Communication pubs.acs.org/crystal
Reductive Solid Phase Epitaxy of Layered Y2O2Bi with Bi2− Square Net from (Y, Bi) Powders and Y2O3 Amorphous Thin Film Ryosuke Sei,† Tomoteru Fukumura,*,†,‡ and Tetsuya Hasegawa†,‡ †
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan CREST, Japan Science and Technology Agency (JST), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
‡
S Supporting Information *
ABSTRACT: Ingenious solid phase epitaxy of ThCr2Si2-type Y2O2Bi thin film with a Bi2− square net was developed. A Y2O3 amorphous thin film with Y and Bi powders was heated in two steps under reductive atmosphere on a nonoxide latticematched CaF2 substrate. This procedure was indispensable for the epitaxial growth of Y2O2Bi, circumventing the formation of the C-rare earth type (Y,Bi)2O3 with Bi3+. Highly oxidizable Y metal was a key to accomplish the unusual reductive state of Bi2−. The lattice-matched CaF2 substrate possessing a similar structure to the Y2O22+ unit in Y2O2Bi promoted the spontaneous c-axis orientation of the thin film. film is desired rather than the polycrystalline powder specimens presently available. A polycrystalline R2O2Bi powder has been synthesized by utilizing two-step heating: a mixture of stoichiometric amounts of Bi, R, and R2O3 was sealed in an evacuated silica tube by first heating at low temperature (500 °C), followed by second heating at high temperature (750 °C).2 A rare earth bismuthide (RBi) is considered to be formed in the first heating step according to the R−Bi binary phase diagram,8 followed by the formation of an R2O2Bi phase in the second step heating. This is because polycrystalline R2O2Bi and analogous R2O2Sb have also been synthesized from a mixture of RBi (Sb), R2O3, and Bi (Sb).9,10 In order to perform epitaxial growth of R2O2Bi epitaxial thin film, solid phase epitaxy (SPE) is one promising method because SPE is a powerful technique for epitaxial growth of various layered oxide thin films.11,12 The main difficulty in SPE is to achieve unusual valence of Bi2− in R2O2Bi; thus, the SPE process under a highly reductive condition will be needed as in previous studies. In this Communication, we report on development of reductive SPE for the thin film growth of ThCr2Si2-type Y2O2Bi with Bi2−. Y2O2Bi epitaxial thin films were obtained by the solid phase reaction between Y and Bi powders and Y2O3 amorphous films on lattice-matched CaF2 substrates, utilizing two-step heating.
1. INTRODUCTION In the emerging field of oxide electronics, naturally and artificially layered oxides are one of the key functional materials such as high temperature superconductor cuprates and high electron mobility SrTiO3/LaAlO3 interfaces.1 Recently, a series of layered ThCr2Si2-type compounds R2O2Bi (R = rare earth or Y) were synthesized in the form of polycrystalline powder.2 The crystal structure of R2O2Bi is the same as that of AFe2As2 (A = Ba, Sr, Ca),3 which is a parent compound of Fe-based high temperature superconductors. In R2O2Bi, the Bi atoms form a square net structure with the unusual valence Bi2− (Figure 1).
Figure 1. Crystal structure of the ThCr2Si2-type R2O2Bi using CIF data in ref 2 drawn with the computer program VESTA.7 The blue rectangle denotes the unit cell.
2. EXPERIMENTAL SECTION
The presence of the Bi square net results in fascinating electronic properties such as the metal−insulator transition driven by chemical pressure in R2O2Bi,2 the anisotropic Dirac Fermions in SrMnBi2,4 and the superconductivity associated with heavy and light carriers in CeNi0.8Bi2.5 In addition, R2O2Bi is expected to serve as a topological insulator due to the large spin−orbit interaction of Bi.6 In order to investigate the intrinsic properties of R2O2Bi, a single crystal or epitaxial thin © 2014 American Chemical Society
Pulsed laser deposition of a Y2O2Bi thin film from a multiphase pellet composed of Bi, Y2O3, and Y2O2Bi on various lattice-matched (001)oriented substrates always resulted in the formation of a cubic C-rare earth type (Y,Bi3+)2O3 epitaxial thin film even in high vacuum (
