Structure of the mixed-metal carbonate KAgCO_3 revisited: order–disorder (OD) polytypism and allotwinning

摘要

Crystals of KAgCO_3 belong to an order–disorder (OD) family of structures composed of layers of two kinds. There are two polytypes with a maximum degree of order [MDO_1: Pccb; MDO_2: Ibca, doubled a-axis compared with MDO_1], which are both realised to a different extent in two crystals under investigation [volume fraction MDO_1:MDO_2 in 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 KAgCO_3 was 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.