Flux syntheses can select for different products than analogous solid state reactions due to the increased role of solvation and kinetics over thermodynamics, altered redox potentials, and/or stabilization of preferred intermediates. Understanding and harnessing the most relevant factors in these syntheses to rationally synthesize new phases requires extensive experimental exploration of synthetic parameters, a framework for extending experimental results into theories for how the system works, and knowledge of structure-property relationships to ultimately identify target phases. Here, I present results following this approach.

We recently showed the usefulness of the hydroflux (a roughly 1:1 mixture of H₂O and alkali hydroxide) to target new alkali copper tellurates with varied dimensionality, stoichiometries, and magnetic motifs. Phase diagrams as a function of flux composition for multiple alkali were developed to elucidate the reactions governing the alkali-dependent product formation. Trends in these phase diagrams allowed us to target new phases with structural two- or three-dimensionality and either Te⁴⁺ or Te⁶⁺ scaffolding. Ultimately, these factors also select for magnetic interactions in one, two, or three dimensions as subtle changes in bond angles and connectivity can either limit or enhance magnetic interactions between Cu²⁺ ions. Using this knowledge, we have discovered seven new phases and identified a phase boundary between Te⁴⁺ and Te⁶⁺ containing products; this region remains of interest for synthesis of novel mixed-valent phases.

The altered kinetics and low temperatures of hydrothermal syntheses not only yield novel metastable phases, they also allow for otherwise inaccessible modifications to existing crystal structures. Guided by the superconductivity occurring near a charge density wave and suppression of the lone pair effect (LPE) in the BaBiO₃ family of superconductors, we have pursued synthesis of phases involving LPE suppression in mixed valent alkali tellurates. Under hydrothermal conditions, the mixed-valent rhombohedral pyrochlore phase CsTe₂O₆, which natively suppresses the LPE, transforms to a cubic phase with a controllably smaller unit cell. The charge density wave is suppressed in the cubic phase, and the tunable unit cell size indicates deintercalation or substitution of Cs ions and thus a tuning of electron-phonon interactions. While the system does not yet superconduct, our strategy and progress will be discussed.

Brandon Wilfong

Appearances