Crystal structure of the compound Bi 2 Zn 2/3 Nb 4/3 O 7

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Juan C. Nino Center for Dielectric Studies, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802

Terrell A. Vanderah Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

Ian M. Reaney Engineering Materials, University of Sheffield, Western Bank, Sheffield S10 2TN, United Kingdom

Clive A. Randall and Michael T. Lanagan Center for Dielectric Studies, Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 24 August 2001; accepted 18 March 2002)

The crystal structure of Bi2Zn2/3Nb4/3O7 was determined using a combination of electron, x-ray, and neutron powder diffraction. The compound crystallizes with a monoclinic zirconolite-like structure [C2/c (No.15) space group, a ⳱ 13.1037(9) Å, b ⳱ 7.6735(3) Å, c ⳱ 12.1584(6) Å, ␤ ⳱ 101.318(5)°]. According to structural refinement using neutron diffraction data, Nb preferentially occupies six-fold coordinated sites in octahedral sheets parallel to the (001) planes, while Zn is statistically distributed between two half-occupied (5 + 1)-fold coordinated sites near the centers of six-membered rings of [Nb(Zn)O6] octahedra. The Nb/Zn cation layers alternate along the c-axis with Bi-layers, in which Bi cations occupy both eight- and seven-fold coordinated sites. The eight-fold coordinated Bi atoms exhibited strongly anisotropic thermal displacements with an abnormally large component directed approximately along the c-axis (normal to the octahedral layers).

I. INTRODUCTION

Ternary oxides in the Bi2O3-ZnO-Nb2O5 system exhibit high dielectric constants (⑀ ⳱ 80–150), relatively low dielectric losses, and chemically tunable temperature coefficients of capacitance (␶c).1–4 Such properties, combined with sintering temperatures of less than 950 °C, make these materials attractive candidates for capacitor and high-frequency filter applications in multilayer structures co-fired with silver electrodes. Additionally, Bi2O3–ZnO–Nb2O5 thin films deposited on Pt-electrodes on silicon exhibit potential for use as integrated microwave components and decoupling capacitors.5 Two ternary compounds in the Bi2O3–ZnO–Nb2O5 system have been reported to occur; namely, Bi1.5ZnNb1.5O7 (⑀ ⳱ 150, ␶c ≈ −400 ppm/°C), and Bi2Zn2/3Nb4/3O7 (⑀ ⳱ 80, ␶c ≈ +200 ppm/°C).3–5 Because these compounds exhibit a)

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J. Mater. Res., Vol. 17, No. 6, Jun 2002 Downloaded: 17 Mar 2015

opposite signs of ␶c, their mixture provides temperaturecompensated ceramics.6 Despite a growing interest in the dielectric properties of the bismuth zinc niobates, their crystal structures and the exact stoichiometries remain uncertain. Bi1.5ZnNb1.5O7 has been reported to crystallize with a cubic pyrochlore structure, suggesting that Zn ions occupy the A-sites (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7.2– 4 However, our recent studies revealed that these specimens contain small