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Reports: ND1052053-ND10: High Pressure Properties of Materials

Janice Lynn Musfeldt, University of Tennessee

Program scope

The goal of our research program is to establish a fundamental understanding of the mechanisms underlying the interplay between charge, structure, and magnetism in complex materials. This insight facilitates the development of tunable multifunctional solids and nanomaterials, which are scientifically and technologically important. Our main strategy involves investigating the dynamic response of functional materials under external stimuli such as high magnetic fields, under unusual chemical and photochemical activation, and at very small sizes where quantum confinement becomes apparent. This allows us to learn about the relationships between different ordered and emergent states, explore the dynamic aspects of coupling, and gain insight into the generality of these phenomena and their underlying mechanisms. In addition to broadening the understanding of novel solids under extreme conditions, multifunctional materials and their assemblies are of interest for light harvesting, spintronic, and solid state lubrication applications.

Recent progress

This ACS-PRF New Directions initiative extends our program on “materials under extreme conditions” to include pressure as a complementary tuning parameter, thus allowing us to examine coupling phenomena that were previously out of reach. Two examples in which pressure triggers a magnetic crossover in quantum magnets are detailed below. We are also working to integrate high pressure synchrotron-based spectroscopy into our regularly funded NSF program on molecular quantum magnets, and we acquired our own diamond anvil cell to set up high pressure capabilities in our home laboratory rather than relying solely on the facilities at the Brookhaven synchrotron.

Pressure-driven high-to-low spin transition in the bimetallic quantum magnet [Ru2(O2 CMe)4]3[Cr(CN)6]: Synchrotron-based infrared and Raman spectroscopies were brought together with  diamond anvil cell techniques and an analysis of the magnetic properties to investigate the pressure-induced high → low spin transition in [Ru2(O2CMe)4]3 [Cr(CN)6]. The extended nature of the diruthenium wave function combined with coupling to chromium-related local lattice distortions changes the relative energies of the π∗ and d∗ orbitals and drives the high → low spin transition on the mixed-valence diruthenium complex. This is a rare example of an externally controlled metamagnetic transition in which both spin-orbit and spin-lattice interactions contribute to the mechanism. The resulting phase diagram, with its many competing states, is shown in Fig. 1.

Figure 1: Schematic temperature-pressure-magnetic field phase diagram for [Ru2(O2 CMe)4]3[Cr(CN)6]. Different magnetic states are revealed in response to various external stimuli. The regimes obtained from an analysis of local lattice distortions are also represented.

Pressure-induced hydrogen bonding changes drive magnetic crossover in pentacoordinate CuF2(H2O)2(3Cl-pyrd): Hydrogen bonding plays a foundational role in the life, earth, and chemical sciences, with its richness and strength depending on the situation. In molecular materials, these interactions determine assembly mechanisms, control superconductivity, and even permit magnetic exchange. In spite of its long-standing importance, exquisite control of hydrogen bonding in molecule-based magnets has only been realized in limited form and remains as one of the major challenges. Recently, we revealed how a pressure-driven magnetic crossover in a quantum magnet like CuF2(H2O)2(3-chloropyridine) is, at heart, a change in the dimensionality of the hydrogen bonding network (Fig. 2). Looking at it another way, pressure (and probably strain) control network dimensionality which determines whether the antiferromagnetic or ferromagneticstate is exposed. Similar pressure- and strain-driven crossover mechanisms involving coordinated motion of hydrogen bond networks may play out in other quantum magnets. These findings motivate our continuing work on control of hydrogen bonding, magnetic crossover transitions, and piezomagnetism in molecular quantum magnets.

Figure 2: Schematic view of the pressure-driven antiferromagnetic → ferromagnetic transition in CuF2(H2O)2(3Cl-pyd) which is driven by the change in dimensionality of the hydrogen bonding network. Combined Raman and infrared measurements under compression reveal that the magnetic crossover involves reversible formation of -OH...Cl intermolecular hydrogen bonds that act as magnetic exchange linkages in the third direction. There is also a higher pressure transition that corresponds to a local lattice distortion around the pentacoordinate Cu site