Jahn–Teller Ordering Dynamics in the Paraelectric BiMn7O12 Phase: 57Fe Probe Mössbauer Diagnostics

The electrical hyperfine interactions of the ⁵⁷Fe probe nuclei stabilized in the structure of the BiMn₇O₁₂ manganite are studied by Mössbauer spectroscopy. Mössbauer spectra are measured in the para-electric temperature range, which includes the structural phase transitions I2/m ↔ Im\(\bar {3}\) (T₁ ≈ 600 K) and Im ↔ I2/m (T₂ ≈ 450 K). The calculation of the parameters of the electric field gradient tensor with allowance for the dipole contributions of Bi³⁺ cations in the range of the first phase transition allowed us to confirm a random orientation of the dipole moments p_Bi in the cubic phase of the manganite (Im\(\bar {3}\)). Based on an analysis of the Mössbauer spectra recorded at T₂ < T < T₁, we considered various scenarios for the manifestation of the dynamic Jahn–Teller effect, which leads to the “melting” of the orbital order in the manganese sublattice, in terms of a two-level relaxation model.