Sr4AlNbO8: A new crystal structure type determined from powder X-ray data

摘要

Sr4AlNbO8 was synthesized at 1500 °C in air. The crystal structure was initially determined from powder X-ray diffraction data, and later refined with combined X-ray and neutron diffraction data (P21/c; a=7.17592(2) ?, b=5.80261(2) ?, c=19.7408(1) ?; β=97.5470(1)°, V=814.869(3) ?3, Z=4, Rp/Rwp=10.04%/13.18% for X-ray data, 4.40%/5.67% for neutron data, and 7.71%/10.74% in total with χ2 of 3.76, 23 °C). The crystal structure is a new structure type and may be described as a three-dimensional polyhedral network resulting from the corner-sharing of NbO6 and Sr1O6 octahedra and AlO4 tetrahedra. Also, the other strontium atoms (Sr2, Sr3, and Sr4) occupy the larger cavities surrounded by oxygen atoms to form nine, eight, and 11 coordination, respectively. Considering that Sr, Al, and Nb atoms are crystallographically distinct in terms of interatomic distances and polyhedral coordination, Sr4AlNbO8 can be regarded as a stoichiometric compound.