Identification of the chromophore in the apatite pigment [Sr10(PO4)6(Cu(x)OH(1-x-y))2]: linear OCuO- featuring a resonance Raman effect, an extreme magnetic anisotropy, and slow spin relaxation

Diffusion of Copper Ions in the Lattice of Substituted Hydroxyapatite during Heat Treatment

The doping of hydroxyapatite with various substituent ions can give this material new and useful properties. Nonetheless, local distortions of structure after doping can change the properties of the material. In this work, the thermal stability of copper-substituted hydroxyapatite synthesized by the mechanochemical method was investigated. In situ diffraction analyses showed that copper ion diffusion during the heating of Cu-substituted hydroxyapatite promotes phase transformations in the substituted hydroxyapatite. The behavior of copper ions was studied in samples with ratios (Ca + Cu)/P = 1.75 and 1.67. It was found that in both cases, single-phase Cu-substituted hydroxyapatite with the general formula Ca_10-xCu_x(PO₄)_6-y(CO₃)_y(OH)_2-yO_y is formed by the mechanochemical synthesis. When heated at approximately 600-700 °C, the lattice loses copper cations, but at higher temperatures, CuO diffusion into the hydroxyl channel takes place. Cuprate-substituted hydroxyapatite with the general formula Ca₁₀(PO₄)₆(OH)_2-2x(CuO₂)_x forms in this context. At 1200 °C, the sample is single-phase at (Ca + Cu)/P = 1.75. Nonetheless, slow cooling of the material leads to the emergence of a CuO phase, as in the case of (Ca + Cu)/P = 1.67, where the material contains not only CuO but also Cu-substituted tricalcium phosphate. In the manufacture of ceramic products from Cu-substituted hydroxyapatite, these structural transformations must be taken into account, as they alter not only thermal but also biological properties of such materials.

Dy3+ single ion magnet in the extended inorganic solid Ca(Y,Dy)AlO4

Confined in the extended solid with a widespread K2NiF4 structure type, cation Dy3+ exhibits magnetic bistability under a zero field.

Quenching of the Eu3+ Luminescence by Cu2+ Ions in the Nanosized Hydroxyapatite Designed for Future Bio-Detection

The hydroxyapatite nanopowders of the Eu3+-doped, Cu2+-doped, and Eu3+/Cu2+-co-doped Ca10(PO4)6(OH)2 were prepared by a microwave-assisted hydrothermal method. The structural and morphological properties of the products were investigated by X-ray powder diffraction (XRD), transmission electron microscopy techniques (TEM), and infrared spectroscopy (FT-IR). The average crystal size and the unit cell parameters were calculated by a Rietveld refinement tool. The absorption, emission excitation, emission, and luminescence decay time were recorded and studied in detail. The 5D0 → 7F2 transition is the most intense transition. The Eu3+ ions occupied two independent crystallographic sites in these materials exhibited in emission spectra: one Ca(1) site with C3 symmetry and one Ca(2) sites with Cs symmetry. The Eu3+ emission is strongly quenched by Cu2+ ions, and the luminescence decay time is much shorter in the case of Eu3+/Cu2+ co-doped materials than in Eu3+-doped materials. The luminescence quenching mechanism as well as the schematic energy level diagram showing the Eu3+ emission quenching mechanism using Cu2+ ions are proposed. The electron paramagnetic resonance (EPR) technique revealed the existence of at least two different coordination environments for copper(II) ion.

Tb-based silicate apatites showing slow magnetization relaxation with identical parameters for the Tb3+ and Dy3+ counter ions

Tb-diluted and Tb-rich apatite-type silicates with compositions Y7.75Tb0.25Ca2(SiO4)6O2 and Tb8Ca2(SiO4)6O2, respectively, exhibit field induced multiple slow relaxation of magnetization. The former reveals two slow relaxation paths, the latter only one with a longer relaxation time of several seconds. The relaxation features of the Tb-diluted one are comparable with those of analogue compounds, where Tb is replaced by Dy, as well as with those of a Tb-doped calcium phosphate apatite. The relaxation parameters of the Tb-rich compound virtually match those of the Dy-based analogue Dy8Ca2(SiO4)6O2. The latter represents the first instance of independence of magnetization relaxation on the nature of a paramagnetic rare-earth metal ion in single ion magnet like materials.

All-Inorganic Single-Ion Magnets in Ceramic Matrices

All-inorganic single-ion magnets representing paramagnetic ions incorporated in a crystalline diamagnetic matrix are reviewed. Key results and advantages of this approach in comparison with the common strategy based on molecular metal-organic complexes are considered, and some unsolved problems and future perspectives are discussed.

Influence of ligand field on magnetic anisotropy in a family of pentacoordinate CoII complexes

A family of mononuclear penta-coordinated CoII complexes, [Co(L)Cl2]·CH3OH (1), [Co(L)Br2] (2) and [Co(L)(NCS)2] (3) (where L is 1-mesityl-N,N-bis(pyridin-2-ylmethyl)methanamine) were synthesized and characterized. In these complexes, the neutral non-planar ligand, L, binds to three coordination sites around the metal center while two others are bound by anionic halide/pseudo halide ligands. The coordination geometry of the complexes is dictated by the coordinated anionic ligands. Thus, the coordination geometry around the metal ion is distorted trigonal bipyramidal for complexes 1 and 3, while it is distorted square pyramidal for complex 2. Ab initio CASSCF/NEVPT2 calculations on the complexes reveal the presence of an easy plane magnetic anisotropy with the D and E/D values being, 13.3 and 0.14 cm-1 for 1; 36.1 and 0.24 cm-1 for 2 and ±8.6 and 0.32 cm-1 for 3. These values are in good agreement with the values that were extracted from the experimental DC data. AC magnetic measurements reveal the presence of a field-induced slow relaxation of magnetization. However, clear maxima in the out-of-phase susceptibility curves were not observed for 1 and 3. For complex 2, peak maxima were observed when the measurements were carried out under an applied field of 1400 Oe which allowed an analysis of the dynamics of the slow relaxation of magnetization. This revealed that the relaxation is mainly controlled by the Raman and direct processes with the values of the parameters found to be: B = 0.77(15) s-1 K-6.35, n = 6.35(12) and A = 3.41(4) × 10-10 s-1 Oe-4 K-1 and m = 4 (fixed). The ab initio calculation which showed the multifunctional nature of the electronic states of the complexes justifies the absence of zero-field SIM behaviour of the complexes. The magnitude and sign of the D and E values and their relationship with the covalency of the metal-ligand bonds was analysed by the CASSCF/NEVPT2 as well as AILFT calculations.

Multiple slow relaxation of magnetization in Dy3+ confined in the crystal matrix of rare-earth-calcium silicates with the apatite structure

Apatite-type silicates Y7.75Dy0.25Ca2(SiO4)6O2 and Dy8Ca2(SiO4)6O2 were prepared by high-temperature solid state synthesis. In the crystal lattice, Dy3+ partially substitutes Ca2+, preferably at the 6h Ca2-site, and forms a short bond of 2.2 Å with the intra-channel O2-. The imposed strong ligand field anisotropy provides large magnetic anisotropy, which manifests itself as slow relaxation of magnetization at low temperatures. The magnetic dynamics is characterized by three or two characteristic values of relaxation time, respectively, which may be attributed to a single Dy3+ center. A phenomenological model is proposed which explains this response in terms of single paramagnetic center multiple relaxation.

Crystal Chemistry and Antibacterial Properties of Cupriferous Hydroxyapatite

Copper-doped hydroxyapatite (HA) of nominal composition Ca₁₀(PO₄)₆[Cu_x(OH)_2-2xO_x] (0.0 ≤ x ≤ 0.8) was prepared by solid-state and wet chemical processing to explore the impact of the synthesis route and mode of crystal chemical incorporation of copper on the antibacterial efficacy against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) strains. Apatites prepared by solid-state reaction showed unit cell volume dilation from 527.17 ų for copper-free HA to 533.31 ų for material of the putative composition Ca₁₀(PO₄)₆[Cu_0.8(OH)_0.4O_0.8] consistent with Cu⁺ insertion into the [001] hydroxyapatite channel. This was less pronounced (528.30 ų to 529.3 ų) in the corresponding wet chemical synthesised products, suggesting less complete Cu tunnel incorporation and partial tenancy of Cu in place of calcium. X-ray absorption spectroscopy suggests fast quenching is necessary to prevent oxidation of Cu⁺ to Cu²⁺. Raman spectroscopy revealed an absorption band at 630 cm⁻¹ characteristic of symmetric O-Cu⁺-O units tenanted in the apatite channel while solid-state ³¹P magic-angle-spinning nuclear magnetic resonance (MAS NMR) supported a vacancy-Cu⁺ substitution model within the apatite channel. The copper doping strategy increases antibacterial efficiency by 25% to 55% compared to undoped HA, with the finer particle sizes and greater specific surface areas of the wet chemical material demonstrating superior efficacy.

TbO+ in a calcium apatite matrix featuring a triple trigger-type relaxation of magnetization

Tb for Ca substituted hydroxyapatite ceramic samples with composition Ca10-xTbx(PO4)6(OH1-x/2-δ)2, where x = 0.1, 0.5, were synthesized by solid-state reaction at 1300 °C in air, and their crystal structure, vibrational spectra, luminescence, and magnetic properties were studied. Implanting Tb3+ into the calcium apatite crystal lattice results in formation of an effective TbO+ ion which displays a short terbium-oxygen bond of 2.15 Å and a stretching vibration at 534 cm-1. The Tb3+ electronic structure has been revealed by analyzing the luminescence spectra and dc/ac magnetization data. Accordingly, the ground state represents a pseudo doublet with MJ = ±6 and the first exited level is by 112 cm-1 higher in energy. The ion exhibits field induced magnetic bistability with the magnetization reversing over the first exited state. Three paths of magnetization relaxation with field-temperature controlled switching between the paths have been identified.

Manifestation of strong magnetic and giant Raman anisotropy in single crystals of Cu for H substituted strontium hydroxyapatite

The linear [O–Cu–O]⁻ reveals a strong easy axis magnetic anisotropy with a high energy of 4 × 10² cm⁻¹ probed by magnetization measurements and polarized Raman spectroscopy of a single crystal.

Impact of fluoride for hydroxide substitution on the magnetic properties of a Co-based single-ion magnet imbedded in the barium apatite crystal lattice

The atomic group O–Co–O persists in the apatite channel and retains a high energy barrier for magnetization reversal.

Single Ion Magnets from 3d to 5f: Developments and Strategies

Single-ion magnets (SIMs), exhibiting slow magnetization relaxation in the absence of the magnetic field, originate from their single spin-carrier centre. In pursuit of high-performance magnetic properties, such as high spin-reversal barrier and high blocking temperature, various metal centres were investigated to establish SIMs, including 3d and 5d transition metal ions, 4f lanthanide ions, and 5f actinide ions, which possess unique zero-field splitting and magnetic properties. Therefore, proper ligand field is of great importance to different types of metals. In the given great breakthroughs since the first SIM, [Pc₂ Tb]^- (Pc=dianion of phthalocyanine), was reported, strategies of ligand field design have emerged. In this review, the developments of SIMs with different metal centres are summarized, as well as the possible strategies.

Slow Spin Relaxation in Dioxocobaltate(II) Anions Embedded in the Lattice of Calcium Hydroxyapatite

Pure-phase cobalt-doped calcium hydroxyapatite ceramic samples with composition Ca₁₀(PO₄)₆[(CoO₂)_x(OH)_1-2x]₂, where x = 0-0.2, were synthesized by high-temperature solid-state reaction, and their crystal structures, vibrational spectra, and magnetic properties were studied. Co atoms are found to enter into the apatite trigonal channel formally substituting H atoms and forming bent dioxocobaltate(II) anions. The anion exhibits single-molecule-magnet (SMM) behavior: slow relaxation of magnetization below 8 K under a nonzero magnetic field with an energy barrier of 63 cm^-1. The barrier value does not depend on the concentration of Co ions, virtually coincides with the zero-field-splitting energy as determined from direct-current magnetization, and is very close to the value obtained earlier for cobalt-doped strontium hydroxyapatite. Moreover, the vibration frequencies of the dioxocobaltate(II) anion are found to be the same in calcium and strontium apatite matrixes. The very weak dependence of the SMM parameters on the matrix nature in combination with good chemical and thermal stabilities of the compounds provides wide opportunities to exploit the intrinsic properties of such a SMM-like anion.

Cobalt-Based Single-Ion Magnets on an Apatite Lattice: Toward Patterned Arrays for Magnetic Memories

Single-ion magnets (SIMs) that can maintain magnetization direction on an individual transition metal atom represent the smallest atomic-scale units for future magnetic data storage devices and molecular electronics. Here we present a robust extended inorganic solid hosting efficient SIM centers, as an alternative to molecular SIM crystals. We show that unique dioxocobaltate(II) ions, confined in the channels of strontium hydroxyapatite, exhibit classical SIM features with a large energy barrier for magnetization reversal (U_eff) of 51-59 cm^-1. The samples have been tuned such that a magnetization hysteresis opens below 8 K and U_eff increases by a factor of 4 and can be further enhanced to the highest values among 3d metal complexes of 275 cm^-1 when Ba is substituted for Sr. The SIM properties are preserved without any tendency toward spin ordering up to a high Co concentration. At a maximal Co content, a hypothetical regular hexagonal grid of SIMs with a 1 nm interspacing on the (001) crystal facet would allow a maximal magnetic recording density of 10⁵ Gb/cm².

A Co-based single-molecule magnet confined in a barium phosphate apatite matrix with a high energy barrier for magnetization relaxation

A Co-ion introduced into the trigonal channel of an apatite-type lattice forms a magnetically anisotropic two-coordinated Co-complex with a record-high spin-reversal energy barrier.

Rich diversity of single-ion magnet features in the linear OCu(III)O(-) ion confined in the hexagonal channels of alkaline-earth phosphate apatites

Following our recent discovery of slow spin relaxation in the unique [OCu(III)O](-) anion located in the apatite-type pigment A10(PO4)6(CuxOH1-x-y)2, where A = Sr, we present the magnetic behavior of this anion for the cases of A = Ca and Ba, which provides evidence for a cation field impact on the properties of a single-ion magnet molecular anion.