Lanthanoid silicates (Ln(9.33)Si(6)O(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 (500-700 degrees 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(9.33+x/3)Al(x)Si(6-x)O(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 (apatite + NdAlO(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]. (C) 2011 Elsevier By. All rights reserved.