Crystal structures of betaine phosphate/arsenate mixed crystal

Single crystals of a betaine-phosphate/arsenate mixed system (30% P was rep laced with As randomly) were investigated by means of X-ray diffraction.. T he crystal structures of room temperature and of low temperature (20 K) wer e determined. At room temperature, the least-squares calculation converged at the R factor 3.6% with 1038 reflections. Tile structure is isomorphous t o pure betaine phosphate. For the low-temperature structure the R factor co nverged at 4.3% with 4896 reflections (3.7% with 2987 normal reflections an d 9.6% with 1909 superlattice reflections). The structure analysis clarifie s that the hydrogen atoms uf tile phosphoric acid between neighboring phosp hate tetrahedra are ordered ill the low temperature phase resulting the sup erstructure (the unit cell dimension along the a-axis is doubled). By repla cing P with As up to 30% in P(As)O-4 tetrahedra, the unit cell structure is changed little; only the mean size of the tetrahedra is enlarged by 2.2%.