Structure, Optical Properties, and Magnetism of the Full Zn1–xCuxWO4 (0 ≤ x ≤ 1) Composition Range

Abstract

Microcrystalline and submicrometer powders of Zn(1-x)Cu(x)WO(4) (0 ≤ x ≤ 1) have been prepared by a solid-state synthesis from stoichiometric quantities of the constituent d-block metal oxide and tungsten oxide as well as from a Pechini sol-gel synthesis starting from the d-block metal nitrate and ammonium metatungstate. The stoichiometry of the product is confirmed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis. X-ray diffraction shows that for the entire range of compositions, a single-phase product crystallizes in the wolframite structure, with a symmetry-lowering transition from P2/c to P1[overline] at x = 0.20, concomitant with the first-order Jahn-Teller distortion of Cu(2+). Far-IR spectroscopy corroborates that symmetry lowering is directly related to the tetragonal distortion within the CuO(6) octahedra, with the Zn-O A(u) symmetry mode at 320 cm(-1) (x = 0) splitting into two stretches at 295 and 338 cm(-1) (x = 0.3). UV-vis-NIR spectroscopy shows an optical absorption edge characteristic of an indirect band gap that linearly decreases in energy from 3.0 eV (x = 0) to 2.25 eV (x = 1). SQUID magnetometry shows that Zn(1-x)Cu(x)WO(4) (0.1 ≤ x ≤ 1) has an effective moment of 2.30 ± 0.19 μ(B) per mol copper, typical of Cu(2+) in extended solids. For high concentrations of copper (x ≥ 0.8), two transitions are observed: one at high-temperature, 82 K (x = 1.0) that decreases to 59 K (x = 0.8), and the Néel temperature, 23.5 K (x = 1.0) that decreases to 5.5 K (x = 0.8). For x < 0.8, no long-range order is observed. A physical 1:1 mixture of both CuWO(4):ZnWO(4) shows magnetic ordering identical to that of CuWO(4).