Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.     

Growth of rare earth silicides on silicon

Dysprosium metal layers deposited on (100)Si substrates were annealed in situ at 300 °C for different time intervals in order to produce DySi_2−x layers at several stages of growth. Electron microscopy work showed that silicon reacts with Dy metal through the simultaneous growth of an amorphous interlayer and a polycrystalline phase to form DySi_2−x. Compressive stresses due to volumetric changes during the reaction deform the produced silicide layer and induce roughness at the surface and interface.     

Electron spectroscopic studies of nanowires formed by (GaMn)As growth on GaAs(111)B

Valence band photoemission with photon energies around the Mn2p excitation threshold has been used to study the development of nanowires catalyzed by MnAs particles. A gradual change in the spectra with increasing nanowire length is observed, such that the resonant photoemission eventually dominates over the Auger decay channel. The change is ascribed to dilution of Mn, showing that Mn is transferred from the MnAs particles into the nanowires.     

Surface core level shifts in photo-electron spectra from the Ca, Sr and Ba titanates

Photoelectron binding energy (BE), shifts observed in core level spectra of Ca, Sr and Ba present at the outer most surface with respect to the bulk in the respective single crystal titanates can be correlated to the inverse of their respective electro-negativities. Such trends can be explained within the context of the charge potential model. This model, which does not account for final state effects, indicates that (a) the direction of the BE shift results from chemical environment variations (an inter-atomic effect sometimes referred to in Madulung constant terms), and (b) BE shift variations result from electron density variations on the atom the photo-electron emanated from (an intra-atomic effect, which can also be relayed in ionic radii and electro-negativity terms).     

Curie temperatures and modified de Gennes factors of rare earth nitrides

The Curie temperatures T_C of the nitrides of the rare earths (Gd, Tb, Dy, Ho, and Er), including binary systems, were investigated. T_C was found to be approximately proportional to the de Gennes factor, ξ=(g−1)²J(J+1), where g is the Landé g-factor and J is the total angular momentum quantum number of a trivalent rare earth (RE). This proportionality was significantly improved by introducing a modified de Gennes factor, ξ_bi. The conventional de Gennes factor ξ indicates the exchange interaction given by the inner product of the effective spin components of ions of the same kind, whereas our modified de Gennes factor ξ_bi also considers interactions between different kinds of ions and statistical factors calculated on the basis of the binomial distribution. The good proportionality obtained between T_C and ξ_bi indicates that the spin components of RE ions interact with each other. This interaction is considered to be responsible for the ferromagnetism of these nitrides (including binary systems). These considerations were supported by the synthesis of and magnetization measurements on Gd_xEr_1−xN (x=0, 0.25, 0.5, 0.75, 1) samples.     

Valence and C 1s core level photoelectron spectra of camphor

The valence photoelectron spectrum of camphor has been recorded with 95 eV synchrotron radiation, with better definition than previous He I spectra. The spectrum is interpreted by comparison with these He I results and with the aid of an outer-valence Green’s Function calculation of the orbital ionization energies. These calculations closely reproduce the observed vertical ionization energies in the outer valence region. A core level spectrum of the C 1s region (hν=357.9 eV) is also presented and reveals a marked shift of the carbonyl carbon relative to all others in the molecule.     

Problems in experimental charge density modelling of rare earth atom complexes: the case of gadolinium

Difficulties to model the charge density of a gadolinium atom coordinated to organic ligands are discussed. Refinements using the available scattering factors did not lead to satisfactory results. This can be due to the inaccuracy of the wave functions calculation or to inherent difficulties in the treatment of f electrons whose specific nature would justify the use of a more realistic starting electron density model before performing the electron density calculations.     

The white line inL absorption spectra of rare earth oxides

The method of curve fitting of the experimentally obtained absorption edge has been developed and successfully adopted to study the precise variation in the density of states responsible for the occurrence of the white line associated with theL _II andL _III absorption discontinuities of rare earth (RE) in their sesquioxides. The theoretical equation and the curve fitting routine is flexible enough to include all the transitions responsible for XANES as well as other phenomena such as, screening effect, variation in the potential around the metal ion due to surrounding ligand ion, charge transfer etc. An erratum to this article is available at .     

Dense-Kondo antiferromagnetism in (CePd3)8Ge

We report the existence of (CePd₃)₈Ge and (LaPd₃)₈Ge, and find them to have the same cubic superstructure as reported for (CePd₃)₈T (T=Ga, In, Sn, Sb, Pb and Bi) by Gordon and co-workers [1 and 2]. The cerium ions in (CePd₃)₈Ge are trivalent and undergo an antiferromagnetic transition below 5 K with a dense-Kondo-lattice behaviour.     

Crystal field and exchange interactions in rare earth transition metal salen compounds containing bipyridine, pyrazole complexes

Crystal field parameters for Pr³⁺ in {[Ni(salen)Pr-(hfac)₃](H₂O)} (noted as NiPr) and {[Ni(salen)Pr(hfac)₃(pyr)]-(CHCl₃)} (noted as NiPrpyr) have been found from a fit to the thermal variation in the magnetic susceptibility of NiPr and NiPrpyr. The nature of exchange interaction in [Cu(salen)Pr(hfac)3(pyr)] (noted as CuPrpyr), {[{Cu(salen)Pr(hfac)₃}₂(pyz)](H₂O)₃} (noted as Cu₂Pr₂pyz) and {[{Cu(salen)Pr(hfac)₃}₂(bpy)]-(CHCl₃)₂} (noted as Cu₂Pr₂bpy, bpy=4,4_-bipyridine) have been found using the derived results for NiPr and NiPrpyr. All the exchange interactions give significant contribution to the thermal variation in magnetic susceptibility below 50 K. The contribution due to Pr-Cu interaction is positive while that of the Cu-Cu and Pr-Pr interactions are negative. The behaviors below 10 K for Pr-Cu and Pr-Pr are difficult to explain, and point to a possible change in structure of CuPrpyr, Cu₂Pr₂pyz and Cu₂Pr₂bpy below 10 K. The theoretical thermal variations in the magnetic specific heat of NiPr and NiPrpyr are computed and discussed.     

Quantum critical behaviour in doped Y bB12 studied by ab initio calculations

The Kondo insulator Y bB₁₂ is known to undergo a transition to the metallic state with doping or under an external magnetic field. Within the virtual crystal approximation (VCA), we calculated the occupation of the Yb 4f and 5d shells, and , as a function of doping of Y bB₁₂ with the rare earths Tm and Lu. We found that exhibits an anomalous change at the critical concentration of the dopant, in agreement with experiment ( for Y b_1−xLu_xB₁₂ and for Y b_1−xTm_xB₁₂). We suggest that the critical behaviour seems to be strictly connected with the change of and in consequence the change of the Yb valency.     

Antiferromagnetism and the Kondo behaviour in Ce(Pt1−xPdx)Ga

We present the results of lattice parameters at room temperature, the static magnetic susceptibility and the magnetic resistivity between 1.8 and 300 K, and the low-temperature specific-heat measurements for the series Ce(Pt_1−xPd_x)Ga, (x=0.0, 0.2, 0.5, 0.8 and 1.0). Two maxima in the temperature dependence of the magnetic resistive curve for each sample are observed, one above 100 K, and another at around 4 K, which due to an interplay between crystal-field effect and the Kondo effect. As determined from the peak values of the temperature dependence of the specific heat data C(T), all samples exhibit antiferromagnetic ordering from 1.3 K for CePdGa to 3.4 K for CePtGa. The large reduction of entropy for each sample below T_N is associated with the Kondo effect.     

High-pressure Raman spectra of L-isoleucine crystals

Raman spectroscopy investigations of L-isoleucine crystals under high pressures have been carried out up to 7.3 GPa. From this study it was possible to observe modifications on bands associated to both rocking vibrations of r(NH₃⁺) and r(CO₂⁻) as well as to lattice modes at about 2.3 and 5.0 GPa. These modifications were correlated to either conformational change of molecules or to a solid–solid phase transition undergone by the crystals involving the hydrogen bonds that maintain the molecules held in the unit cell. A comparison with a few results on other amino acid crystals is also given.     

Enhancing the phase stability and ionic conductivity of scandia stabilized zirconia by rare earth co-doping

Effect of co-doping Yb, Gd and Ce in scandia stabilized zirconia (SSZ) on the phase stability, high temperature aging behavior and ionic conductivity was studied. Both binary (10 mol% SSZ) and the ternary (co-doped) compositions were found to be in single cubic phase in the as-processed condition. However, the binary composition exhibited the rhombohedral ‘β’ phase after sintering whereas the ternary compositions remained in the single cubic phase. The sintered pellets were aged at 900 °C for 500 h in air to study the phase stability at high temperature. Transmission electron microscopy revealed that the aged samples of Yb and Gd co-doped compositions contain small amount of the tetragonal phase which resulted in considerable degradation in conductivity (more than 20%). The Ce co-doped sample, on the other hand, was in single cubic phase after aging and this ensured that conductivity reduction was minimal in this composition. The co-doped samples however, showed higher conductivity before and after aging compared to the binary composition. The rhombohedral ‘β’ phase was absent in all the co-doped ternary compositions even after high temperature aging.     

Unusual ferromagnetism in nanoparticles of doped oxides and manganites

The observation of unusually large ferromagnetism in the nanoparticles of doped oxides and enhanced ferromagnetic tendencies in manganite nanoparticles have been in focus recently. For the transition metal doped oxide nanoparticles a phenomenological ‘charge transfer ferromagnetism’ model has been recently proposed by Coey et al. From a microscopic calculation with charge transfer between the defect band and mixed valent dopants, acting as reservoir, we show how the unusually high ferromagnetic response develops. The puzzle of nanosize-induced ferromagnetic tendencies in manganites is also addressed within the same framework where lattice imperfections and uncompensated charges at the surface of the nanoparticle are shown to reorganize the surface electronic structures with enhanced double exchange.     

Control of luminescence and conductivity of delafossite-type CuYO2 by substitution of rare earth cation (Eu, Tb) and/or Ca cation for Y cation

Delafossite-type oxides of CuTb_yY_1−yO₂, CuEu_yY_1−yO₂, CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ have been prepared by solid state reactions. The lattice-parameter dependence on the composition implies substitution of the Tb³⁺, Eu³⁺ and Ca²⁺ cations for the Y³⁺ site. Noticeable sharp emission lines due to the f-f transitions (⁵D₄→⁷F_J, J=3-6) of Tb³⁺ or due to the f-f transitions (⁵D₀→⁷F_J, J=0-4) of Eu³⁺ are observed at room temperature. Electrical conductivities of CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ are larger than those of CuTb_yY_1−yO₂ and CuEu_yY_1−yO₂, indicating the increase of the hole concentration caused by the substitution of Ca²⁺ for the Y³⁺ site. These results indicate the controllability of the luminescence and conductivity in CuCa_xTb_yY_1−x−yO₂ and CuCa_xEu_yY_1−x−yO₂ delafossite-type oxides by simultaneous substitution of the rare earth Tb³⁺ or Eu³⁺ cation and the Ca²⁺ cation for the Y³⁺ site.     

Linear and nonlinear optical susceptibilities of orthorhombic rare earth molybdates RE2(MoO4)3

The dielectric responses (i.e. the refractive indices and the second order nonlinear susceptibilities) of all orthorhombic rare earth molybdates have been studied on the basis of the relationship between dielectric responses and the average atomic number of constituent atoms of crystals. Both the linear and second order nonlinear optical responses at 1.064 μm decrease with increasing atomic number from La to Lu.     

Investigation of Lin, H-Lin and O-Lin interactions in fcc(100) and fcc(110) surface symmetries by molecular dynamic simulation method

We have investigated the structure and energies of lithium microclusters containing 3-10 atoms in the fcc(100) and fcc(110) surface symmetries, and the interaction of an oxygen and hydrogen atom with these lithium microclusters for the on-top, open and bridge sites approaches. The calculations have been performed with molecular dynamics simulation methods (MDSM) at 1 K temperature and the results were compared with the literature.     

Synthesis process and the luminescence properties of rare earth doped NaLa(WO4)2 nanoparticles

Rare earth doped NaLa(WO₄)₂ nanoparticles have been prepared by a simply hydrothermal synthesis procedure. The X-ray diffraction (XRD) pattern shows that the Eu³⁺-doped NaLa(WO₄)₂ nanoparticles with an average size of 10-30 nm can be obtained via hydrothermal treatment for different time at 180 °C. The luminescence intensity of Eu³⁺-doped NaLa(WO₄)₂ nanoparticles depended on the size of the nanoparticles. The bright upconversion luminescence of the 2 mol% Er³⁺ and 20 mol% Yb³⁺ codoped NaLa(WO₄)₂ nanoparticles under 980 nm excitation could also be observed. The Yb³⁺-Er³⁺ codoped NaLa(WO₄)₂ nanoparticles prepared by the hydrothermal treatment at 180 °C and then heated at 600 °C shows a 20 times stronger upconversion luminescence than those prepared by hydrothermal treatment at 180 °C or by hydrothermal treatment at 180 °C and then heated at 400 °C.