Doctoral Dissertations

Date of Award

5-2009

Degree Type

Dissertation

Degree Name

Master of Science

Major

Chemistry

Major Professor

Janice L. Musfeldt

Abstract

In this thesis, I present spectroscopic studies on the family of Kagomé staircase compounds M₃V₂O₈ (M=Ni and Co). These materials display extremely rich and complex temperature-magnetic field phase diagrams. Although the compounds have identical space group and atom-atom connectivity, they present small differences in the local structure, opening the door to potential tunability of the physical properties. Each displays a unique H-T phase diagram and distinctive magnetic ordering-induced lattice distortions that set important differences among the analogs. I measured the temperature dependence of vibrational properties of Ni₃V₂O₈ and Co₃V₂O₈ to elucidate local structure changes through the series of low-temperature magnetic ordering transitions. Magnetoelastic coupling is evidenced by frequency shifts of the vibrational modes at different magnetic ordering temperatures.I analyzed the frequency shift trends in concert with displacement patterns determined by dynamics calculations. Ni₃V₂O₈ shows a significant pre-transitional effect attributed to large superexchange fluctuations due to Ni center displacements. The PM [right arrow] HTI transition affects the highest frequency a-polarized mode; which is the driver of the transition and involves large oxygen c-directed displacements. The transition to the LTI phase involves a complex local distortion of both Ni[subscript spine] and Ni[subscript crosstie] centers. I found that one mode with B₂[subscript u] symmetry and two modes with B₃[subscript u] symmetry are the most relevant. The B₂[subscript u] mode was expected based on a previous theoretical model. However, participation of B₃[subscript u] modes in the transition to the ferroelectric phase is new, and opens the door to consideration of new mechanisms for spin-phonon coupling.My work on Co₃V₂O₈ shows that it has a softer lattice and less strong spin-lattice coupling than Ni₃V₂O₈. Within our sensitivity limit, b-polarized modes do not show significant frequency shifts in the low-temperature range. Comparison with Ni₃V₂O₈ suggests that this could be a reason for no multiferroic behavior in Co₃V₂O₈. I also measured the phonons as a function of the magnetic field. Two b-polarized modes, b₄ at ~280 cm⁻¹ and b₁₁ at ~880 cm⁻¹, show a remarkably strong applied magnetic field dependence; on the order of 20 % at 18 T for the b₄ mode. Through frequency shifts and spectral weight, I show how these modes are sensitive to the magnetic state when increasing the field.

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