161Dy Mössbauer study of the endohedral metallofullerenes Dy@C n (n = 80, 82, 84)

Mössbauer studies of Dy@C _n (n = 80, 82, 84) metallofullerenes were performed at 4.2, 9.6 and 78 K with the ¹⁶¹Dy (25.6 keV) resonance. The observed spectra consist of two subspectra splitted by magnetic hyperfine fields near to the full moment value of trivalent Dy. Paramagnetic relaxation is observed even at 4.2 K. The observed isomer shift is consistent with the Dy³⁺ state and indicates a full charge transfer to the fullerene cage.     

Decay of 161m1,m2Dy isomers under conditions of a resonance environment (Mössbauer Screen)

The half-lives of the isomers ^161m1Dy and ^161m2Dy (E = 25.6 keV and T _1/2 ～ 30 ns for the former and E = 74.6 keV and T _1/2 ～ 3 ns for the latter) placed in a ¹⁶⁰Gd₂O₃ crystal lattice at T = 300 K and surrounded by stable ¹⁶¹Dy nuclei in the composition of ¹⁶¹Dy₂O₃ were measured by the method of (β-γ) coincidences in the beta-decay process ¹⁶¹Tb → ¹⁶¹Dy. Nuclei of ^161m1,m2Dy were obtained according to the chain ¹⁶⁰Gd(n, γ)¹⁶¹Gd → ¹⁶¹Tb → ¹⁶¹Dy from ¹⁶⁰Dy₂O₃ weighted portions irradiated at the PWR-M reactor of the Petersburg Nuclear Physics Institute (PNPI, Gatchina, Russia). The T _1/2 value observed for the isomer ^161m1Dy was found to be correlated with the number of surrounding ¹⁶¹Dy nuclei. The presence of this correlation in ^161m1Dy can be explained by the multiple resonance scattering of photons from isomer decay within the sample used. No such correlation was observed for ^161m2Dy. The half-lives measured for the isomers ^161m1Dy and ^161m2Dy in the absence of the above environment are 29.2(1) and 3.50(1) ns, respectively.     

Temperature‐dependent Raman scattering studies of the geometrically frustrated pyrochlores Dy2Ti2O7, Gd2Ti2O7 and Er2Ti2O7

The temperature‐dependent Raman studies of A₂Ti₂O₇(A = Dy, Er, Gd) were performed on single crystals and polycrystalline samples in the 4.2–295 K temperature range. The Raman spectra showed softening of the majority of phonon modes upon cooling in the whole temperature range studied and large decrease of linewidths. These changes have been analyzed in terms of strong third‐order phonon–phonon anharmonic interactions. Moreover, the 312 and 330 cm⁻¹ modes of Er₂Ti₂O₇(Gd₂Ti₂O₇) showed hardening upon cooling down to about 130 K (100 K) and then anomalous softening below this temperature. The observed anomalous behavior of the Raman modes indicates that some important changes occur in these materials at low temperatures. However, the origin of this behavior is still not clear. Copyright © 2008 John Wiley & Sons, Ltd.     

Backscattering mirrors for X-rays and Mössbauer radiation

Observation of exact backscattering of X-rays and studies of its energy and angular dependences; test of the validity of the dynamical theory of diffraction in the extreme case of exact backscattering; backscattering high-energy-resolution monochromators; backscattering interferometers, in particular of the Fabry–Pérot interferometer type; and precise, up to 5·10^—9 Å, measurements of crystal lattice parameters: these are central topics of the paper. Special attention is paid to the selection of crystals to be used as backscattering mirrors. Noncubic crystals like Al₂O₃, SiC, etc., allow backscattering for X-rays with practically any energy above 10 keV. Feasibility of backscattering mirrors for Mössbauer radiation of ⁵⁷Fe (14.4 keV), ¹⁵¹Eu (21.5 keV), ¹¹⁹Sn (23.9 keV), and ¹⁶¹Dy (25.6 keV) nuclei is demonstrated by Al₂O₃ crystals. A concrete design of a sapphire Fabry–Pérot–Bragg étalon is presented.

     

Goldanskii effect in quadrupole Mössbauer spectra of the 89 keV gamma ray of 156Gd

Unusual relative line intensities in the recoilless absorption spectra of the 89 keV gamma ray of ¹⁵⁶Gd in Gd₂Ti₂O₇ have been observed at various temperatures. The results prove that the unusual intensities are mainly due to lattice vibrational anisotropy.     

Magnetic relaxation in spinel Mo-ferrite and Ti substituted Mo-ferrite

A compensation temperature of 138 K was observed in the temperature-dependent magnetization curves of MoFe₂O₄. Relatively slow magnetization relaxation characterized the transitions between different spin states (compensated and uncompensated). Large magnetic after effect was found in time-dependent magnetization curves after heating or cooling from different characteristic temperatures for different spin states. The magnetic relaxation was nearly independent on magnetic field, supporting the presence of spin states and no involvement of domain structure. For the Ti substituted Mo_0.6Ti_0.4Fe₂O₄ sample, there were a compensation at ∼ 100 K and a maximum of magnetization at ∼ 175 K. Similar results of anomalous magnetic relaxation was observed in Ti substituted Mo-ferrite (Mo_0.6Ti_0.4Fe₂O₄). If the Mo_0.6Ti_0.4Fe₂O₄ sample was heated from 100 K to 235 K, the time-dependent magnetization curve could be considered as a combination of two magnetic relaxation processes. However, if the sample was heated from 100 K to 295 K, the time- dependent magnetization curve became complex. Received 30 October 2001 and Received in final form 21 January 2002     

Magnetism in Ti6O11

Measurements of the magnetization of Ti₆O₁₁ show transitions of 119± 1 K and 147 ±1K. Above 147 K we attribute the paramagnetism to d¹ electrons on Ti³⁺ ions. Below 147 K there are contributions from both Ti⁷⁺₂ dimers and Ti³⁺ ions. The gradual increase in magnetic moment with increasing temperature to 119 K we interpret as an increase in spins due to the break up of Ti⁶⁺₂ dimers to Ti³⁺ ions.     

Determination of Oxygen partial free energy for non-stoichiometric TiO by E.M.F. measurements: Thermodynamics of TiO at 1323 K

The equilibrium oxygen partial pressures with non-stoichiometric TiO and the two phase regions TiOTi₂O₃ and Ti₂OTiO have been determined at 1323 K by electromotive force measurements with ThO₂Y₂O₃ solid electrolyte. These pressures are included between 10^?25.6atm and 10^?34.3atm. The results take into account the PO₂ dependence of the ionic transference number of our electrolytes. This dependence has been determined down to 10^?35 atm by using a UUO₂ reference electrode and applying a model proposed earlier by Schmalzried for the conductivity of ThO₂Y₂O₃.We had previously measured ΔH(O₂) for the same range of composition and at the same temperature. Hence a set of new thermodynamic data relative to ΔG(O₂), ΔH(O₂) and ΔS(O₂) is available for TiO at 1323 K.     

Tuning of phonon anharmonicity in pyrochlore titanates: temperature‐dependent Raman studies of Sm2Ti2‐xZrxO7 (x=0, 1/2, 3/4, and 2) and stuffed spin‐ice Ho2 + xTi2 − xO7 − x/2 (x=0, 1/3, and 2/3)

Anomalous temperature dependence of Raman phonon wavenumbers attributed to phonon–phonon anharmonic interactions has been studied in two different families of pyrochlore titanates. We bring out the role of the ionic size of titanium and the inherent vacancies of pyrochlore in these anomalies by studying the effect of replacement of Ti^{4 +} by Zr^{4 +} in Sm₂Ti₂O₇ and by stuffing Ho^{3 +} in place of Ti^{4 +} in Ho₂Ti₂O₇ with appropriate oxygen stoichiometry. Our results show that an increase in the concentration of the larger ion, i.e. Zr^{4 +} or Ho^{3 +}, reduces the phonon anomalies, thus implying a decrease in the phonon–phonon anharmonic interactions. In addition, we find signatures of coupling between a phonon and crystal field transition in Sm₂Ti₂O₇, manifested as an unusual increase in the phonon intensity with increasing temperature. Copyright © 2011 John Wiley & Sons, Ltd.     

Effective hyperfine temperature in frustrated Gd 2Sn 2O 7: two level model and 155Gd Mössbauer measurements

Using ¹⁵⁵Gd M?ssbauer spectroscopy down to 27 mK, we show that, in the geometrically frustrated pyrochlore Gd₂Sn₂O₇, the Gd³⁺ hyperfine levels are populated out of equilibrium. From this, we deduce that the hyperfine field, and the correlated Gd³⁺ moments which produce this field, continue to fluctuate as T ↦ 0. With a model of a spin 1/2 system experiencing a magnetic field which reverses randomly in time, we obtain an analytical expression for the steady state probability distribution of the level populations. This distribution is a simple function of the ratio of the nuclear spin relaxation time to the average electronic spin-flip time. In Gd₂Sn₂O₇, we find the two time scales are of the same order of magnitude. We discuss the mechanism giving rise to the nuclear spin relaxation and the influence of the electronic spin fluctuations on the hyperfine specific heat. The corresponding low temperature measurements in Gd₂Ti₂O₇ are presented and discussed. Received 17 October 2001 Published online 6 June 2002     

First-principle calculations for electronic structure and bonding properties in layered Na<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>7</Subscript>

First-principles calculations of Na₂Ti₃O₇ have been carried out with density-functional theory (DFT) and ultrasoft pseudopotentials. The electronic structure and bonding properties in layered Na₂Ti₃O₇ have been studied through calculating band structure, density of states, electron density, electron density difference and Mulliken bond populations. The calculated results reveal that Na₂Ti₃O₇ is a semiconductor with an indirect gap and exhibits both ionic and covalent characters. The stability of the (Ti₃O₇)²⁻ layers is attributed to the covalent bonding of strong interactions between O 2p and Ti 3d orbitals. Furthermore, the O atoms located in the innerlayers interact more strongly with the neighboring Ti atoms than those in the interlayer regions. The ion-exchange property is due to the ionic bonding between the Na⁺ and (Ti₃O₇)²⁻ layers, which can stabilize the interlayers of layered Na₂Ti₃O₇ structure.     

Comparative analysis of crystal field effects and energy level scheme of six-fold coordinated Cr ion in the pyrochlores, Y2B2O7 (B=Ti, Sn)

The electronic energy levels of the six-fold coordinated Cr⁴⁺ ion in the pyrochlores Y₂B₂O₇ (B=Sn⁴⁺, Ti⁴⁺), have been computed using the exchange charge model of crystal field theory. The calculated Cr⁴⁺ energy levels and their trigonal splitting are in good agreement with experimental spectra. Calculations of the crystal field parameters show that the higher crystal field strength in Y₂Sn₂O₇ (in comparison with Y₂Ti₂O₇) arises from increased orbital overlap effects between the Cr⁴⁺ ion and the nearest oxygen ions, which are located at the 48f crystallographic position of the pyrochlore lattice. The increased overlap in Y₂Sn₂O₇ occurs despite the fact that the Cr⁴⁺-O^2- bond distance in Y₂Sn₂O₇ is longer than in Y₂Ti₂O₇. This is attributed to a lack of hybridization (covalent bonding) between the filled 2p orbital of oxygen ion occupying the 48f site of the pyrochlore lattice and the filled Sn⁴⁺ 4d¹⁰ orbital. As a result, a stronger crystal field is experienced by Cr⁴⁺ ions in Y₂Sn₂O₇, even if the Cr⁴⁺-O²⁻ distances are greater in this case, when compared to those in Y₂Ti₂O₇.     

Single-Ion Mechanism of Weak Ferromagnetism and a Spin-Flop Transition in One- and Two-Position Antiferromagnets

Nuclear resonance reflectivity from a [Dy₁₉Gd₁₉] × 20 superlattice has been measured utilizing the 25.652 keV nuclear level of ¹⁶¹Dy. The measured time spectra of nuclear resonance reflectivity make it possible to reveal the variation of the hyperfine magnetic field B_hf on dysprosium nuclei in the temperature range of 4–110 K and to determine the relaxation time of the hyperfine field, using the decay speed-up of the excited state of ¹⁶¹Dy nuclei.     

Mössbauer studies of YBa2Cu3O7−δ-type high-T c superconductors

The local magnetic, electronic, and structural properties of (RE)Ba₂Cu₃O_7?δ supercondcutors (RE=Gd, Dy, and Eu) were studied by Mössbauer spectroscopy using the resonances of¹⁵⁵Gd,¹⁶¹Dy,¹⁵¹Eu, and⁵⁷Eu. In GdBa₂Cu₃O_7?δ, different magnetic ordering behaviors of the Gd sublattice are found for the orthorhombic (superconducting) and tetragonal (non-superconducting) phases. In DyBa₂Cu₃O_7?δ, the magnetic moments of the respective CEF ground states in the orthorhombic and tetragonal phases are derived from paramagnetic hyperfine splittings at 1.4 K. In both DyBa₂Cu₃O_7?δ and EuBa₂Cu₃O_7?δ, anomalies connected with the superconducting transitions in isomer shift, recoil-free fraction, and relaxation behavior were looked for, but not found. The electric-quadrupole splittings observed for both systems are discussed in connection with the lattice EFGs derived for the Gd system. In GdBa₂ (Cu_0.995Fe_0.005)₃O_7?δ, the local properties of the various Fe sites are investigated over a wide temperature range in both the orthorhombic and tetragonal phase. The magnetic ordering of the Gd sublattice in the orthorhombic phase and of the Cu(2) sublattice in the tetragonal phase, respectively, is monitored via the magnetic splittings at the various Fe sites. Possible assignments of Cu(1) and Cu(2) sites as well as different oxygen configurations around the substituted Fe ions are discussed.     

Ion-impact chemistry in the system titanium-oxygen (studies on bombardment-enhanced conductivity—III)

The bombardment of TiO₂, whether poly- or single crystalline, with Kr ions leads to an altered surface layer having the following characteristics. It exhibits a high electrical conductivity, has the diffraction pattern of finely polycrystalline Ti₂O₃, is on the average 110 ± 20 Å thick (for 30 keV Kr), and is indefinitely stable in air at room temperature. The formation of the layer is favored by increasing the target temperature. Formation is half complete at (6 ± 2) × 10¹⁶ ions/cm², hence at a dose substantially greater than that for the half completion of sputter equilibrium ([7 ± 2] × 10¹⁵ ions/cm²). One model which could lead to Ti₂O₃ can be excluded fairly readily: this is thermal-spike stimulated vaporization, as the relevant vapor pressures are too low. More satisfactory is a model in which, due to either preferential oxygen sputtering or internal precipitation of oxygen, Ti₂O₃ nuclei are formed and grow. The reason that the stoichiometry is precisely Ti₂O₃ can be rationalized by an argument based on surface binding energies (E_b), in the sense that E_b for TiO₂ to sputter congruently is 6.4 eV, to yield nuclei of Ti₃O₅ is 5.7, to yield nuclei of Ti₂O₃ is 5.1, and to yield TiO is 6.4. A similar rationalization holds also for impact-induced chemical changes observed or inferred with AgBr, CuO, Fe₂O₃, MoO₃, U₃O₈ and V₂O₅, except that here thermal-spike stimulated vaporization cannot be excluded.     

(Bi,La)4Ti3O12-Sr(Bi,La)4Ti4O15共生结构铁电材料性能研究

采用固相烧结工艺，制备了不同La掺杂量(x=0.00，0.25，0.50，0.75，1.00，1.25和1.50) 的(Bi, La)₄Ti₃O₁₂-Sr(Bi, La)₄Ti4O₁₅ (SrBi_8-xLa_xT i₇O₂₇)共生结构铁电陶瓷样品.用x射线衍射对其进行微结构分析 ，并测量铁 关键词： 4Ti₃O₁₂-SrBi₄Ti₄O₁₅')" href="#">Bi₄Ti₃O₁₂-SrBi₄Ti_{4O15 La掺杂 铁电性能 居里温度 弛豫铁电}     

Electrical conductivity and dielectric relaxation behavior of AgFeP2O7 compound

In the present study, AgFeP₂O₇ was prepared by a solid-state reaction method. Rietveld refinement of the X-ray diffraction pattern suggests the formation of the single phase desired compound with monoclinic structure at room temperature. Not only were the impedance spectroscopy measurements of our compound carried out from 209 Hz to 5 MHz over the temperature range of 553 K–698 K but its AC conductivity as well as the dielectric relaxation were evaluated. Impedance measurements show AgFeP₂O₇ an ionic conductor being the conductivity 1.04?×?10^–?5(Ω^–?1cm^–?1) at 573 K. The conductivity and modulus formalisms provide nearly the same activation energies for electrical relaxation of mobile ions revealing that transport properties in this material appear to be due to an ionic hopping mechanism dominated by the motion of the Ag⁺ ions along tunnels presented in the structure of the investigated material.     

Lattice energy calculations for Ti4O7 using a shell model

The lattice energies of Ti₄O₇ containing Ti³⁺-Ti³⁺ pairs at temperatures below 120 K have been calculated by the polarizable point ion shell model. The positions of ions which minimize the total energy of the crystal have first been determined and then the lattice energies have been obtained using these ion positions. The defect energy is found to be 65.79 eV per eliminated oxygen ion. The ion positions determined in the calculation are, in general, found to move toward the positions in the real structure from their initial positions in the ideal structure. The lattice energies of the Ti₄O₇ structure in which the Ti³⁺ ions are redistributed hypothetically at the sites adjacent to the crystallographic shear planes have been also calculated. This was found to have a defect energy lower than the Ti₄O₇ structure with the distribution of valences observed experimentally. This result is discussed in terms of the quantum effect due to the nonmagnetic bond formed by electrons trapped on the Ti³⁺-Ti³⁺ pairs.     

High Pressure Spectroscopy of Ti Doped Al2O3 and YAIO3 Host Crystals

We report high-pressure luminescence spectroscopy studies of Ti^3? in Al₂O₃ and YAlO₃. High-pressure luminescence spectra were measured for Al₂O₃:Ti^3? up to 90kbar and for YAlO₃:Ti^3? up to 181 kbar. In both cases, a blue shift of the luminescence peak with pressure was observed and is attributed to an increase in the octahedral crystal-field strength (10 Dq) with pressure. In the case of YAlO₃;Ti^3?, an additional luminescence peak was observed at 181 kbar and below ～ 200 K, and is attributed to a metastable state. The metastability of the ²E excited state was induced by pressure that removes the equivalency of three energy minima of the ²E state coupled to the ? mode.     

Influence of sintering conditions and doping on the dielectric relaxation originating from the surface layer effects in CaCu3Ti4O12 ceramics

Detailed investigations into the dielectric dispersion phenomenon in the giant dielectric constant material CaCu₃Ti₄O₁₂ (CCTO) around room temperature revealed the existence of two successive dielectric relaxations. In the temperature domain, a new dielectric relaxation was clearly observed around 250 K, in addition to the well-investigated dielectric relaxation close to 100 K. The effect of sintering and doping (La³⁺) on the strength of these dielectric relaxations were studied in detail. The sintering temperature as well as its duration was found to have tremendous influence on the dielectric relaxation that was encountered around 250 K. This Maxwell-Wagner (M-W) type of relaxation was found to be originating from the surface layer containing the Cu-rich phase, which was ascribed to the difference in the oxygen content between the surface and the interior of the sample. Interestingly, this particular additional relaxation was not observed in La_2/3Cu₃Ti₄O₁₂, a low dielectric constant member of the CCTO family, in which the segregation of Cu-rich phase on the surface was absent. Indeed the correlation between the new relaxation and the presence of Cu-rich phase in CCTO ceramics was further corroborated by the absence of the same after removing the top and bottom layers.