Magnetic behavior of the alloys CeCuyGa4−y and Ce1−xLaxCuGa3

The results of magnetic susceptibility (x), electrical resistivity (ρ) and heat-capacity (C) measurements of the alloys CeCu_yGa_4−y (y = 0.5, 1.0, 1.25 and 1.5) and Ce_1−xLa_xCuGa₃ (x = 0.2 and 0.9), crystallizing in the BaAl₄-type tetragonal structure, are reported. The Kondo effect tends to dominate with increasing y. All the data for the y = 0.5 alloy are consistent with the onset of ferromagnetic ordering at 6 K. For y = 1.0 and 1.25, though ρ gradually drops below 6 K, the C data do not show the existence of magnetic ordering above 2 K; presumably, for y = 1.0, 1.25 and 1.5, long-range magnetic ordering sets in below 2 K. Above all, C tends to increase with decreasing temperature before the onset of long range magnetic ordering and the values of C/T per Ce mol are considerably diminished for x = 0.9. It is proposed that this C/T enhancement lies in the magnetic precursor effects and not on the heavy-fermion behaviour.     

Photoconductivity and photodielectric effect in LiY1 − x Lu x F4 crystals doped with Ce3+ and Yb3+ ions

Time and spectral dependences of the dielectric permittivity of the LiY_{1 ? x} Lu _x F₄ (x = 0, 0.5, and 1) crystals doped with Ce³⁺ and co-doped with Yb³⁺ ions under UV laser excitation were studied by the 8-mm microwave resonant technique at room temperature. The obtained photoconductivity spectrum in 240–310 nm spectral range was interpreted as a stepwise photoionization spectrum of the Ce³⁺ ions due to sequential 4f–5d and 5d–6s transitions. Average lifetimes of free and defect trapped (color centers) charge carriers were estimated.     

Low-temperature phase transitions and dynamics of ammonium in (NH4)3H(SO4)2 and [(NH4)1−x Rbx]3H(SO4)2 crystals

The (NH₄)₃H(SO₄)₂ and [(NH₄)_0.82Rb_0.18]₃H(SO₄)₂ crystals are investigated by dielectric spectroscopy, inelastic incoherent neutron scattering (IINS), and neutron powder diffraction. A comparative analysis of the data obtained is given. It is shown that the phase transitions II ? III, III ? IV, IV ? V, and V ? VII in the (NH₄)₃H(SO₄)₂ crystal are accompanied by changes in the orientation ordering of the NH ₄ ⁺ ions. In the [(NH₄)_0.82Rb_0.18]₃H(SO₄)₂ crystal, these phase transitions are completely suppressed and the long-range order inherent in the II phase is retained over the entire temperature range covered (6–300 K). It is revealed that this crystal at the temperature T _g≈70 K undergoes a transition to the dipole glass phase, which is attended by “freezing” the orientation disordering of the ammonium ions.     

Defects and transport in SrFe1−xCoxO3−δ

The non-stoichiometry and chemical diffusion coefficient of SrFe_1−xCo_xO_3-δ have been measured by steady state and transient thermogravimetry in the temperature range 750–1200 °C at different oxygen partial pressures. At high oxygen partial pressures, the chemical diffusion coefficient was in the range 1·10⁻⁴ – 7·10⁻⁴ cm²/s. This, combined with high concentration of disordered vacancies make these materials perhaps the fastest solid oxygen ion diffusers known at high temperatures and high oxygen partial pressures. However, due to the high concentration of defects in SrFe_1−xCo_xO_3-δ the compound transforms from a cubic (disordered) perovskite to a brownmillerite type of structure under reduced oxygen partial pressures below approx. 900 °C. Due to this phase transition, the mobility of oxygen vacancies in SrFe_1−xCo_xO_3-δ decreases up to about an order of magnitude at 850 °C. We also observe an ordering effect at 1000 °C, although smaller in size, and this is suggested to be due to short range ordering of four-coordinated polyhedra of Fe. For possible use as oxygen separation membranes, phase stability against sulphur and carbon containing atmospheres is also discussed with respect to the formation of carbonates and sulphates. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Magnetic ordering and the Kondo effect in the alloys,Ce2Co1−xPdxSi3

The compound Ce₂CoSi₃, crystallizing in a AlB₂-derived hexagonal structure, has been recently identified as a Kondo lattice with a non-magnetic ground state. Here, we report the influence of gradual replacement of Co by Pd on the magnetic behaviour in the pseudo-ternary solid solution, Ce₂Co_1−xPd_xSi_3, by magnetization (2–300 K), electrical resistivity (2–300 K) and heat-capacity (0.7–30 K) measurements to bring out a transformation from non-magnetic to magnetic ordering. Distinct features attributable to the existence of a competition between the Kondo effect and magnetic ordering with varying x and temperature are observed in the electrical resistivity data. The results reveal that small substitutions of Pd (x=0.2–0.3) are sufficient to induce magnetic ordering of the Ce ions at low temperatures. The strength of the Kondo interaction as indicated by the paramagnetic Curie temperature decreases monotonically with increasing Pd content. A notable finding is that there are qualitative changes in the isothermal magnetization data in the magnetically ordered state, as if there are modifications in the magnetic structure with changes in the Co/Pd composition. The importance of electronic structure relative to unit-cell volume in deciding magnetic characteristics of this class of compounds is brought out taking into account the trends in the magnetic behaviour of isostructural Ce compounds.     

Domain structure and phase transitions of LiKSO4:EPR of SO−4 centers

Analysis of the EPR of SO⁻₄ centers in LiKSO₄ crystals indicates that the SO⁻₄ centers are associated with local positive-ion vacancies. Twin-domains of the 〉110〈 types are observed to be temperature-dependent. The anomaly at 215 K on cooling is associated with a sudden growth of the twins and the anomaly at 190 K is associated with a p6₃ to p6₃P6,mc structural phase transition.     

Phase transitions and energy storage properties of some compositions in the (1−x)Li2SO4–xNa2SO4 system

In the binary system (1?x)Li₂SO₄–xNa₂SO₄, the solid–solid phase transitions and energy storage properties of Li₂SO₄, Na₂SO₄, the binary compound LiNaSO₄ and two eutectoids (E₁: 0.726Li₂SO₄–0.274Na₂SO₄; E₂: 0.03Li₂SO₄–0.97Na₂SO₄) were investigated by X-ray diffraction and differential scanning calorimetry. Li₂SO₄ has a solid–solid phase transition at 578 °C with the transition enthalpy 252 J g^?1. The binary compound LiNaSO₄ gives a slightly lower enthalpy value, 214 J g^?1 and its transition temperature is clearly reduced to 514 °C. The transition enthalpy of the eutectoid E₁ is maintained to 177 J g^?1 and its transition temperature is further reduced to 474 °C. Li₂SO₄, LiNaSO₄ and the eutectoid E₁ are applicable phase transition materials because of their large transition enthalpies. The enthalpies of Na₂SO₄ and the eutectoid E₂ are not very high (～45 J g^?1), but their transition temperatures are quite low (～250 °C); thus their transition properties may be applied at such low temperatures.     

Comparative study of aerogels nanostructured catalysts: Ni/ZrO2–SO4 2− and Ni/ZrO2–Al2O3–SO4 2−

Ionics - A series of Ni/ZrO2–SO4 2− and Ni/ZrO2–Al2O3–SO4 2− catalysts were prepared in one step by the sol–gel method and dried in hypercritical conditions of...     

Synthesis and thermoluminescence characteristics of γ-irradiated K3Ca2(SO4)3F:Eu or Ce fluoride

New halophosphor K₃Ca₂(SO₄)₃F activated by Eu and Ce has been synthesized by a co-precipitation method and characterized according to its thermoluminescence. The formation of traps in rare earth doped K₃Ca₂(SO₄)₃F and the effects of γ-radiation dose on the glow curve are discussed. The glow curve of K₃Ca₂(SO₄)₃F:Ce shows a prominent single peak at 150°C, whereas K₃Ca₂(SO₄)₃F:Eu and K₃Ca₂(SO₄)₃F:Ce,Eu at 142°C and 192°C, respectively. A single glow peak indicates that there is only one set of trap being activated within the particular temperature range. The presented phosphors are also studied because of its fading, reusability and trapping parameters. There was just 2% fading during a period of 10 days, indicating no serious fading problem. Trapping parameters such as order of kinetics (b), activation energy (E) and frequency factor (S) were calculated by using Chen's half-width method. The observations presented in this paper are good for lamp phosphors as well as solid-state dosimeter.     

High-resolution infrared Fourier transform spectroscopy of SO in the X3Σ− and a1Δ electronic states

The v = 1 ← 0 vibration-rotation absorption bands of ³²S¹⁶O, ³⁴S¹⁶O, and ³²S¹⁸O in the ground electronic state, X³Σ⁻, and the v = 1 ← 0 vibration-rotation band of ³²S¹⁶O in the first excited electronic state, a¹Δ, were measured at 0.004 cm⁻¹ unapodized resolution with a high-resolution Fourier transform spectrometer coupled to a long path absorption cell. The v = 2 ← 0 vibration-rotation band of ³²S¹⁶O in the X³Σ⁻ state was also observed. Line positions for P- and R-branch transitions up to N = 44 for ³²S¹⁶O have been measured and analyzed yielding improved molecular parameters. The present measurements are compared with previous infrared and microwave measurements.     

First-principles calculations and Bader analysis of oxygen-deficient induced magnetism in cubic BaTiO3−x and SrTiO3−x

The possibility of oxygen deficient induced magnetism in cubic BaTiO₃ (BTO) and SrTiO₃ (STO) perovskites is investigated by first-principles calculations, using the projector-augmented-wave method, within the generalised gradient approximation (GGA) and generalised gradient approximation with on-site effect (GGA + U), for the exchange correlation potential. For non-stoichiometric BaTiO_3?x and SrTiO_3?x, supercells were created in order to have two vacancy concentrations, i.e. x?=?0.125, 0.083. Spin charge distributions and magnetic moments associated with each ion, including local density of states projected in Bader atoms, were analysed by performing a full Bader charge analysis. Results show that oxygen vacancies could induce magnetism in BaTiO_3?x with x?=?0.125 and x?=?0.083 under GGA and GGA + U approximations. For SrTiO_3?x with x?=?0.125, ferromagnetism is induced with GGA, whereas with GGA + U the non-magnetic state is retained. On the other hand, with x?=?0.083, ferromagnetism is induced under GGA and GGA + U.     

Relationship between structural phase transition and carrier concentration in La2−x−ySrxRyCuO4 (R = Ce or Tb)

We have studied the relationship between the crystal structure and the carrier concentration in La_2?x?ySr_xCe_yCuO₄ by low-temperature X-ray diffraction method. The analysis for the [1 1 0]_t peak of the tetragonal index confirms that the high-temperature tetragonal phase changes to the low-temperature orthorhombic one in both La_1.89Sr_0.11CuO₄ and La_1.88Sr_0.11Ce_0.01CuO₄. We have also examined the effects of Tb substitution for La-site on the superconductivity and the structure in La_2?x?ySr_xTb_yCuO₄. A dip of the critical temperature T_c(x) due to the 1/8 anomaly and a maximum of T_c(x) at the optimum carrier concentration do not depend on the Tb concentration. This result suggests the possibility that Tb is introduced as the trivalent ion for x = 0.07–0.18.     

Supertransferred Hyperfine Magnetic Fields at 111Cd Impurity Sites in Cd x Fe3−x O4 and Zn x Fe3−x O4

The hyperfine magnetic field at ¹¹¹Cd impurities substituting iron in the mixed spinels Cd _x Fe_3−x O₄ and Zn _x Fe_3−x O₄ has been determined by means of the Perturbed Angular Correlation technique. Compounds with different concentrations x were investigated as a function of temperature. The possibility of determining the lattice location of probes at octahedral or tetrahedral sites through the magnitude of the electric field gradient is analyzed. The measured hyperfine magnetic field at impurities in tetrahedral sites is discussed in terms of the populations of magnetic ions in the nearest neighbor sites. This revised version was published online in September 2006 with corrections to the Cover Date.     

Thermoelectric properties of the Nd2−x Ce x CuO4−y system

The temperature dependence of the Seebeck coefficient (in region 300–900 K) and the lattice constants of Nd_2–x Ce_xCuO_4–y (x=0, 0.05, 0.10, 0.15) with different oxygen content were measured. The value of S is always negative and it decreases with both Ce content and oxygen non-stoichiometry. At a certain concentration of Ce and oxygen in the material, the Seebeck coefficient becomes temperature-independent.The authors would like to thank Professor J. Horák of University of Chemical Technology, Pardubice for helpful discussions.     

Anomaly in the upper critical magnetic field common to YBa2Cu3O7−δ , HoBa2Cu3O7−δ , and Nd1.85Ce0.15CuO4−δ irradiated by helium ions

The high-temperature superconductors YBa₂Cu₃O_7−δ , HoBa₂Cu₃O_7−δ , and Nd_1.85Ce_0.15CuO_4−δ are found to possess a common anomaly in the variation of the upper critical magnetic field owing to irradiation by helium ions. While the resistivity ρ increases by many times, the increase in H _c2 from scattering by radiation defects typical of ordinary superconductors does not occur. In terms of the ordinary mechanism, the anomaly may be explained by a significant reduction in the conduction electron density resulting from a loss of oxygen, which causes a significant rise in ρ with a small change in the scattering. Fiz. Tverd. Tela (St. Petersburg) 41, 1372–1376 (August 1999)     

Producing μ−d and μ−t in vacuum

After the feasibility of vacuum isolated ^–d production was demonstrated at TRIUMF in 1989, development was begun on a target system that would take advantage of the process to aid in the understanding of the muon catalyzed fusion cycle. Minimal neutron backgrounds, the ability to use silicon detectors, and compatibility with tritium were considered important for a very versatile target system. The advantages which the target gives in isolating CF process will be outlined.