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Cobalt-doped anatase Ti1–x Co x O2 (0 < x ≤ 0.04) nanopowders (with a particle size of 30–40 nm) were produced by the hydrothermal synthesis method. Morphology, structure, and thermal stability of the synthesized compounds were examined using transmission electron microscopy, infrared spectroscopy, and X-ray diffraction analysis. Using X-ray photoelectron spectroscopy, cobalt ions are shown to have an oxidation state of 2+, with titanium ions having a tetravalent state of Ti4+. In the as-prepared state, all investigated compounds of Ti1–x Co x O2 are paramagnetic, with the value of paramagnetic susceptibility growing in proportion to cobalt content; with the spin of cobalt ion equal to S = 3/2. Analysis of the electron paramagnetic resonance spectra reveals that doping TiO2 with cobalt (up to 2%) is accompanied by a significant increase in the concentration of F+ centers. Further growth of the cobalt content results in a relatively wide line (nearly 600 Oe) in the spectrum, with a g-factor of about 2.005, demonstrating exchange-coupled regions being formed, the fraction of which increases with cobalt content, while the intensity of F+-center signals is reduced appreciably. Annealing of Ti0.96Co0.04O2 in vacuum at 1000 K is shown to have resulted in the substantial localization of cobalt atoms in the subsurface layers, resulting in an approximately 3-fold increase in the Co atoms content on the surface of nanoparticles as compared with that in the bulk. This is shown to be accompanied by appearance of spontaneous magnetization at room temperature, the value of which depends on the cobalt content in TiO2 nanopowders. The value of magnetic moment per Co atom decreases monotonically down to a value of ≃1 μB with cobalt content increasing. A core–shell model proposed to be the most adequate for describing the magnetic properties of TiO2:Co after the reducing annealing. A hypothesis is put forward suggesting that the defect surface enriched with Co atoms and vacancies is described with itinerant type magnetism, allowing for the delocalized nature of electrons near vacancies.

In this work, we report the results of comprehensive experimental and theoretical study of magnetic properties of TiO 2 nanoparticles (20 nm) doped with Fe at various concentrations ranging from 0.1 to 4.6 at. %. Our electron paramagnetic resonance and magnetic measurements data evidence the mixed magnetic state, where paramagnetic Fe 3+ ions coexist with short-range antiferromagnetic correlations caused by negative exchange interaction between neighboring Fe 3+ ions. A quantum mechanical model of the Fe-based magnetic cluster represented as a set of dimers with strong ∼(100−300) K and weak (∼1 K) exchange interactions has been proposed. Our model was found to provide a good description of magnetic properties of TiO 2 :Fe nanopowders. Density-functional theory (DFT) calculations revealed Fe 3+ oxidation state of the iron center in the vicinity of an oxygen vacancy in the crystal structure of anatase. DFT calculations confirmed that two types of Fe 3+ spin-pairs with weak and strong exchange interactions can be formed in the vicinity of an oxygen vacancy. Accumulation of magnetic moment carriers and formation of magnetic clusters in TiO 2 nanoparticles with anatase structure were found to be a general tendency for all studied TiO 2 :Fe nanopowders.

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During the synthesis of ceramics based on Y 2 O 3 (yttria), it is common practice to use dioxides of tet ravalent ions (ThO 2 , ZrO 2 , HfO 2 ) as sintering addi tives, which simultaneously favor an increase in the transparency of ceramics. In particular, Yttralox-a ceramic material created by General Electric Co. (United States)-consists of 90 mol % Y 2 O 3 and 10 mol % ThO 2 and is close to glass with respect to transmission in the visible spectral range [1]. Russian specialists [2,3] have developed a technology of yttria based ceramics with additives of 10 mol % ZrO 2 or HfO 2 , which are characterized by a transmission of 86 or 82%, respectively, at a wavelength of 6 μm. These ceramics are used in bulbs for high pressure lamps of elevated brightness. Ceramics activated by rare earth ions can also be used as active elements of solid state lasers. Highly transparent modified Yttralox ceramics (Y 2 O 3 with additives of 10 mol % ThO 2 and 1 mol % Nd 2 O 3 ) [4] was used to create the first polycrystalline ceramic lasers [5], although their efficiency was relatively low. Investigations aimed at creating more effective laser ceramics were continued, and new yttria based com positions with ZrO 2 and HfO 2 additives were created [6][7][8]. It was found [7] that the optimum ZrO 2 content in these ceramics for highly effective lasing is about 3 mol %. However, it was a priori believed that additive metal ions (Th, Zr, Hf) occur in the lattice in tetrava lent states not forming centers of luminescence and absorption.In recent years, ZrO 2 and HfO 2 additives have been introduced into yttria activated by rare earth ions in order to produce disordering of the crystal structure of ceramic grains, which ensures broadening of the amplification band of activator ions. These ceramics are promising materials for the active media of lasers generating ultrashort radiation pulses. In particular, it was shown [8,9] ) 0.12 as compared to that in the analogous zirconia free ceramics [9]. However, to the best of the authors' knowledge, no data were reported on the possible presence of trivalent zirco nium and hafnium ions in high transparency yttria based ceramics with additives of these elements.The aim of the present work was to use the electron paramagnetic resonance (EPR) method for determin ing the presence of trivalent zirconium and hafnium ions in high transparency yttria based ceramics.