Thermoelectric power of (Me4N)2Ag13I15 and (Et4N)2Ag13I15

Thermoelectric power using reversible silver electrodes and electrical conductivity on the compressed pellets of (Me₄N)₂Ag₁₃I₁₅, and (Et₄N)₂Ag₁₃I₁₅ have been measured between room temperature and below 160°C. The results of θ can be expressed by the equations:?θ = 0.115 (10³/T)+0.2905VK^?1 and ?θ = 0.150 (10³/T) + 0.305mV K^?1; and those of conductivity by the equations; σ = 28.7 exp (?0.17eV/kT) ohm^?1cm^?1 and $\text{σ = 216.6 exp (?0.24e}\text{V}\text{kT}\text{)}$ ohm^?1cm^?1; respectively for Me- and Et-electrolytes. The results are discussed and compared with those of previous authors.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Emission features of Ho ion in Nb2O5, Ta2O5 and La2O3 mixed Li2O-ZrO2-SiO2 glasses

Li₂O-ZrO₂-SiO₂: Ho³⁺ glasses mixed with three interesting d-block elemental oxides, viz., Nb₂O₅, Ta₂O₅ and La₂O₃, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb₂O₅ and Ta₂O₅ mixed Li₂O-ZrO₂-SiO₂ glasses (free of Ho³⁺ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta⁵⁺ and Nb⁵⁺ ions in the glass network. The Judd-Ofelt theory was successfully applied to characterize Ho³⁺ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β_r and the radiative lifetime τ_r, for ⁵S₂ emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for ⁵S₂ level of Ho³⁺ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La₂O₃ mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho³⁺ ions.     

Adsorption-desorption studies using ESD of CCl2F2, C2H2F2 and C2F6 on tungsten

The behavior of the desorbing F⁺ ion current from electron bombarded CCl₂F₂, C₂H₂F₂ and C₂F₆ adsorbed on tungsten has been used to investigate the processes of adsorption and desorption of these gases. For tungsten near room temperature, measurements of the F⁺ ion current as a function of electron bombardment time indicated very similar or even identical F⁺-yielding adsorbed species resulting from adsorption of either CCl₂F₂ or C₂H₂F₂ and widely different species from C₂F₆. Cl⁺ ions were also observed to desorb from CCl₂F₂ ad-layers. The behavior of the Cl⁺ ion current with time during electron bombardment indicated electronic conversion between adsorbed binding modes. Complementary investigations on the interaction of CCl₂F₂, C₂H₂F₂ and C₂F₆ with tungsten were carried out by thermal desorption experiments in which the F⁺ ion signal was used to observe the coverage decrease of the F⁺-yielding species. The experiments were performed at tungsten temperatures in the 1200–1600 K range. It was concluded that the F⁺-yielding adsorbed species from CCl₂F₂ and C₂H₂F₂ were strongly bound to the tungsten surface. The F⁺-yielding species from C₂F₆ were found to be weakly bound. From a comparison of the ESD and thermal desorption results, the possibility of dissociative adsorption as well as the nature of the adsorbed species is discussed.     

Magnetic transition and large reversible magnetocaloric effect in EuCu1.75P2 compound

The magnetocaloric effect(MCE) in EuCu1.75P2 compound is studied by the magnetization and heat capacity measurements.Magnetization and modified Arrott plots indicate that the compound undergoes a second-order phase transition at TC ～ 51 K.A large reversible MCE is observed around TC.The values of maximum magnetic entropy change(-△SxMma) reach 5.6 J·kg-1·K-1 and 13.3 J·kg-1·K-1 for the field change of 2 T and 7 T,respectively,with no obvious hysteresis loss in the vicinity of Curie temperature.The corresponding maximum adiabatic temperature changes(△Tadmax) are evaluated to be 2.1 K and 5.0 K.The magnetic transition and the origin of large MCE in EuCu1.75P2 are also discussed.     

Structural study of the Lu2Si2O7-Sc2Si2O7 system

Different compositions in the Lu₂Si₂O₇-Sc₂Si₂O₇ system have been synthesized following the ceramic method. All XRD patterns are compatible with the thortveitite structure (β-RE₂Si₂O₇ polymorph). Unit cell parameters change linearly with composition, which indicates a complete solid solubility of Sc₂Si₂O₇ in Lu₂Si₂O₇. ²⁹Si MAS NMR spectra show a decrease of the ²⁹Si chemical shift with increasing Sc content. A correlation reported in the literature to predict ²⁹Si chemical shifts in silicates is applied here to obtain the theoretical variation in ²⁹Si chemical shift values in the system Lu₂Si₂O₇-Sc₂Si₂O₇ and the results compare favourably with the values obtained experimentally. The FWHM values of the ²⁹Si MAS NMR curves indicate a random distribution of Lu and Sc in the structure of the intermediate members. Finally, the IR study of the system confirms the solubility of Sc₂Si₂O₇ in Lu₂Si₂O₇, showing the splitting of several modes in the intermediate members and a linear shift of the frequency on going from one end-member to the other.     

High temperature phase transition in NH4H2PO4

NH₄H₂PO₄ (ADP) has been investigated by infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis and d.c. conductivity in the temperature range of 25–180° C. Sharp reversible changes were observed in the region from 400 to 500 cm^?1 of the infrared spectra in the temperature range of 138–174° C. Similar and supportive data were obtained with DSC, TGA and DC conductivity measurements. The results clearly suggest a high temperature phase transition for ADP before its melting point.     

High field electrical properties of CdF2 crystals doped with rare earth ions

The electrical properties of CdF₂ crystals doped with Eu³⁺ and Gd³⁺ are described. A reversible “forming” effect is observed under high electric fields. The effect seems to be connected with a reversible modification of the potential barrier at the metal-insulator contact.     

Emission analysis of Dy and Pr:Bi2O3-ZnF2-B2O3-Li2O-Na2O glasses

In this paper, we present the spectral results of Dy³⁺ and Pr³⁺ (1.0 mol%) ions doped Bi₂O₃-ZnF₂-B₂O₃-Li₂O-Na₂O glasses. Measurements of X-ray diffraction (XRD), differential scanning calorimetry (DSC) profiles of these rare-earth ions doped glasses have been carried out. From the DSC thermograms, glass transition (T_g), crystallization (T_c) and melting (T_m) temperatures have been evaluated. The direct and indirect optical band gaps have been calculated based on the glasses UV absorption spectra. The emission spectrum of Dy³⁺:glass has shown two emission transitions ⁴F_7/2→⁶H_15/2 (482 nm) and ⁴F_7/2→⁶H_13/2 (576 nm) with an excitation at 390 nm wavelength and Pr³⁺:glass has shown a strong emission transition ¹D₂→³H₄ (610 nm) with an excitation at 445 nm. Upon exposure to UV radiation, Dy³⁺ and Pr³⁺ glasses have shown bright yellow and reddish colors, respectively, from their surfaces.     

Polymorphism in Bi2Sn2O7

Single crystals of Bi₂Sn₂O₇ were grown in a Bi₂O₃ flux. Phase transitions were identified at about 90 and 680° using X-ray, SHG, DSC, dielectric, and optical data. γ-Bi₂Sn₂O₇, which exists above 680°C is centric and cubic with a = 10.73 Å at 700°, and it probably has the ideal pyrochlore structure. β-Bi₂Sn₂O₇, which exists between 680° and about 90°C, is acentric but remains cubic with a = 21.40 Å. α-Bi₂Sn₂O₇, which exists from about 90°C to below room temperature, is acentric and noncubic, probably tetragonal with a = 21.328 and c = 21.545 Å. The α-β transition is first order, and the β-γ transition appears to be second order. Substitutions of Pb²⁺ or Cd²⁺ for Bi³⁺ and of Ga³⁺, Rh³⁺ Sc³⁺, In³⁺, Sb⁵⁺ Nb⁵⁺ or Ta⁵⁺ for Sn⁴⁺ lower the α-β transition temperature.     

Upconversion properties of Er^3＋/Yb^3＋ co-doped TeO2-Nb2O5-Li2O glasses

The Er^3＋/Yb^3＋ co-doped TeO2-Nb2O5-Li2O glass is prepared by conventional melting method, and its upconversion spectra are measured. The intense green upconversion luminescence upon excitation with a 976 nm laser diode is observed with the naked eyes. The dependence of luminescence intensity on the ratio of Yb^3＋/Er^3＋ is discussed in detail, and the relationship between the ratio of green luminescence intensity to red luminescence intensity and the ratio of Yb^3＋/Er^3＋ is also studied, The luminescence intensity increases with the ratio of Yb^3＋/Er^3＋ increasing. The ratio of Yb^3＋/Er^3＋ plays a more important role than the concentration of Er^3＋ in determining the upconversion luminescence intensity. The ratio of green luminescence intensity to red luminescence intensity reaches a maximum when ratio of Yb^3＋/Er^3＋ is 3. Thus the glass could be one of the potential candidates for LD pumping solid-state lasers.     

Effect of Ce on the spectroscopic properties in Er doped TeO2-GeO2-Nb2O5-Li2O glasses

The Ce³⁺ ion was introduced into Er³⁺ doped TeO₂-GeO₂-Nb₂O₅-Li₂O (TGNL) glass to improve the 1.5 μm fluorescence characteristics. As increasing of Ce³⁺ concentration, the lifetime of Er³⁺:⁴I_11/2 level is shortened form 360 to 225 μs, while the Er³⁺:⁴I_13/2 level remains unchanged. Accordingly, the upconversion fluorescence (blue, green and red) was quenched. Improved 1.5 μm emission is obtained and the reason is ascribed to the increase of nonradiative rate between the ⁴I_11/2 and ⁴I_13/2 level of the Er³⁺ ions.     

Comparative study of NiSb2 and FeSb2 as negative electrodes for Li-ion batteries

Crystalline FeSb₂ powder prepared by ceramic route is examined as negative electrodes for lithium-ion batteries. The complete reaction mechanism of FeSb₂ is investigated by ¹²¹Sb and ⁵⁷Fe Mössbauer spectroscopy as well as magnetic measurements and the results are correlated with a previous in situ XRD characterization. On the first discharge the reaction with Li proceeds through a biphasic process transforming FeSb₂ into a new Li_xFe_ySb₂ phase, and this ternary phase is then converted into fcc Li₃Sb and metallic Fe nanoparticles. The combination of Mössbauer spectroscopy and magnetic analyses leads i) to a better understanding of the FeSb₂ → ternary phase reaction and concomitantly allowed ii) to specify the stoichiometry of the new ternary phase. On charge, the extrusion of lithium includes the back conversion of the Li₃Sb/Fe mixture into both Li₄Fe_0.5Sb₂ and metallic Sb, which are the main active species for the following cycles, responsible for the poor cycling life of the FeSb₂ electrode. The nature of these resulting products is quite different from that previously observed for the isotype NiSb₂ electrode which is characterized by a highly reversible mechanism.     

Single color upconversion emission in Ho/Yb and Tm/Yb doped P2O5-MgO2-Sb2O3-MnO2-AgO glasses

The Ho³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ doped P₂O₅-MgO₂-Sb₂O₃-MnO₂-AgO glasses were prepared by high temperature melting method. Under a 975 nm laser diode (LD) excitation, the single red and single blue upconversion (UC) emissions were observed in Ho³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ doped samples, respectively. By studying the spontaneous radiative and multiphonon relaxation probabilities, we find that the multiphonon relaxation probability of ⁵I₆ (Ho³⁺) state is very large (1.39 × 10⁶ s^− 1), which is helpful to the population of ⁵I₇ state. The multiphonon relaxation probability of ³H₅ and ³F_2,3 (Tm³⁺) is also very large, which results in lots of population in ³F₄ and ³H₄ states. The results are that the red UC emission of Ho³⁺ and the blue UC emission of Tm³⁺ are stronger.     

Pseudo potential calculations for Na+2, K+2, Rb+2 and Cs+2

A Hellmann type pseudopotential, is used to calculate the six lowest Σ potential energy curves of Na⁺₂, K⁺₂, Rb⁺₂ and Cs⁺₂ molecular ions.     

Study of the Ru sublattice magnetic structure in the magnetic superconductor RuSr2GdCu2O8

In order to investigate the Ru sublattice magnetic structure, a study of the field dependence of the ^99,101Ru nuclear magnetic resonance (NMR) has been carried out on the magnetic superconductor RuSr₂GdCu₂O₈. It is found that the ^99,101Ru NMR signal intensity increases significantly with applied magnetic field up to ≈3 kOe, beyond which, it progressively decreases. In addition, a shift of the NMR peaks to lower frequency is observed to begin at ≈1.3 kOe. These behaviors are shown to be accompanied by a field-induced Ru moment spin-flop in the ab planes, and are understood in terms of a previously proposed type-I antiferromagnetic ordering for the Ru sublattice. Based on this model, the inter-plane antiferromagnetic exchange coupling is determined to be ≈1.8 kOe along with a reversible in-plane spin-flop which is characterized by a field ≈0.6 kOe.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

Analysis of ν2 of D2S

The infrared spectrum of ν₂ of D₂S was recorded from 740 to 1100 cm^?1 on the University of Denver 50-cm FTIR spectrometer system. We have assigned 655 transitions from D₂³²S and 129 from D₂³⁴S, and have analyzed them using Watson's A-reduced Hamiltonian evaluated in the I^r representation. We used the recently published D₂³²S and D₂³⁴S ground state Hamiltonian constants [C. Camy-Peyret, J. M. Flaud, L. Lechuga-Fossat and J. W. C. Johns, J. Mol. Spectrosc.109, 300–333 (1985)]. Upper state Hamiltonian constants were obtained from a fit of the ν₂ transitions, keeping the ground state constants fixed while varying the upper state constants. The standard deviation of the D₂³²S ν₂ fit is 0.0025 cm^?1. The standard deviation of the D₂³⁴S ν₂ fit is 0.0041 cm^?1.     

Preparation and luminescence properties of Y2(WO4)3: Eu3+

A red-emitting Y₂(WO₄)₃: Eu³⁺ phosphor (orthorhombic high temperature phase, anhydride) is prepared by two different methods: the firing of mixtures of constituent oxides and that of precipitates from aqueous solutions. After optimizing preparation conditions, the cathodoluminescence brightness reaches 56% that of Y₂O₂S: Eu³⁺, a commercial red phosphor for color TV. Formation of a high temperature phase below the reported transition temperature is noted in the fired precipitates. This phase occurrence is shown to depend on the treatment of the precipitates to be fired. Reflection difference measurement of Eu-doped and undoped samples assigns an excitation band of about 245 nm to the Eu-O charge transfer band. Different by-products in the two preparation methods are identified by measuring emission spectra under selective excitation. Reversible hydration-dehydration of the phosphor is demonstrated by successively measuring photoluminescence first in vacuum and then in air at various temperatures. No deterioration of luminescence efficiency is observed after repeating this reversible structural change.     

Synthesis and crystal structures and luminescent properties of divalent europium-doped Ba2ZnSi2O7 and BaZn2Si2O7

The monoclinic Ba₂ZnSi₂O₇:Eu²⁺ blue-green-emitting phosphor and the orthorhombic BaZn₂Si₂O₇:Eu²⁺ green-emitting phosphor were prepared by combustion-assisted synthesis method as the fluorescent materials for ultraviolet-light-emitting diodes (UV-LEDs) performed as a light source. The crystallinity and luminescence were investigated using X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. Pure monoclinic Ba₂ZnSi₂O₇ and orthorhombic BaZn₂Si₂O₇ crystallize completely at 1100 °C. The doped Eu²⁺ ions did not cause any significant change in the host structure. The emission spectra presented an emission position red shift of up to 16 nm from Ba₂ZnSi₂O₇:Eu²⁺ to BaZn₂Si₂O₇:Eu²⁺. The excitation spectra of Ba₂ZnSi₂O₇:Eu²⁺ and BaZn₂Si₂O₇:Eu²⁺ were broad-banding, extending from 260 to 465 nm, which match the emission of UV-LEDs.