Synthesis and Ionic Conductivity Properties of Oxynitride Perovskites A_xLa_(2/3)Ta_2O_(6-x)N_x(A=Na,Li) with La_(2/3)Ta_2O_6 Type Structure and a New Ruddlesden-Popper Phase Li_2LaTa_2O_6N

摘要

Oxynitride tantalum perovskites A_xLa_(2/3)Ta_2O_(6-x)N_x with A = Na or Li have been prepared, by reacting La_(2/3)Ta_2O_6 with NaNO_3 or Li(OH), respectively, at 600-950℃ in NH_3 gas atmosphere, and characterised b X-ray powder diffraction, thermogravimetry and impedance measurements. The incorporation of sodium or lithium into La_(2/3)Ta_2O_6 resembles an intercalation and is achieved by a simultaneous replacement of O~(2-) by N~(3-). The compounds formed exhibit the same type of unit cell as La_(2/3)Ta_2O_6 (P4/mmm, a ～ 3.92 , c ～7.91 A). Rietveld refinements of atom site occupancy factors for the sodium containing phases show a preferential occupancy of sodium and lantanum atoms in alternate layers perpendicular to the c axis. Available data suggest that analogous phases with lithium are formed. The latter are lithium ionic conductors with activation energies E_a ≈ 0.8 eV and σ_(max) 1.3X10~(-4) SXcm~(-1) at 300℃. A new oxynitride compound Li_2LaTa_2O_6N has been prepared by reaction of Li_2CO_3, La(OH)_3 and Ta_2O_5 in NH_3 atmosphere at 800-900℃. The crystal structure related to a member in the Ruddlesden-Popper series of perovskite-like phases Sr_(n+1)Ti_nO_(3n+1) with n = 2 and has been refined by the Rietveld method to R_F = 9%, with space group I4/mmm, a = 3.9506(5), c = 18.456(6) A, using Guinier-Haegg Cu/Kα_1 powder diffraction data. The compound is a moderate lithium ion conductor with σ= 3.5 X 10~(-6) S X cm~(-1) at 300℃ and an activation energy E_a = 0.66 eV.