Alkali Cation Diffusion in the Channel System of Hexagonal Nepheline(K,Na,o)Na3[Al3(Al,Si)Si4O16]

Sodium-rich nepheline, (K0.47Na0.48o0.05)Na3[Al3.95Si4.05O16] (o = vacancy), has been synthesized from fused salts. Electron microprobe analyses and X-ray diffraction Rietveld refinement were used to ascertain the composition and the K:Na = 1:1 occupation, leaving 5% vacancies in the large channel||c. Guinier-Lenné powder X-ray diffraction from 100 to 895°C showed no phase transitions and yielded expansivities of a [Å] = 9.971(1) + 2.18(1)·10–4·T [°C] and c [Å] = 8.377(8) + 0.94(1)·10–4·T [°C]. Using polycrystalline pellets, impedance spectra (100–10^7 Hz) were registered at constant temperatures from 100 to 370°C. The dispersion of the permittivity, electric modulus, and impedance functions revealed three features with distinct Arrhenius behavior. Considering (i) anisotropic bulk diffusion within the grains, (ii) aggregation of these grains at random orientation in air, and (iii) intergrain and pellet-electrode contacts, models were developed to simulate the spectra. Agreement with the observed data is obtained for the following assignments: a high-frequency feature (Ea ~ 0.68 eV) corresponds to Na+ movement along the large channel, which is fast but occluded between K+ ions; a low-frequency electric modulus feature (Ea ~ 1.05 eV) corresponds to the movement of the blocking ion (K+) along the large channel; and a low-frequency impedance feature (Ea ~ 1.32 eV) corresponds to Na+ movement within the ab plane. Due to the presence of K+, the near dc conductivity ||c is only (ca. 10 times (at 231°C) higher than ||ab, in contrast with the high anisotropy (ca. 10^4) known for pure sodium nepheline. The accessible volume and its distribution within the framework were calculated for Na+ and K+ and shown to aid in understanding of the diffusion mechanism.