Structure/property relationships of the thermoelectric oxyselenides (Bi 1-x A x CuOSe) (A=Ba and Ca)

The crystal structures, solid solution limit, and powder X-ray reference diffraction patterns for two ‘natural superlattice’ series Bi1-xBaxCuOSe (x = 0.05, 0.075, 0.1, 0.2, and 0.3), and Bi1-xCaxCuOSe (x = 0, 0.05, 0.075, 0.1, 0.2 and 0.3) have been determined. The structure/property relationships of these thermoelectric materials are summarized. As the ionic radius of Ba2+ is greater than that of Bi3+, the unit cell volume, V, of Bi1-xBaxCuOSe increases progressively from x = 0 to x = 0.2 (from 137.868 (5) A3 to 141.194 (10) A3, respectively). However, even though the ionic radius of Ca2+ is smaller than that of Bi3+, the unit cell volumes, V, of Bi1-xCaxCuOSe also show an increasing trend as a function of x (137.868 (5) A3 to 139.295 (12) A3 from x = 0 to 0.3, respectively) due to the relatively large increase in c parameter. The structure of Bi1-xAxCuOSe (A = Ba and Ca) can be considered as built from [Bi2(1-x)A2xO2]2(1−x)+ layers normal to the c-axis alternating with fluorite-like [Cu2Se2]2(1−x)- layers in the c-direction. The substitutions of Ba and Ca on the Bi site of Bi1-xAxCuOSe lead to the weakening of the ‘bonding’ between the [Bi2(1-x)A2xO2]2(1−x)+ and the [Cu2Se2]2(1−x)- layers (a decrease of Columbic force), resulting in an increase of the c-axis parameter and V. Powder X-ray diffraction patterns of Bi1-xAxCuOSe were submitted for inclusion in the Powder Diffraction File (PDF).