Crystal Structures of Betaine Phosphate/Arsenate Mixed Crystal

Single crystals of a betaine-phosphate/arsenate mixed system (30% P was replaced with As randomly) were investigated by means of X-ray diffraction. The crystal structures of room temperature and of low temperature (20 K) were determined. At room temperature, the least-squares calculation converged at the R factor 3.6% with 1038 reflections. The structure is isomorphous to pure betaine phosphate. For the low-temperature structure the R factor converged at 4.3% with 4896 reflections (3.7% with 2987 normal reflections and 9.6% with 1909 superlattice reflections). The structure analysis clarifies that the hydrogen atoms of the phosphoric acid between neighboring phosphate tetrahedra are ordered in the low temperature phase resulting the superstructure (the unit cell dimension along the a-axis is doubled). By replacing 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%