John E. Greedan's Research Group: Search for New Materials

Brockhouse Institute for Materials Research

Research Interests

I. METAL-INSULATOR TRANSITIONS

II. GEOMETRICALLY FRUSTRATED MAGNETIC MATERIALS

III. SEARCH FOR NEW MATERIALS

Research Interests

New Materials

topic of recent interest in the group concerns complex oxides which contain the 4d and 5d transition elements and their electronic properties (magnetism and electrical transport). While the crystal chemistry and properties of materials based on the 3d transition metals have been studied intensively over the past decades, 4d- and 5d-based compounds represent a much less explored area in transition metal chemistry, but one which is potentially vast and which is becoming increasingly important. One expects very different crystal chemistry and structure-property correlations in the 4d and 5d groups vis-a-vis the 3d group due to several factors, including the more extended nature of the 4d and 5d orbitals, the predominance of higher oxidation states and the tendency to form metal-metal bonded clusters. Recent examples from our group include the quasi-one-dimensional R₃MO₇ compounds (R = rare earth element; M = Ru, Re and Os), illustrated by Pr₃RuO₇ (Figure 1), where the isolated chains of corner-sharing RuO6 octahedra are emphasized.


	Another example is provided by the newly discovered "pillared" perovskites of general composition R₅[M₂O₁₀][M′ M″O₆]. The structure of this latter phase is illustrated by La₅Re₃MnO₁₆, (La₅[Re₂O₁₀][ReMnO₆]) (Figure 2), where layers of Re^VMn^IIO₆ units, consisting of corner-sharing octahedra (the definitive structural motif of perovskites) which are separated or "pillared" by Re^V₂O₁₀ dimeric units in which a Re=Re double bond is present. An exploration of the crystal chemistry of this new structure type is currently underway in which substitutions are being attempted in both the layer and dimer sites. The magnetic properties of these new materials are very surprising as exemplified by La₅[Re₂O₁₀] (Figure 3). We have shown just recently, using neutron diffraction, that the sharp spike at ~ 160 K is due to a complex type of magnetic order involving the weak coupling of the [ReMnO₆] layers through the diamagnetic [Re₂O₁₀] dimers. That the transition occurs at such a relatively high temperature, ~ 160 K, is surprising given that the interlayer separation is > 10 Å and this phenomenon is being explored in greater detail.
Fig-1 structure of Pr₃RuO₇



Fig-2 structure of La₅Re₃MnO₁₆	Fig-3 magnetic susceptibility vs. temperature for La₅Re₃MnO₁₆

top of page