High pressure NaAlSiO₄: The first silicate calcium ferrite isotype

Abstract

Nepheline (NaAlSiO4)has been transformed at about 1000° C and pressures greater than 180 kbar to a new phase possessing the calcium ferrite structure in a diamond‐anvil press employing YAG laser heating. The unit‐cell dimensions of the high‐pressure phase at STP are ao = 8.740 ± 0.007, bo = 2.746 ± 0.002, Co = 10.206 ± 0.008 Å for indexing as space group Pnma with Z=4. The zero‐pressure volume change associated with nepheline → calcium ferrite structure is −31.9%, and that with Jadeite + α‐NaAlO2 (the intermediate phase assemblage) → calcium ferrite structure is −11.9%. The calcium ferrite‐type NaAlSiO4 is the first example of a silicate with this structure and is thus the most closely packed A2SiO4 compound. Mg2SiO4 may possibly adopt this structure in the earth's mantle, but in any case the new phase is the most likely host for sodium in the earth's lower mantle.

High‐pressure transformations in MgAl2O4

“…There need not be a discrepancy between these findings, however, as Kesson et al relied entirely on TEM, which probes only a small fraction of the sample. It is also possible that the incorporation of Si and Na into the A1-rich phase [Irifune and Ringwood, 1993;Kesson et al, 1994] may enhance the stability of the CaFe20 n structure because NaAISiO n adopts this structure above 18 GPa [Liu, 1977]. By combining TEM (necessarily on quenched samples) with X ray diffraction both at high pressures (and ideally high temperatures) and after quenching to ambient conditions, it should be possible to clarify the phase relations for MORB throughout the pressure range of the lower mantle.…”

Crystal chemistry of six-coordinated silicon: a key to understanding the earth's deep interior

“…High-pressure studies of NaAIGeO4 (Ringwood & Major, 1967a; and NaA1SiO4 (Liu, 1977c(Liu, , 1978aYamada, Matsui & Ito, 1983) revealed that these compounds adopt the orthorhombic calcium ferrite (CaFe204) structure in which all Si and AI are in octahedral coordination. Yamada et al (1983), who synthesized NaAISiO4 at pressures above 24 GPa, used X-ray diffraction to identify their polycrystalline product and propose atomic coordinates.…”

A first-principles study on Si24 as an anode material for rechargeable batteries

“…Moreover, high-pressure induced phase transformations of several electrode materials, e.g., Li x MPO 4 (M ¼ Fe and Co), 8,9 16 In addition to these high-pressure polymorphs of current electrode materials, many novel alkali compounds with an open-channel structure have been derived at high pressure. 12,[17][18][19] It has been proven that these channels likely provide a diffusion path for the alkali ions and lead to high ionic conductivities. 12,18 Although these materials have potential application for batteries, related investigations are rare.…”