Alkali Cation Diffusion in the Channel System of Hexagonal Nepheline  (  K  , Na ,  □  ) Na3 [ Al3 ( Al , Si ) Si4 O 16 ]

Abstract

Sodium-rich nepheline, (K 0.47 Na 0.48 ᮀ 0.05 )Na 3 [Al 3.95 Si 4.05 O 16 ] (ᮀ ϭ 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 ϭ 1:1 occupation, leaving 5% vacancies in the large channel ||c. Guinier-Lenné powder X-ray diffraction from 100 to 895ЊC showed no phase transitions and yielded expansivities of a [Å] ϭ 9. 971(1) ϩ 2.18(1)и10 Ϫ4 иT [ЊC] and c [Å] ϭ 8.377(8) ϩ 0.94(1)и10 Ϫ4 иT [ЊC]. Using polycrystalline pellets, impedance spectra (100-10 7 Hz) were registered at constant temperatures from 100 to 370ЊC. 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 (E a Ϸ 0.68 eV) corresponds to Na ϩ movement along the large channel, which is fast but occluded between K ϩ ions; a low-frequency electric modulus feature (E a Ϸ 1.05 eV) corresponds to the movement of the blocking ion (K ϩ ) along the large channel; and a low-frequency impedance feature (E a Ϸ 1.32 eV) corresponds to Na ϩ movement within the ab plane. Due to the presence of K ϩ , the near dc conductivity ||c is only (ca. 10 times (at 231ЊC) 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 ϩ and K ϩ and shown to aid in understanding of the diffusion mechanism.

25 Conducting solids, covering ionic and electronic conductors

“…Sodium-rich nepheline (K 0.47 Na 0.48 & 0.05 )Na 3 [Al 3.95 Si 4.05 O 16 ] has been synthesized from fused salts and fast Na motion along the large channel has been observed. 104 The K ions block the pathway of the Na ions however.…”

The influence of inclusions on phase transformations of framework aluminosilucates

“…Stepen Al-Si uređenja zavisi od stehiometrije, kada je Si/Al > 1 javlja se delimična neuređenost [37], dok je u nefelinima sa odnosom Al/Si = 1, moguće potpuno uređenje. Pokazano je da su nefelini jednodimenzionalni jonski provodnici [38,39], pa je najveći broj današnjih ispitivanja nefelina usmeren u pravcu njihove primene. Čist natrijumski nefelin (Na 8-r r Al 8-r Si 8+r O 32 ; 0≤r<2) zadržava osnovnu nefelinsku strukturu P6 3 za široki opseg vrednosti parametra sastava r. Za r ~ 0, detektovane su strukturne promene pri kojima dolazi do tripliranja c-ose jedinične ćelije koja je normalna na sloj šestočlanih tetraedarskih prstenova.…”

“…nefelinskih polimorfa [32,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], mehanizam transformacije nt-Carn → Neph nije u potpunosti razjašnjen. R. Dimitrijević et al [36] je prijavio transformaciju niskotemperaturnog karnegita u osnovni heksagonalni nefelin, pri čemu su refleksije niskog intenziteta detektovane u difraktogramu nefelinske faze, pripisane fazi nisko-temperaturnog karnegita.…”