Single crystal growth of apatite-type Al-doped neodymium silicates by the floating zone method

Lanthanoid silicates (Ln _9.33Si ₆O ₂₆) adopt the hexagonal apatite structure and show potential as solid oxide fuel cell (SOFC) electrolytes due to their high oxide ion conductivity at intermediate temperatures (500700 °C). Ions migrate preferentially along the crystallographic c-axis, and can be improved by introducing lower valent elements at the silicon sites. To better understand this phenomenon, single crystals of aluminium-doped neodymium silicate Nd _9.33x/3Al _xSi _6-xO ₂₆ (0≤x≤1) were synthesised using the floating-zone method in an inert environment at a growth rate of 5 mm/L. The products with x<1.5 were homogeneous, transparent and crack-free crystals. For x=1.5 the monophasic crystal contained minor cracks after cooling and for x=2 a two phase (apatiteNdAlO ₃) polycrystalline mass was obtained. The quality of the 0≤x≤1.5 crystals were confirmed by neutron diffraction and synchrotron X-ray rocking curve diffraction. The incorporation of aluminium into apatite at nominal concentrations was independently established by energy dispersive X-ray spectroscopy (EDX), and electron backscatter diffraction (EBSD) found the crystallographic habit showed extension along [001].