Theme III: Low loss microwave dielectrics with simultaneously high magnetic permeability
Theme Leader: Stemmer

Wireless communication devices, such as cell phones and radar systems, require dielectric materials that simultaneously possess large, temperature-independent dielectric constants and low dielectric losses at microwave frequencies. Currently there are only a few materials, such as Ba₃Zn₂TaO₉ that meet these requirements. Nonetheless, from a commercial point of view the ancillary properties of Ba₃Zn₂TaO₉ leave much to be desired. Processing must be done at high temperatures where zinc volatility becomes problematic[1], and the widespread proliferation of cell phones has driven up the cost of tantalum to an undesirable level. To find alternate materials it is necessary to develop a better understanding of the structure-bonding-property relationships that give rise to an optimal combination of properties. In particular, the factors that control the temperature dependence of the dielectric constant are not well understood. Studies have shown that cation size ratios[2], order-disorder effects[3], and octahedral tilting distortions[4] all play a role, and lattice vibrations, thermal expansion and phase transitions are also believed to be critical features. We will begin by determining the relationships between structural features, phase transitions and dielectric properties of various structures containing perovskite building blocks. Our focus will be on compounds containing octahedrally coordinated early d⁰ transition metal ions that are prone to undergo the types of static and dynamic distortions that give rise to high permittivity (i.e. Ti⁴⁺, Nb⁵⁺, Ta⁵⁺, W⁶⁺, etc.). Through the use of chemical and external pressure the lattice vibrations and phase transitions will be altered and the impact on the dielectric properties will be observed. Chemical pressure will be investigated through study of different compounds containing similar structural features. For example Ti⁴⁺ centered octahedra will be investigated by studying ternary perovskites (i.e. SrTiO₃, CaTiO₃), ordered perovskites (i.e. La₂MgTiO₆), layered perovskites (i.e. Sr₂TiO₄, Sr₃Ti₂O₇, Bi₄Ti₃O₁₂) and pyrochlores (Y₂Ti₂O₇). External pressure will be imposed through synthesis and characterization of superlattice thin film architectures.

While there is a fair amount of data available for ternary and some ordered perovskites, there has been little work in this area on layered perovskites or other structure types. Woodward will extend his previous work on octahedral tilting distortions in 3D perovskites[5,6,7] to include layered perovskites. This will be accompanied by diffraction studies of phase transitions in these compounds, and variable temperature measurements of the dielectric properties of ceramics. Stemmer and Seshadri will extend their studies of pyrochlores and other geometrically constrained frameworks. Spaldin will perform first-principles calculations of the dielectric properties of the materials under study. To incorporate the effects of disorder, models will be parameterized using first-principles results, and then Monte Carlo and molecular dynamics simulations will be used to simulate disordered systems[8,9]. Stemmer and Salvador will deposit films of the most promising materials and investigate the role of interfacial stresses in superlattices. Stemmer's expertise in characterizing dielectric materials in both bulk and thin film form will be utilized to its fullest extent.

In addition to the above project on low loss and temperature-independence, we will also direct our efforts towards designing better tunable dielectrics. Tunable dielectrics promise to revolutionize high-frequency electronics. For example, modern multi-mode cell-phones must duplicate the front-end functions for each desired frequency band or modulation format. Dielectrics with a permittivity that can be controlled by an applied electric or magnetic field could allow these functions to be consolidated. Currently, only ferroelectrics in their paraelectric phase with the perovskite structure are being considered for these applications. Such materials show very large tunabilities but also relatively high losses. Our goal is the discovery of new solids that show a high tunability of the dielectric constant combined low dielectric losses.

On full funding of the center in Phase II, the incorporation of additional functionalities, such as tuning by a magnetic field will be explored, for example by incorporating magnetic ions in the pyrochlore structure. Since low loss is important, as in the magnetic ferroelectrics, only transition metal ions that are robust to oxidation and reduction are promising. Woodward and Seshadri will start by exploring ferrites which have high permeability but are strongly insulating, and therefore are good candidates.

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