Structure of the mixed-metal carbonate KAgCO3revisited: order–disorder (OD) polytypism and allotwinning

Abstract

Crystals of KAgCO3belong to an order–disorder (OD) family of structures composed of layers of two kinds. There are two polytypes with a maximum degree of order [MDO1:Pccb; MDO2:Ibca, doubleda-axis compared with MDO1], which are both realised to a different extent in two crystals under investigation [volume fraction MDO1:MDO2in crystal (I): 0.0216:0.9784 (3) and in crystal (II): 0.9657:0.0343 (3)]. Sharp diffraction spots and the absence of diffuse scattering indicate highly ordered macroscopic domains. The structure of KAgCO3was refined concurrently against all reflections using an allotwin model (addition of the intensities of both domains). It is shown that a disorder model refined against reflections of only one domain can lead to a significant overestimation of the volume fraction of this domain.