Origins of the odd optical observables in plutonium and americium tungstates

A series of trivalent f-block tungstates, MW(2)O(7)(OH)(H(2)O) (M = La, Ce, Pr, Nd, and Pu) and AmWO(4)(OH), have been prepared in crystalline form using hydrothermal methods. Both structure types take the form of 3D networks where MW(2)O(7)(OH)(H(2)O) is assembled from infinite chains of distorted tungstate octahedra linked by isolated MO(8) bicapped trigonal prisms; whereas AmWO(4)(OH) is constructed from edge-sharing AmO(8) square antiprisms connected by distorted tungstate trigonal bipyramids. PuW(2)O(7)(OH)(H(2)O) crystallizes as red plates; an atypical color for a Pu(iii) compound. Optical absorption spectra acquired from single crystals show strong, broadband absorption in the visible region. A similar feature is observed for CeW(2)O(7)(OH)(H(2)O), but not for AmWO(4)(OH). Here we demonstrate that these significantly different optical properties do not stem directly from the 5f electrons, as in both systems the valence band has mostly O-2p character and the conduction band has mostly W-5d character. Furthermore, the quasi-particle gap is essentially unaffected by the 5f degrees of freedom. Despite this, our analysis demonstrates that the f-electron covalency effects are quite important and substantially different energetically in PuW(2)O(7)(OH)(H(2)O) and AmWO(4)(OH), indicating that the optical gap alone cannot be used to infer conclusions concerning the f electron contribution to the chemical bond in these systems.