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

摘要

Five structures of yttrium oxalate trihydrate and its deuterated isomorphs have been determined from leasthyphen;squares refinement of neutron diffraction data. Three structures with different Hndash;D isotopic ratios were determined from 25thinsp;deg;C data and two structures with the same isotopic ratios were determined from data collected at 60 and minus;155thinsp;deg;C. The cell dimensions for the structures are as follows: YHOX 25thinsp;deg;C,a0= 8.6962(6) Aring;,c0= 12.824(3) Aring; YDOX 25thinsp;deg;C,a0= 8.6989(7) Aring;,c0= 12.818(3) Aring; YHDOX 25thinsp;deg;C,a0= 8.704(2) Aring;,c0= 12.832(2) Aring; YHDOX 60thinsp;deg;C,a0= 8.708(2) Aring;,c0= 6.435(3) Aring; YHDOX minus;155thinsp;deg;C,a0= 8.677(4) Aring;,c0= 12.751(7) Aring;. In each of the structures a proton or deuteron is on a twofold symmetry axis and nearly centered between two H2O molecules forming an H5O2ion. The Ondash;Hndash;O bond is symmetrical but not linear; thezparameters for H and O are slightly different. The Ondash;Hndash;O distance in the diaquo ion is 2.442(2) Aring;, and the Ondash;Dndash;O distance is 2.454(2) Aring;. The four other protons in the diaquo ion (paired by symmetry) form nonlinear hydrogen bonds with O atoms in the oxalate molecules lsqb;2.643(2) and 2.680(2) Aring;rsqb;. The respective Ondash;Hndash;O angles are 178.2(3) thinsp;deg; and 167.9(3) thinsp;deg;. There is a nonequilibrium temperature dependence of the partitioning of hydrogen and deuterium among the available proton positions. The centered proton in the H5O2is the preferred site for hydrogen and the H/D ratio increases with decreasing temperature. Between 155 and 25thinsp;deg;C the structure of YH(C2O4)2sdot;3H2O becomes progressively disordered with respect to the H5O2+ion. At approximately 53thinsp;deg;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 fromP42/ntoP4/n;c0is halved. The progressive disorder is superimposed upon theP4/ndisorder.