Praseodymium diiodide, PrI2, revisited by synthesis, structure determination and theory

Praseodymium diiodide, PrI2, is obtained from the triiodide, PrI3, by reduction with praseodymium metal at elevated temperatures. The two modifications, PrI2-IV and -V, are obtained in different ratios upon fast and slow cooling, respectively. PrI2-IV crystallizes with the CdCl2 type of structure (trigonal, R-3m, a = 426.5(1), c = 2247.1(8) pm) while PrI2-V (cubic, F-43m, a = 1239.9(2) pm) represents an own structure type that may be considered as a structural variant of the CdCl2 type with tetrahedral Pr-4 clusters. To elucidate the electronic properties of the modifications of PrI2 first principles electronic band structure calculations have been carried out using the tight-binding linear-muffin-tin-orbital method (LMTO) as well as the full potential augmented plane wave method (FP-LAPW). The band structure and the bonding were analysed in terms of projections of the bands onto orthogonal orbitals. It was especially focussed on Pr-Pr interactions by crystal orbital Hamiltonian population (COHP) analysis. The calculations show accordingly that a configurational crossover between a [Xe]6s(0)5d(0)4f(n) and a [Xe]6s(0)5d(1)4f(n-1) configuration can be observed in the case of PrI2, depending upon the structure adopted. A higher d orbital contribution results in stronger Pr-Pr interactions. Thus, the driving force appears to be an optimisation of bonding. (C) 2004 Elsevier B.V. All rights reserved.