Electron density and electrostatic potential of KMnF3: a phase-transition study

Area of Science:

• Solid-state chemistry
• Crystallography
• Materials science

Background:

• Potassium manganese trifluoride (KMnF3) is a cubic perovskite exhibiting a phase transition to a tetragonal structure at 186 K.
• Understanding the electron density and chemical bonding is crucial for elucidating phase-transition mechanisms.

Purpose of the Study:

• To investigate the temperature dependence of electron density in KMnF3.
• To correlate critical point parameters with the phase-transition mechanism.
• To analyze chemical bond rearrangements preceding the phase transition.

Main Methods:

• Three accurate X-ray diffraction experiments were performed at 190, 240, and 298 K.
• Data were analyzed using the Hansen-Coppens multipole model, including anharmonicity up to the fourth level.
• Topological analysis of electron density and electrostatic potentials was conducted.

Main Results:

• At room temperature, KMnF3 exhibits Mn-F and K-F bonds. At lower temperatures, K-F bonds weaken, and new Mn-K bonds form.
• Mn-K bonds strengthen as temperature approaches 186 K, indicating a precursor effect starting 50-60 K above the transition.
• The effective one-particle potential of fluorine atoms shifts from a single minimum to four minima at low temperatures.

Conclusions:

• The observed rearrangement of chemical bonds, particularly the formation and strengthening of Mn-K bonds, is a significant precursor to the KMnF3 phase transition.
• Electron density topology indicates closed-shell interactions for K-F and Mn-K, and intermediate interactions for Mn-F.
• The study provides insights into the microscopic mechanisms driving the phase transition in KMnF3.