Li ion diffusion in Li4Ti5O12 thin film electrode prepared by PVP sol-gel method

Li₄Ti₅O₁₂ thin films for rechargeable lithium batteries were prepared by a sol-gel method with poly(vinylpyrrolidone). Interfacial properties of lithium insertion into Li₄Ti₅O₁₂ thin film were examined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and potentiostatic intermittent titration technique (PITT). Redox peaks in CV were very sharp even at a fast scan rate of 50 mV s⁻¹, indicating that Li₄Ti₅O₁₂ thin film had a fast electrochemical response, and that an apparent chemical diffusion coefficient of Li⁺ ion was estimated to be 6.8×10⁻¹¹ cm² s⁻¹ from a dependence of peak current on sweep rates. From EIS, it can be seen that Li⁺ ions become more mobile at 1.55 V vs. Li/Li⁺, corresponding to a two-phase region, and the chemical diffusion coefficients of Li⁺ ion ranged from 10⁻¹⁰ to 10⁻¹² cm² s⁻¹ at various potentials. The chemical diffusion coefficients of Li⁺ ion in Li₄Ti₅O₁₂ were also estimated from PITT. They were in a range of 10⁻¹¹-10⁻¹² cm² s⁻¹.     

Synthesis and properties of the Co7Se8−xSx and Ni7Se8−xSx solid solutions

We report the synthesis and elementary properties of the Co₇Se_8−xS_x (x=0-8) and Ni₇Se_8−xS_x (x=0-7) solid solutions. Both systems form a NiAs-type structure with metal vacancies. In general, the lattice parameters decrease with increasing x, but in the Ni₇Se_8−xS_x system c increases on going from x=5 to 7. Magnetic susceptibility measurements show that all samples exhibit temperature-independent paramagnetism from 25-250 K. Samples within the Co₇Se_8−xS_x system, as well as Ni₇Se₈ and Ni₇SeS₇, were found to be poor metals with resistivities of ∼0.20 and ∼0.06 mΩ cm at 300 K, respectively. The Sommerfeld constant (γ) was determined from specific heat measurements to be ∼13 mJ/mol_CoK² and ∼7 mJ/mol_NiK² for Co₇Se_8−xS_x and Ni₇Se_8−xS_x, respectively.     

Thermoelectric properties of Ag-doped n-type (Bi2Te3)0.9-(Bi2−xAgxSe3)0.1 (x=0-0.4) alloys prepared by spark plasma sintering

Ag-doped n-type (Bi₂Te₃)_0.9-(Bi_2−xAg_xSe₃)_0.1 (x=0-0.4) alloys were prepared by spark plasma sintering and their physical properties evaluated. When at low Ag content (x=0.05), the temperature dependence of the lattice thermal conductivity follows the trend of (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1; while at higher Ag content, a relatively rapid reduction above 400 K can be observed due possibly to the enhancement of scattering of phonons by the increased defects. The Seebeck coefficient increases with Ag content, with some loss of electrical conductivity, but the maximum dimensionless figure of merit ZT can be obtained to be 0.86 for the alloy with x=0.4 at 505 K, about 0.2 higher than that of the alloy (Bi₂Te₃)_0.9-(Bi₂Se₃)_0.1 without Ag-doping.     

Effect of sintering on the ionic conductivity of garnet-related structure Li5La3Nb2O12 and In- and K-doped Li5La3Nb2O12

Garnet-structure related metal oxides with the nominal chemical composition of Li₅La₃Nb₂O₁₂, In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ and K-substituted Li_5.5La_2.75K_0.25Nb₂O₁₂ were prepared by solid-state reactions at 900, 950, and 1000 °C using appropriate amounts of corresponding metal oxides, nitrates and carbonates. The powder XRD data reveal that the In- and K-doped compounds are isostructural with the parent compound Li₅La₃Nb₂O₁₂. The variation in the cubic lattice parameter was found to change with the size of the dopant ions, for example, substitution of larger In³⁺(r_CN6: 0.79 Å) for smaller Nb⁵⁺ (r_CN6: 0.64 Å) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) Å at 1000 °C. Samples prepared at higher temperatures (950, 1000 °C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 °C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K⁺ for La³⁺ and In³⁺ for Nb⁵⁺ in Li₅La₃Nb₂O₁₂ exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25-300 °C. Among the compounds investigated, the In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ exhibits the highest bulk lithium ion conductivity of 1.8×10⁻⁴ S/cm at 50 °C with an activation energy of 0.51 eV. The diffusivity (“component diffusion coefficient”) obtained from the AC conductivity and powder XRD data falls in the range 10⁻¹⁰-10⁻⁷ cm²/s over the temperature regime 50-200 °C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li₅La₃Nb₂O₁₂ was found to be unsuccessful under the investigated conditions.     

Exfoliated nanoplatelets of an Aurivillius phase, Bi3.25La0.75Ti3O12: Characterisation by X-ray diffraction and by high-resolution electron microscopy

Nanoparticles of the Aurivillius phase La-substituted BTO (Bi_4−xLa_xTi₃O₁₂, with x=0.75) were obtained through a chemical lithiation process. They have been characterised by X-ray diffraction and transmission electron microscopy (diffraction and imaging at high resolution). The defect-free particles are platelet-shaped with the c large axis perpendicular to the plane. From high-resolution images, it is clear that the delamination process occurs at the level of the (Bi₂O₂)²⁺ intermediate layer and is destructive for this layer. The smallest thickness measured corresponds to one cell parameter (3.3 nm) but a large range of thicknesses have been observed: this suggests that the lithium insertion does not take place in all (Bi₂O₂)²⁺ layers, despite a large excess of lithium and a long reaction time. This is confirmed by ICP analysis, which leads to a formula Li_0.99Bi_3.25La_0.77Ti_3.00O₁₂ for the lithiated compound. This behaviour towards lithium intercalation differs from those observed with BTO in literature, where lithium insertion is reported as occurring in every (Bi₂O₂)²⁺ layer. Possible explanations for this difference are advanced based on microstructural and structural considerations.     

Lithium insertion into manganese dioxide polymorphs in aqueous electrolytes

The electrochemical behaviour of the spinel-like LiMn₂O₄ was studied in non-aqueous and aqueous saturated alkali nitrate electrolytes in comparison with the layered manganese dioxide δ-MnO₂. The results obtained by galvanostatic and cyclic voltammetry techniques showed that the insertion of Li⁺/e^– or H⁺/e^– depends on both the host lattice and the type of electrolyte. The spinel-like LiMn₂O₄ preferably allowed the insertion of Li⁺/e^– in non-aqueous and aqueous saturated LiNO₃ electrolytes, as observed from the similarity of the electrochemical behaviour in these electrolytes and the stability of the structure. This was explained by the presence of a three-dimensional network of vacant tetrahedral and half-filled octahedral sites in LiMn₂O₄, which guarantee high mobility of Li⁺ ions. The layered manganese dioxide could be inserted by Li⁺/e^– only in non-aqueous electrolytes. The work described herein was carried out at the Institut für Anorganische und Analytische Chemie, Technische Universit?t Berlin, Germany     

Crystal and magnetic structures of the brownmillerite compound Ca2Fe1.039(8)Mn0.962(8)O5

The crystal and magnetic structures of the brownmillerite material, Ca₂Fe_1.039(8)Mn_0.962(8)O₅ were investigated using powder X-ray and neutron diffraction methods, the latter from 3.8 to 700 K. The compound crystallizes in Pnma space group with unit cell parameters of a=5.3055(5) Å, b=15.322(2) Å, c=5.4587(6) Å at 300 K. The neutron diffraction study revealed the occupancies of Fe³⁺ and Mn³⁺ ions in both octahedral and tetrahedral sites and showed some intersite mixing and a small, ∼4%, Fe excess. While bulk magnetization data were inconclusive, variable temperature neutron diffraction measurements showed the magnetic transition temperature to be 407(2) K below which a long range antiferromagnetic ordering of spins occurs with ordering wave vector k=(000). The spins of each ion are coupled antiferromagnetically with the nearest neighbors within the same layer and coupled antiparallel to the closest ions from the neighboring layer. This combination of intra- and inter-layer antiparallel arrangement of spins forms a G-type magnetic structure. The ordered moments on the octahedral and tetrahedral sites at 3.8 K are 3.64(16) and 4.23(16) μ_B, respectively.     

Crystal chemistry and crystallography of the Aurivillius phase Bi5AgNb4O18

Bi₅AgNb₄O₁₈ is a new phase, which was discovered during the phase equilibrium study of the Bi₂O₃-Ag₂O-Nb₂O₅ system. Bi₅AgNb₄O₁₈ was prepared at 750°C and is stable in air up to its melting temperature of 1160.1±5.0°C (standard error of estimate). Results of a Rietveld refinement using neutron powder diffraction confirmed that Bi₅AgNb₄O₁₈ is isostructural with Bi₃TiNbO₉, Bi₅NaNb₄O₁₈, and Bi₅KNb₄O₁₈. The structure was refined in the orthorhombic space group A2₁am, Z=2, and the lattice parameters are a=5.4915(2) Å, b=5.4752(2) Å, c=24.9282(8) Å, and V=749.52(4) Å³. The structure can be described as the m=2 member of the Aurivillius family, (Bi₂O₂)²⁺ (A_m−1B_mO_3m+1)²⁻ (where A=Bi and B=Ag, Nb), which is characterized by perovskite-like (A_m−1B_mO_3m+1)²⁻ slabs regularly interleaved with (Bi₂O₂)²⁺ layers. The octahedral [NbO₆] units are distorted with Nb-O distances ranging from 1.856(4) to 2.161(2) Å and the O-Nb-O angles ranging from 82.6(3)° to 98.5(3)°. These octahedra are tilted about the a- and c-axis by about 10.3° and 12.4°, respectively. Ag was found to substitute exclusively into the Bi-site that is located in the layer between the two distorted [NbO₆] units. Although the Ag substitutes into the Bi-site with the Bi:Ag ratio of 1:1, the existence of a superlattice was not detected using electron diffraction. A comparison of (Bi₂O₂)²⁺(A_m−1Nb_mO_3m+1)²⁻ structures (where A=Ag, Na, and K) revealed a relation between the pervoskite tolerance factor, t, and structural distortion. The reference pattern for Bi₅AgNb₄O₁₈ has been submitted to the International Centre for Diffraction Data (ICDD) for inclusion in the Powder Diffraction File.     

Synthesis, structural characterization and Li ion conductivity of a new vanado-molybdate phase, LiMg3VMo2O12

A new vanado-molybdate LiMg₃VMo₂O₁₂ has been synthesized, the crystal structure determined an ionic conductivity measured. The solid solution Li_2−zMg_2+zV_zMo_3−zO₁₂ was investigated and the structures of the z=0.5 and 1.0 compositions were refined by Rietveld analysis of powder X-ray (XRD) and powder neutron diffraction (ND) data. The structures were refined in the orthorhombic space group Pnma with a∼5.10, b∼10.4 and c∼17.6 Å, and are isostructural with the previously reported double molybdates Li₂M₂(MoO₄)₃ (M=M²⁺, z=0). The structures comprise of two unique (Li/Mg)O₆ octahedra, (Li/Mg)O₆ trigonal prisms and two unique (Mo/V)O₄ tetrahedra. A well-defined 1:3 ratio of Li⁺:Mg²⁺ is observed in octahedral chains for LiMg₃VMo₂O₁₂. Li⁺ preferentially occupies trigonal prisms and Mg²⁺ favours octahedral sheets. Excess V⁵⁺ adjacent to the octahedral sheets may indicate short-range order. Ionic conductivity measured by impedance spectroscopy (IS) and differential scanning calorimetry (DSC) measurements show the presence of a phase transition, at 500-600 °C, depending on x. A decrease in activation energy for Li⁺ ion conductivity occurs at the phase transition and the high temperature structure is a good Li⁺ ion conductor, with σ=1×10⁻³-4×10⁻² S cm⁻¹ and E_a=0.6 to 0.8 eV.     

The ionic conductivity and local environment of cations in Bi9ReO17

The influence of temperature on the structure of Bi₉ReO₁₇ has been investigated using differential thermal analysis, variable temperature X-ray diffraction and neutron powder diffraction. The material undergoes an order-disorder transition at ∼1000 K on heating, to form a fluorite-related phase. The local environments of the cations in fully ordered Bi₉ReO₁₇ have been investigated by Bi L_III- and Re L_III-edge extended X-ray absorption fine structure (EXAFS) measurements to complement the neutron powder diffraction information. Whereas rhenium displays regular tetrahedral coordination, all bismuth sites show coordination geometries which reflect the importance of a stereochemically active lone pair of electrons. Because of the wide range of Bi-O distances, EXAFS data are similar to those observed for disordered structures, and are dominated by the shorter Bi-O bonds. Ionic conductivity measurements indicate that ordered Bi₉ReO₁₇ exhibits reasonably high oxide ion conductivity, corresponding to 2.9×10⁻⁵ Ω⁻¹ cm^-1 at 673 K, whereas the disordered form shows higher oxide ion conductivity (9.1×10⁻⁴ Ω⁻¹ cm⁻¹ at 673 K).     

Preparation of a new pyrochlore-type compound Na0.32Bi1.68Ti2O6.46(OH)0.44 by hydrothermal reaction

A new pyrochlore-type Na_0.32Bi_1.68Ti₂O_6.46(OH)_0.44 with the cubic cell of a=10.339(5) Å was prepared by hydrothermal reaction using TiO₂ (anatase) and Bi₂O₃ in NaOH solution. This compound was obtained when the molar ratio of NaOH/TiO₂ was above 2 and the reaction temperature was above 240 °C. The TG-curve of as-prepared sample showed a mass loss of 0.8 mass% which was caused by release of OH group. This compound decomposed to a pyrochlore-type compound and a layered-type Na_0.5Bi_4.5Ti₄O₁₅ above 800 °C. The optical band gap of Na_0.32Bi_1.68Ti₂O_6.46(OH)_0.44 was estimated to be 2.5 eV.     

Carbon coated Na3V2(PO4)3 as novel electrode material for sodium ion batteries

A Na₃V₂(PO₄)₃ sample coated uniformly with a layer of 6 nm carbon has been successfully synthesized by a one-step solid state reaction. This material shows two flat voltage plateaus at 3.4 V vs. Na⁺/Na and 1.63 V vs. Na⁺/Na in a nonaqueous sodium cell. When the Na₃V₂(PO₄)₃/C sample is tested as a cathode in a voltage range of 2.7-3.8 V vs. Na⁺/Na, its initial charge and discharge capacities are 98.6 and 93 mAh/g. The capacity retention of 99% can be achieved after 10 cycles. The electrode shows good cycle performance and moderate rate performance. When it is tested as an anode in a voltage range of 1.0-3.0 V vs. Na⁺/Na, the initial reversible capacity is 66.3 mAh/g and the capacity of 59 mAh/g can be maintained after 50 cycles. These preliminary results indicate that Na₃V₂(PO₄)₃/C is a new promising material for sodium ion batteries.     

Synthesis, structure and electrical properties of Cu3.21Ti1.16Nb2.63O12 and the CuOx-TiO2-Nb2O5 pseudoternary phase diagram

Subsolidus phase relations in the CuO_x-TiO₂-Nb₂O₅ system were determined at 935 °C. The phase diagram contains one new phase, Cu_3.21Ti_1.16Nb_2.63O₁₂ (CTNO) and one rutile-structured solid solution series, Ti_1−3xCu_xNb_2xO₂: 0<x<0.2335 (35). The crystal structure of CTNO is similar to that of CaCu₃Ti₄O₁₂ (CCTO) with square planar Cu²⁺ but with A site vacancies and a disordered mixture of Cu⁺, Ti⁴⁺ and Nb⁵⁺ on the octahedral sites. It is a modest semiconductor with relative permittivity ∼63 and displays non-Arrhenius conductivity behavior that is essentially temperature-independent at the lowest temperatures.     

Kinetic analysis of the thermal stability of lithium silicates (Li4SiO4 and Li2SiO3)

The kinetics describing the thermal decomposition of Li₄SiO₄ and Li₂SiO₃ have been analysed. While Li₄SiO₄ decomposed on Li₂SiO₃ by lithium sublimation, Li₂SiO₃ was highly stable at the temperatures studied. Li₄SiO₄ began to decompose between 900 and 1000 °C. However, at 1100 °C or higher temperatures, Li₄SiO₄ melted, and the kinetic data of its decomposition varied. The activation energy of both processes was estimated according to the Arrhenius kinetic theory. The energy values obtained were −408 and −250 kJ mol⁻¹ for the solid and liquid phases, respectively. At the same time, the Li₄SiO₄ decomposition process was described mathematically as a function of a diffusion-controlled reaction into a spherical system. The activation energy for this process was estimated to be −331 kJ mol⁻¹. On the other hand, Li₂SiO₃ was not decomposed at high temperatures, but it presented a very high preferential orientation after the heat treatments.     

Li and Li MAS NMR Studies on Fast Ionic Conducting Spinel-Type Li2MgCl4, Li2-xCuxMgCl4, Li2-xNaxMgCl4, and Li2ZnCl4

⁶Li and ⁷Li MAS NMR spectra including 1D-EXSY (exchange spectroscopy) and inversion recovery experiments of fast ionic conducting Li₂MgCl₄, Li_2-xCu_xMgCl₄, Li_2-xNa_xMgCl₄, and Li₂ZnCl₄ have been recorded and discussed with respect to the dynamics and local structure of the lithium ions. The chemical shifts, intensities, and half-widths of the Li MAS NMR signals of the inverse spinel-type solid solutions Li_2-xM^I_xMgCl₄ (M^I=Cu, Na) with the copper ions solely at tetrahedral sites and sodium ions at octahedral sites and the normal spinel-type zinc compound, respectively, confirm the assignment of the low-field signal to Li^tet of inverse spinel-type Li₂MgCl₄ and the high-field signal to Lioct as proposed by Nagel et al. (2000). In contrast to spinel-type Li_2-2xMg_1+xCl₄ solid solutions with clustering of the vacancies and Mg²⁺ ions, the Cu⁺ and Na⁺ ions are randomly distributed on the tetrahedral and octahedral sites, respectively. The activation energies due to the various dynamic processes of the lithium ions in inverse spinel-type chlorides obtained by the NMR experiments are E_a=6.6-6.9 and ΔG^*>79 KJ mol⁻¹ (in addition to 23, 29, and 75 kJmol-1 obtained by other techniques), respectively. The largest activation energy of >79 KJ mol⁻¹ corresponds to hopping exchange processes of Li ions between the tetrahedral 8a sites and the octahedral 16d sites. The smallest value of 6.6-6.9 KJ mol⁻¹, which was derived from the temperature dependence of both the spin-lattice relaxation times T₁ and the correlation times τ_C of Li^tet, reveals a dynamic process for the Li^tet ions inside the tetrahedral voids of the structure, probably between fourfold 32e split sites around the tetrahedral 8a site.     

Synthesis and crystal structure of Bi6.4Pb0.6P2O15.2: A new polymorph in the series Bi6+xM1−xP2O15+y

Bi_6.4Pb_0.6P₂O_15.2 is a polymorph of structures with the general stoichiometry Bi_6+xM_1−xP₂O_15+y. However, unlike previously published structures that consist of layers formed by edge sharing OBi₄ tetrahedra bridged by PO₄ and TO₆ (T=transition metal) tetrahedra and octahedra the title compound's structure is more complex. It is monoclinic, C2, a=19.4698(4) Å, b=11.3692(3) Å, c=16.3809(5) Å, β=101.167(1)°, Z=10. Single-crystal X-ray diffraction data were refined by least squares on F² converging to R₁=0.0387, wR₂=0.0836 for 7023 intensities. The crystal twins by mirror reflection across (001) as the twin plane and twin component 1 equals 0.74(1). Oxygen ions are in tetrahedral coordination to four metal ions and the O(BiPb)₄ units share corners to form layers that are part of the three-dimensional framework. Eight oxygen ions form a cube around the two crystallographically independent Pb ions. Pb-O bond lengths vary from 2.265(14) to 2.869(14) Å. Pairs of such cubes share an edge to form a Pb₃O₂₀ unit. The two oxygen ions from the unshared edges are part of irregular Bi polyhedra. Other oxygen ions of Bi polyhedra are part only of O(BiPb)₄ units, and some oxygen ions of the polyhedra are also part of PO₄ tetrahedra. One, two, three and or four PO₄ moieties are connected to the Bi polyhedra. Bi-O bond lengths ?3.1 Å vary from 2.090(12) to 3.07(3) Å. The articulations of Pb cubes, Bi polyhedra and PO₄ tetrahedra link into the three-dimensional structure.     

Crystal structure and physical properties of the quaternary manganese-bearing pavonite homologue Mn1.34Sn6.66Bi8Se20

The quaternary manganese tin bismuth selenide, Mn_1.34Sn_6.66Bi₈Se₂₀ was synthesized by combining constituent elements at 723 K. Single crystal structure determination revealed that Mn_1.34Sn_6.66Bi₈Se₂₀ is isostructural to the mineral pavonite, AgBi₃S₅, crystallizing in the monoclinic space group C2/m (#12) with a=13.648(3) Å; b=4.175(1) Å; c=17.463(4) Å; β=93.42(3)°. In the structure, two kinds of layered modules, denoted A and B, alternate along [0 0 1]. Module A consists of paired chains of face-sharing monocapped trigonal prisms (around Bi/Sn) separated by a single chain of edge-sharing octahedra (around Mn/Sn). Module B represents a NaCl-type fragment of edge-sharing [(Bi/Sn)Se₆] octahedra. Mn_1.34Sn_6.66Bi₈Se₂₀ is an n-type narrow gap semiconductor with E_g∼0.29 eV. At 300 K, thermopower, electrical conductivity and lattice thermal conductivity values are −123 μV/K, 47 S/cm and 0.6 W/m K, respectively. Mn_1.34Sn_6.66Bi₈Se₂₀ is paramagnetic at high temperatures and undergoes antiferromagnetic transition at T_N=10 K.     

Structure and magnetic order in the series BixRE1−xFe0.5Mn0.5O3 (RE=La,Nd)

The influence of Bi³⁺ on the structural and magnetic properties of the rare-earth-containing perovskites REFe_0.5Mn_0.5O₃ (RE=La,Nd) was studied, and the limit of bismuth substitution was determined to be x≤0.5 in Bi_xRE_1−xFe_0.5Mn_0.5O_3+δ (RE=La,Nd) at ambient pressure. Crystal structures in both La and Nd series were determined to be GdFeO₃-type Pnma with the exception of the Bi_0.3La_0.7Fe_0.5Mn_0.5O₃ sample, which is monoclinic I2/a in the a⁻b⁻b⁻ tilt scheme. The samples undergo a transition to G-type antiferromagnetic order along with a weak ferromagnetic component, mixed with cluster-glass type behavior. The substitution of bismuth into the lattice results in a drop in T_N relative to the lanthanide end-members. Long range ordering temperatures T_N in the range 240-255 K were observed, with a significantly lower ordered magnetic moment in the case of lanthanum (M∼1.7-1.9 μ_B) than in the case of neodymium (M∼2.1 μ_B).     

Electronic structure and photocatalytic properties of ABi2Ta2O9 (A=Ca, Sr, Ba)

A new series of layered perovskite photocatalysts, ABi₂Ta₂O₉ (A=Ca, Sr, Ba), were synthesized by the conventional solid-state reaction method and the crystal structures were characterized by powder X-ray diffraction. The results showed that the structure of ABi₂Ta₂O₉ (A=Ca, Sr) is orthorhombic, while that of BaBi₂Ta₂O₉ is tetragonal. First-principles calculations of the electronic band structures and density of states (DOS) revealed that the conduction bands of these photocatalysts are mainly attributable to the Ta 5d+Bi 6p+O 2p orbitals, while their valence bands are composed of hybridization with O 2p+Ta 5d+Bi 6s orbitals. Photocatalytic activities for water splitting were investigated under UV light irradiation and indicated that these photocatalysts are highly active even without co-catalysts. The formation rate of H₂ evolution from an aqueous methanol solution is about 2.26 mmol h^-1 for the photocatalyst SrBi₂Ta₂O₉, which is much higher than that of CaBi₂Ta₂O₉ and BaBi₂Ta₂O₉. The photocatalytic properties are discussed in close connection with the crystal structure and the electronic structure in details.     

Thermoelectric properties of p-type pseudo-binary (Ag0.365Sb0.558Te)x-(Bi0.5Sb1.5Te3)1−x (x=0-1.0) alloys prepared by spark plasma sintering

In this paper, pseudo-binary (Ag_0.365Sb_0.558Te)_x-(Bi_0.5Sb_1.5Te₃)_1−x (x=0-1.0) alloys were prepared using spark plasma sintering technique, and the composition-dependent thermoelectric properties were evaluated. Electrical conductivities range from 7.9×10⁴ to 15.6×10⁴ Ω⁻¹ m⁻¹ at temperatures of 507 and 318 K, respectively, being about 3.0 and 8.5 times those of Bi_0.5Sb_1.5Te₃ alloy at the corresponding temperatures. The optimal dimensionless figure of merit (ZT) of the sample with molar fraction x=0.025 reaches 1.1 at 478 K, whereas that of the ternary Bi_0.5Sb_1.5Te₃ alloy is 0.58 near room temperature. The results also reveal that a direct introduction of Ag_0.365Sb_0.558Te in the Bi-Sb-Te system is much more effective to the property improvement than naturally precipitated Ag_0.365Sb_0.558Te in the Ag-doped Ag-Bi-Sb-Te system.