Investigation and model of the symmetry-changing phase transition in Keggin anion salts

Strand, Jacob

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https://hdl.handle.net/2142/133277 Copy

Description

Title

Investigation and model of the symmetry-changing phase transition in Keggin anion salts

Author(s)

Strand, Jacob

Issue Date

2026-05-05

Date of Ingest

2026-05-15T11:31:10-05:00

Keyword(s)

• Polyoxometalate
• Keggin Anion
• Barium Phosphomolybdate
• Phase Transition
• Frenkel-Kontorova Model
• Density Functional Theory
• Raman Spectroscopy

Language

eng

Abstract

This thesis presents a computational investigation of the temperature-dependent structural phase transition in Keggin anion salts, focused on barium phosphomolybdate, Ba1.36H0.28PMo12O40· ~30H2O. The phase transition observed at 160 K, characterized by a change in space group from Fd¯3 to F d¯3m and an increase in phosphomolybdate symmetry from T to Td with increasing temperature, is studied using molecular simulation and structural analysis, and compared to variable temperature Raman spectroscopy. A modified version of the Frenkel-Kontorova model using a double Morse potential and Morse-type interatomic springs is used to simulate the motion of the twelve molybdenum atoms in an isolated Keggin anion. The simulation supports a multistep, sequential inversion mechanism between left and right-handed Keggin anion configurations as opposed to a single-step inversion process. The alternating handedness observed in the low temperature crystal structure is attributed to van der Waals strain between terminal oxygen atoms at the nearest-neighbor molecular junctions. This strain is incorporated into the model via a Lennard-Jones potential. Lattice expansion and increased thermal motion with rising temperature are proposed to relieve this strain, driving the transition to the Td structure. Density functional theory calculations at the B3LYP/LANL2DZ level are used to assign vibrational modes of the phosphomolybdate anion, with the Raman-active mode at 73 cm-1 being associated with the phase transition. The disappearance of this mode above the phase transition temperature is attributed to the anharmonic environment of the molybdenum atoms in the Td structure.

Type of Resource

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