Fine structure in the low temperature luminescence of Zn2SiO4:Mn and Mg4Ta2O9:Mn

The luminescence of powder samples of the well-known green-emitting Zn₂SiO₄:Mn and the red-emitting Mg₄Ta₂O₉:Mn phosphor shows a considerable fine structure at 4° K in appropriately prepared samples containing a sufficiently low Mn concentration. For (Zn_1-xMn_x)₂SiO₄ (0.0005?x?0.05) two sharp lines were found which are interpreted as due to zero phonon transitions between the ⁴T₁ and ⁶A₁ levels of Mn²⁺ ions on the two crystallographically different zinc sites. The remaining structure is ascribed to vibronic sidebands. The decay times of the luminescence bands associated with the two sites differ; they are 12 and 15 ms for the high and low energy bands respectively. The experimental results of Vlam are confirmed by our data. In addition some (Zn_1-yBe_y)₂SiO₄:Mn (0.025? y ?1) samples were investigated. In Mg₄Ta₂O₉:Mn two zero phonon lines could be identified, indicating that in this material Mn²⁺ is distributed over two inequivalent Mg sites. Most of the phonon replicas were found at intervals of 15 meV. Raman scattering experiments showed that this energy corresponds to one of the lattice vibrations. The decay time of this luminescence band is 1.0 ms.     

Indication from NMR of Grotthuss mechanism for proton conduction in H2Sb4O11·nH2O

We have measured the second moment, the linewidth and the relaxation times T₁ and T₂ of the ¹H magnetic resonance signal from 4.2 to 380 K in the fact proton conductors H₂Sb₄O₁₁·nH₂O. Our results reveal that the high ionic conductivity of these materials is due to a Grotthuss-type proton diffusion mechanism with succession of molecular reorientations of H₃O⁺ ions or H₂O molecules and of proton jumps from H₃O⁺ to H₂O.     

Magnetic resonance in relaxed excited states AX and AT of Ga+ in alkali halides

The optical detection technique is used to observe magnetic resonance in the excited states ³T_1u of Ga⁺ in KI, KC? and NaC?. The relaxed excited states A_X and A_T display trigonal and tetragonal symmetry respectively, as a result of a static Jahn-Teller effect. Furthermore the coupling to E_g and T_2g modes is found to be nearly equal in KI : Ga⁺, as already observed in KBr : Ga⁺.     

Structural and magnetic properties and hyperfine interaction in La3.5Ru4O13 compound

Structural, magnetic and hyperfine interaction measurements have been carried out on the novel compound La_3.5Ru₄O₁₃ prepared under two different atmospheres (air and oxygen flow). This compound is formed in the orthorhombic structure (space group Pmmm, # 47). The coexistence of the triple-layered perovskite-type planes (quasi-2D structure) and the rutile-like slabs (1D structure) leads to interesting magnetic and electronic properties in this compound. The magnetic susceptibility of this system shows a peak at T~47 K associated with antiferromagnetic interactions. The Curie-Weiss behaviour of the susceptibility provides an effective magnetic moment consistent with Ru ions in low-spin state. Perturbed angular correlation measurements carried out with ¹¹¹Cd probe in the temperature range 10-60 K reveal only quadrupole interactions and indicate the occurrence of structural distortions for T<40 K.     

An optical study of the stretching absorption band near 3 microns from OH- defects in LiNbO3

A near infrared absorption band (λ = 2.87 μ), identified as the 0–1 stretching vibrational band of OH^- defects in LiNbO₃, has been observed at 6, 77, and 300 K. This band was completely polarized perpendicular to the crystallographic c-axis within experimental error and two peaks, separated by approximately 13 cm^?1, were resolved. Both the structure and optical anisotropy of this band were independent of stoichiometry and sample treatments and may be understood in terms of a simple model where protons are captured by O^2- anions forming OH^- defects whose orientations are determined by neighboring O^2- ions.     

Structural and magnetic studies of the alloy system GdInxAg1-x

The structural and magnetic studies below room temperature of the alloy system GdIn_xAg_1-x(0?x?1) are reported. It has been found that alloys with x?0.5 crystallize in CsCl type cubic structure, but the distortion of the unit cell starts for alloys with x#62;0.5 and for these alloys the unit cell becomes tetragonal. No phase change is evident from the low temperature (295-8 K) structural studies. However, a break appears in the X^-1_m vs. T linear plot of each alloy of this system at a specific temperature (designated as break temperature T_B). The variation of T_B with x is similar to the variation of phase transition temperature with x reported for LaIn_xAg_1-x. Close agreement has been found in the values of effective magneton number (p), magnetic ordering [Néel (T_N) or Curie (T_C)] and paramagnetic Curie (θ_p) temperature for materials studied by us and earlier workers. The variation of magnetization with applied field strength (2.5-65)×10⁵ Am^-1) at 4.2 K has also been reported for ferromagnetic of this system. It has been concluded that alloys with 0.4?x?0.6 are simple ferromagnets with parallel alignment of magnetic moment in the ground state. The angular arrangement of the magnetic moment starts appearing in the ground state for alloys with x?0.4 for x#62;0.6 and continues till x becomes closure to 0.17 or 0.84. The alloys with x=0.17 or x=0.84 have θ_p and T_C equal to zero and appear paramagnetic. Angular arrangement in spins again appear for alloys with x?0.17 or x#62;0.84, however all materials with 0?x?0.17 or 0.84?x?1 remain antiferromagnetic.     

Electron paramagnetic resonance and optical spectroscopy of K3Na(CrO4)2 ferroelastics activated by MnO 4 2− molecular impurity ions

Crystals of a proper ferroelastic K₃Na(CrO₄)₂ containing molecular impurity ions MnO ₄ ^2? are studied using electron paramagnetic resonance (EPR) and optical spectroscopy. The EPR spectrum of the Mn⁶⁺ ion contained in the molecular impurity ion MnO ₄ ^2? is identified at low temperatures (T ≤ 20 K). The intensity of this spectrum decreases unusually fast as the temperature increases. A broad IR luminescence band with a vibronic structure well resolved at a temperature of 8 K is revealed. Theoretical treatment of the Mn⁶⁺ ion involved in the molecular impurity ions MnO ₄ ^2? of the K₃Na(CrO₄)₂ ferroelastic crystal suggests that an important role in this case is played by the pseudo-Jahn-Teller. The pseudo-Jahn-Teller effect offers an explanation for the appearance of a fine structure in the vibronic replicas in the luminescence spectrum, on the one hand, and accounts for the fast decrease in the intensity of the EPR spectrum of K₃Na(CrO₄)₂: MnO ₄ ^2? with increasing temperature, on the other.     

Magnetoelectric behavior of ferrimagnetic BixCo2−xMnO4 (x=0, 0.1 and 0.3) thin films

Thin films of Bi_xCo_2−xMnO₄ (x=0, 0.1 and 0.3) were grown on quartz, LaAlO₃ (LAO) and YBa₂Cu₃O₇ (YBCO) buffer layer coated LAO substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD) and Raman scattering measurements showed that the thin films exhibit single phase polycrystalline cubic spinel structure on all the substrates. Near edge X-ray absorption fine structure (NEXAFS) studies confirmed the octahedral occupancy of the substituted Bi³⁺ ions. Temperature dependent zero-field cooled (ZFC) magnetization measurements show the ferrimagnetic (FM) behavior (T_C∼186 K) and magnetization undergoes a crossover from positive to negative, owing to the opposite contributions of magnetic moments from Co and Mn ions. A weak ferroelectric property exhibited by the films above room temperature was evidenced through the capacitance-voltage (C-V) and dielectric measurements. Magnetoelectric coupling was found to be maximum just below FM-T_C.     

Mössbauer and magnetization studies of the U1−xNpxO2 fluorites

²³⁷Np Mössbauer effect and magnetic susceptibility measurements of the U_1?Np_xO₂ fluorite solid solution have been performed in the composition range 0.15 ? x ? 0.75. For x = 0.15 and 0.25, the Np ions order magnetically at a lower temperature T₀ than the bulk material (T_N) (T₀ ～ 19 K, T_N ～ 27 K for x = 0.15). For x = 0.50, T₀ ～ 10 K (T_N ～ 12 K from recent neutron diffraction measurements). For x = 0.75, T₀ ～ T_N ～ 9 K. The Np (induced) ordered moment is ～ 0.5 μ_B. The ²³⁷Np Mössbauer isomer shift shows that the Np ions are in a IV charge state.     

119Sn and 57Fe M?ssbauer study of the local structure of perovskite-type ferrites CaFe2 ? x N x O5 ( N = Sc, Al) and manganite CaMn7O12

Anion-deficient substituted ferrites Ca₂Fe_{2 − x} N _x O₅ (N = Sc³⁺, Al³⁺) and mixed manganite CaMn₇O₁₂ have been investigated by ¹¹⁹Sn and ⁵⁷Fe probe M?ssbauer spectroscopy. The mechanism of charge compensation for heterovalent impurity Sn⁴⁺ ions in the structure of the ferrite Ca₂Fe₂O₅ has been established. The presence of nonequivalent crystallographic positions of manganese cations, caused by their charge ordering in the structure of the manganite CaMn₇O₁₂, is shown. Magnetic ordering of Mn³⁺ and Mn⁴⁺ cations in the octahedral sublattice of CaMn₇O₁₂ at T < T _M2 ≈ 90 K is established. Original Russian Text ? A.V. Sobolev, I.A. Presnyakov, K.V. Pokholok, V.S. Rusakov, T.V. Gubaidulina, A.V. Baranov, G. Demazeau, 2007, published in Izvestiya Rossiiskoi Akademii Nauk Seriya Fizicheskaya, 2007, Vol. 71, No. 9, pp. 1347–1354.     

Magnetic properties of the quasi-one-dimensional system BaMn2V 2O8

Magnetic properties of BaMn₂V ₂O₈ are investigated by means of susceptibility, magnetization, and heat capacity measurements. Our experimental results show that BaMn₂V ₂O₈ is a one-dimensional canted antiferromagnet. The antiferromagnetic transition at a relatively high T_N of 37 K may be due to an enhancement of interchain interaction in the system, since Mn²⁺ ions have large spin moment of S=5/2. Weak ferromagnetism in BaMn₂V ₂O₈ may be due to Dzyaloshinskii-Moriya interactions arising from its noncentrosymmetric crystal structure.     

Critical current density and flux pinning in La2−xPrxCa2x Ba2Cu4+2xOz (x=0.1-0.5) superconductors

Polycrystalline La_2−xPr_xCa_2xBa₂Cu_4+2xO_z (LPCaBCO) compounds with x=0.1-0.5 were synthesized by solid-state reaction method and studied by room temperature X-ray diffraction, dc resistivity, dc magnetization and iodometry. The superconducting transition temperatures in these tetragonal triple perovskite compounds increases from 32 to 62 K (T_c^onset values) with increasing dopant concentration. The mixing of rare earth La³⁺ and Pr^3+/4+ ions at rare earth site (La³⁺) along with substitution of divalent Ca²⁺ results in the shrinkage of unit cell volume. The contraction of unit cell volume due to larger ion being substituted by smaller ions, gives rise to creation of pinning centres in the unit cell leading to increase in critical current density and flux pinning.     

EPR powder spectra of Co(II) in ZnRh2O4 spinel matrix

The EPR powder spectra of spinel solid solutions Co_xZn_1-xRh₂O₄ (x ? 0.10) have been studied in the temperature range 6–77 K. The spectra show that Co²⁺ ions occupy distorted tetrahedral sites. As the cobalt concentration increases, the spectrum of the isolated ions is gradually replaced by a strong absorption produced by antiferromagnetic exchange coupled clusters of Co²⁺ ions.     

Structural and magnetic properties of Mg(In2−xMnx)O4 system

We synthesized the Mn-doped Mg(In_2−xMn_x)O₄ oxides with 0.03?x?0.55 using a solid-state reaction method. The X-ray diffraction patterns of the samples were in a good agreement with that of a distorted orthorhombic spinel phase. Their lattice parameters and unit-cell volumes decrease with x due to the substitution of the smaller Mn³⁺ ions to the larger In³⁺ ions. The undoped MgIn₂O₄ oxide presents diamagnetic signals for 5 K?T?300 K. The M(H) at T=300 K reveals a fairly negative-sloped linear relationship. Neither magnetic hysteresis nor saturation behavior was observed in this parent sample. For the Mn-doped samples, however, positive magnetization were observed between 5 and 300 K even if the x value is as low as 0.03. The mass susceptibility enhances with Mn content and it reaches the highest value of 1.4×10⁻³ emu/g Oe (at T=300 K) at x=0.45. Furthermore, the Mn-doped oxides with x=0.06 and 0.2, respectively, exhibit nonlinear magnetization curves and small hysteretic loops in low magnetic fields. Susceptibilities of the Mn-doped samples are much higher than those of MnO₂, Mn₂O₃ oxides, and Mn metals. These results show that the oxides have potential to be magnetic semiconductors.     

V centers produced in KI crystals under KrCl excimer laser irradiation

Abstract

Irradiation of KI near 150 K with KrCl excimer laser irradiation (hv=5. 58 eV) produces V centers causing V₂ and V₃ bands. The two bands exhibit 100-type dichroism. In KI containing V centers, the 111 cm^?1 Raman signal attributed to I_3- molecular ions is observed. Under KrCl excimer laser irradiation at low temperatures, resonance Raman scattering effects have been also studied for KI, NaI and LiI.     

Isotopeneffekt in der paraelastischen relaxation

The kinetics of the reorientation of the molecular impurities¹⁶O₂ ^? and¹⁸O₂ ^? in the host lattices KCl, KBr and KJ has been investigated by means of ESR as a function of uniaxial stress in the temperature range between 1,3°K and 4,5°K. Below 4°K the reorientation rate is proportional to the temperature, indicating that one-phonon processes dominate at low temperatures. ForT>4°K the reorientation through excited librational levels becomes significant. The large isotope effect in the reorientation rate of¹⁶O₂ ^? and¹⁸O₂ ^? confirms that tunneling plays an important role. The analogy between the diffusion of polarons and the reorientation of molecules in solids is discussed in detail. The evaluation and interpretation of the experimental results in terms of the theory of Gosar and Pire yields values for the tunneling matrix elements, the potential barrier between two equilibrium orientations, the libration frequencies and an effective moment of inertia. The theory accounts well for the contribution of the one-phonon processes to the relaxation rate for stress along [001]. For stress along [111] the agreement is only qualitative.     

A new approach to the phase transition in NpO2: Model with trivalent Np ions

The hypothesis is advanced that in NpO₂ the cation is trivalent and, consequently, the oxygen 2p bands contain one hole per unit cell. Two models are constructed to describe the phase transition occuring at T₀ = 25K. In each of the models the holes are in spatially localized states at all temperatures in accordance with the semiconducting behaviour of NpO₂. In one model, the phase transition is an antiferromagnetic ordering of the spins of the holes localized in that half of the anion cubes of the fluorite structure which is devoid of Np. In the other model O^? and O^2? ions are assumed to form an alternating lattice below T₀, and no inherent magnetic moment exists either below or above T₀. To check these possibilities, experiments are proposed.     

Preparation and magnetic and electrical properties of EuBaMn<Subscript>2</Subscript>O<Subscript>6−δ</Subscript> (δ=0,1)

The EuBaMn₂O₆ composition prepared in air at T=1500°C is shown to be a cubic perovskite exhibiting spin-glass properties below T_f=40 K. The reduced composition, EuBaMn₂O₅, crystallizes in a perovskite-like, YBaCuFeO₅-type layered structure with a tetragonal unit cell. EuBaMn₂O₅ with crystallographically ordered Eu³⁺ and Ba²⁺ ions is a ferromagnet with T_N=160 K. The anomalous behavior of the paramagnetic susceptibility is assumed to be due to a partial ordering of the Mn²⁺ and Mn³⁺ ions. EuBaMn₂O₅ oxidized in air at 900°C to EuBaMn₂O₆ has a magnetic ordering temperature T_M=260 K, near which the magnetoresistance reaches a peak value. X-ray diffraction measurements show the long-range order in the Eu³⁺ and Ba²⁺ ion arrangement to persist in the oxidized EuBaMn₂O₆ sample.     

非晶锂离子导体B2O3-0.7Li2O-0.7LiCl-xAl2O3晶化初期的核磁共振研究

通过真空密封热处理、避免了样品晶化后吸水引起的误差,采用脉冲法在293K和77K测量了晶化过程初期三种非晶锂离子导体B₂O₃-0.7Li₂O-0.7LiCl-xAl₂O₃(x=0.15,0.10和0.05)的⁷Li核磁共振谱。发现在低温(77K)只有固相锂离子对应的自旋-自旋弛豫时间T₂=87μs,严格按高斯函数衰减。在室温下固相锂离子对应的T_2s=127μs,仍是高斯型;但液相锂离子对应的T₂却按洛仑兹函数衰减。这反映出锂离子导体的固-液二相性。三种非晶B₂O₃-0.7Li₂O-0.7LiCl-xAl₂O₃(x=0.15,0.10和0.05)分别在热处理温度401,388和381℃附近,其液相锂离子对应的T_2l都剧增,其吸收谱线宽都变窄。由此再次验证了非晶母体与微晶之间的两相界面效应的物理图象。 关键词：