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Re®nements of ®ve crystals in the Cs 2x Rb 2À2x -Ti 2 O 2 As 2 O 8 series, caesium rubidium titanyl arsenate, with x = 0.07, 0.31, 0.58, 0.71 and 0.86, which are compositional analogues of KTiOPO 4 (KTP), have been completed at 293 K and two (x = 0.71, 0.86) at low temperature. All the structures are found to be orthorhombic (space group Pna2 1 ) and are isostructural with KTP, although there is evidence of some Cs disorder over additional sites in the framework, particularly at the Cs-rich end of the series, as discussed in Part II . Acta Cryst. B54, 645±651]. Unusually large U 33 parameters for shared Cs/Rb sites are observed and are shown to be the result of the existence of separate sites for Cs and Rb within the structural framework, although the coordinates of these sites cannot be resolved convincingly. The structural changes in the TiO 6 /AsO 4 framework required to accommodate an increasing fraction of the larger Cs cation across the series and under cooling to 120 K are elucidated. Finally, the deviations of the roomtemperature and low-temperature structures from the high-temperature prototypic structure (space group Pnan) are examined and suggest that the phasetransition temperature should increase linearly from CsTiOAsO 4 to RbTiOAsO 4 .

The problems of refinements of structures in a polar space group, such as Pna21, are illustrated by the example of crystals in the Cs2x Rb2−2x Ti2O2As2O8, caesium rubidium titanyl arsenate, series, which are isostructural with the well known non-linear optical crystal KTiOPO4. It is shown in particular that errors in the data collection and/or refinement tend to contribute positive residual electron density in sites which are related by an inversion operation to the real Cs/Rb sites. This phenomenon is a consequence of the presence of two minima in least-squares space, representing here the possible coordinate sets x,y,z and x,y,−z, for a polar structure and is exacerbated in these structures by the presence of strong inversion psuedo-symmetry. It is shown that in the worst case, the positive residual electron density can be eliminated and an improved agreement to the observed data obtained by refinement of a partially micro-twinned disordered structure in which the Cs/Rb occupancy is split between the main sites and the inversion-related sites. It is argued that the true structure is micro-twinned, including partial occupancy of the additional sites.

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