We determined the structure of the hydrated Cu(II) complex by both neutron diffraction and first-principles molecular dynamics. In contrast with the generally accepted picture, which assumes an octahedrally solvated Cu(II) ion, our experimental and theoretical results favor fivefold coordination. The simulation reveals that the solvated complex undergoes frequent transformations between square pyramidal and trigonal bipyramidal configurations. We argue that this picture is also consistent with experimental data obtained previously by visible near-infrared absorption, x-ray absorption near-edge structure, and nuclear magnetic resonance methods. The preference of the Cu(II) ion for fivefold instead of sixfold coordination, which occurs for other cations of comparable charge and size, results from a Jahn-Teller destabilization of the octahedral complex.

The full set of partial structure factors for the prototypical network glass GeSe2 was measured using the method of isotopic substitution in neutron diffraction. The basic building block of the network is the Ge(Se(1/2))(4) tetrahedron in which 34(5)% of the Ge reside in edge-sharing configurations. The intrinsic chemical order of the glass is, however, broken with a maximum of 25(5)% Ge and 20(5)% Se being involved in homopolar bonds at distances of 2.42(2) and 2.32(2) A, respectively.

The partial structure factors of bulk-quenched glassy GeSe2 were measured by using the method of isotopic substitution in neutron diffraction to enable the first detailed comparison at the partial pair distribution function level of a covalently bonded network system in both its glassy and liquid phases. The results show that the basic building block of the glass is the Ge(Se1/2)4 tetrahedron in which 34(5)% of the Ge atoms reside in edge-sharing configurations. The intrinsic chemical order of the glass is, however, broken with a maximum of 25(5)% Ge and 20(5)% Se being involved in homopolar bonds at distances of 2.42(2) and 2.32(2) Å, respectively, which is consistent with the existence of these features in the liquid phase of GeSe2. Like for the liquid, concentration fluctuations in the glass are found to extend over distances characteristic of the intermediate-range atomic ordering as manifested by the appearance of a first sharp diffraction peak at 1.00(2) Å−1 in the Bhatia–Thornton concentration–concentration partial structure factor. A comparison is made between the measured partial structure factors and recent first principles molecular dynamics simulations for the glassy and liquid phases. It is found that the most significant disagreement between experiment and simulation occurs with respect to the Ge–Ge correlations and that the simulated results for the glass are too liquid-like, reflecting the use of a quench time greatly in excess of that achieved experimentally.

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The partial structure factors and pair distribution functions for liquid GeSe at 727(2) °C were measured by using the method of isotopic substitution in neutron diffraction. The results show that the liquid retains little memory of the high-temperature crystalline phase of GeSe. On melting, there is a reduction in the Ge-Se coordination number from 6 to 3.2(2) and both Ge-Ge and Se-Se homopolar bonds become features of the molten state with contact distances of 2.36(2) and 2.34(2) Å and coordination numbers of 0.8(1) and 0.22(3) respectively. The results are discussed by reference to the structures of molten CuSe and CuBr, which contain electronegative species of the same or similar size. The structure is also compared with that of the glass-forming network melt GeSe2 and it is found that, unlike the latter, there is no strong indication of intermediate-range atomic ordering as manifest by a prominent first sharp diffraction peak in one of the measured partial structure factors.