Research Accomplishments

Alexandra Navrotsky’s research interests lie at the intersection of solid state chemistry, geochemistry, and materials science. The fundamental question that gives unity to a diverse set of studies (over two hundred papers) on materials ranging from oxide superconductors to silicates deep in the Earth’s mantle is “why does a given structure form for a specific composition, pressure and temperature?” The “why” involves relating thermodynamic properties, structural parameters, and chemical bonding in a systematic fashion. At Arizona State University in the 1970’s and 80’s, at Princeton from 1985 to 1997, and at UC Davis since 1997, Navrotsky has built a unique high temperature calorimetry facility, designed and improved on the instrumentation, and developed and applied methods for measuring the energetics of crystalline oxides, of glasses, amorphous, and nanophase and porous materials, of hydrous phases and carbonates, and nitrides and oxynitrides. The thermochemical data obtained are essential to understanding materials compatibility and reactivity in both technological and geological application, but, more fundamentally, the energetics offer insight into chemical bonding, order-disorder reactions, and phase transitions.

Navrotsky’s career started with spinels; for her Ph.D. she developed a thermodynamic model to describe the cation distribution between octahedral and tetrahedral sites. This model has been used extensively by many workers since then, and has been extended and refined in a series of papers from Navrotsky’s group in the mid 1980’s. The occurrence of silicate spinels, and the olivine-to-spinel transition in geologic systems under pressure, led Navrotsky into geophysical applications. Her calorimetric work quantified phase relations in the Earth’s mantle and showed the importance of disorder and configurational entropy in determining phase stability. She first pointed out the importance of entropy stabilization” in crystalline ceramic phases with positive heats of formation.

The perovskite structure has been another focus of attention. Ranging from MgSiO3 at depths of 700 km in the earth, to the YBCO superconductors, to PbTiO3, PbZrO3, and BaTiO3- based electronic ceramics, this structure is ubiquitous but highly variable in its distortions, electronic properties and energetics. Navrotsky’s calorimetry has provided original and essential thermochemical data for a variety of perovskite-related phases. Calorimetric study of perovskites with cations in formal high oxidation states (nominal Cu3+ in high Tc materials, Ni, Co, Fe, and Mn in alkaline-earth-doped La2MO4 and LaMO3) provides direct energetic parameters to constrain models of defect chemistry and to understand electronic properties. In a wide range of perovskites, the role of cation size, expressed by empirical radius ratios and tolerance factors, has been quantitatively correlated with enthalpies and entropies of formation. These arguments, extended from ceramic perovskites to the Earth’s mantle, predicted in 1980 a negative pressuretemperature slope for the transition to MgSiO3 perovskite. This negative Clausius-Clapeyron slope, since verified by phase equilibria, by calorimetry, and by lattice vibrational modeling, appears to have major consequences for convection and evolution on a planetary scale.

The chemistry of silicate, aluminosilicate, and borosilicate glasses and melts is governed by a series of charge balanced substitutions, for example Si4+ = Al3+ + Na+. Navrotsky’s work has quantified their energetics, correlated the energetics with both empirical and ab initio calculations of bonding, and with spectroscopic data. Out of this systematic study emerges a picture which stresses the importance of short-range-order and nearest and next-nearest neighbor interactions in determining stability. This picture is easily transferable to other materials with limited longrange order, especially zeolites, mesoporous surfactant-assembled inorganic materials such as the MCM-41 silicas, and nanophase materials. Navrotsky’s work has shown that many zeolitic and mesoporous phases have energies only slightly higher than those of their stable dense polymorphs.

Another area of research focuses on the heats of formation of nitrides and oxynitrides. Calorimetric methods has been invented and improved. Similar questions relating energetics, phase stability, and coupled substitutions can be asked for nitrides as for oxides, but both data and unifying concepts are much less developed. Recent work in Navrotsky’s lab focuse on silicon nitrides, sialons, phosphorus oxynitrides and polymer derived ceramics. A new area is the energetics of sulfides and other chalcogenides.

Interest in nanomaterials, high surface area oxides, and hydrous oxides is a current focus of the UC D avis Peter A. Rock Thermochemistry Laboratory. Direct calorimetric measurement has proven what was long suspected, that different surface energies of polymorphs (e.g., g -Al 2O3 and a -Al 2O3) can stabilize, at high surface area, a phase which is metastable in the bulk. Work on a number of important environmental and soil minerals is in progress. The thermochemistry is part of a larger interdisciplinary initiative at UC Davis on Nanomaterials in the Environment, Agriculture, and Technology (NEAT) led by Navrotsky.

Another area of interest is the energetic of organic –inorganic composite materials. These include organic- templated mino- and meso- porous materials, silver (alkane) thiolates, clathrates, and hest guest compounds. In situ calorimetry is being used to study the direct synthesis and transformation of such materials. Metal organic framework materials (MOF’s), which extend the porosity seen in zeolites to much higher levels, are also a major research interest. The thermochemistry of actinides materials, related to nuclear energy and radioactive waste disposal are an ongoing interest as are carbonates and CO2 sequestration. Work on solid electrolytes with the fluoride structure and materials with variable valence with battery applications is part of an increasing focus on energy and sustainability.

Navrotsky has educated a large number of Ph.D. students and postdocs. Several of them hold faculty positions in which they utilize their understanding of thermochemistry: Peter Davies at University of Pennsylvania; Paul McMillan at University of London; Richard Hervig at Arizona State University; Masaki Akaogi at Gakushuin University, Tokyo, Japan; Nancy Ross at Virginia Tech; Rebecca Lange at University of Michigan; Jackie Ying at Singapor; Sophie Guillemet-Fritsch at University of Toulous; T. R. S. Prasanna at IIT, Bombay, India; Pamela Burnley at University of Nevada Las Vegas; Claire Fialips at Newcastle University; Juraj Majzlan at Jena University; Martin Wilding at University of Wales; Tori Forbes at University of Iowa; and Di Wu and Xiaofeng Guo at Washington State University. Many others work in federal labs and in the private sector. Over the past few years, the Peter A. Rock Thermochemistry Laboratory has averaged about 20 people (6 graduate students, 6-8 postdocs, visitors, staff, undergrads). It is thus very visible and active in promoting solid state chemistry, thermodynamics and calorimetry.

Navrotsky has developed and improved instrumentation (Calvet microcalorimeters) and techniques (solvents, gas bubbling techniques) for oxide melt solution calorimetry. Whereas 20 years ago the technique required 50 mg of sample and was unable to handle ultra-refractory and volatile-containing materials, it now requires 5 mg (or less) and is of much broader applicability.

Recent interests emphasize the application of thermochemistry to problems of energy. The group is active in two areas: the thermodynamics of perovskite- and fluorite-based materials relevant to solid oxide fuel cells, and the thermodynamics of materials containing uranium and other actinides relevant to nuclear energy and nuclear waste disposal.

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