Structural behavior and thermoelectric properties of the brownmillerite system Ca2(ZnxFe2−x)O5

The Ca₂(Zn_xFe_2−x)O₅ series was synthesized and characterized to determine the influence of zinc dopant on the brownmillerite structure for thermoelectric applications. All single-phase compounds exhibited Pnma symmetry at room temperature up to the solubility limit at x=0.10. High-temperature X-ray powder diffraction was used to show that the nature of the Pnma-Imma(0 0 γ)s00 transition in Ca₂Fe₂O₅ is modified by the presence of zinc. While the Zn-free composition transitions to an incommensurate phase, the Zn-containing phases transition instead to a commensurate phase, Imma(0 0 γ)s00 with γ=1/2. Both the Néel temperature and the onset temperature of the Pnma-Imma(0 0 γ)s00 phase transition decreased with increasing zinc concentration. Rietveld analysis of the in situ diffraction pattern for the x=0 sample at 1300 °C demonstrates that the structure contains statistically disordered chain orientations as described by space group Imma. Thermoelectric properties were analyzed in air from 100 to 800 °C. The positive Seebeck coefficient revealed hole-type conduction for all compositions. Doped samples exhibited electrical conductivities up to 3.4 S/cm and thermal conductivity of 1.5 W/mK. Transport analysis revealed thermally activated mobility consistent with polaron conduction behavior for all compositions.     

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.     

Bi2/3Ce1/3Rh2O5: A new mixed-valent Rh oxide with hitherto unknown structure

The new compound Bi_2/3Ce_1/3Rh₂O₅ has been discovered. It is currently the only known compound in the Bi-Ce-Rh-O system, and it crystallizes in a previously unknown structure type. The structure was established from single crystal X-ray diffraction data. Interatomic distances indicate the oxidation states as Bi_2/3³⁺Ce_1/3⁴⁺Rh₂^3.33+O₅. The structure indicates no ordering between Rh³⁺ and Rh⁴⁺. The lack of charge ordering is consistent with the metallic properties determined from electrical conductivity, Seebeck coefficient, and magnetic susceptibility measurements.     

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.     

Improvement of thermoelectric properties of alkaline-earth hexaborides

Thermoelectric (TE) and transport properties of alkaline-earth hexaborides were examined to investigate the possibility of improvement in their TE performance. As carrier concentration increased, electrical conductivity increased and the absolute value of the Seebeck coefficient decreased monotonically, while carrier mobility was almost unchanged. These results suggest that the electrical properties of the hexaboride depend largely on carrier concentration. Thermal conductivity of the hexaboride was higher than 10 W/m K even at 1073 K, which is relatively high among TE materials. Alloys of CaB₆ and SrB₆ were prepared in order to reduce lattice thermal conductivity. Whereas the Seebeck coefficient and electrical conductivity of the alloys were intermediate between those of CaB₆ and SrB₆ single phases, the thermal conductivities of the alloys were lower than those of both single phases. The highest TE performance was obtained in the vicinity of Ca_0.5Sr_0.5B₆, indicating that alloying is effective in improving the performance.     

High-temperature thermoelectric studies of A11Sb10 (A=Yb, Ca)

Large samples (6-8 g) of Yb₁₁Sb₁₀ and Ca₁₁Sb₁₀ have been synthesized using a high-temperature (1275-1375 K) flux method. These compounds are isostructural to Ho₁₁Ge₁₀, crystallizing in the body-centered, tetragonal unit cell, space group I4/mmm, with Z=4. The structure consists of antimony dumbbells and squares, reminiscent of Zn₄Sb₃ and filled Skutterudite (e.g., LaFe₄Sb₁₂) structures. In addition, these structures can be considered Zintl compounds; valence precise semiconductors with ionic contributions to the bonding. Differential scanning calorimetry (DSC), thermogravimetry (TG), resistivity (ρ), Seebeck coefficient (α), thermal conductivity (κ), and thermoelectric figure of merit (zT) from room temperature to at minimum 975 K are presented for A₁₁Sb₁₀ (A=Yb, Ca). DSC/TG were measured to 1400 K and reveal the stability of these compounds to ∼1200 K. Both A₁₁Sb₁₀ (A=Yb, Ca) materials exhibit remarkably low lattice thermal conductivity (∼10 mW/cm K for both Yb₁₁Sb₁₀ and Ca₁₁Sb₁₀) that can be attributed to the complex crystal structure. Yb₁₁Sb₁₀ is a poor metal with relatively low resistivity (1.4 mΩ cm at 300 K), while Ca₁₁Sb₁₀ is a semiconductor suggesting that a gradual metal-insulator transition may be possible from a Ca_11−xYb_xSb₁₀ solid solution. The low values and the temperature dependence of the Seebeck coefficients for both compounds suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.     

Crystallographic and magnetic characterisation of the brownmillerite Sr2Co2O5

Sr₂Co₂O₅ with the perovskite-related brownmillerite structure has been synthesised via quenching, with the orthorhombic unit cell parameters a=5.4639(3) Å, b=15.6486(8) Å and c=5.5667(3) Å based on refinement of neutron powder diffraction data collected at 4 K. Electron microscopy revealed L-R-L-R-intralayer ordering of chain orientations, which require a doubling of the unit cell along the c-parameter, consistent with the assignment of the space group Pcmb. However, on the length scale pertinent to NPD, no long-range order is observed and the disordered space group Imma appears more appropriate. The magnetic structure corresponds to G-type order with a moment of 3.00(4) μ_B directed along [1 0 0].     

Oxygen-ion and electron conductivity in Sr2(Fe1−xGax)2O5

High-temperature electrical conductivity measurements, structural data from powder X-ray diffraction and ⁵⁷Fe Mössbauer spectroscopy were combined to study the interrelationship of oxygen ion transport and p- and n-type transport in Sr₂(Fe_1−xGa_x)₂O₅, where x=0, 0.1 and 0.2. Although gallium substitution generally decreases the total ion-electron transport, the transition of the orthorhombic brownmillerite structure to a cubic phase on heating results in the recurrence of the conductivity to the same high level as in the parent ferrite (x=0). The changes of the partial contributions to the total conductivity as a function of x are shown to reflect a complicated interplay of the disordering processes that develop in the oxygen sublattice on heating in response to replacement of iron with gallium.     

PrCo2Al8 and Pr2Co6Al19: Crystal structure and electronic properties

The praseodymium cobalt aluminides, PrCo₂Al₈ and Pr₂Co₆Al₁₉, were prepared by reaction of the elemental components in an arc-melting furnace, followed by heat treatment at 900 °C for several days. Their chemical composition was checked by scanning electron microscopy and energy dispersive spectroscopy, and their crystal structure was refined from single crystal X-ray diffraction data. PrCo₂Al₈ adopts the CaCo₂Al₈ type of structure, crystallizing with the orthorhombic space group Pbam, with four formula units in a cell of dimensions at room temperature: , , . Pr₂Co₆Al₁₉ crystallizes in the monoclinic space group C2/m, with four formula units in a cell of dimensions at room temperature: , , and β=103.903(1)°. Its structure belongs to the U₂Co₆Al₁₉ type. The crystal structures of both compounds studied can be viewed as three-dimensional structures resulting from the packing of Al polyhedra centred by the transition elements. Along the c-axis, the coordination polyhedra around the Pr atoms pack by face sharing to form strands, which are separated one from another by an extended Co-Al network. Magnetic measurements have revealed that PrCo₂Al₈ orders antiferromagnetically at , with a clear metamagnetic transition occurring at a critical field H_c=0.9(1) T. The temperature dependence of the susceptibility of Pr₂Co₆Al₁₉ does not provide any evidence for long-range magnetic ordering in the temperature domain 1.7-300 K. At low temperatures (T<10 K), the susceptibility saturates in a manner characteristic of a non-magnetic singlet ground state. At high temperatures, the magnetic susceptibility of each compound follows a Curie-Weiss law, with the effective magnetic moment per Pr atom of 3.48(5)μ_B and 3.41(2)μ_B for PrCo₂Al₈ and Pr₂Co₆Al₁₉, respectively. These values are close to the theoretical value of 3.58μ_B expected for a free Pr³⁺ ion and exclude any contribution due to the Co atoms. Both compounds exhibit in the temperature range 5-300 K metallic-like electrical conductivity, and their Seebeck coefficient is of the order of several μV/K.     

Structural and magnetic properties of perovskite Ca3Fe2WO9

A complex perovskite with composition Ca₃Fe₂WO₉ has been synthesised, and the temperature evolution of nuclear and magnetic structures investigated by neutron powder diffraction. It was shown that at room temperature this compound adopts a monoclinic perovskite structure belonging to space group P12₁/n1 (, , ), β=90.04(2)°). The partial B-site ordering, of the Fe⁺³ and W⁺⁶ cations, at (2c) and (2d) sites was determined. At low temperatures the magnetic diffraction peaks were registered and a possible model for the magnetic structure was proposed in accordance with the ferrimagnetic properties of the title compound. The magnetic structure is defined by a propagation vector k=(1/2,1/2,0) and can be described as an array of ferromagnetic (20−1) layers, which couple antiferromagnetically to each other. All the Fe moments within a layer are aligned parallel (or anti-parallel) to the c-axis. The structural and magnetic features of this compound are discussed and compared with those of some other quaternary oxides A₃Fe₂WO₉ (A=Ba, Sr, Pb).     

Crystal structure and electronic properties of the new compounds, U6Fe16Si7 and its interstitial carbide U6Fe16Si7C

The new compounds U6Fe16Si7 and U6Fe16Si7C were prepared by arc-melting and subsequent annealing at 1500 °C. Single-crystal X-ray diffraction showed that they crystallize in the cubic space group (No. 225), with unit-cell parameters at room temperature a=11.7206(5) Å for U6Fe16Si7 and a=11.7814(2) Å for U6Fe16Si7C. Their crystal structures correspond to ordered variants of the Th6Mn23 type. U6Fe16Si7 adopts the Mg6Cu16Si7 structure type, whereas U6Fe16Si7C crystallizes with a novel “filled” quaternary variant. The inserted carbon is located in octahedral cages formed by six U atoms, with U-U interatomic distances of 3.509(1) Å. Insertion of carbon in the structure of U6Fe16Si7 has a direct influence on the U-Fe and Fe-Fe interatomic distances. The electronic properties of both compounds were investigated by means of DC susceptibility, electrical resistivity and thermopower. U6Fe16Si7 is a Pauli paramagnet. Its electrical resistivity and thermopower point out that it cannot be classified as a simple metal. The magnetic susceptibility of U₆Fe₁₆Si₇C is best described over the temperature range 100-300 K by using a modified Curie-Weiss law with an effective magnetic moment of 2.3(2) μ_B/U, a paramagnetic Weiss temperature, θ_p=57(2) K and a temperature-independent term χ₀=0.057(1) emu/mol. Both the electrical resistivity and thermopower reveal metallic behavior.     

Synthesis, structure and physical properties of reduced barium titanate Ba2Ti13O22

Polycrystalline sample of the reduced barium titanate Ba₂Ti₁₃O₂₂ was synthesized by solid state reaction at 1523 K in Ar atmosphere for the first time. The Rietveld refinement using the powder X-ray diffraction data confirmed the sample to be main phase of Ba₂Ti₁₃O₂₂ having the orthorhombic crystal system, space group Bmab and the lattice parameters of a=11.67058(11) Å, b=14.12020(13) Å and c=10.06121(9) Å, and V=1657.995(20) ų. The valence state of Ti was evaluated by both the Ti–O bond distance analysis and the Ti K-edge XANES analysis. The magnetic susceptibility was nearly temperature independent in the range of 60–300 K, suggesting the Van Vleck Paramagnetism. The electrical conductivity at 300 K is approximately 320.50 S/cm, and a semiconducting behavior was observed below room temperature. The Seebeck coefficient showed a negative value of −1.25 μV/K at 300 K, indicating n-type behavior. These facts were confirmed by the results of the present theoretical calculations by the FLAPW method.     

Synthesis and high-temperature thermoelectric properties of Ni and Ti substituted LaCoO3

The effect of Ti and Ni substitution in LaCoO_3−δ was investigated by means of electrical resistivity and Seebeck coefficient properties in a broad temperature range. The studied compounds crystallize in a rhombohedral crystal structure within the whole substitution range. The Seebeck coefficient of most of the studied compounds is positive indicating predominant hole-type charge carriers. The electrical resistivity decreases with increasing temperature for all compositions. Increasing the Ni content results in a decrease of the electrical resistivity, while the resistivity increases with increasing Ti content. The power factor, PF, for the Ni substituted samples is PF=1.42×10⁻⁴ W/m² K for x=0.10 at and decreases with temperature. The LaCo_1−xTi_xO_3±δ compounds reveal an enhancement of the power factor with increasing temperature. Ti substitution leads to a higher power factor compared to that of Ni substitution at .     

Structural, electrical and reflectance studies of the system ZnMn2-2xNixTixO4

X-ray, electrical conductivity and reflectance studies of the system ZnMn_2-2xNi_xTi_xO₄ have been carried out. The system is tetragonal in the range 0 ≤x ≤ 0.25 and cubic in the range 0.5 ≤x ≤ 1. Electrical resistivity temperature behaviour obeys Wilson’s law for all the compounds and the thermoelectric coefficient values vary between 325 and-290 μV/K. The activation energy and p_RT decrease gradually with increase in concentration of charge carriers atB-site except for ZnNiTiO₄. Reflectance spectral studies indicate the presence of Ni²⁺ at the octahedral site.     

Heavy doping of H ion in 12CaO·7Al2O3

12CaO·7Al₂O₃ (C12A7, mayenite), which has a nanoscale porous structure that can accommodate extraframework species such as hydride (H⁻), oxide (O²⁻), hydroxide (OH⁻) ions, and electrons, has been doped with H⁻ ions to investigate its effects as dominant extraframework species. Chemical doping with CaH₂ enables the concentration of H⁻ ions to reach almost the theoretical maximum. The concentration of H⁻ ions is characterized by optical absorption intensity ascribed to photoionization of H⁻ ions, and ¹H magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Persistent electron generation, which is accompanied by the formation of an F⁺ absorption band and electrical conductivity, by irradiation with ultraviolet light at room temperature increases as the H⁻ ion doping increases until it reaches half the theoretical maximum and then decreases as the H⁻ ion concentration increases further. This dependence indicates that both H⁻ and O²⁻ ions are necessary for the generation of persistent electrons.     

Thermal properties of the pyrochlore, Y2Ti2O7

The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y₂Ti₂O_7, have been determined over the temperature range 2 K?T?300 K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θ_D, of ca. 970 K) and an assignment of the remaining 63 optic modes, as well as a correction for C_p−C_v. From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15 K)−H₀=34.69 kJ mol⁻¹ and S(T=298.15 K)−S₀=211.2 J K⁻¹ mol⁻¹. The thermal conductivity shows a peak at ca. θ_D/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10 μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y₂Ti₂O₇ is 2.8 W m⁻¹ K⁻¹, close to the calculated theoretical thermal conductivity, κ_min, for fully coupled phonons at high temperatures.     

Synthesis, crystal structure and magnetic properties of the Sr2Al0.78Mn1.22O5.2 anion-deficient layered perovskite

A new layered perovskite Sr₂Al_0.78Mn_1.22O_5.2 has been synthesized by solid state reaction in a sealed evacuated silica tube. The crystal structure has been determined using electron diffraction, high-resolution electron microscopy, and high-angle annular dark field imaging and refined from X-ray powder diffraction data (space group P4/mmm, a=3.89023(5) Å, c=7.8034(1) Å, R_I=0.023, R_P=0.015). The structure is characterized by an alternation of MnO₂ and (Al_0.78Mn_0.22)O_1.2 layers. Oxygen atoms and vacancies, as well as the Al and Mn atoms in the (Al_0.78Mn_0.22)O_1.2 layers are disordered. The local atomic arrangement in these layers is suggested to consist of short fragments of brownmillerite-type tetrahedral chains of corner-sharing AlO₄ tetrahedra interrupted by MnO₆ octahedra, at which the chain fragments rotate over 90°. This results in an averaged tetragonal symmetry. This is confirmed by the valence state of Mn measured by EELS. The relationship between the Sr₂Al_0.78Mn_1.22O_5.2 tetragonal perovskite and the parent Sr₂Al_1.07Mn_0.93O₅ brownmillerite is discussed. Magnetic susceptibility measurements indicate spin glass behavior of Sr₂Al_0.78Mn_1.22O_5.2. The lack of long-range magnetic ordering contrasts with Mn-containing brownmillerites and is likely caused by the frustration of interlayer interactions due to presence of the Mn atoms in the (Al_0.78Mn_0.22)O_1.2 layers.     

The effect of Ag addition on electrical properties of the thermoelectric compound Ca3Co4O9

Ca₃Co₄O₉/Ag composites incorporating different amounts of Ag were synthesized by solid-state reaction. Scanning electron microscopy revealed Ag particles dispersed among and combined with Ca₃Co₄O₉ grains several times larger in size. The electrical resistivity (ρ) of the composites is favorably lower than that of Ca₃Co₄O₉ alone and decreases with increasing Ag content. It can thus be inferred that the highly conductive Ag particles between the oxide grains contribute to the reduction of ρ. Although minimal in smaller amounts, the addition of Ag also seems to have a negative impact on the Seebeck coefficient (S) of the composites due to its poor S. Since the reduction of ρ is more significant than the degradation of S, the power factor is found to be improved by the addition of 10 wt% Ag.     

Magnetic properties of Ca2F2-xMnxO5

Magnetic susceptibility of Ca₂F_2-xMn_xO₅ members crystallizing in two different structures, one having octahedral (O), tetrahedral (T) and square-pyramidal (SP) coordination of transition metal atoms (OTSP structure) and the other having octahedral and tetrahedral coordination (OT structure), has been investigated. Susceptibility behaviour of the oxides with OTSP structure is different from that of the oxides with OT structure. Ca₂Fe_1-33Mn_0-67O₅ with OTSP structure shows an antiferromagnetic ordering while the corresponding oxide with OT structure shows weak ferromagnetism. Contribution No. 398 from the Solid State and Structural Chemistry Unit     

Synthesis, structure, and magnetic and electronic properties of Cs2Hg2USe5

The compound Cs₂Hg₂USe₅ was obtained from the solid-state reaction of U, HgSe, Cs₂Se₃, Se, and CsI at 1123 K. This material crystallizes in a new structure type in space group P2/n of the monoclinic system with a cell of dimensions a=10.276(6) Å, b=4.299(2) Å, c=15.432(9) Å, β=101.857(6) Å, and V=667.2(6) Å³. The structure contains layers separated by Cs atoms. Within the layers are distorted HgSe₄ tetrahedra and regular USe₆ octahedra. In the temperature range of 25-300 K Cs₂Hg₂USe₅ displays Curie-Weiss paramagnetism with μ_eff=3.71(2) μ_B. The compound exhibits semiconducting behavior in the [010] direction; the conductivity at 298 K is 3×10⁻³ S/cm. Formal oxidation states of Cs/Hg/U/Se may be assigned as +1/+2/+4/− 2, respectively.