The geometrical isotope effect in CrOOH/CrOOD

Torbjörn Gustafsson,¹ Mizuhiko Ichikawa² and Takeshi Tsuchida³

¹Department of Materials Chemistry, Ångström Laboratory,Uppsala University, Box 538, SE-751 21 Uppsala, Sweden

²Division of Physics, Graduate School of Science, Hokkaido University, 060-0810 Sapporo, Japan

³Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, 060-8628 Sapporo, Japan

The CrOOH system is well known for its reported unusually large geometrical isotope effect on deuteration. The material has a layered structure with chromium ions occupying octahedral holes in pairs of close packed layers of oxide ions. The chromium oxygen sandwiches are perpendicular to the hexagonal c-axis and connected by hydrogen bonds. All hydrogen bonds in the structure are thus parallel and oriented along the c- direction. It is known from previous diffraction studies that the hydrogen and deuterium compounds are isostructural so the geometrical isotope effect is directly visible in the elongation of the crystallographic c-axis. The hydrogen bond in this compound is about 2.5Å. The hydrogen atom is usually centred for shorter bonds and off centred for bonds longer than ~2.5Å. It is thus possible that the deuteration induces a change from a centred hydrogen bond to a non-centred for this compound. This is supported by the large changes observed in the IR-spectrum on deuteration.

Recent neutron powder diffraction data indicate a super-lattice formation comprising a doubling of the hexagonal a-axis for the deuterated compound. There is not any indication of a super-lattice formation in the neutron diffraction data from the hydrogen containing compound. On the other hand, X-ray diffraction does not show super-lattice formation for any of the two compounds.

These observations are compatible with an isotope induced super-lattice formation. Since the super-lattice is not visible in the X-ray data it is probable that an ordering of the deuterium atoms causes it. A satisfactory model for the deuterated compound is still to be found.