Identification of the Chromophore in the Apatite Pigment [Sr₁₀(PO₄)₆(Cu_xOH_1−x−y)₂]: Linear OCuO⁻ Featuring a Resonance Raman Effect, an Extreme Magnetic Anisotropy, and Slow Spin Relaxation

Abstract

A new chromophore has been identified in copper-doped apatite pigments having the general composition [Sr(10)(PO(4))(6)(Cu(x)OH(1-x-y))(2)], in which x=0.1, 0.3 and y=0.01-0.42. By using X-ray absorption spectroscopy, low-temperature magnetization measurements, and synchrotron X-ray powder structure refinement, it has been shown that the oxygenated compounds contain simultaneously diamagnetic Cu(1+) and paramagnetic Cu(3+) with S=1. Cu(3+) is located at the same crystallographic position as Cu(1+), being linearly coordinated by two oxygen atoms and forming the OCuO(-) anion. The Raman spectroscopy study of [A(10)(PO(4))(6)(Cu(x)OH(1-x-y))(2)], in which A=Ca, Sr, Ba, reveals resonance bands at 651-656 cm(-1) assigned to the symmetric stretching vibration (ν(1)) of OCuO(-). The strontium apatite pigment exhibits a strong paramagnetic anisotropy with an unprecedentedly large negative zero-field splitting parameter (D) of ≈-400 cm(-1). The extreme magnetic anisotropy causes slow magnetization relaxation with relaxation times (τ) up to 0.3 s at T=2 K, which relates the compounds to single-ion magnets. At low temperature, τ is limited by a spin quantum-tunneling, whereas at high temperature a thermally activated relaxation prevails with U(eff)≈48 cm(-1). Strong dependence of τ on the paramagnetic center concentration at low temperature suggests that the spin-spin relaxation dominates in the spin quantum-tunneling process. The compound is the first example of a d-metal-based single-ion magnet with S=1, the smallest spin at which an energy barrier arises for the spin flipping.

Color Tuning in Garnet Oxides: The Role of Tetrahedral Coordination Geometry for 3 d Metal Ions and Ligand–Metal Charge Transfer (Band‐Gap Manipulation)

“…The Zn II ions were substituted by Co II ,N i II ,a nd Cu II ions. Thus,s ingle-phase garnets were obtained for Ca 3 The tetrahedral Ga/CoÀOd istances have an average value of 1.873 .T he OÀGa/CoÀOb ond angles exhibit values of 114.92 and 99.058 (Figure 3b), which indicates that the Ga/CoO 4 tetrahedra are distorted. Similard istortions of the tetrahedral sites in the garnet structure have been observed earlier.…”

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

“…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.)…”

“Isolated” DyO+ Embedded in a Ceramic Apatite Matrix Featuring Single‐Molecule Magnet Behavior with a High Energy Barrier for Magnetization Relaxation

“…Them agnetization of the Dy-doped apatites at low temperature will be basically determined by the ground doublet with j M J j= 15/2 (calculated g z = 19.85), as the first exited doublet is > 300 cm À1 above.Asaconsequence,under am agnetic field of several Te sla, the powder average magnetization should asymptotically approach av alue of half the saturation, [5] (1/4)g z m B = 4.96 m B .E xperimental data are consistent with this estimate (Figure 6insert). The cT(T) dependence calculated from the modeled electronic structure of 1 is close to the experimental one ( Figure 6).…”

“…[3] Recently,w er eported that in the apatite lattice La may substitute Ca, while it shifts to the trigonal channel center of the apatite structure forming aLaO + unit with avery short LaÀOb ond of 2.1 . [5,6] These findings have motivated us to prepare and study alkaline-earth phosphate-hydroxyapatites doped with Dy 3+ ,m aterials which indeed show pronounced SMM behavior. [5,6] These findings have motivated us to prepare and study alkaline-earth phosphate-hydroxyapatites doped with Dy 3+ ,m aterials which indeed show pronounced SMM behavior.…”

“…[4] On the other hand, we have shown that Cu and Co ions inserted into the apatite channels behave like individual mononuclear SMMs. [5,6] These findings have motivated us to prepare and study alkaline-earth phosphate-hydroxyapatites doped with Dy 3+ ,m aterials which indeed show pronounced SMM behavior.…”