A neutron diffraction study of the diaquohydronium ion in yttrium oxalate trihydrate

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Abstract
Five structures of yttrium oxalate trihydrate and its deuterated isomorphs have been determined from least‐squares refinement of neutron diffraction data. Three structures with different H–D isotopic ratios were determined from 25 °C data and two structures with the same isotopic ratios were determined from data collected at 60 and −155 °C. The cell dimensions for the structures are as follows: YHOX 25 °C, a0 = 8.6962(6) Å, c0 = 12.824(3) Å YDOX 25 °C, a0 = 8.6989(7) Å, c0 = 12.818(3) Å YHDOX 25 °C, a0 = 8.704(2) Å, c0 = 12.832(2) Å YHDOX 60 °C, a0 = 8.708(2) Å, c0 = 6.435(3) Å YHDOX −155 °C, a0 = 8.677(4) Å, c0 = 12.751(7) Å. In each of the structures a proton or deuteron is on a twofold symmetry axis and nearly centered between two H2O molecules forming an H5O2 ion. The O–H–O bond is symmetrical but not linear; the z parameters for H and O are slightly different. The O–H–O distance in the diaquo ion is 2.442(2) Å, and the O–D–O distance is 2.454(2) Å. The four other protons in the diaquo ion (paired by symmetry) form nonlinear hydrogen bonds with O atoms in the oxalate molecules [2.643(2) and 2.680(2) Å]. The respective O–H–O angles are 178.2(3) ° and 167.9(3) °. There is a nonequilibrium temperature dependence of the partitioning of hydrogen and deuterium among the available proton positions. The centered proton in the H5O2 is the preferred site for hydrogen and the H/D ratio increases with decreasing temperature. Between 155 and 25 °C the structure of YH(C2O4)2⋅3H2O becomes progressively disordered with respect to the H5O2+ ion. At approximately 53 °C, there is a dimorphic transition in the structure and both the independent water molecule and the diaquo ion become disordered and the space group changes from P42/n to P4/n; c0 is halved. The progressive disorder is superimposed upon the P4/n disorder.