Composition- and temperature-dependent phase transitions in 1:3 ordered perovskites Ba 4- x Sr x NaSb 3 O 12

A series of 25 members of the 1:3 ordered perovskite family of the type Ba4-xSrxNaSb3O12 has been synthesized and their structures determined using synchrotron X-ray and neutron powder diffraction techniques. At room temperature the sample Ba4NaSb3O12 has a cubic structure in space group Im over(3, -) m with a=8.2821(1) Å, where the Na and Sb cations are ordered in the octahedral sites but there is no tilting of the (Na/Sb)O6 octahedra. As the average size of the A-site cation decreases, through the progressive replacement of Ba by Sr, tilting of the octahedra is introduced firstly lowering the symmetry to tetragonal in P4/mnc then to orthorhombic in Cmca and ultimately a monoclinic structure in P21/n as seen for Sr4NaSb3O12 with a=8.0960(2) Å, b=8.0926(2) Å, c=8.1003(1) Å and β=90.016(2)°. The powder neutron diffraction studies show that the orthorhombic and tetragonal phases in Cmca and P4/mnc co-exist at room temperature for samples with x between 1.5 and 2