Thin films of BaTiO3 have been epitaxially grown on Pt(001)/MgO(100) substrates by reactive evaporation. Structural and electrical properties were investigated as a function of film thickness. In situ reflection high-energy electron diffraction and cross-sectional transmission electron microscope observations have revealed that the BaTiO3 films are epitaxially grown on Pt/MgO substrates from the initial stage without any other phase formation. From the images of an atomic force microscope, it has been found that islands of BaTiO3 are present on the bare Pt surface at the initial stage of deposition; the island structure changes to a continuous layer above 1.2 nm in thickness and BaTiO3 grows in a two-dimensional mode. The lattice parameters and the dielectric properties are dependent on the film thickness. Thermodynamic theory was introduced to explain the thickness dependence of the relative dielectric constant εr. Good agreement between the experimental results and the theoretical calculations leads to the conclusion that the thickness dependencies of the lattice parameters and the dielectric constants are caused by the two-dimensional stress due to the lattice mismatch between Pt and BaTiO3 and/or the difference in the thermal expansion coefficients of BaTiO3 and the MgO substrate.
The nonstoichiometric compound [Ba]x[(Pt,Cu)O3] (x = 1.317) has a composite crystal structure with
RP31 c . DR3mI I~.-is, a=5.817(2), c=4.233(1)A, k=(a*+ b*)/3 + 1.519 (1)c*, V = 124.0 A 3, Z1 = 2, Z2 = 3, Dx =7.46Mgm-3, MoKa, CuKfl, /.t(MoKa)= 42.9 mm-l. The structure was analyzed by a newly developed computer program based on the theory reported in a previous paper [Yamamoto (1992).Acta Cryst. A48, 476--483]. Final R factors: 0.075 (overall), 0.041 (main), 0.108 (first-order satellite), 0.102 (second-order satellite) and 0.224 (third-order satellite). The first subsystem, Ba, is located in the channels of the second columnar subsystem, (Pt,Cu)O3, parallel to the c axis. Atomic position and Pt/Cu-atom occupancies are modulated along the c axis by the interaction between the two subsystems. Ba atoms are displaced helically while Pt/Cu atoms have a displacive modulation only along the c axis with rotational displacement of the coordinated 03 atom clusters around the c axis. The first subsystem has a trigonal structure but with rhombohedral modulation because of the rhombohedral subsystem of (Pt,Cu)O3. The possibility of a phase transition specific to the composite crystals is discussed.
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