A neutron and X-ray refinement of the crystal structure of CaHAsO4.H2O (haidingerite)

Abstract

A neutron-diffraction analysis has been carried out on the crystal structure of the mineral haidingerite, CaHAsO4. Ha0, based on space group Penb (D~,), in accord with three recent X-ray analyses. Positions of the three independent hydrogen atoms have been located by a Fourier difference synthesis with neutron data collected at 2 = 1.15 A. For positional and thermal parameters of all atoms, full-matrix leastsquares refinement led to a final R-value of 0-057 over 518 independent neutron reflexions. Estimated standard deviations of the positional coordinates of the hydrogen atoms are 001 A, and about 0005 A for the other atoms. Further full-matrix least-squares refinements of the X-ray data indicated that surprisingly large apparent discrepancies between the neutron and the original X-ray refinement could be attributed largely to differences in refinement procedure. The anionic hydrogen, H(1), is 1.00 (1)A from O(1), which has the longest As-O bond of 1.729 (5)/~, and is engaged in a 2.63 A long non-linear hydrogen bond to 0(3). The water molecule has an unusually large H-O-H angle of 113 (1) ° and forms markedly non-linear bonds of 2.80 and 2.82 .& to arsenate oxygen atoms. With the nearest calcium ion 041 A out of the H-O-H plane and 10 ° off the line of the lone-pair bisector, the environment of the water molecule may be regarded as intermediate between one in which either one lone pair or the lonepair bisector is directed towards the metal ion.