Cation Distribution and Coordination Chemistry of Cu(II) in Zn(II) Hydroxide Nitrate Solid Solutions: A Structural and Spectroscopic Study

Abstract

The X-ray powder diffraction analysis of the appearing phases in hydroxide nitrate compounds (ZnxCu1-x) (OH)2-y(NO3H)y · zH2O is reported. Single-phase solid solutions are detected only for 0 < x < 0.30 (structural type Cu2(OH)3NO3, phase I) and for 0.50 < x < 0.65 (structural type Zn3(OH)4(NO3)2, phase I'). In the interval 0.3 < x < 0.5 the structurally closely related phases I and I′ coexist. For 0.65 < x ≤ 1.0 a second two-phase region with I′ and a third structure type Zn5(OH)8(NO3)2 · 2H2O (phase II-b) was observed. From the intensities of the X-ray reflections and the dependence of the lattice parameter b′ on x it was deduced that only a small fraction of Cu2+ enters phase II-b. Using the structural data and the EPR spectra together with calculations within the angular overlap model (AOM) the observed EPR signals could be correlated with the occupation of the various Zn2+ sites by Cu2+ in the three phases. It was also possible to derive the changes of the Zn2+ host site geometries induced by substituting Cu2+ ions due to vibronic coupling. The nicely resolved EPR spectra at high x values originate from small amounts of Cu2+ in phase II-b. They display patterns of tetragonally elongated and—in the dynamical average—compressed Cu(OH)6 octahedra at higher temperatures, which are assigned to the Zn(2) and Zn(1) sites, respectively. At 5 K the latter polyhedron freezes into a geometry with a strong orthorhombic distortion component due to vibronic Jahn-Teller coupling. The EPR spectra further indicate that the bulk of Cu2+ enters phases I and I′. Powder reflectance spectra at 5 K are in support of these conclusions. The bonding parameters of Cu2+ in phase II-b have been determined from the well-resolved hyperfine structure in the EPR spectra.