortions that cannot be described by the direct group-subgroup paths, for example, in LaMn 7 O 12 (Im3 ⇒ (Immm) ⇒ I2/m), 43 to the effects of the lone electron pair of Bi 3+ . 46 But the ferroelectricity of BiMn 7 O 12 has not been demonstrated yet. The parent structure of A 2 A′A″B 4 O 12 has the space group P4 2 /nmc (no.…”

“…Im ⇒ I1),46 and there is one such composition-driven distortion in Cu 2+x Ta 4 O 12 (Im3 ⇒ (Pm3) ⇒ Pmmm),47 where space groups in parentheses give group-subgroup links. Despite a large number of AA′ 3 B 4 O 12 -type compounds, proper polar distortions are realized only in one case of BiMn 7 O 12 due…”

Group-theoretical analysis of 1:3A-site-ordered perovskite formation

“…Im ! P1 phase transitions at T = 460 K and 290 K, respectively (Imamura et al, 2008;Belik, Matsushita, Kumagai et al, 2017;Sławiń ski et al, 2017;Okamoto et al, 2010).…”

“…The entry into the full set of the OPs of the irrep k 12 10 (À 4 À ) leads to the possibility of the appearance of ferroelectric properties in this phase. From the crystal chemical point of view, the stabilization of the polar phase is associated with the stereochemical effect induced by the presence of Bi 3+ ions with the 6s 2 one pair of electrons (Belik, Matsushita, Kumagai et al, 2017;Sławiń ski et al, 2017;Mezzadri et al, 2009).…”