Structural analysis of lanthanum gallate
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ce the discovery of high Tc oxide superconductors,1 extensive application of these materials to microelectronics has been expected. These applications, most importantly, require the growth of single crystal thin films. Therefore, it is necessary to use a substrate that can be lattice-matched with high Tc oxide superconductors. To date, a (100) SrTiO3 single crystal, which had a lattice mismatch of 2.2% and 0.3% with a- and 6-axes of a superconducting YBa2Cu3O^ (YBCO) at room temperature, respectively, was commonly used as the substrate for YBCO single crystal thin film growth (lattice constants of SrTiO3 are a - b = c = 3.905 A, while those of YBCO are a = 3.820 A, b = 3.892 A, and c = 11.688 A). Most superconducting thin films grown on SrTiO3 substrates are polycrystalline and fully oriented with the c-axis perpendicular to the substrate. On the other hand, it has been pointed out that SrTiO3 has an extremely high dielectric constant and loss tangent at 77 K. Therefore, the SrTiO3 substrate is not suitable for microwave applications. Recently, lanthanum gallate (LaGaO3) crystal has been attracting much attention as a substrate for epitaxial growth of superconducting YBCO thin films instead of SrTiO3 (Ref. 2) because LaGaO3 has better lattice match. Lattice constants of LaGaO3 are a = 5.496 A, b = 5.524 A, and c = 1.1%1 A as an orthorhombic cell.3 The lattice mismatch of the Ga-O-Ga distance in the a-b plane of LaGaO3 (Va 2 + b2 /2 = 3.896 A) with a- and fo-axes of YBCO is estimated to be 0.9% and 0.01%, respectively. Furthermore, LaGaO3 has an extremely lower dielectric constant and loss tangent at 77 K than SrTiO3.2 According to the above concept, Koren et al. attempted to grow a superconducting YBCO film on LaGaO3 substrate.4 However, the critical current density in superconducting thin film was poorer than that grown on the SrTiO3 substrate. Sandstrom et al. discovered phase transition at —150 °C.2 More recently, Miyazawa found a surface roughening of a polished J. Mater. Res., Vol. 6, No. 1, Jan 1991 http://journals.cambridge.org
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substrate caused by a ferroelastic-like phase transformation at 140-150 °C.5 We believe that low critical density is induced by this phase transition. As for the phase transition of LaGaO3, Geller reported first that there was only a second order transition from the orthorhombic to the rhombohedral structure at 875 °C.3 On the other hand, O'Bryan et al reported that there was a first order transition from the orthorhombic to the rhombohedral structure at —150 °C and suggested that the phase transition at 875 °C did not exist.6 In this matter, the phase transition of LaGaO3 is confused to some extent. This paper reports, in detail, the phase transition of LaGaO3 analyzed by high temperature x-ray diffractometry (HT-XRD) and differential scanning calorimetry (DSC). II. EXPERIMENTAL PROCEDURE
The LaGaO3 sample examined for the HT-XRD was prepared by mixing a commercially available La2O3 (99.9% purity) and j6-Ga2O3 (99.9% purity) in the stoichiometric ratio. The m
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