59th Annual Report on Research 2014

Introduction: Functional properties of noncentrosymmetric (NCS) compounds include piezoelectricity and ferroelectricity (FE), which can enhance catalytic conversion of fossil fuels into consumable resources. Presently, compounds belonging to the ABO₃ perovskite family, such as lead-zirconium titanate (PZT) and barium titanate (BTO), have found utility as catalysts for methane combustion and tar reforming owing to electric polarization dependent chemical absorption. While work to date has focused primarily on perovskite oxides, fluorine and mixed-anion (i.e. oxyfluoride) compounds are also technologically interesting for catalyzing chemical reactions owing to the changes in electronic structure derived from the more strongly localized fluorine states. Oxy/fluoro-perovskites have more ionic metal-ligand interactions compared with oxides. In mixed anion systems, this competition between ionic and covalent bonding can favor polar crystal classes which support electric polarizations. Nonetheless, few have studied the microscopic mechanisms stabilizing NCS structures in fluorides and oxyfluorides. Here we use ab initio electronic structure methods to explore the phase stability of oxy/fluoride cryolites with an emphasis on structural phase transitions and identify promising avenues to stabilize functional polar phases.

Results: Towards the discovery of polar oxy/fluorides, we first examined the effect of different mechanical boundary conditions on cryolite flourides with chemical formula A₃BF₆. The fluoromanganate Na₃MnF₆ undergoes an unusual isosymmetric phase transition induced by hydrostatic pressure. Our theoretical study (submitted for publication during this period) uncovered the microscopic origin of the transition. We found that at a critical pressure (2.15 GPa) the phonon modes associated with the Jahn-Teller distortion of the Mn-F bond are renormalized, which leads to a large change in the electronic orbital polarization between isostructural monoclinic phases. Interestingly, this is a rare example where the Jahn-Teller distortions persist both before and after the isosymmetric transition. This understanding provided the basis for our study of the manganite cryolites as thin film materials under biaxial strain.

We then investigated the effect of biaxial strain in the cryolites, Na₃MnF₆ and Na₃ScF₆, to examine if the previously described transition can be obtained in a more realistic geometry, e.g. in a thin film coherently strained on various substrates. We found the isosymmetric transition in Na₃MnF₆ can be induced with modest tensile strains > 1% (Figure 1). No transitions associated with Jahn-Teller orbital degeneracies occur in the fluoroscandate because it contains a Jahn-Teller inactive d⁰ cation. In addition, we found a second monoclinic variant of Na₃MnF₆ that is stable at strains larger than 3.5%, but not found to exist under hydrostatic pressure.

Prior to this work we were unaware of experimental reports of either fluoride cryolites or elpasolite with polar crystal structures. None of the known stoichiometric A₂BB’F₆ cryolite/elpasolites are compatible with second-order Jahn-Teller active d⁰ cations that undergo cooperative polar displacements. To that end over the last period, we focused on uncovering mechanisms that would stabilize polar phases in these structures. In the case of Na₃ScF₆, while the compound contains a d⁰ transition metal cation, Sc out-of-center distortions have yet to be observed. The absence of these distortions is understood to be a consequence of its low formal valence and large ionic radius. Nonetheless, we found polar lattice instabilities in Na₃ScF₆ high temperature cubic phase (Figure 2, inset), which largely consist of Na and not Sc cation displacements. The Na cations displace to improve the under-bonding environment of the 12-fold coordination site they occupy. Our calculations found that the polar displacements are compatible with the a⁰a⁰c⁺ rotation pattern but not with the a^-a^-c⁰ tilt of the ScF₆ polyhedra.

In an attempt to find a phase that might compete with the non-polar ground state we computed the total energy with respect to epitaxial strain, relaxing the atomic structure at different compressive (negative) and tensile (positive) strain values, for two structures with the a⁰a⁰c⁺: one without the polar displacements (P4/mnc) and one containing them (Pmn2₁). We find that the polar Pmn2₁ phase is more stable than the P4/mnc up to 2.5% compressive strain and all tensile strain values explored (Figure 2); however, the total energy of the non-polar P2₁/n ground state is lower by more than 400 meV/formula unit (f.u.) at 0% strain. We anticipate that the stabilization of the polar phase in Na₃ScF₆ will require strains that are much larger than what is considered experimentally accessible. Nonethelesss, we highlight the use of tensile strain as a useful route to increase bond stresses on the Na cation and drive cooperative polar displacements.

The financial support from the Petroleum Research fund has been critical to furthering a structure-driven electronic functionality paradigm within my group. It has also led to a new understanding of isosymmetric phase transitions and mechanisms for cation off-centering distortions in fluorine compounds. In year 2, the project has partially funded 1 full-time PhD student working on fluoride cryolites. It also supported efforts in the design of NCS fluoride crystals, which may find use in non-linear optical applications, e.g., to be used in systems for the study of interfacial electrochemical energy conversion reactions. In addition, the PhD student began to develop a “medium” throughput technique for generating candidate oxyfluoride polymorph structures based on tiling the anion sub-lattice within a desired stoichiometry. Travel funds enabled the PhD student to gain experiences invaluable to his future research career. In the past year the student has attended the annual March Meeting of the American Physical Society and international workshops on phenomena in electronic crystals and oxide materials. These experiences have allowed him the opportunity to broaden the scope of his scientific knowledge as well as establish potential collaborators for present and future research endeavors.