Crystal structures and reference powder patterns of BaR2ZnO5 (R = La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, and Tm)

Reference x-ray powder patterns and the crystal structures of the lanthanid e compounds, BaR2ZnO5, in which R = La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, or T m, were determined by the x-ray Rietveld refinement technique. A structural trend was confirmed for this series of compounds. The compounds with small er ionic radii (R = Sm, Eu, Gd, Dy, Ho, Y, Er, or Tm) are isostructural to the orthorhombic "green phase" (BaY2CuO5). The lattice parameters for compo unds with R = Tm to Sm range from a = 7.01855(9) Angstrom to 7.20452(14) An gstrom, b = 12.25445 (17) Angstrom to 12.5882(2) Angstrom, and c = 5.6786(1 4) Angstrom to 5.81218(11) Angstrom, respectively. R is sevenfold coordinat ed inside a monocapped trigonal prism. These prisms share edges to form wav e-like chains parallel to the long b-axis. The BaR2ZnO5 compounds which con tain larger size R (La and Nd) crystallize in the tetragonal space group 14 /mcm. The lattice parameters are a = 6.90982(10) and c = 11.5977(2) Angstro m for BaLa2ZnO5, and a = 6.75979(5) Angstrom and c = 11.54560(12) Angstrom for BaNd(2)ZnO5. The structure consists of ZnO4 tetrahedra (instead of plan ar CuO4 groups as found in BaR2CuO5) with 10-fold coordinated bicapped squa re prismatic Ba and 8-fold coordinated bicapped trigonal prismatic R ions b etween them. The reference x-ray powder patterns will be submitted to the P owder Diffraction File (PDF).