CRYSTAL STRUCTURE AND PHYSICAL PROPERTIES OF COMPOUNDS IN THE PHASE-SYSTEM Ln2O3-ReO2-Re2O7
H. Ehrenberg1,2, T. Hartmann1, G. Wltschek1, R. Doyle2, H. Fuess1
1 Materials Science, Darmstadt
University of Technology, D-64287 Darmstadt, Germany
2 IRCS, University of Cambridge CB3
0HE, UK
Keywords: Rare-earth rhenium oxides (crystal structures, magnetic properties, electrical conductivity), metallic bonds between metal ions, dimerization in one-dimensional metals.
The interest in physical properties of compounds in the phase diagram Ln2O3-ReO2-1/2Re2O7, with Ln any rare-earth element or yttrium, is based on the different formal oxidation states of rhenium, which vary from +4 to +7 and include half-integer values. A summary is presented of all compounds, which have been structurally characterized in detail, classified by the formal oxidation state of rhenium and the Ln:Re ratio. The underlying crystal structures suggest various extremely interesting physical behaviours, especially in respect of the magnetic and conducting properties. The rhenium oxides ReO2 and ReO3 exhibit metallic conductivity, and metal-insulator transitions are expected for some of the rare-earth rhenium oxides.
The oxidation state of rhenium is of primary importance for the physical properties: Re(+7) is diamagnetic. Further electrons cannot be removed easily and thereby contribute significantly to the electrical conductivity. To illustrate this, we have studied a single crystal of Dy3ReO8 with the fluorite-type structure with a random distribution of Dy or Re on one site. Semiconducting properties with an energy gap of 160 meV are deduced from the temperature dependence of resistivity between 200 and 300 K. All of the compounds of the form Ln3ReO8 with Ln = Sm to Yb are paramagnetic down to 1.8 K. Gd is the exception, and this compound displays weak ferromagnetism below 10 K.
Lower oxidation states of rhenium should be accompanied by permanent magnetic moments. However, the extra electrons (in addition to the Re(+7) core) are expected to be mobile. This should have striking consequences for both the magnetic moment per ion as well as the electrical conductivity. A common feature observed is metallic bonds between rhenium-rhenium pairs. This is reported for oxidation states varying from +4 to +5.5, e.~g.~{}[1,2]. However the formal oxidation state is not a sufficient condition for the formation of metallic bonds and a ''low'' Ln:Re ratio is also required. In particular, the ratio has to be lower than 3:1 for Re(+5) since no such rhenium-rhenium pairs exist in Sm3ReO7[3]. In Ln5Re2O12 the formal oxidation state of rhenium is +4.5, and metallic bonds exist. The crystal structure was solved by synchrotron X-ray diffraction on a twinned crystal of Ln = Tm. This revealed two domains, related to each other by a rotation through 180o about the a*-axis, based on the standard setting for space group C2/m. Distorted ReO6 octahedra form chains with alternating Re-Re distances of 2.455(1) A and 3.220(1) A. Twinning has also been reported for Ln = Dy [4] and Ho [5], and we propose that this kind of twinning is due to a transition from a high-temperature phase (b) without monoclinic distortion, formed during syntheses, into the above described a-phase, which is stable at room temperature. In the b-phase only one intermediate rhenium-rhenium distance is expected within the chains of ReO6 octahedra and some of the electrons are mobile. This one-dimensional metal transforms into a dimerized state with alternating short and long distances, as realized in the a-phase and similar to a Peierls instability.
On the other hand, isolated ReO6 octahedra exist in the newly established compounds Ln6ReO12 for Ln = Ho to Lu with rhenium in its formal oxidation state of +6. These compounds are isotypical with Ln6MoO12, Ln6WO12 and Ln6UO12 [6], but have to be prepared using very different methods. Deduced from the maximum in the temperature dependence of magnetization at constant field, magnetic ordering above 1.8 K was only observed for Ln = Yb.