Doctoral Dissertations

Date of Award

12-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Janice Musfeldt

Committee Members

Alexei Sokolov, David Mandrus, Ziling Xue

Abstract

Molecule-based materials offer unique opportunities to explore the interplay between charge, spin, and lattice across quantum phase transitions. With their flexible architectures and overall low energy scales, quantum phases can be induced at experimentally realizable conditions. In this dissertation, I present a spectroscopic study of three important families of multiferroics and quantum magnets with a variety of tuning parameters to unravel the mechanisms required to reach distinct non-equilibrium phases. The exploration of spin-lattice coupling and local lattice distortions across magnetic quantum phase transitions is the unifying theme of this work. As our first platform for investigation, we explore the coupling between ferroic orders in the metal-organic framework [(CH₃)₂NH₂]M(HCOO)₃ (M=Mn,Co,Ni) family. The formate bend links the ferroelectric and magnetic quantum phase transition in the Mn analog. Strikingly, B-site substitution drastically alters this mechanism. The Ni material behaves similarly to the Mn analog but at much higher energy scales, whereas the Co system utilizes formate stretches. B-site substitution is thus a powerful tool for developing structure-property relations within chemically analogous materials, providing control of electronic and magnetic properties as well as energy scales. Copper coordination polymers provide a second platform with which to extend our work. Magneto-infrared spectra of [Cu(pyz)₂(2-HOpy)₂](PF₆)₂ and [Cu(pyz)₁.₅(4-HOpy)₂](ClO₄)₂, combined with prior work of other copper complexes, allow for the investigation of spin-lattice coupling across magnetic quantum phase transitions as a function of structural and magnetic dimensionality. Spin-phonon coupling strength versus magnetic dimensionality reveals that coupling is maximized in the ladder complex. These findings are applicable to other materials with field-induced transitions from the antiferromagnetic to fully saturated state. Multiferroic (NH₄)₂[FeCl₅(H₂O)] is our final test case, sporting a complex network of hydrogen and halogen bonds. The high-field polarization change is quenched at the quasicollinear- to collinear-sinusoidal magnetic reorientation, collapsing before magnetic saturation. Remarkably, nearly all low-frequency modes distort to facilitate the development of the magnetic quantum phase, entirely different than most other molecule-based magnets. Signatures of electron-phonon coupling emerge through magneto-infrared measurements. Together, these findings elucidate quantum phase transitions, spin-lattice coupling, and structure-property relations in molecular multiferroics and quantum magnets, motivating further exploration of non-equilibrium phases in these materials.

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