A neutron diffraction study of the crystal structure of calcium bis(dihydrogen arsenate), Ca(H2AsO4)2

Abstract

The crystal structure of Ca(H2AsO4)2 (triclinic, PT, with two formula units per cell) has been refined by single-crystal neutron-diffraction analysis to give an R value of 0.025 over 930 independent neutron reflexions. For the non-hydrogen atoms, bond lengths, angles and thermal parameters agree very closely with the results of a recent X-ray analysis. Difference Fourier syntheses, before and after anisotropic leastsquares refinement of all atomic parameters, enabled the four hydrogen atoms to be located unequivocally. In one anion, two half-hydrogens, located at centres of symme:ry, are believed to be engaged in symmetrical hydrogen bonds, 0(5) . . .0(5 ' )= 2.436 A, 0(8) . . .0(8 ' )= 2.444 /~. By contrast, another hydrogen bond is relatively long (2.875 + 0.002 ,~) and non-linear [angle 0(7). . . H(3)... 0(3')= 150-2 + 0-3°]. The As-O bonds carrying the half hydrogen atoms are 1677 + 0-002 .~ long, intermediate in length between As-O bonds with full hydrogens (1-727 + 0.002/~) and thase with no hydrogen atom attached (1.651 + 0.002 A) (averaged values).