The influence of partial substitution of phosphorus by arsenic in monoclinic CsH2PO4. X-ray single crystal, vibrational and phase transitions in the mixed CsH2(PO4)0.72(AsO4)0.28

Abstract

Partial substitution of P by As, leading to the solid solution CsH 2 (PO 4 ) 1− x (AsO 4 ) x , with x =0.28 (abbreviated as CDAP) has been shown. The structural characteristics of the crystals were analyzed by means of X-ray diffraction, which revealed that the new title compound is nearly isomorphous with the monoclinic phase of CsH 2 PO 4 (CDP). The structure was solved from 796 independent reflections with R 1 =0.0292 and Rw 2 =0.0702, refined with 59 parameters. The following results have been obtained: space group P2 1 , a =4.9250(4) Å, b =6.4370(3) Å, c =7.9280(6) Å, β =107.316(3)°, V =239.94(3) Å 3 , Z =2 and ρ cal =3.349 g cm −3 . The hydrogen bonds are clearly distinguished in the electron density maps which display distributions corresponding to order of protons. The shorter bond (2.452(4) Å), links the phosphate–arsenate groups into chains running along the b -axis and the longer bond (2.531(3) Å), crosslinks the chains to form (001) layers. The Raman and infrared spectra of CDAP recorded at room temperature in the frequency ranges 15–1200 cm −1 and 400–4000 cm −1 , respectively, confirm the presence of PO 3− 4 and AsO 3− 4 groups in the crystal. Differential scanning calorimetry traces show three phase transitions at 333, 449 and 490 K in this material, which are characterized by X-ray powder diffraction at high temperature. Graphic