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

Abstract

We describe the synthesis, crystal structures, and optical absorption spectra/colours of 3d transition‐metal‐substituted lyonsite‐type oxides Li3Al1–xMIIIx(MoO4)3 (0 < x ≤ 1.0; MIII = Cr, Fe) and Li3–xAl1–xMII2x(MoO4)3 (0 < x ≤ 1.0; MII = Co, Ni, Cu). The oxides were readily synthesized by the solid‐state reaction of stoichiometric mixtures of the constituent binaries at around 700 °C in air. The crystal structures determined by Rietveld refinement of PXRD data revealed that in the smaller trivalent‐metal‐substituted lyonsite oxides, the MIII ions occupy the octahedral (8d, 4c) sites and the lithium ions occur exclusively at the trigonal‐prismatic (4c) site in the orthorhombic (Pnma) structure. On the other hand, larger divalent‐metal (CoII/CuII)‐substituted derivatives show occupancy of the CoII/CuII ions at both the octahedral and trigonal‐prismatic sites. We have investigated the colours and optical absorption spectra of Li3Al1–xMIIIx(MoO4)3 (MIII = Cr, Fe) and Li3–xAl1–xMII2x(MoO4)3 (MII = Co, Ni, Cu) and interpreted the results in terms of the average crystal field strengths experienced by MIII/MII ions in multiple coordination geometries. We have also identified the role of metal‐to‐metal charge transfer (MMCT) from the partially filled transition metal 3d orbitals to the empty Mo 4d orbitals in the resulting colours of these oxides.

Exploring the Corundum Structure as a Host for Colored Compounds – Synthesis, Structures, and Optical Studies of (MM′)3TeO6 (M = Mg, Mn, Co, Ni, Zn; M′ = Mg, Mn, Co, Ni, Cu)

“…Similar differences were also noted for the absorption edges/band gaps of the divalent wolframite tungstates AWO 4 (A = Mg, Mn, Co, Ni, Cu, Zn) and the lyonsite molybdates Li 3 Al(MoO 4 ) 3 and Li 2 Mg 2 (MoO 4 ) 3 as well as their transition‐metal (TM) analogues . The smaller charge‐transfer energies observed for TM wolframite tungstates and lyonsite molybdates compared with those of their non‐transition‐metal analogues were assigned to the MMCT transitions in which the partially occupied 3d orbitals of the TMs become the highest occupied molecular orbital (HOMO), and the empty 4d/5d states of molybdate/tungstate become the lowest unoccupied molecular orbital (LUMO).…”

“…As part of the present study, we explored the possible substitution of the Te 6+ site in the Ni 3 TeO 6 structure by Mo 6+ ions. It has been suggested that the presence of empty 4d orbitals near the partially filled 3d orbitals would promote the transfer of 3d electrons into the empty 4d orbitals , . Our efforts towards the substitution of Mo 6+ ions in place of Te 6+ ions were partially successful, as we were limited to x = 0.25 in these compounds.…”

“…Of the many studies, the observations of the blue color in Mn 3+ ‐substituted YInO 3 and Co 2+ ‐substituted ZnMoO 4 as well as the different colors in copper‐substituted apatites and transition‐metal‐substituted MgTi 2 O 5 have shown much promise. Recently, we explored the distorted coordination geometries of transition elements in different hosts, which resulted in uniquely colored compounds . Continuing our efforts in this direction, we have now investigated the corundum‐related structure of the general formula (MM′) 3 TeO 6 (M = Mg, Mn, Co, Ni, Zn; M′ = Mg, Mn, Co, Ni, Cu).…”

Visible‐Light‐Activated C−C Bond Cleavage and Aerobic Oxidation of Benzyl Alcohols Employing BiMXO5 (M=Mg, Cd, Ni, Co, Pb, Ca and X=V, P)

“…Similar differences were also observed for the absorption edges of the divalent wolframite tungstates AWO 4 (A = Mg, Mn, Co, Ni, Cu, Zn) [24] and the lyonsite molybdates Li 3 Al(MoO 4 ) 3 and Li 2 Mg 2 (MoO 4 ) 3 as well as their transition-metal (TM) analogues. [27] The smaller charge-transfer energies observed for TM wolframite tungstates and lyonsite molybdates compared with those of their non-transition-metal analogues were assigned to the metal-metal charge transfer (MMCT) transitions. In MMCT transitions, the partially occupied 3d orbitals of the TMs become the highest occupied molecular orbital (HOMO), and the empty 4d/5d states of molybdate/tungstate become the lowest unoccupied molecular orbital (LUMO).…”

Structural evolution of transition metal orthoborates (Zn3B2O6−Co3B2O6) with the Kotoite mineral structure: Synthesis, structure and properties

“…[a] Δ is the polyhedral distortion parameter defined by Δ= 1/N ∑ [{(r i −r)/r} 2 ]10 3 ; where N is the number of bonds and r i and r are individual and average bond lengths, respectively [12,32] …”