Hydrothermal Synthesis, Structure Investigation, and Oxide Ion Conductivity of Mixed Si/Ge-Based Apatite-Type Phases

Apatite-type oxides ([AI4]­[AII6]­[(BO4)6]­O2), particularly those of the rare-earth silicate and
germanate systems, are among the more promising materials being considered
as alternative solid oxide fuel cell electrolytes. Nonstoichiometric
lanthanum silicate and germanate apatites display pure ionic conductivities
exceeding those of yttria-stabilized zirconia at moderate temperatures
(500–700 °C). In this study, mixed Si/Ge-based apatites
were prepared by hydrothermal synthesis under mild conditions rather
than the conventional solid-state method at high temperatures. Single-phase
and highly crystalline nanosized apatite powders were obtained with
the morphology changing across the series from spheres for the Si-based
end member to hexagonal rods for the Ge-based end member. Powder X-ray
and neutron analysis found all of these apatites to be hexagonal (P63/m). Quantitative X-ray microanalysis
established the partial (<15 at%) substitution of La3+ by Na+ (introduced from the NaOH hydrothermal reagent),
which showed a slight preference to enter the AI 4f framework
position over the AII 6h tunnel site. Moreover, retention
of hydroxide (OH–) was confirmed by IR spectroscopy
and thermogravimetric analysis, and these apatites are best described
as oxyhydroxyapatites. To prepare dense pellets for conductivity measurements,
both conventional heat treatment and spark plasma sintering methods
were compared, with the peculiar features of hydrothermally synthesized
apatites and the influence of sodium on the ionic conductivity considered.