Fluorite and Pyrochlore Phases in the HfO 2 -La 2 O 3 -Gd 2 O 3 Systems: Characterization and Calorimetric Study of Samp
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1122-O01-07
Fluorite and Pyrochlore Phases in the HfO2 -La2O3 -Gd2O3 Systems: Characterization and Calorimetric Study of Samples Quenched From Melts Formed by Laser Heating and Aerodynamic Levitation Sergey V. Ushakov1, Alexandra Navrotsky1 and Jean A. Tangeman2† 1 Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, CA 95616 2 Containerless Research, Inc., Evanston, IL 60202 ABSTRACT New experimental results on pyrochlore and defect fluorite phases in HfO2-La2O3 and HfO2-Gd2O3 systems are summarized. Fluorite Hf0.5Gd0.5O1.75 was formed by containerless melting and quenching. Melts with 25-65 mol% La2O3 did not produce any fluorite-type phases, but pyrochlores with cell parameters 10.74 to 10.86 Å. The fluorite phase of Hf0.5La0.5O1.75 can be formed on crystallization of an amorphous precursor from aqueous precipitation. Both Laand Gd- fluorite phases transform to ordered pyrochlore on annealing at 1450 °C. The enthalpies of formation from oxides are -107 ±5 kJ/mol for Hf2La2O7 and -49 ±5 kJ/mol for Hf2Gd2O7 as measured by high-temperature solution calorimetry. Further experiments are needed to elucidate the nature of stabilization of fluorite phase in thin films and powders. Occurrence of disordered phases in thin films, nanoparticles and radiation damaged materials is discussed. INTRODUCTION Crystalline lanthanum and gadolinium hafnates are used in various applications, from coatings in turbine engines [1] to neutron absorbers [2], however the original impetus for this study was the search for novel high-k dielectric materials. In that race, one of the much desired features, besides a higher dielectric constant than SiO2, was that the material remains in the amorphous state after short-duration exposure to a temperature of ~900-1000 °C during dopant drive-in annealing. Amorphous materials are preferred over crystalline ones for gate dielectric structures because amorphous films can be deposited over large areas without grain boundaries, which would inevitably serve as leak pathways for the electric current. A similar problem is faced by geologists during selection of a nuclear waste repository, where crack systems have a greater influence on radionuclide transport than transport within the rocks themselves. Many rare earth oxides as well as hafnia and zirconia have high enough dielectric constant and suitable band gap for consideration as novel gate dielectrics, however none of them can be sustained in an amorphous state at the necessary temperatures. We found that the HfO2La2O3 system demonstrates remarkable thermal stability in the amorphous phase [3], while HfO2-Y2O3 does not. Lanthanum hafnate with an amorphous or defect fluorite structure could be formed by a low-temperature synthesis route or after radiation damage [4]. Containerless quenching from the melt state inevitably produced pyrochlore Hf2La2O7, however defect fluorite phase was quenched in the HfO2-Gd2O3 system [5].
†currently at Corporate Research Materials Laboratory 3M Center, St. Paul, MN < jtangeman
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