A. A. Rodionov

A study of hydroxyapatite nanocrystals by the multifrequency EPR and ENDOR spectroscopy methods

Specimens of powders of hydroxyapatite (Ca10(PO4)6(OH)2) with average crystallite sizes in the range of 20–50 nm synthesized by the wet precipitation method have been investigated by the multifrequency (9 and 94 GHz) electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) methods. In specimens subjected to X-ray irradiation at room temperature, EPR signals that are caused by nitrogen compounds have been observed. Numerical calculations performed in terms of the density functional theory show that the observed EPR signal is caused by the occurrence of paramagnetic centers, the structure of which is NO32− and which replace the positions of PO43− in the hydroxyapatite structure.

Superhyperfine structure of the EPR spectra of impurity ions in the LiYF4 : Nd3+ system doped by 143Nd isotopes

Results of measuring the EPR spectra in the LiYF4: Nd3+ system doped by 143Nd isotopes with the nuclear spin I = 7/2 have been presented. We have succeeded to observe superhyperfine splitting of the EPR spectra of the samples under investigation at the orientation of the magnetic field B along the symmetry axis c of the crystal and at B ⊥ c on the lines corresponding to both even neodymium isotopes and 143Nd.

Broadening of paramagnetic resonance lines by charged point defects in neodymium-doped scheelites

We study paramagnetic resonance linewidth in a series of CaWO4 and CaMoO4 crystals with different concentrations of neodymium ions (0.0031–0.81 at %). Experimental data are interpreted in the framework of the statistical theory of line broadening by charged point defects. In our calculations, three different contributions are singled out: arising from the local electric fields, electric field gradients and magnetic fields of the nearby point defects. The interaction parameters are determined from the spectroscopic data available for Nd:CaWO4 crystal. Direct calculations of the linewidth are performed for different crystal orientations with respect to external magnetic field. We conclude that major contribution to the broadening comes from the interactions with random electric fields produced by neodymium and charge compensator ions.

EPR studies of the mechanochemically Er3+-activated fluorite nanoparticles

In the present work we show with EPR spectroscopy that the simultaneous grinding of the crystalline CaF2 and ErF3 salts leads to the mechanochemical doping of the single Er3+ ions in the CaF2 host. Dependence of the EPR spectra intensity on the grain size indicates clearly that the Er3+ centers are created at the surface of the particles. Dominant part of the observed paramagnetic centers represent the Er3+ ions substituting the regular quasicubic Ca2+ lattice site (g = 6.7) perturbed by the vicinity of the surface. Fine particles of CaF2 reveal also the nearly-isotropic EPR signal at g ~ 1.97 that originates from the surface defects and can be used for characterization of the obtained samples with respect of the average grain size. Grinding of the of the CaF2 and ErF3 mixture in the ratio of 100:1 during 12 hours gives the surface density of the Er3+ ions of ~ 0.1 nm−2, or ~ 104 Er3− ions per the 190-nm size particle.

Superhyperfine structure of the ESR spectra of Gd 3+ impurity ions in LiYF 4 double fluoride

Electron spin resonance spectra of Gd3+ ions forming a small (~0.0001 at %) impurity in a LiYF4 single crystal have been investigated in a wide temperature range from liquid helium to room temperature. A number of the fine-structure components of the spectrum exhibit a pronounced superhyperfine structure depending on the orientation of the external magnetic field with respect to the crystallographic axes. The superhyperfine structure was not observed in earlier ESR studies of double fluorides with a Gd3+ impurity.

A Study of Mechanochemical Doping of Fluoride Crystals with a Fluorite Structure by Er3+ Ions via Electron Paramagnetic Resonance Spectra

Using electron paramagnetic resonance (EPR) spectroscopy, we have shown that, upon mecha- noactivated doping of powders of compounds CaF2, SrF2, and BaF2 with Er3+ ions, impurity centers of single erbium ions with cubic symmetry are formed. Investigations of dependences of EPR spectra intensities on the particle size show that the process of mechanochemical doping with Er3+ ions proceeds differently for CaF2, SrF2, and BaF2 host matrices. In the case of CaF2, impurity centers are localized in a very thin near-surface layer of CaF2 particles, in SrF2, the impurity is distributed over the volume of particles, while, in BaF2, there is a layer of a finite thickness for which the probability of doping in the course of mechanosynthesis is very small and the impurity of the rare-earth element is localized in the core of large particles. These data can be explained assuming that the result of mechanosynthesis of particles of fluorides with a fluorite structure doped with Er3+ ions at room temperature is governed by two processes—mechanoactivated diffusion of rare-earth ions into particles and segregation of impurity ions at grain boundaries. In this case, the typical scales for compounds CaF2, SrF2, and BaF2 considerably differ from each other.

The transition from dynamics to statics in the electron-spin-resonance spectra of impurity Mn2+ ions in strontium titanate

The electron-spin-resonance (ESR) spectra of SrTiO3:Mn single crystals have been investigated. Results unambiguously indicate that the impurity center formed by an Mn2+ ion has a dynamic nature. In the high temperature range (T > 100 K), ESR spectra of Mn2+ ions reveal cubic symmetry; the spectrum is found to broaden significantly with a decrease in temperature. Upon cooling to T < 10 K, low-symmetry centers of Mn2+ ions with a strong orientational dependence emerge in the spectra. Temperature evolution of the ESR spectrum can be described within the model of a dynamic off-center Mn2+ ion substituting for the Sr2+ ion, with a transition to the static regime at low temperatures with an average localization energy of ∼2.4 ± 0.4 meV for Mn2+ centers due to random deformations.

EPR study of clusters of rare-earth ions in mixed fluoride crystals

The EPR spectra of the (BaF2)1 − x(CeF3)x system are studied for the concentrations x = 0, 0.001, 0.002, 0.005, 0.01, and 0.02. The appearance of new tetragonal centers is detected beginning from x = 0.002, the intensity of these centers being maximal at x = 0.01. The (CaF2)1 − x − y(CeF3)x(YF3)y double solutions with x = 0.001 and y from 0 to 0.02 are also studied. In addition to the ordinary tetragonal center, beginning from y = 0.001, a new tetragonal center appears with the same structure as in the previously studied mixed crystals based on BaF2—namely, the Ce3+-□-R3+ chain elongated along the fourfold axis substitutes the Ca2+-Ca2+-Ca2+ and Ba2+-Ba2+-Ba2+ chains in regular CaF2 and BaF2 crystals (□ is the cation vacancy, and R3+ is the Ce3+, La3+, or Y3+ trivalent ion).

Superhyperfine Structure of the EPR Spectra of Nd 3+ Impurity Ions in Fluorite CaF 2

EPR spectra of a CaF2 single crystal that was grown from melt containing a small addition of NdF3 were studied. Signals corresponding to tetragonal centers of Nd3+ ions and cubic centers of Er3+ and Yb3+ ions were found. Superhyperfine structure (SHFS) in the spectra of the Nd3+ ions was observed for the first time in this crystal; parameters of the superhyperfine interaction of the Nd3+ ions with the nearest nine fluorine ions were determined. The dependence of the resolution of the Nd3+ EPR spectrum SHFS on the incident microwave power at the temperature of T ≈ 6 K was studied. Obtained results are discussed and compared with the literature data.