Exploring the Color of Transition Metal Ions in Irregular Coordination Geometries: New Colored Inorganic Oxides Based on the Spiroffite Structure, Zn_2–xM_xTe₃O₈ (M = Co, Ni, Cu)

Abstract

We describe the synthesis, crystal structures, and optical absorption spectra of transition metal-substituted spiroffite derivatives, Zn(2-x)M(x)Te3O8 (M(II) = Co, Ni, Cu; 0 < x ≤ 1.0). The oxides are readily synthesized by solid state reaction of stoichiometric mixtures of the constituent binaries at 620 °C. Reitveld refinement of the crystal structures from powder X-ray diffraction (XRD) data shows that the Zn/MO6 octahedra are strongly distorted, as in the parent Zn2Te3O8 structure, consisting of five relatively short Zn/M(II)-O bonds (1.898-2.236 Å) and one longer Zn/M(II)-O bond (2.356-2.519 Å). We have interpreted the unique colors and the optical absorption/diffuse reflectance spectra of Zn(2-x)M(x)Te3O8 in the visible, in terms of the observed/irregular coordination geometry of the Zn/M(II)-O chromophores. We could not however prepare the fully substituted M2Te3O8 (M(II) = Co, Ni, Cu) by the direct solid state reaction method. Density Functional Theory (DFT) modeling of the electronic structure of both the parent and the transition metal substituted derivatives provides new insights into the bonding and the role of transition metals toward the origin of color in these materials. We believe that transition metal substituted spiroffites Zn(2-x)M(x)Te3O8 reported here suggest new directions for the development of colored inorganic materials/pigments featuring irregular/distorted oxygen coordination polyhedra around transition metal ions.

Unique Colours of 3d‐Transition‐Metal‐Substituted Lyonsite Molybdates and Their Derivatives: The Role of Multiple Coordination Geometries and Metal‐to‐Metal Charge Transfer

“…The assignments were made by comparing the ligand‐field transitions for M III O 6 and M II O 6 chromophores given in ref , , , …”

“…Inorganic pigments find wide application in paints, inks, glasses, plastics and coloured glazes in which the thermal and chemical stabilities as well as weather‐resistance properties of the pigment materials play a crucial role Most inorganic pigments consist of transition metals (TMs) substituted in the positions of non‐transition‐metal/non‐metal atoms in various colourless metal oxide hosts. Thus, Cu‐substituted apatites, [A 10 (PO 4 ) 6 (Cu x OH 1– x – y ) 2 ] (A = Ca, Sr, Ba), which produce magenta/violet/blue colours, 3d‐TM‐substituted spiroffite, Zn 2 Te 3 O 8 , which yields purple/yellow/green colours, and Co 2+ ‐, Ni 2+ ‐ and Cu 2+ ‐substituted diphosphate, SrZnP 2 O 7 , which give orange, cyan, green and red colours, are some of the TM‐containing pigment materials reported in recent times , . The colours of inorganic pigments mainly arise from the electronic transitions within the partially filled d shell of the TM ion surrounded by the oxide (ligand) ions of the host; the crystal structure of the host provides specific/characteristic ligand‐field geometries (octahedral, tetrahedral, trigonal bipyramidal etc.)…”

“…Isomorphous karrooite (MgTi 2 O 5 ) substituted with TM ions also yields several bright‐coloured ceramic pigments . Similarly, divalent TM ion substitution at the distorted octahedral Zn II sites of Zn 2 Te 3 O 8 produces purple, yellow and parrot‐green colours . Recently, Dey et al investigated the origin of the unusual colours of wolframite‐type AWO 4 (A = Mg, Mn, Co, Ni, Cu, Zn) oxides.…”

Temperature resistant red and purple ceramic pigments based on the solid solution series BaZn2−xNixSi2O7 and BaMg2−xNixSi2O7

“…Furthermore, the type of ligands also affects the colour. New colours up to now not reported in literature can also be obtained by the incorporation of chromophore ions in an irregular coordination shell [22].…”

Exploring the Colour of 3d Transition‐Metal Ions in Trigonal Bipy­ramidal Coordination: Identification of Purple‐Blue (CoO5) and Beige‐Red (NiO5) Chromophores in LiMgBO3Host

“…[10][11][12] Similarly, substitution of 3d metal ions at the distorted octahedral coordination site of Zn II in Zn 2 Te 3 O 8 produces purple, yellow, and parrot-green coloured oxides. [13] Five-coordination can result in two geometries; namely, square pyramid (SP) and trigonal bipyramid (TBP). Substitution of transition-metal ions in SP geometry has been studied in SrZnP 2 [14] SP geometry around Cu II in R 2 Cu 2 O 5 and R 2 BaCuO 5 (R = rare earth metals) gives intense green materials.…”