Betaine arsenate, $\mathrm{(CH_3)_3NCH_2COO · D_3AsO_4}$: structure refinement using multiple-wavelength synchrotron radiation data

The crystal structure of deuterated betaine arsenate, (CH3)3NCH2COO · D3AS3O4, has been re-investigated from X-ray diffraction data collected with synchrotron radiation at three different wavelengths (0.98 Å, 1.05 Å, 1.15 Å) in the vicinity of the As K-absorption edge (1.04497 Å). Each data set was refined individually, but a joint refinement of all data simultaneously resulted in substantially decreased standard deviations of atomic parameters and derived quantities. The results of a previous structure analysis by Schildkamp, Schäfer and Spilker [Z. Kristallogr. 168 (1984) 187–195] are essentially confirmed.The both paraelectric and ferroelastic room-temperature phase of betaine arsenate crystallizes in space group P21/n [Z = 4, a = 9.556(4) Å, b = 12.897(6) Å, c = 8.128(3) Å, β = 91.23(2)°, V = 1001.5(7) Å3]. Its crystal structure consists of AsO43−-tetrahedra which are linked by disordered hydrogen bonds into chains along the c-axis. The betaine zwitterions are attached to these chains in a herring-bone fashion via ordered dimeric hydrogen bonds. These two different hydrogen-bonding schemes are reflected in the AsO43− geometry: there are two shorter and two longer As–O bonds. The shorter distances [1.650(2) Å, 1.653(2) Å] to the oxygens involved in the arsenate chains are typical for double bonded As=O; the longer distances [1.675(2) Å, 1.691(2) Å] correspond with As–OH single bonds