title: Structure of the mixed-metal carbonate KAgCO₃ revisited: order-disorder (OD) polytypism and allotwinning.
abstract: Crystals of KAgCO3 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 [MDO1: Pccb; MDO2: Ibca, doubled a-axis compared with MDO1], which are both realised to a different extent in two crystals under investigation [volume fraction MDO1:MDO2 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 KAgCO3 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.
authors.affiliation: X-ray Centre, Vienna University of Technology, Austria.
authors.forename: Philipp
authors.initials: P
authors.lastname: Hans
authors.affiliation: X-ray Centre, Vienna University of Technology, Austria.
authors.forename: Berthold
authors.initials: B
authors.lastname: Stöger
authors.affiliation: Institute of Chemical Technologies and Analytics, Vienna University of Technology, Austria.
authors.forename: Matthias
authors.initials: M
authors.lastname: Weil
authors.affiliation: Institute of Chemical Technologies and Analytics, Vienna University of Technology, Austria.
authors.forename: Erich
authors.initials: E
authors.lastname: Zobetz
keywords: allotwinning; order–disorder; polytypism