Synthesis and Properties of Epitaxial Thin Films of c-axis Oriented Metastable Four-Layered Hexagonal BaRuO 3
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INTRODUCTION The recent discovery of superconductivity in Sr2 RuO4 compound without any copper and doping' has led to renewed interest in the physical properties, particularly magnetism, and structural chemistry of the ruthenium based oxides (ARuO 3 : A = Ba, Sr, Ca). Among all ruthenates, only BaRuO 3, which has the largest A-site cation in the series, possesses a hexagonal structure in the bulk, while others have a GdFeO 3-typed orthorhombic structure instead.2 The structural chemistry of ruthenates can be described in terms of hexagonal and cubic close packing of A0 3 layers. If all A0 3 layers are cubic close packed, the RuO 6 octahedra form a three dimensional array by sharing only one oxygen to give the cubic-like structure, such as cubic, tetragonal and orthorhombic. In contrast, if A0 3 layers are entirely hexagonal close packed, the RuO 6 octahedra are shared by three oxygens to form the hexagonal structure. Owing to the two basic packing forms, it is recognized that the bulk BaRuO 3 ceramic has three different crystal structures. They are (1) the nine-layered rhombohedral structure (9R) with a = 5.75 A and c = 21.6 A, 3 (2) the six-layered hexagonal structure (6H) with a = 5.71 A and c = 14 A, 4 and (3) the four-layered hexagonal structure (4H) with a = 5.73A and c = 9.5 A,4 depending on the amount of hexagonal and cubic close packing of the BaO3 layers. The 9R phase, which has been considered as the most stable, corresponds to a stacking sequence of CHHCHHCHH, where "C" and "H" represent cubic and hexagonal close packing, respectively. The 6H phase, which has been found mainly as a result of high-pressure bulk synthesis, possesses a stacking sequence of CCHCCH with more cubic close packing. Finally the 4H stacking sequence is CHCH, which contains equal amount of hexagonal and cubic close packing. Like its 6H counterpart, the 4H structure containing pure Ba-Ru-O has not been synthesized reproducibly under ordinary conditions, so it has been considered as metastable. The first bulk 4H Ba-Ru-O ceramic was synthesized under 55 Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
high pressure (between 15 and 30 kbars) by Longo et al.,4 and its single crystal form was grown using a BaC12 flux with first refinement of crystal structure.5 More recently, the electrical and magnetic properties of bulk single crystal form have been characterized, 6 although this phase is still regarded as not reproducible. In this letter, we report the synthesis, electrical transport, and magnetic properties of epitaxial thin films of metastable 4H BaRuO 3 . EXPERIMENT In order to match the in-plane symmetry and lattice of (0001) 4H BaRuO3 , the following substrates have been chosen for the study: (1) (0001) A120 3 with a lattice parameter d0oo of 4.121 A and a lattice mismatch eloo of +20.41 %, (2) (111) SrTiO 3 with d1lo = 5.523 A and £10 = +3.73%, (3) (111) MgA120 4 with d220 = 5.715 A and -220 = +0.24%, and (4) (111) MgO with d 107 = 5.962 A and EIl0 = -3.9 1%. The films were grown by the 90' off-axis sputte
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