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Unformatted text preview: pinel structure (Fe'"), [Fe"Fe"']004.
( 1977). '' 250 Aluminium, Gallium, Indium and Thallium Figure 7.15 Crystal structure of Na-B-alumina (see
text). This section, which is a plane parallel to the c-axis, does not show the
closest Na-Na distance.
plane; it also explains the very low resistivity of
the order of 30ohmcm. The structure can also
accommodate supernumerary Na ions, and the
compound, even in the form of single crystals,
is massively defective, having typically 20-30%
more Na than indicated by the idealized formula;
this is probably compensated by additional AI
vacancies in the "spinel blocks" adjacent to the
mirror planes, e.g. Na2.58A121.8034.
Sodium-B-alumina can be prepared by heating Na2C03 (or NaNO3 or NaOH) with any
modification of A1203 or its hydrates to -1500"
in a Pt vessel suitably sealed to avoid loss of
Na2O (as N a + 0 2 ) . In the presence of NaF
or A1F3 a temperature of 1000" suffices. Na,&alumina melts at -2000" (probably incongruently) and has d 3 . 2 5 g ~ m - ~ . Na can
be replaced by Li, K, Rb, CUI, Ag', Ga', In'
or T1' by heating with a suitable molten salt,
and Ag' can be replaced by NO+ by treatment with molten NOCVAlCl3. The ammonium
compound is also known and H3Of-B-alumina
can be prepared by reduction of the Ag compound. Similarly, Al"' can be replaced by Gau'
or Fe"' in the preparation, leading to compounds of (idealized) formulae Na20.11Ga203,
Na2O.l lFe203, K20.11Fe203, etc. Altervalent
substitution is also possible, e.g. in Na-B"alumina, Nal+xMxAlll-xO17r in which M is a Ch. 7 divalent cation such as Mg, Ni or Zn. A typical composition is Nal.67Mgo.67Allo.33017 and the
excess Na charge is compensated for by substituting the divalent or univalent cation into the
lattice sites normally occupied by Al.(58)
Apart from the intriguing structural implications of these fast-ion solid-state conductors, Nap-alumina and related phases have been extensively used as permeable membranes in the NdS
battery system (p. 678): this requires an air-stable
membrane that is readily permeable to Na ions
but not to Na atoms or S, that is non-reactive with
molten Na and S, and that is not an electronic
conductor. Not surprisingly, few compounds have
been found to compete with Na-,%alumina in this
field, although Na-B"-alumina has the remarkable additional property of enabling the rapid
diffusion of a large proportion of cations in the
periodic table (whereas Na-B-alumina itself is
restricted mainly to univalent cations). Indeed,
the p"-aluminas are the first family of high conductivity solid electrolytes which permit fast ion
transport of multivalent cations in solids.(58)
Unrelated to the B- and B"-aluminas are a
group of white, hygroscopic sodium-rich aluminates which have recently been prepared by
heating Na2O and A1203 in appropriate stoichiometric ratios at 700°C for 18-24 hours.(59)
Na5A104, which is isostructural with Na~Fe04,
contains isolated [A1041 tetrahedra with A1-0
176-179pm. Na7A1308 features a novel ring
structure made up of six A104 tetrahedra sharing corner...
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