Thermoelectric properties of homologous p- and n-type boron-rich borides

Thermoelectric properties of a series of layered homologous rare-earth boron carbonitrides: HoB₁₇CN, REB₂₂C₂N (RE=Y,Er,Lu), and YB_28.5C₄, were investigated. Samples for measurements were prepared in the form of hot pressed or isostatically pressed and annealed single phase polycrystalline powder. This series of compounds has structures where B₆ octahedral and rare-earth atomic layers reside between an increasing number of B₁₂ icosahedral and C-B-C chain layers, and has structural analogy to boron carbide. Interestingly, a variation from p-type thermoelectric behavior for YB_28.5C₄ to n-type for REB₂₂C₂N and HoB₁₇CN was observed. This is the first non-doped compound among the boron-rich borides in which n-type thermoelectric behavior has been observed. Similar to other boron cluster compounds low values of the thermal conductivity κ were found. The origins of the low κ in such compounds has not been fully explained, but comparison among the homologous series shows that the thermal conductivity appears to increase as the number of boron cluster layers increases. This result indicates that the heavy rare-earth atoms residing in the boron matrix may play a role in depressing thermal conductivity in addition to other features common to boron cluster compounds. Although the absolute values of the determined figures of merit ZT are not large for hot pressed samples, the Seebeck coefficients and power factors for both n-type and p-type in this series show an increase at temperatures exceeding 1000 K.     

Effect of transition element doping on crystal structure of rare earth borosilicides REB44Si2

On a previous study on samples of doped-YB₄₄Si₂, an improvement of thermoelectric properties has been achieved. Regarding the interesting effect of the doping of transition elements on the thermoelectric properties, a single crystal study has been carried out on Zn doped, Rh doped and Ni doped samples to assess how the transition element doping affects the crystal structure. Refinements were carried out based on the structural model solution of YB₄₄Si₂ reported in a previous study. Variations in the silicon contents were found in the doped single crystals. Splitting of partially occupied sites has also been detected for some of the doped samples. In this paper we present differences in the partial occupations of boron and silicon sites. Possibility of transition elements insertions based on the differences in crystal structures will be presented.     

Solar photocatalytic activities of porous Nb-doped TiO2 microspheres prepared by ultrasonic spray pyrolysis

Ultrasonic spray pyrolysis method was used to prepare Nb-doped TiO₂ porous microspheres with an average diameter of 500 nm for solar photocatalytic applications. The effect of Nb-doping on morphology, structure, surface area, as well as spectral absorption properties of TiO₂ microspheres was investigated with SEM, TEM, XRD, Raman spectra, BET, and UV-Vis absorption spectra. The Nb-doping decreased the grain size of TiO₂ porous microsphere, and influenced its surface area and pore size distribution dependent on the doping concentration, but changed negligibly the morphology and size of TiO₂ microspheres. Moreover, the Nb-doping was observed to extend the spectral absorption of TiO₂ into visible spectrum, and the absorption onset was red-shifted for about 88 nm at a doping level of 5% compared to pristine TiO₂ microspheres. Under solar or visible irradiation, Nb-doped TiO₂ microspheres showed higher photocatalytic activity for methylene blue degradation compared with TiO₂ microspheres, which could be ascribed to the extended light absorption range and the suppression of electron-hole pair recombination.     

Synthesis, structure and properties of the new rare-earth Zintl phase Yb11GaSb9

A new rare-earth rich Zintl phase Yb₁₁GaSb₉ was synthesized by direct fusion of the corresponding elements, and large single crystals of the compound were obtained from high temperature flux synthesis. Its crystal structure was determined by single-crystal X-ray diffraction to be orthorhombic in the non-centrosymmetric space group Iba2 (No. 45), Z=4 (R₁=3.24%, wR₂=6.40%) with , , measured at 90(3) K. The structure belongs to the Ca₁₁InSb₉-type and can be viewed as built of isolated Sb₄-tetrahedra centered by Ga, Sb-dimers and isolated Sb anions, which are separated by Yb²⁺ cations. Electron count according to the Zintl formalism suggests that the phase is electron-precise and charge-balanced, which is supported by the virtually temperature-independent magnetization for Yb₁₁GaSb₉. Electrical resistivity data from 2 to 400 K confirm that Yb₁₁GaSb₉ is a small band-gap semiconductor with room temperature resistivity , and low-temperature resistivity at 2 K . As such, Yb₁₁GaSb₉ and related compounds might be promising materials for thermoelectric applications, and currently, efforts to synthesize new members of this family and test their thermoelectric performance are under way.     

The low-temperature form of Rb2KCrF6 and Rb2KGaF6: The first example of an elpasolite-derived structure with pentagonal bipyramid in the B-sublattice

The crystal structure of the low-temperature forms of Rb₂KCrF₆ and Rb₂KGaF₆ has been solved on single crystal. The symmetry is tetragonal with F4/m space group; the unit cell parameters are: , for Rb₂KCrF₆ at and , for Rb₂KGaF₆ at . The relationships between the parameters of the prototype cubic elpasolite, which is stable at high temperature, and the tetragonal superlattice of the low temperature form have been established. Considering the general formulation A₂BB′F₆, the cationic positions in the A and (B,B′) sublattices remain identical in the two allotropic varieties. The main originality of the structure concerns the environment of 4/5 of the potassium atoms (B sublattice) which is transformed from octahedra into pentagonal bipyramids sharing edges with adjacent B′F₆ octahedra containing Cr or Ga. The displacive phase transition is simply explained by the rotation of 45° in the (a,b) plane of 1/5 of the B′F₆ (B′=Cr, Ga) octahedra. The similarity of this phase transition and the transformation of perovskite into tetragonal tungsten bronze (TTB) will be discussed.     

Large magnetoresistance in a ferromagnetic cobaltite: The 12H Ba0.9CoO2.6

The magnetic and transport properties of a hexagonal cobaltite related to the perovskite structure have been studied. By combining transmission electron microscopy, X-ray powder diffraction and iodometric titration, it is found that Ba_0.9CoO_2.6 crystallizes in the 12H structure [P6₃/mmc, a=5.6612 (1) Å and c=28.4627(8) Å]. Interestingly, this compound is a ferromagnet with a Curie temperature T_C=50 K and a saturation magnetization . This value is smaller than expected from the effective paramagnetic moment, μ_eff=3.7μ_B/Co, corresponding to an average spin per Co, from which one would expect . This suggests either a canted structure or a strong local magnetic anisotropy related to the crystal field of the CoO_n polyhedra. A clear transition in the electrical resistivity is found at T_C consistent with a spin scattering reduction as the sample becomes ferromagnetic. The spin-charge coupling is evidenced by the large negative magnetoresistance effect optimum near T_C=50 K, with .     

Structures, phase transitions and microwave dielectric properties of the 6H perovskites Ba3BSb2O9, B=Mg, Ca, Sr, Ba

We present a complete temperature-composition phase diagram for Ba₃BSb₂O₉, B=Mg, Ca, Sr, Ba, along with their electrical behavior as a function of B. These compounds have long been recognized as 6H-type perovskites, but (with the exception of B=Mg) their exact structures and properties were unknown due to their low symmetries, temperature-dependent phase transitions, and difficulties in synthesizing pure samples. The full range of possible space group symmetries is observed, from ideal hexagonal P6₃/mmc to monoclinic C2/c to triclinic . Direct second-order transitions between these phases are plausible according to group theory, and no evidence was seen for any further intermediate phases. The phase diagram with respect to temperature and the effective ionic radius of B is remarkably symmetrical for B=Mg, Ca, and Sr. For B=Ba, a first-order phase transition to a locally distorted phase allows a metastable hexagonal phase to persist to lower temperatures than expected before decomposing around 600 K. Electrical measurements revealed that dielectric permittivity corrected for porosity does not change significantly as a function of B and is in a good agreement with the values predicted by the Clausius-Mossotti equation.     

X-ray powder diffraction studies and thermal behaviour of NaK2B9O15, Na(Na.17K.83)2B9O15, and (Na.80K.20)K2B9O15

The crystal structures of NaK₂B₉O₁₅ (, , , β=94.080(1)°, R_p=0.047, R_wp=0.059, R_B=0.026), Na(Na_.17K_.83)₂B₉O₁₅ (, , , β=94.228(2)°, R_p=0.053, R_wp=0.068, R_B=0.026), and (Na_.80K_.20)K₂B₉O₁₅ (, , , β=94.071(1)°, Z=4, R_p=0.041, R_wp=0.052, R_B=0.023) were refined in the monoclinic space groups P2₁/c(Z=4) using X-ray powder diffraction data and the Rietveld method. These nonaborates are isostructural to K₃B₉O₁₅. Their crystal structure consists of a three-dimensional open framework built up from three crystallographically independent triborate groups. The alkali metal cations are located on three different sites in the voids of the framework. High-temperature X-ray diffraction studies show that NaK₂B₉O₁₅ decomposes at about 700 °C in accordance with the peritectic reaction NaK₂B₉O₁₅↔K₅B₁₉O₃₁+liquid. The thermal expansion of NaK₂B₉O₁₅ and Na(Na_.17K_.83)₂B₉O₁₅ is highly anisotropic. A similarity of the thermal and compositional (Na-K substitution) deformations of NaK₂B₉O₁₅ is revealed: heating of NaK₂B₉O₁₅ by 1 °C leads to the same deformations of the crystal structure as increasing the amount of K atoms in (Na_1−xK_x)₃B₉O₁₅ by 0.04 at% K.     

The effect of a cation radii on structural, magnetic and electrical properties of doped manganites La0.6−xPrxSr0.4MnO3

Structural, magnetic and transport properties of La_0.6−xPr_xSr_0.4MnO₃ with x=0.0, 0.03, 0.06, 0.18, 0.3, 0.42, 0.54 and 0.6 are studied. The system exhibits a rhombohedrally distorted perovskite structure for x?0.3. A rhombohedral-orthorhombic (Pnma) structure transition is detected in the doping range from x=0.42 to 0.6. The structure refinement by Rietveld analysis of the X-ray powder diffraction data shows that the average distance Mn-O increases in the rhombohedral phases and decreases in the orthorhombic phases. Results show that the Curie temperature decreases from 374 to 310 K when 〈r_A〉 varies from 1.254 to 1.231 Å. Electrical measurements show that all samples exhibit a metallic to semiconducting transition with increasing temperature. Meanwhile, the size of the resistivity ρ increases near T_C. This phenomenon is interpreted as a gradual bending of the Mn-O-Mn bond angle, with decreasing 〈r_A〉, which causes the narrowing of the electronic bandwidth and the effect of the A-site variance σ².     

A Raman spectroscopic study of the phase transition of BaZr(PO4)2: Evidence for a trigonal structure of the high-temperature polymorph

We have studied the structural evolution of monoclinic BaZr(PO₄)₂ during heating up to 835 K by Raman spectroscopy. In agreement with previous studies we found a first-order phase transition at about 730 K during heating while upon cooling the reverse transition occurs at 705 K. However, some disagreement about the crystal structure of the high-temperature polymorph occurs in the literature. While the space group has not yet been determined, the X-ray diffraction pattern of the high-temperature phase has been indexed on either an orthorhombic or a hexagonal unit cell. We found that the number of Raman active internal PO₄ vibrational modes decrease from nine to six during the transition. A group theoretical survey through all orthorhombic, trigonal, and hexagonal factor groups revealed that the observed number of vibrations would only be consistent with the Ba and Zr atoms located at a site, the P and two O atoms at a C_3v(3m), and six O atoms at a C_s(m) site in the D_3d factor group. Based on our Raman data, the space group of the high-temperature polymorph is thus either , , or .     

Crystal structures, physical properties and NMR experiments on the ternary rare-earth metal silicide boride compounds RE5Si2B8 (RE=Y, Sm, Gd, Tb, Dy, Ho)

The ternary rare-earth metal silicide borides RE₅Si₂B₈ (RE=Y, Sm, Gd, Tb, Dy Ho) were prepared by arc melting the elemental components and subsequent annealing up to . The crystal structure was determined for each term of the series from single-crystal X-ray data: tetragonal symmetry, space group P4/mbm, Z=2; unit cell parameters a=7.2616(3), and a=7.1830(2), for Sm₅Si₂B₈ and Ho₅Si₂B₈, respectively. The structure is a new type and can be structurally described as an intergrowth of ThB₄-like and U₃Si₂-like slabs of composition REB₄ and RE₃Si₂, respectively, alternating along the c direction. The boron and silicon substructures are wholly independent and well ordered. The magnetic properties are as follows: Y₅Si₂B₈ is a Pauli-type paramagnet above 1.8 K, Gd₅Si₂B₈ undergoes a weak (canted) ferromagnetic-like order at 70 K followed by a colinear antiferromagnetic spin alignment at 44 K. Tb₅Si₂B₈ and Dy₅Si₂B₈ order antiferromagnetically at a Néel temperature of T_N=45 and 28 K, respectively. In the paramagnetic regime, the effective moments are in good accord with the theoretical RE³⁺ free ion moments. The temperature dependence of the electrical resistivities for the Y, Gd, Tb, and Dy containing samples corroborates with the metallic state of the nonmagnetic (Y) and the magnetically ordered compounds. ¹¹B, ²⁹Si and ⁸⁹Y nuclear magnetic resonance (NMR) spectroscopy on nonmagnetic Y₅Si₂B₈ shows different signals, which correspond to the expected number of distinct crystallographic sites in the structure. ¹¹B NMR on Y₅Si₂B₈ indicates that the local magnetic susceptibilities are substantially different from the ones observed in the related compound YB₄.     

Physical properties of layered homologous RE-B-C(N) compounds

Physical properties of a series of homologous RE-B-C(N) B₁₂ cluster compounds REB₁₇CN, REB₂₂C₂N, and were investigated. The structures of the compounds are layer-like along the c-axis, with rare earth and B₆ octahedral layers separated by B₁₂ icosahedral and C-B-C chain layers whose number increases successively from two B₁₂ layers for the REB₁₇CN compound to four for the REB_28.5C₄ compound. The rare earth atoms are configured in two triangular flat layers which are stacked on top of one another in AB stacking where the nearest-neighbor rare earth directions are the three atoms forming a triangle in the adjacent layer. The series of homologous compounds exhibit a spin glass transition with T_f shifting in correspondence with variations of the basal plane lattice constants, consistent with the magnetic interaction being effective in the basal planes. The isothermal remanent magnetization shows a stretched exponential decay . Exponents determined for the different homologous compounds were scaled as a function of T_r=T/T_f and found to follow the empirical dependency determined for typical spin glasses. It is indicated that a mixture of disorder originating from the partial occupancy of the rare earth sites and frustration of interactions due to the unique configuration is responsible for the manifestation of spin glass transitions in these homologous systems.     

Ag3Ni2O4—A new stage-2 intercalation compound of 2H-AgNiO2 and physical properties of 2H-AgNiO2 above ambient temperature

Ag₃Ni₂O₄ was obtained as single crystals from a mixture of 2H-AgNiO₂ and Ag₂O in oxygen high-pressure autoclaves (P6₃/mmc (no. 194), a=2.9331(6), c=28.313(9) Å, Z=2). It may be regarded as a stage-2 intercalation compound of the host 2H-AgNiO₂ and is the first staging compound constituted of alternating subvalent and Ag⁺ sheets, inserted between NiO₂⁻ slabs. From a structural point of view, Ag₃Ni₂O₄ represents an intermediate between AgNiO₂ and the recently reported Ag₂NiO₂. The electronic structures of 2H-AgNiO₂ and Ag₃Ni₂O₄ have been investigated based on DFT band structure calculations. The high-temperature characteristics of the starting material 2H-AgNiO₂ were investigated. The inverse magnetic susceptibility, electrical resistivity and differential scanning calorimetry (DSC) show a phase transition in the temperature range of .     

Thermoelectric and structural properties of a new Chevrel phase: Ti0.3Mo5RuSe8

The new Chevrel phase Ti_0.3Mo₅RuSe₈ has been synthesized and characterized by quantitative microprobe analysis, powder X-ray diffraction, and high-temperature thermoelectric properties measurements. The thermoelectric properties of this compound are compared to the previously reported data for other related Chevrel phases. We report also the results of Rietveld analysis of powder X-ray diffraction data for Ti_0.3Mo₅RuSe₈. This compound adopts the rhombohedral Chevrel phase structure (space group , Z=3) with hexagonal lattice constants a=9.75430(25) Å and c=10.79064(40) Å. The low level of incorporation and low scattering power of Ti precluded the identification of the Ti positions, and Rietveld refinement was carried out only for the Mo₅RuSe₈ framework of Ti_0.3Mo₅RuSe₈ (R_p=10.5%, R_wp=14.6%). Rietveld analysis was also used to refine the structure of the unfilled phase Mo₅RuSe₈ (, Z=3, a=9.63994(8) Å, c=10.97191(11) Å, R_p=8.0%, R_wp=10.5%). Comparisons between the two structures are made.     

Crystal growth, transport, and magnetic properties of Ln3Co4Sn13 (Ln=La, Ce) with a perovskite-like structure

Ln₃Co₄Sn₁₃ (Ln=La, Ce) have been synthesized by flux growth and characterized by single crystal X-ray diffraction. These compounds adopt the Yb₃Rh₄Sn₁₃-type structure and crystallize in the cubic space group (No. 223) with Z=2. Lattice parameters at 298 K are , , and , for the La and Ce analogues, respectively. The crystal structure consists of an Sn-centered icosahedron at the origin of the unit cell, which shares faces with eight Co trigonal prisms and 12 Ln-centered cuboctahedra. Magnetization data at 0.1 T show paramagnetic behavior down to 1.8 K for Ce₃Co₄Sn₁₃, with per Ce³⁺, while conventional type II superconductivity appears below 2.85 K in the La compound. Electrical resistivity and specific heat data for the La compound show a corresponding sharp superconducting transition at T_c∼2.85 K. The entropy and resistivity data for Ce₃Co₄Sn₁₃ show the existence of the Kondo effect with a complicated semiconducting-like behavior in the resistivity data. In addition, a large enhanced specific heat coefficient at low T with a low magnetic transition temperature suggests a heavy-fermionic character for the Ce compound. Herein, the structure and physical properties of Ln₃Co₄Sn₁₃ (Ln=La, Ce) are discussed.     

Synthesis and crystal structure of MgB12

Single crystals of MgB₁₂ were synthesized from the elements in a Mg/Cu melt at 1600 °C. MgB₁₂ crystallizes orthorhombic in space group Pnma with , and . The crystal structure (Z=30, 5796 reflections, 510 variables, R₁(F)=0.049, wR₂(I)=0.134) consists of a three dimensional net of B₁₂ icosahedra and B₂₁ units in a ratio 2:1. The B₂₁ units are observed for the first time in a solid compound. Mg is on positions with partial occupation. The summation reveals the composition MgB_12.35 or Mg_0.97B₁₂ , respectively. This is in good agreement with the value of MgB_11.25 as expected by electronic reasons to stabilize the boron polyhedra and .     

Structural investigation of Sr2LiReO6. Evidence for a continuous tetragonal-cubic phase transition

The presence of a continuous and reversible -I4/m phase transition in a polycrystalline sample of the ordered double perovskite Sr₂LiReO₆ is described. The transition that occurs near 300 °C is a consequence of in-phase tilting of the BO₆ octahedra. The temperature dependence of both the lattice parameters and the spontaneous strains are consistent with a second order phase transition as would be expected for a soft mode transition.     

Improving the cycling performance,rate capacity,and thermal stability of LiCoO<Subscript>2</Subscript> by doping high-valence ions into the Li<Superscript>+</Superscript> site

The poor rate performance and low discharge capacity of LiCoO₂ limit its applications. Therefore, in this work, we synthesized Li_1-xM_xCoO₂ (M = Na, Zr, Nb) via solid-phase synthesis to improve its properties. X-ray diffraction (XRD) results suggest that the doping elements were successfully doped into LiCoO₂. The electrochemical properties showed that the samples doped with the high-valence elements Zr and Nb had a higher capacity, better cycle stability, and better rate performance than those doped with the low-valence element Na. In particular, the capacity retention of LiCoO₂, Li_0.97Na_0.03CoO₂, Li_0.99Zr_0.01CoO₂, and Li_0.99Nb_0.01CoO₂ was 68, 42, 85, and 87%, respectively, after 80 cycles at a rate of 10 C at 55 °C. However, doping of Zr and Nb into the Li⁺ site of LiCoO₂ will reduce the content of Li⁺. And, less Li⁺ extracted in the cathode material resulting in low discharge capacity under low current density. The larger radius of Na⁺ is incorporated into the Li slab and enlarged the interlayer spacing of the (003) plane. The larger (003) interplanar spacing can significantly facilitate the lithium diffusion and is also favorable to the rate capability. The differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis results demonstrated that the Zr-doped and Nb-doped LiCoO₂ had a higher thermal stability in the charged state than the Na-doped LiCoO₂. Additionally, the resistances of the Zr-doped and Nb-doped electrodes were much lower than that of the undoped electrode. Our research results indicate that doping with high-valence elements is a very effective strategy for optimizing the electrochemical performance of LiCoO₂ and that this method can also be extended to other cathode materials.

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