"Unveiling Metal-Like Phenomena and Novel Electronic States in Mixed-Valence and Mixed-Dimensionality Halide Perovskites in Extreme Conditions"
As materials chemists, it is necessary to understand how metal-ligand coordination environments on local (molecular unit) scales affect electronic structure of extended solids. Halide perovskites are highly crystalline, semiconducting extended solids that exhibit highly tunable chemical compositions and connectivity of their metal halide frameworks. This tunability provides unique opportunity to discover novel electronic states through employing a multitude of metal site identities and dimensionalities of electronic connectivity. Indeed, their vast arrays of compositions and structural connectivity provide widely variable electronic structures that bridge insulators to low-bandgap semiconductors.
Their tunable bandgaps, facile solution-state synthesis, and photoluminescence properties have encouraged research into their applications in energy storage and conversion as well as visible light emission. However, we have limited understanding of structure-property relationships to inform optimization of new material compositions and structures for application, because systematic studies of these relationships are largely unexplored. My doctoral research has focused on probing structural transitions of varied halide perovskite compositions to understand how their local and average structure influence charge transfer processes. I will focus my defense discussion on my work surrounding a mixed-valence Au2+–Au3+ halide perovskite that exhibits intervalence charge transfer (IVCT), when electron transfer between the metal centers through bridging halides composes the band edges. Through performing structural, optical, and electronic characterization of this material with varied temperature, pressure, and magnetic fields, I have worked to systematically map structural transitions to property trends. Through this work, I have unveiled composition-structure relationships that result in novel magnetic states and emergent electronic structure phenomena related to changes in its charge-transfer processes.
Through these studies, I have discovered novel metal-like transitions when the coordination environments around the Au metal centers become very similar, but not identical. At this structural transition, the material remains semiconducting with a very low absorption onset of <10 meV. However, it exhibits metal like properties, such as dramatically enhanced conductivity on par with metals (~102 S•cm–1 ) and Drude features in reflectivity, due to the thermal barrier for conductivity dropping below ambient energy at room temperature (25 meV) such that ambient energy can promote free carriers. This high-pressure phase transition in this material results in a transition to a Class II-III mixed-valence material, which has never been experimentally observed in extended solids to our knowledge.
The coexistence of magnetism (from Au2+) and high conductivity prompted us to perform the first studies of magnetoresistance and Hall effect in a halide perovskite at high pressures. We observe a 9% increase in magnetoresistance at 2 K between 0 T and 9 T, and a two-carrier dependence of the Hall resistance that becomes electron-dominant at high pressures. We globally fit the magnetoresistance and Hall resistance to a two-carrier model to extract carrier densities and mobilities. At 2 K, the carrier density increases from 1015 cm‒3 to 1018 cm‒3 and mobility increases from 250 cm2 V‒1 s‒1 to 350 cm2 V‒1 s‒1 with pressure from 16 GPa to 27 GPa, with higher densities and lower mobilities for holes than those of electrons. However, there are slight deviations from magnetoresistance curvature we typically expect for fitting to a two-carrier model in the data at 2 K, that does not occur at higher temperatures. This indicates possible contributions from the magnetic center to the magnetoresistance at 2 K, which we attribute to coexistence of local (unpaired) and itinerant electrons, reminiscent of a Kondo lattice. At 2 K, the temperature is low enough that ambient thermal energy (~0.2 meV) is approximately 1/5 of the lowest thermal conductivity barrier (~1 meV), resulting in a Boltzmann distribution of electrons when some overcome the thermal conductivity barrier to become itinerant, and others remain localized on Au2+ sites. Thus, although this perovskite remains a semiconductor from 0 – 55 GPa, compression enables metal-like transport and magnetoresistance, driven by a transition from a localized (Class II) to a partially delocalized (Class II-III) mixed-valence electronic structure.