A novel approach for identifying and synthesizing highdielectric materials
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A novel approach for identifying and synthesizing high dielectric materials J.-H. Parka) CHiPR, Department of Chemistry, State University of New York, Stony Brook, New York 11794-2100
J.B. Parise CHiPR, Department of Geosciences, State University of New York, Stony Brook, New York 11794-2100
P.M. Woodward Department of Chemistry, The Ohio State University, Columbus, Ohio 43210-1185
I. Lubomirsky and O. Stafsudd Department of Electrical Engineering (IV), University of California, Los Angeles, California 90024 (Received 8 January 1999; accepted 20 May 1999)
Modern telecommunications require materials with high dielectric constants (). The number of suitable elements ultimately limits one approach to the discovery of new materials, targeting compositions with high atomic polarizabilities (␣). By decreasing the molar volume of compositions with high ␣, however, we anticipated dramatic increases in and demonstrated that this approach works. The quenched high-pressure perovskite polymorph of Na2MTeO6 (M ⳱ Ti, Sn) showed a twofold increase in , compared to the ilmenite form. This result suggested the highest values of occur for compositions with high ␣, which form quenchable compounds at high pressures and temperatures. Recent progress in microwave-integrated systems for telecommunication and satellite television applications has necessitated the development of a variety of hyperfrequency devices, for use as filters and frequencystabilized oscillators. The dielectric properties of materials used to construct such devices must meet stringent requirements. The important material characteristics include a high dielectric constant ( >20) to miniaturize the size of the device, a low-loss factor (tan ␦ 1 GPa, theapproach we describe here requires some predictive modeling to increase the success rate.6 Shannon’s polarizability table provides a method for identifying compositions that should have high dielectric constants. Structural modeling, empiricism, or intuition can then be used to identify those compositions that are expected to have high-pressure polymorphs that can be retained on return to room pressure conditions. (Diamond is an excellent example of a technologically important material quenchable at room pressure.) As a test of this philosophy we started with two compounds, Na2MTeO6 (M ⳱ Ti, Sn), which adopt ilmeniterelated structures.7 It is well known that application of pressure can induce transformation from ilmenite to the more densely packed perovskite. (There are known mineral examples such as the phase transition of SrGeO3 from wollastonite, garnet to perovskite, of PbO2 from rutile to columbite or of MgSiO3 from pyroxene, ilmenite to perovskite.) We also note that the ilmenite and perovskite structures are prevalent among high dielectric constant materials (see Table I). These two factors were responsible for our initial choice of test compound. Both Na2TiTeO6 and Na2SnTeO6 ilmenites transform to distorted perovskites at 7 GPa/950 °C. Of critical importanc
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