Neutron powder diffraction study of tetragonal Li7La3Hf2O12 with the garnet-related type structure

We have successfully synthesized a polycrystalline sample of tetragonal garnet-related Li-ion conductor Li₇La₃Hf₂O₁₂ by solid state reaction. The crystal structure is analyzed by the Rietveld method using neutron powder diffraction data. The structure analysis identifies that tetragonal Li₇La₃Hf₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.106(2) Å and c=12.630(2) Å with a cell ratio of c/a=0.9637. The crystal structure of tetragonal Li₇La₃Hf₂O₁₂ has the garnet-type framework structure composed of dodecahedral La(1)O₈, La(2)O₈ and octahedral HfO₆. Li atoms occupy three types of crystallographic site in the interstices of this framework structure, where Li(1) atom is located at the tetrahedral 8a site, and Li(2) and Li(3) atoms are located at the distorted octahedral 16f and 32g sites, respectively. These Li sites are filled with the Li atom. The present tetragonal Li₇La₃Hf₂O₁₂ sample exhibits bulk Li-ion conductivity of σ_b=9.85×10⁻⁷ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=4.45×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.53 eV in the temperature range of 300-580 K.     

New high permittivity tetragonal tungsten bronze dielectrics Ba2LaMNb4O15: M=Mn, Fe

The new phases Ba₂LaMNb₄O₁₅: M=Mn, Fe were prepared by solid state reaction at 1100 °C. They have the tetragonal tungsten bronze structure, space group P4/mbm, at room temperature. The two octahedral sites show partial order of M and Nb with preferential occupancy of the smaller B(1) sites by M. Both phases have high permittivities 90±15 over the range 10-320 K. Ba₂LaFeNb₄O₁₅ is highly insulating with bulk conductivity ?10⁻⁸ ohm⁻¹ cm⁻¹ at 25 °C and tan δ?0.001 over the range 100-320 K and at 10⁵ Hz. Solid solutions between these new phases and the compositionally and structurally related relaxor ferroelectric Ba₂LaTi₂Nb₃O₁₅ show gradual loss of ferroelectric behaviour attributed to replacement of polarisable Ti⁴⁺ by a mixture of (Mn, Fe)³⁺ and Nb⁵⁺.     

Synthesis and structural chemistry of La18Li8Rh4MO39 (M=Ti, Mn, Ru)

Polycrystalline samples of La₁₈Li₈Rh₄MO₃₉ (M=Ti, Mn, Ru) have been prepared by a solid-state method and studied by neutron powder diffraction. They are isostructural with La₁₈Li₈Rh₅O₃₉ and adopt the cubic space group with a ∼12.22 Å. Their structure consists of a La-O framework containing intersecting channels that run along 〈111〉. These channels are occupied by chains made up of alternating, face-sharing trigonal-prismatic and octahedral coordination polyhedra; there are two crystallographically distinct types of octahedral site. The prisms are occupied by Li and the transition metals are disordered over the two octahedral sites.     

Formation, sintering, and electrical conductivity of perovskite Sm(Cr1−xMgx)O3 (0?x?0.23) prepared by the hydrazine method

The presence of SmCrO₄ is experimentally established. In Mg²⁺-substituted SmCrO₃, single-phase perovskite Sm(Cr_1−xMg_x)O₃, where x=0-0.23, are formed at ∼830°C by decomposition of Sm(Cr_1−xMg_x)O₄ which crystallizes at 530-570°C from amorphous materials prepared by the hydrazine method. Sm(Cr_1−xMg_x)O₃ solid solution powders consisting of submicrometer-size particles are sinterable; dense materials can be fabricated by sintering for 2 h at 1700°C in air. The relative densities, grain sizes, and electrical conductivities increase with increased Mg²⁺ content. Sm(Cr_0.77Mg_0.23)O₃ materials exhibit an excellent direct current electrical conductivity of 2.2×10³ S m⁻¹ at 1000°C.     

Structure and photoluminescence properties of Ce-doped novel silicon-oxynitride Ba4−zMzSi8O20−3xN2x (M=Mg, Ca, Sr)

Structural and photoluminescence properties of undoped and Ce³⁺-doped novel silicon-oxynitride phosphors of Ba_4−zM_zSi₈O_20−3xN_2x (M=Mg, Sr, Ca) are reported. Single-phase solid solutions of Ba_4−zM_zSi₈O_20−3xN_2x oxynitride were synthesized by partial substitutions of 3O²⁻→2N³⁻ and Ba→M (M=Mg, Ca, Sr) in orthorhombic Ba₂Si₄O₁₀. The influences of the type of alkaline earth ions of M, the Ce³⁺ concentration on the photoluminescence properties and thermal quenching behaviors of Ba₃MSi₈O_20−3xN_2x (M=Mg, Ca, Sr, x=0.5) were investigated. Under excitation at about 330 nm, Ba₃MSi₈O_20−3xN_2x:Ce³⁺ (x=0.5) exhibits efficient blue emission centered at 400-450 nm in the range of 350-650 nm owing to the 5d→4f transition of Ce³⁺. The emission band of Ce³⁺ shifts to long wavelength by increasing the ionic size of M due to the modification of the crystal field, as well as the Ce³⁺ concentrations due to the Stokes shift and energy transfer or reabsorption of Ce³⁺ ions. Among the silicon-oxynitride phosphors of Ba₃MSi₈O_18.5N:Ce³⁺, M=Sr_0.6Ca_0.4 possesses the best thermal stability probably related to its high onset of the absorption edge of Ce³⁺.     

Transport properties and stability of Ni-containing mixed conductors with perovskite- and K2NiF4-type structure

The total conductivity and Seebeck coefficient of a series of Ni-containing phases, including La₂Ni_1−xM_xO_4+δ (M=Co, Cu; x=0.1-0.2) with K₂NiF₄-type structure and perovskite-like La_0.90Sr_0.10Ga_0.65Mg_0.15Ni_0.20O_3−δ and La_0.50Pr_0.50Ga_0.65Mg_0.15Ni_0.20O_3−δ, were studied in the oxygen partial pressure range from 10⁻¹⁸ Pa to 50 kPa at 973-1223 K. Within the phase stability domain, the conductivity of layered nickelates is predominantly p-type electronic and occurs via small-polaron mechanism, indicated by temperature-activated hole mobility and p(O₂) dependencies of electrical properties. In oxidizing conditions similar behavior is characteristic of Ni-containing perovskites, which exhibit, however, significant ionic contribution to the transport processes. The role of ionic conduction increases with decreasing p(O₂) and becomes dominant in reducing atmospheres. All nickelate-based phases decompose at oxygen pressures considerably lower with respect to Ni/NiO boundary. The partial substitution of nickel in La₂Ni(M)O_4+δ has minor effect on the stability limits, which are similar to that of La_0.90Sr_0.10Ga_0.65Mg_0.15Ni_0.20O_3−δ. On the contrary, praseodymium doping enhances the stability of La_0.50Pr_0.50Ga_0.65Mg_0.15Ni_0.20O_3−δ down to p(O₂) values as low as 10⁻¹⁷-10⁻¹⁰ Pa at 1023-1223 K.     

Magnesium doping on brownmillerite Ca2FeAlO5

Ca₂FeAl_1−xMg_xO₅ (x=0, 0.05 and 0.1) compounds adopting the brownmillerite-type structure were prepared by a self-combustion route using two different fuels. Characterisation was performed using X-ray powder diffraction, Mössbauer spectroscopy, magnetisation measurements, chemical analysis, scanning electron microscopy and 4-point dc conductivity measurements. Global results indicate that the solubility limit was reached for x=0.1. An antiferromagnetic behaviour was detected for all studied compositions, with magnetic ordering temperatures of 340 and 290 K for x=0 and 0.05, respectively. Mg doping increases the number of iron cations in tetrahedral sites, which induces magnetisation enhancement at low temperatures through the coupling between octahedral iron cations in different octahedral planes. The compounds exhibit semiconductor behaviour and Mg²⁺ doping yields a significant enhancement of the total conductivity, which can be essentially attributed to the presence of Fe⁴⁺ ions.     

A comparison of the transport properties of lithium-stuffed garnets and the conventional phases Li3Ln3Te2O12

The structures of new phases Li₆CaLa₂Sb₂O₁₂ and Li_6.4Ca_1.4La₂Sb₂O₁₂ have been characterised using neutron powder diffraction. Rietveld analyses show that both compounds crystallise in the space group la3?d and contain the lithium cations in a complex arrangement with occupational disorder across oxide tetrahedra and distorted oxide octahedra, with considerable positional disorder in the latter. Variable temperature neutron diffraction experiments on Li_6.4Ca_1.4La₂Sb₂O₁₂ show the structure is largely invariant with only a small variation in the lithium distribution as a function of temperature. Impedance spectroscopy measurements show that the total conductivity of Li₆CaLa₂Sb₂O₁₂ is several orders of magnitude smaller than related lithium-stuffed garnets with σ=10⁻⁷ S cm⁻¹ at 95 °C and an activation energy of 0.82(3) eV. The transport properties of the conventional garnets Li₃Gd₃Te₂O₁₂, Li₃Tb₃Te₂O₁₂, Li₃Er₃Te₂O₁₂ and Li₃Lu₃Te₂O₁₂ have been evaluated and consistently show much lower values of conductivity, σ≤4.4×10⁻⁶ S cm⁻¹ at 285 °C and activation energies in the range 0.77(4)≤E_a/eV≤1.21(3).     

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.     

Structural and magnetic properties of Pr18Li8Fe5−xMxO39 (M=Ru, Mn, Co)

A polycrystalline sample of Pr₁₈Li₈Fe₄RuO₃₉ has been synthesized by a solid state method and characterized by neutron powder diffraction, magnetometry and Mössbauer spectroscopy; samples of Pr₁₈Li₈Fe_5−xMn_xO₃₉ and Pr₁₈Li₈Fe_5−xCo_xO₃₉ (x=1, 2) have been studied by magnetometry. All these compounds adopt a cubic structure (space group , a₀∼11.97 Å) based on intersecting 〈111〉 chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Pr-O framework. The trigonal-prismatic site in Pr₁₈Li₈Fe₄RuO₃₉ is occupied by Li⁺ and high-spin Fe³⁺. The remaining transition-metal cations occupy the two crystallographically-distinct octahedral sites in a disordered manner. All five compositions adopt a spin-glass-like state at 7 K (Pr₁₈Li₈Fe₄RuO₃₉) or below.     

Synthesis and electrical conductivity of perovskite-type PrCo1−xMgxO3

Oxides in the system PrCo_1−xMg_xO₃ (x=0.0, 0.05, 0.10, 0.15, 0.20, 0.25) were synthesized by citrate technique and characterized by powder X-ray diffraction and scanning electron microscope. All compounds have a cubic perovskite structure (space group ). The maximum ratio of doped Mg in the system PrCo_1−xMg_xO₃ is x=0.2. Further doping leads to the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃. The substitution of Mg for Co improves the performance of PrCoO₃ as compared to the electrical conductivity measured by a four-probe electrical conductivity analyzer in the temperature range from 298 to 1073 K. The substitution of Mg for Co on the B site may be compensated by the formations of Co⁴⁺ and oxygen vacancies. The electrical conductivity of PrCo_1−xMg_xO₃ oxides increases with increasing x in the range of 0.0-0.2. The increase in conductivity becomes considerable at the temperatures ?673 K especially for x?0.1; it reaches a maximum at x=0.2 and 1073 K. From x>0.2 the conductivity of PrCo_1−xMg_xO₃ starts getting lower. This is probably a result of the segregation of Pr₆O₁₁ in PrCo_1−xMg_xO₃ , which blocks oxygen transport, and association of oxygen vacancies. A change in activation energy for all PrCo_1−xMg_xO₃ compounds (x=0-0.25) was observed, with a higher activation energy above 573 K and a lower activation energy below 573 K. The reasons for such a change are probably due to the change of dominant charge carriers from Co⁴⁺ to Vö in PrCo_1−xMg_xO₃ oxides and a phase transition mainly starting at 573 K.     

Mn environment in layered Li[Mg0.5−xNixMn0.5]O2 oxides monitored by EPR spectroscopy

X-band and high-frequency EPR spectroscopy were used for studying the manganese environment in layered Li[Mg_xNi_0.5−xMn_0.5]O₂, 0?x?0.5. Both layered LiMg_0.5Mn_0.5O₂ and monoclinic Li[Li_1/3Mn_2/3]O₂ oxides (containing Mn⁴⁺ ions only) were used as EPR standards. The EPR study was extended to the Ni-substituted analogues, where both Ni²⁺ and Mn⁴⁺ are paramagnetic. For LiMg_0.5−xNi_xMn_0.5O₂ and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂, an EPR response from Mn⁴⁺ ions only was detected, while the Ni²⁺ ions remained EPR silent in the frequency range of 9.23-285 GHz. For the diamagnetically diluted oxides, LiMg_0.25Ni_0.25Mn_0.5O₂ and Li[Li_0.10Ni_0.35Mn_0.55]O₂, two types of Mn⁴⁺ ions located in a mixed (Mn-Ni-Li)-environment and in a Ni-Mn environment, respectively, were registered by high-field experiments. In the X-band, comparative analysis of the EPR line width of Mn⁴⁺ ions permits to extract the composition of the first coordination sphere of Mn in layered LiMg_0.5−xNi_xMn_0.5O₂ (0?x?0.5) and Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ (x>0.2). It was shown that a fraction of Mn⁴⁺ are in an environment resembling the ordered “α,β”-type arrangement in Li_1−δ1Ni_δ1[Li_{(1−2x)/3+δ1}Ni_2x/3−δ1)_α(Mn_(2−x)/3Ni_x/3)_β]O₂ (where and δ₁=0.06 were calculated), while the rest of Mn⁴⁺ are in the Ni,Mn-environment corresponding to the Li_1−δ2Ni_δ2[Ni_1−yMn_y]O₂ () composition with a statistical Ni,Mn distribution. For Li[Li_(1−2x)/3Ni_xMn_(2−x)/3]O₂ with x?0.2, IR spectroscopy indicated that the ordered α,β-type arrangement is retained upon Ni introduction into monoclinic Li[Li_1/3Mn_2/3]O₂.     

Structural and magnetic properties of Nd18Li8Co4−xFexO39−y and Nd18Li8Co4−xTixO39−y

Nd₁₈Li₈Co₃FeO_39−y, Nd₁₈Li₈CoFe₃O_39−y and Nd₁₈Li₈Co₃TiO_39−y have been synthesised and characterised by neutron powder diffraction, magnetometry and Mössbauer spectroscopy. Their cubic structure (Pm3?n, a∼11.9 Å) is based on intersecting <1 1 1> chains comprised of alternating octahedral and trigonal-prismatic coordination sites. These chains lie within hexagonal-prismatic cavities formed by a Nd-O framework. Each compound has an incomplete oxide sublattice (y∼1), with vacancies located around the octahedral sites that lie at the points of chain intersection. These sites are fully occupied by a disordered arrangement of transition-metal cations but only 75% of the remaining octahedral sites are occupied. The trigonal-prismatic sites are fully occupied by lithium except in the case of Nd₁₈Li₈CoFe₃O_39−y where some iron is present. Antiferromagnetic interactions are present on the Nd sublattice in each composition, but a spin glass forms below 5 K when a high concentration of spins is also present on the octahedral sites.     

Transport properties and lithium insertion study in the p-type semi-conductors AgCuO2 and AgCu0.5Mn0.5O2

The transport properties and lithium insertion mechanism into the first mixed valence silver-copper oxide AgCuO₂ and the B-site mixed magnetic delafossite AgCu_0.5Mn_0.5O₂ were investigated by means of four probes DC measurements combined with thermopower measurements and in situ XRD investigations. AgCuO₂ and AgCu_0.5Mn_0.5O₂ display p-type conductivity with Seebeck coefficient of Q=+2.46 and +78.83 μV/K and conductivity values of σ=3.2×10⁻¹ and 1.8×10⁻⁴ S/cm, respectively. The high conductivity together with the low Seebeck coefficient of AgCuO₂ is explained as a result of the mixed valence state between Ag and Cu sites. The electrochemically assisted lithium insertion into AgCuO₂ shows a solid solution domain between x=0 and 0.8Li⁺ followed by a plateau nearby 1.7 V (vs. Li⁺/Li) entailing the reduction of silver to silver metal accordingly to a displacement reaction. During the solid solution, a rapid structure amorphization was observed. The delafossite AgCu_0.5Mn_0.5O₂ also exhibits Li⁺/Ag⁺ displacement reaction in a comparable potential range than AgCuO₂; however, with a prior narrow solid solution domain and a less rapid amorphization process. AgCuO₂ and AgCu_0.5Mn_0.5O₂ provide a discharge gravimetric capacity of 265 and 230 mA h/g above 1.5 V (vs. Li⁺/Li), respectively, with no evidence of a new defined phases.     

Synthesis crystal structure and ionic conductivity of Ca0.5Bi3V2O10 and Sr0.5Bi3V2O10

Two new compounds Ca_0.5Bi₃V₂O₁₀ and Sr_0.5Bi₃V₂O₁₀ have been synthesized in the ternary system: MO-Bi₂O₃-V₂O₅ system (M=M²⁺). The crystal structure of Sr_0.5Bi₃V₂O₁₀ has been determined from single crystal X-ray diffraction data, space group and Z=2, with cell parameters a=7.1453(3) Å, b=7.8921(3) Å, c=9.3297(3) Å, α=106.444(2)°, β=94.088(2)°, γ=112.445(2)°, V=456.72(4) Å³. Ca_0.5Bi₃V₂O₁₀ is isostructural with Sr_0.5Bi₃V₂O₁₀, with, a=7.0810(2) Å, b=7.8447(2) Å, c=9.3607(2) Å, α=106.202(1)°, β=94.572(1)°, γ=112.659(1)°, V=450.38(2) Å³ and its structure has been refined by Rietveld method using powder X-ray data. The crystal structure consists of infinite chains of (Bi₂O₂) along c-axis formed by linkage of BiO₈ and BiO₆ polyhedra interconnected by MO₈ polyhedra forming 2D layers in ac plane. The vanadate tetrahedra are sandwiched between these layers. Conductivity measurements give a maximum conductivity value of 4.54×10⁻⁵ and 3.63×10⁻⁵ S cm⁻¹ for Ca_0.5Bi₃V₂O₁₀ and Sr_0.5Bi₃V₂O₁₀, respectively at 725 °C.     

On AgRhO2, and the new quaternary delafossites AgLi1/3M2/3O2, syntheses and analyses of real structures

Two new quaternary delafossite type oxides with the general formula Ag(Li_1/3M_2/3)O₂, M=Rh, Ir, have been synthesized, and their structures characterized. Based on X-ray and electron diffraction analyses the structural similarity with AgRhO₂ delafossite, has been evidenced. The real structures of the quaternary delafossites have been revealed, which has allowed to fully explain the diffuse scattering as observed in X-ray powder diffraction. AgRhO₂ is thermally stable up to 1173 K, the behavior of the two quaternary compounds AgLi_1/3Rh_2/3O₂ and AgLi_1/3Ir_2/3O₂ is comparable, and they decompose above 950 and 800 K, respectively. AgRhO₂ shows temperature independent paramagnetism, while for the other two an effective magnetic moment of 1.77μ_B for Ir, and 1.70μ_B for Rh were determined, applying the Curie-Weiss law. All compounds are semiconducting with activation energies of 4.97 kJ mol⁻¹ (AgLi_1/3Rh_2/3O₂), 11.42 kJ mol⁻¹ (AgLi_1/3Ir_2/3O₂) and 17.58 kJ mol⁻¹ (AgRhO₂).     

EPR spectroscopic studies in (30 − x) Li2O-xK2O-10CdO-59B2O3-1MnO2 multi-component glass system

EPR studies were carried out in (30 - x) Li₂O-xK₂O-10CdO-59B₂O₃-1MnO₂ multi-component glass system to understand the effect of the variation in the alkali ratios on the EPR parameters. The observed EPR spectra of Mn²⁺ ion exhibits resonances at g = 2.0, 3.3 and 4.3. The resonance at g = 2.0 is due to Mn²⁺ ions in an environment close to the octahedral symmetry, where as the resonances at g = 3.3 & 4.3 are due to the rhombic surroundings of Mn²⁺ ions. Hyperfine splitting constant values at g = 2.0 and number of paramagnetic centers & paramagnetic susceptibility at different observed resonances were evaluated. These parameters show non linear variation with progressive substitution of Li⁺ ion with K⁺ ions may be due to the changes in cation field strengths and local structural variation due to the variation in mixed alkali ion ratios.     

Series of compositions Bi2(M′xM1−x)4O9 (M′, M=Al, Ga, Fe; 0≤x≤1) with mullite-type crystal structure: Synthesis, characterization and O/O exchange experiment

Series of compositions Bi₂(M′_xM_1−x)₄O₉ with x=0.0, 0.1,…, 1.0 and M′/M=Ga/Al, Fe/Al and Fe/Ga were synthesized by dissolving appropriate amounts of corresponding metal nitrate hydrates in glycerine, followed by gelation, calcination and final heating at 800 °C for 24 h. The new compositions with M′/M=Ga/Al form solid-solution series, which are isotypes to the two other series M′/M=Fe/Al and Fe/Ga. The XRD data analysis yielded in all cases a linear dependence of the lattice parameters related on x. Rietveld structure refinements of the XRD patterns of the new compounds, Bi₂(Ga_xAl_1−x)₄O₉ reveal a preferential occupation of Ga in tetrahedral site (4 h). The IR absorption spectra measured between 50 and 4000 cm⁻¹ of all systems show systematic shifts in peak positions related to the degree of substitution. Samples treated in ¹⁸O₂ atmosphere (16 h at 800 °C, 200 mbar, 95% ¹⁸O₂) for ¹⁸O/¹⁶O isotope exchange experiments show a well-separated IR absorption peak related to the M-¹⁸O_c-M vibration, where O_c denotes the common oxygen of two tetrahedral type MO₄ units. The intensity ratio of M-¹⁸O_c/M-¹⁶O_c IR absorption peaks and the average crystal sizes were used to estimate the tracer diffusion coefficients of polycrystalline Bi₂Al₄O₉ (D=2×10⁻²² m²s⁻¹), Bi₂Fe₄O₉ (D=5×10⁻²¹ m²s⁻¹), Bi₂(Ga/Al)₄O₉ (D=2×10⁻²¹ m²s⁻¹) and Bi₂Ga₄O₉ (D=2×10⁻²⁰ m²s⁻¹).     

Syntheses, structures, optical properties, and theoretical calculations of Cs2Bi2ZnS5, Cs2Bi2CdS5, and Cs2Bi2MnS5

Three new compounds, Cs₂Bi₂ZnS₅, Cs₂Bi₂CdS₅, and Cs₂Bi₂MnS₅, have been synthesized from the respective elements and a reactive flux Cs₂S₃ at 973 K. The compounds are isostructural and crystallize in a new structure type in space group Pnma of the orthorhombic system with four formula units in cells of dimensions at 153 K of a=15.763(3), b=4.0965(9), c=18.197(4) Å, V=1175.0(4) Å³ for Cs₂Bi₂ZnS₅; a=15.817(2), b=4.1782(6), c=18.473(3)  Å, V=1220.8(3)  ų for Cs₂Bi₂CdS₅; and a=15.830(2), b=4.1515(5), c=18.372(2) Å, V=1207.4(2) Å³ for Cs₂Bi₂MnS₅. The structure is composed of two-dimensional _∞²[Bi₂MS₅²⁻] (M=Zn, Cd, Mn) layers that stack perpendicular to the [100] axis and are separated by Cs⁺ cations. The layers consist of edge-sharing _∞¹[Bi₂S₆⁶⁻] and _∞¹[MS₃⁴⁻] chains built from BiS₆ octahedral and MS₄ tetrahedral units. Two crystallographically unique Cs atoms are coordinated to S atoms in octahedral and monocapped trigonal prismatic environments. The structure of Cs₂Bi₂MS₅, is related to that of Na₂ZrCu₂S₄ and those of the AMM′Q₃ materials (A=alkali metal, M=rare-earth or Group 4 element, M′= Group 11 or 12 element, Q=chalcogen). First-principles theoretical calculations indicate that Cs₂Bi₂ZnS₅ and Cs₂Bi₂CdS₅ are semiconductors with indirect band gaps of 1.85 and 1.75 eV, respectively. The experimental band gap for Cs₂Bi₂CdS₅ is ≈1.7 eV, as derived from its optical absorption spectrum.     

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.