A neutron powder diffraction study of Na_(1-x)Sr_xTaO_3 (x = 0.2 and 0.3)

摘要

Black powder samples of the reduced oxotantalates Na_(1-x)Sr_xTaO_3 x = 0.2 and 0.3 were prepared via solid state reactions from appropriate amounts of NaTaO_3, Sr_5Ta_4O_(15) and Ta (99.99 percent) in sealed tantalum ampoules at 1500 deg C. They crystallize in variants of the perovskite type structure (a_(per)). According to structural refinements from neutron powder diffraction data Na_(0.8)Sr_(0.2)TaO_3 crystallizes orthorhombic in the GdFeO_3 type structure with unit cell parameters a approx = 2~(1/2) X a_(per), b approx = 2 X a_(per), c approx = 2~(1/2) X a_(per) and space group Pnma. The structure of the more reduced phase Na_(0.7)Sr_(0.3)TaO_3 is tetragonal with unit cell parameters a approx = 2~(1/2) X a_(per), c approx = 2 X a_(per) and space group P4/mbm.