HRTEM, SAED and XRD investigations of La4O4[AsO4]Br and Pr4O4[AsO4]Br

The new oxide-bromide-oxoarsenates (V) La4O4[AsO4]Br and Pr4O4[AsO4]Br were synthesised successfully by a solid state reaction route using a salt flux. Both compounds crystallise with a tetragonal cell. The twinned Pr4O4[AsO4]Br structure was determined from single crystal X-ray diffraction data (P42/m, a = 834.91(9), c = 1321.6(2) pm, V = 0.9213 nm3 and Z = 4). It consists of alternating [Pr4O4]4+ and [BrAsO4]4− layers. The [Pr4O4]+4 layer contains OPr4/4 tetrahedra which share common edges. The AsO43− and Br− anions are located between these layers and form infinite chains along a at z = 0 and along b at z = 1/2. SAED and HRTEM experiments confirmed this well ordered structural model. X-ray powder diffraction data reveal isotypism of La4O4[AsO4]Br: a = 843.07(1), c = 1355.28 pm, V = 0.9633 nm3.

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