Photoluminescence in the mixed valence compound Eu3S4

We have investigated the 5d-4f interband and the intra-4f photoluminescence of Eu₃S₄. An anomalous shift of the interband luminescence can be explained by temperature dependent valence fluctuations. The ferromagnetic ordering of Eu₃S₄ is also verified.     

Magnetic susceptibility and valence fluctuations in Sm3S4

The magnetic susceptibility of Sm₃S₄ single crystals has been measured between 1.8 K and 300 K. At low temperatures the susceptibility is composed of χ_Sm²⁺ + 2χ_Sm³⁺. Near and above 100 K the susceptibility of the Sm³⁺ ions is quenched due to the increasing valence fluctuation rate.     

Pressure-induced valence instability in Sm4Bi3

Sm₄Bi₃ has the anti-Th₃P₄ (cubic) structure with three Sm²⁺ ions and one Sm³⁺ ion. The compound undergoes a first-order, isostructural transition at about 26 kbar. The resistance abruptly decreases by a factor of three and the volume by about 10%. The Sm²⁺ ions undergo a change towards the 3⁺ state in the transition and the material goes from semimetallic to a metallic state. The behavior of the resistance anomaly suggests termination of the phase boundary at a critical point.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

Magnetic susceptibility and valency state in Sm3S4

The lack of pronounced temperature dependence in the magnetic susceptibility of Sm₃S₄ rules out a magnetic ground state for Sm ions. Comparison with semiconducting and metallic phases of SmS suggest the presence of one Sm²⁺ ion for two non-integral valency Sm ions.     

Photoelectron spectra of As4S3, P4S3, P4Se3 and As4O6

The He(I) UV photoelectron spectra of As₄S₃, P₄S₃, P₄Se₃ and As₄O₆ are reported in this paper. The spectra of As₄S₃ and the isostructural molecules P₄S₃ and P₄Se₃ comprise seven broad but clearly defined peaks, whilst the spectrum of As₄O₆ consists of six peaks and two shoulders. By comparing the spectra with one another and with the photoelectron spectra of other arsenic and phosphorus compounds, the character and symmetry species of the highest-energy occupied molecular orbitals of the molecules have been assigned.     

X-ray absorption spectroscopic study of mixed valence systems EuCu2Si2, YbCu2Si2 and Sm4Bi3

X-ray absorption spectroscopy technique is employed to determine the valence of the rare earth ions in EuCu₂Si₂, YbCu₂Si₂ and Sm₄Bi₃. In each case, two absorption peaks corresponding to two different valence states of respective rare earth ions have been observed. Low temperature (77 K) study of EuCu₂Si₂ indicates distinct change in the relative intensities of the absorption peaks compared to those registered at room temperature (300 K). It is inferred from the change in the relative intensities that the population of Eu²⁺ in EuCu₂Si₂ decreases at liquid nitrogen temperature compared to Eu³⁺. Conclusions drawn from these results agree well with those reported by others using different experimental techniques. In Sm₄Bi₃, Sm²⁺ and Sm³⁺ are found to occur in the ratio of 3:1.     

Effect of rare earth (Pr2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3 and Er2O3 ) on the acoustic properties of glass belonging to bismuth-borate system

Bismuth-borate glasses doped with some rare earth ions were studied with respect to the density, molar volume and the elastic moduli, Poisson’s ratio, Debye temperature, microhardness, softening temperature, acoustic impedance, diffusion constant and latent heat of melting. Ultrasonic velocities were measured by the pulse echo overlap technique at a frequency of 10 MHz and at room temperature. From these velocities and density values, various elastic moduli were calculated. The correlation of elastic stiffness, the cross link density, and the fractal bond connectivity of these glasses are discussed. The derived experimental values of shear modulus, bulk modulus, Young’s modulus, and Poisson’s ratio for our glasses are compared with the theoretically calculated values in terms of the bond compression model and Makishima-Mackenize theory.     

Influence of pressure on the inhomogeneous mixed-valent state in Eu3S4

The 21.7-keV nuclear gamma resonance of ¹⁵¹Eu was used for studying the inhomogeneous mixed-valent compound Eu₃S₄ in the presence range 0 to 15 kbar. At room temperature the activation energy for electron hopping between the crystallographically equivalent divalent and trivalent Eu sites was found to decrease with pressure by dE_a/dP = -1.8 ± 0.3 meV/kbar. No change of the mean valence of Eu with pressure was observed.     

Influence of pressure on the inhomogeneous mixed-valent state in Eu3S4

The 21.7 keV nuclear gamma resonance of ¹⁵¹Eu was used for studying the inhomogeneous mixed-valent compound Eu₃S₄ in the pressure range 0–15 kbar. At room temperature the activation energy for electron hopping between the crystallographically equivalent divalent and trivalent Eu sites was found to decrease with pressure by dE_a/dP=-1.8±0.3 meV kbar^-1. No change of the mean valence of Eu with pressure was observed.     

On photoluminescence in HgGa2S4

In this paper, we investigate the kinetics of photoluminescence in excited crystals of HgGa₂S₄ which have recently been proposed for implementing tunable luminescent devices. From photoluminescence experiments, performed at various temperatures and excitation powers, it appears that two kinds of radiative recombination processes take place during crystal excitation. These originate two bands in emission spectra which were resolved by means of a fitting procedure. The dependencies of these bands on temperature and excitation power density are explained by means of a specific kinetic model. A broad band, peaking at about 1.8 eV, is ascribed to electron-hole tunnel recombinations occurring in associated donor-acceptor pairs, according to a Prener-Williams scheme. The second narrow band, peaking at about 2.3 eV, is ascribed to electron-hole recombinations occurring in centres presenting short () and long-life () excited states. At room temperature, owing to thermally activated relaxation from short- to long-life states, these centres saturate under relatively low excitation powers. The tunability of photoluminescence is a consequence of competition between monomolecular and bimolecular recombination processes.     

Inelastic neutron scattering study of acoustical magnons in MnFe2O4

The room-temperature spin wave spectrum in MnFe₂O₄ has been investigated by means of inelastic neutron scattering using the time-of-flight technique. The acoustical branch could be obtained in the [100]- and the [110]-directions up to nearly the Brillouin zone boundary. Values for the effective exchange parameters J_AB and J_BB have been derived. In both symmetry directions some low lying phonon branches could also be observed.     

Thermally and photo-induced changes in the valence states of vapour-deposited As2S3 films: A comparative photoemission study of As2S3 and As4S4

During thermal annealing or light irradiation, the changes in the valence states of vapour-deposited As₂S₃ and As₄S₄ films were observed by UPS. The experimental results reveal that an as-deposited As₂S₃ film contains considerable numbers of As₄S₄ molecular units, which polymerize or cross link to form a As₂S₃ glassy network on annealing or irradiation.     

Inelastic magnetic X-ray scattering from highly correlated electron systems: La1.2Sr1.8Mn2O7, La0.7Sr0.3MnO3 and Fe3O4

Magnetic Compton profiles have been measured for the colossal magnetoresistance manganites La_1.2Sr_1.8Mn₂O₇ and La_0.7Sr_0.3MnO₃, and for magnetite Fe₃O₄, along various crystallographic directions, over a wide range of temperatures and magnetic fields. The experimental results are interpreted via first-principles computations for the double layer manganite, La_1.2Sr_1.8Mn₂O₇, and by using a simple model involving atomic d-orbitals and free electrons for the other two compounds. For all three materials a preference for the occupation of e_g orbitals is found, particularly, for orbitals of dx²−y² symmetry. An itinerant electron contribution is adduced at all temperatures in magnetite; such a contribution also appears in La_1.2Sr_1.8Mn₂O₇, but it is present only at low temperatures in La_0.7Sr_0.3MnO₃.     

Charge ordering in Eu3S4 determined by the valence-difference contrast of synchrotron X-ray diffraction

Synchrotron X-ray diffraction study for single crystals of Eu₃S₄ has revealed that a Th₃P₄-type structure transforms to a charge-ordered one at T_c=188.5 K. The crystal structures of Eu₃S₄ at T=300, 180 and 160 K were determined in the least-squares refinements with the Mo K intensity data. The valence-difference contrast method was applied at the L_II absorption edge of Eu, utilizing a large difference in anomalous scattering factors between Eu²⁺ and Eu³⁺. The cation distribution of Eu²⁺ and Eu³⁺ was determined by crystal-structure analyses based on the intensity data collected at two wavelengths of λ=1.6312 and 1.6298 Å.The least-squares structural refinements suggest that the most plausible atomic arrangement is [Eu³⁺]^4a[Eu²⁺Eu³⁺]^8dS₄. The charge-ordering scheme is that a half of Eu³⁺ ions occupy the whole 4a sites in the crystal structure, while the remaining half of Eu³⁺ ions mix with Eu²⁺ in the 8d sites. The scheme is also supported by the energy dependence of Bragg intensities for 400 and 004 reflections.     

A study of Eu2O3, Sc2O3 co-doped tungsten matrix impregnated cathode

Eu₂O₃ and Sc₂O₃ co-doped W matrix impregnated cathodes have been prepared by the powder metallurgy method. The constitution of active elements on activated cathode surface is analyzed by in-situ Auger electron spectroscopy. It is found that although Eu exists in the matrix, no Eu is found on the cathode surface due to the formation of a stable Eu containing compound. Sc, Ba and O diffuse to the surface of the cathode and form an active surface layer during the activation period whereas the stable Eu-compound cannot liberate free Eu, which can diffuse from the cathode to the surface. The active substance of Sc, Ba and O on the cathode surface contribute to the emission property.     

X-ray photoelectron spectroscopy of Co3O4, Fe3O4, Mn3O4, and related compounds

The metal 2p region spectra of the mixed valence spinels, Co₃O₄, Fe₃O₄, Mn₃O₄, and related compounds were studied. The satellite splittings of Co 2p_{$\text{3}\text{2}$} for the octahedrally coordinated cobaltous ions are 6.2 eV and those for the tetrahedrally coordinated ones are about 5.3 eV. The Co 2p spectrum for Co₃O₄ is considered to be the sum of spectra of magnetic cobaltous ions and low-spin cobaltic ions. In the cases of Fe₃O₄ and Mn₃O₄, the oxidation states were not clearly distinguished because both the divalent and trivalent ions of iron and manganese are high-spin.     

Photoemission study of the density of valence states of amorphous As2S3 and Sb2S3

The densities of valence states of amorphous As₂S₃ and Sb₂S₃ have been investigated by means of X-ray photoemission and ultraviolet photoemission spectroscopy. The spectra are interpreted on the basis of existing band structure calculations.     

Time resolved photoluminescence spectra of a-As2S3

Time resolved photoluminescence (PL) spectra of a-As₂S₃ are reported in the range 0–100 nsec after the excitation light pulse. After a very careful correction for apparatus spectral response, there is no large shift of the PL line as previously reported, but only an increase of the high energy side of the PL spectrum at short times. These results are discussed in terms of models with or without rearranged point defects.     

Density of valence states of As2O3 and Sb2O3

The densities of valence states of As₂O₃ and Sb₂O₃ have been measured by means of X-ray photoemission, ultraviolet photoemission, and soft X-ray emission spectroscopy. The spectra can be interpreted on the basis of existing models for the As and Sb chalcogenides.