Structure–composition sensitivity in “Metallic” Zintl phases: A study of Eu(Ga1−xTtx)2 (Tt=Si, Ge, 0≤x≤1)

Two isoelectronic series, Eu(Ga_1−xTt_x)₂ (Tt=Si, Ge, 0≤x≤1), have been synthesized and characterized by powder and single-crystal X-ray diffraction, physical property measurements, and electronic structure calculations. In Eu(Ga_1−xSi_x)₂, crystal structures vary from the KHg₂-type to the AlB₂-type, and, finally, the ThSi₂-type structure as x increases. The hexagonal AlB₂-type structure is identified for compositions 0.18(2)≤x<0.70(2) with Ga and Si atoms statistically distributed in the polyanionic 6³ nets. As smaller Si atoms replace Ga atoms while the number of valence electrons increases, the lattice parameters, unit cell volumes, and Ga–Si distances in this phase region decrease significantly. Although aspects of X-ray diffraction results suggest puckering of the 6³ nets for the Si-richest example of the AlB₂-type Eu(Ga_1−xSi_x)₂, the complete experimental evidence remains inconclusive. On the other hand, in Eu(Ga_1−xGe_x)₂, six different structural types were observed as x varies. In addition to EuGa₂ (KHg₂-type; space group Imma) and EuGe₂ (own structure type, space group P3¯m1), the ternary phases studied show four different structures: the AlB₂-type for Ga-rich compositions; the YPtAs-type structure for EuGaGe; and two new structures, which are intergrowths of the YPtAs-type EuGaGe and EuGe₂, for Ge-rich compositions. These two Ge-rich phases include: (1) Eu(Ga_0.45(2)Ge_0.55(2))₂ containing two YPtAs-type motifs of EuGaGe plus one EuGe₂ motif; and (2) Eu(Ga_0.40(2)Ge_0.60(2))₂ containing one YPtAs-type motif alternating with a split site at and z=0.4798(2) with ca. 50% site occupancy by Ga and Ge along the c-axis. Magnetic susceptibilities of three Eu(Ga_1−xGe_x)₂ compounds display Curie–Weiss behavior above ca. 100 K, and show effective magnetic moments indicative of divalent Eu with a 4f⁷ electronic configuration, consistent with. X-ray absorption spectra (XAS). Density of states (DOS) and crystal orbital Hamilton population (COHP) analyses, based on first principles electronic structure calculations, rationalize the observed homogeneity ranges of the AlB₂-type phases in both systems and the structural variations as a function of Tt content.     

High-pressure structural evolution of a perovskite solid solution (La1−x,Ndx)GaO3

The structural evolution with pressure of six perovskites in the system La_1−xNd_xGaO₃ with x=0.00, 0.06, 0.12, 0.20, 0.62 and 1.00 have been determined by single-crystal diffraction. At room pressure, all six samples have Pbnm symmetry. The room-pressure bulk moduli vary only slightly with composition, between K_0T=169(4) and 177(2) GPa, with . As pressure is increased there is significant compression of the octahedral Ga–O bonds, the tilts of the GaO₆ octahedra decrease and the structures evolve towards higher symmetry. At room conditions the average Ga–O bond length increases with increasing compositional parameter x. However, the GaO₆ become stiffer with increasing x; the Ga–O bonds thus become stiffer as they become longer. Bond strengths in the octahedra in perovskites are therefore not a simple function of bond lengths but depend also upon the extra-framework cation.Phase transitions to R-3c symmetry occur at 2.2 GPa in end-member LaGaO₃, at 5.5 GPa in the x=0.06 sample, at 7.8 GPa for x=0.12, and at 12 GPa for x=0.20. No evidence of the transition in the x=0.62 or 1.00 samples was found by X-ray diffraction to 9.4 or 8.0 GPa, respectively, or by Raman measurements of NdGaO₃ up to 16 GPa. The transition pressure therefore increases with increasing Nd content (increasing x) at approximately 0.45 GPa per 0.01 increment in x, at least up to x=0.20. Compression of the R-3c phase of LaGaO₃ above the transition results in no significant changes in the tilt angle of the octahedra. The structural behavior of all six samples at high pressures is the result of the GaO₆ octahedra being softer than the extra-framework (La, Nd)O₁₂ site. The results therefore demonstrate that the evolution of solid-solution perovskites at high pressures follow the same general principles recently elucidated for end-member compositions.     

Phase equilibria and crystal structure of the solid solution LaFe1−xNixO3−δ (0 ≤ x ≤ 1)

Phase equilibria in the LaFeO₃–“LaNiO₃” were studied at 1100 °C in air. The samples were synthesized by standard ceramic and/or solution route via nitrate or citrate precursors. According to the results of XRD it was found that the homogeneity ranges of LaFe_1−xNi_xO_3−δ solid solution lay within 0.0 ≤ x ≤ 0.4 (sp.gr. Pbnm) and 0.6 ≤ x ≤ 0.8 (sp.gr. ). The structural parameters (bond lengths, atom coordinates) for the single-phase samples were refined using Rietveld analysis. The unit cell parameters versus LaFe_1−xNi_xO_3−δ composition are presented.     

Synthesis, thermal stability and magnetic properties of the Lu1−xLaxMn2O5 solid solution

Polycrystalline samples of the Lu_1−xLa_xMn₂O₅ solid solution system were synthesized under moderate conditions for compositions with x up to 0.815. Due to the large difference in ionic size between Lu³⁺ and La³⁺, significant changes in lattice parameters and severe lattice strains are present in the solid solution. This in turn leads to the composition dependent thermal stability and magnetic properties. It is found that the solid solution samples with x≤0.487 decompose at a single well defined temperature, while those with x≥0.634 decompose over a temperature range with the formation of intermediate phases. For the samples with x≤0.487, the primary magnetic transition occurs below 40 K, similar to LuMn₂O₅ and other individual RMn₂O₅ (R=Bi, Y, and rare earth) compounds. In contrast, a magnetic phase with a 200 K onset transition temperature is dominant in the samples with x≥0.634.     

Thermoelectric properties of Cu-doped n-type (Bi2Te3)0.9–(Bi2−xCuxSe3)0.1(x=0–0.2) alloys

n-Type (Bi₂Te₃)_0.9–(Bi_2−xCu_xSe₃)_0.1 (x=0–0.2) alloys with Cu substitution for Bi were prepared by spark plasma-sintering technique and their structural and thermoelectric properties were evaluated. Rietveld analysis reveals that approximate 9.0% of Bi atomic sites are occupied by Cu atoms and less than 4.0 wt% second phase Cu_2.86Te₂ precipitated in the Cu-doped parent alloys. Measurements show that an introduction of a small amount of Cu (x0.1) can reduce the lattice thermal conductivity (κ_L), and improve the electrical conductivity and Seebeck coefficient. An optimal dimensionless figure of merit (ZT) value of 0.98 is obtained for x=0.1 at 417 K, which is obviously higher than those of Cu-free Bi₂Se_0.3Te_2.7 (ZT=0.66) and Ag-doped alloys (ZT=0.86) prepared by the same technologies.     

Positron lifetime study of Pr1−xCaxMnO3 (x=0.5, 0.3) during magnetic transition

We measured the positron lifetime in perovskite manganites Pr_1−xCa_xMnO₃ (x=0.3, 0.5). Two lifetime components were observed for each compound; they were attributed to the annihilation of free positrons and positrons trapped at the A-site vacancies. The positron lifetime at the A-site vacancies changed significantly during the antiferromagnetic transition in both the compounds, whereas it was constant around the charge-ordering transition. This change indicates that the electron distribution at the vacancies changed possibly due to the change in the electron distribution of neighboring oxygen atoms. This result indicates that positron lifetime measurements can provide unique information on electronic states during a spin-related phase transition in various oxide materials.     

Structural and magnetic properties of the solid solution () YMn1−x(Cu3/4Mo1/4)xO3

Recently, the ferroelectromagnet YMnO₃ has been the focus of interest because it exhibits both antiferromagnetism (Néel temperature 80 K) and ferroelectricity (Curie temperature 914 K). There have been no reports of complete YMn_1−xM_xO₃ solid solutions in which substitution of the foreign M cation preserves the hexagonal P6₃cm structure. In contrast there exist several homeotypic phases with the general formula, Ln_1+nCu_nMO_3+3n (n=1 (M=Ti), 2 (M=V) and 3 (M=Mo); Ln: lanthanide). Several YMn_1−x(Cu_3/4Mo_1/4)_xO₃ compounds have been synthesized. The solid solution, from YMnO₃ (x=0) to YCu_3/4Mo_1/4O₃ (x=1) has been characterized by X-ray diffraction and transmission electron microscopy study. For 0<x<0.9, the compounds are found to crystallize in the non-centrosymmetric structure, space group P6₃cm, of YMnO₃. The Mn-free end member, x=1, crystallizes in a complex multiple cell, the superstructure being associated to Cu³⁺/Mo⁶⁺ cationic ordering. Dilution of the Mn³⁺ magnetic array by the paramagnetic (Cu²⁺) and diamagnetic (Mo⁶⁺) cations is found to decrease the antiferromagnetic ordering temperature and it becomes undetectable for x0.5 compositions.     

Hydrothermal synthesis, thermal, structural, spectroscopic and magnetic studies of the Mn5−xCox(HPO4)2(PO4)2(H2O)4 (x=1.25, 2, 2.5 and 3) finite solid solution

The Mn_5−xCo_x(HPO₄)₂(PO₄)₂(H₂O)₄ (x=1.25, 2, 2.5, 3) finite solid solution has been synthesized by mild hydrothermal conditions under autogeneous pressure. The phases crystallize in the C2/c space group with Z=4, belonging to the monoclinic system. The unit-cell parameters obtained from single crystal X-ray diffraction are: a=17.525(1), b=9.0535(6), c=9.4517(7) Å, β=96.633(5) ° being R1=0.0436, wR2=0.0454 for Mn75Co25; a=17.444(2), b=9.0093(9), c=9.400(1) Å, β=96.76(1) ° being R1=0.0381, wR2=0.0490 for Mn60Co40; a=17.433(2), b=8.9989(9), c=9.405(1) Å, β=96.662(9) ° being R1=0.0438, wR2=0.0515 for Mn50Co50 and a=17.4257(9), b=8.9869(5), c=9.3935(5) Å, β=96.685(4) ° being R1=0.0296, wR2=0.0460 for Mn40Co60. The structure consists of a three dimensional network formed by octahedral pentameric entities (Mn,Co)₅O₁₆(H₂O)₆ sharing vertices with the (PO₄)³⁻ and (HPO₄)²⁻ tetrahedra. The limit of thermal stability of these compounds is, approximately, 165 °C, near to this mean temperature the phases loose their water content in two successive steps. IR spectra show the characteristic bands of the water molecules and the phosphate and hydrogen-phosphate oxoanions. The diffuse reflectance spectra are consistent with the presence of MO₆ octahedra environments in slightly distorted octahedral geometry, except for the M(3)O₆ octahedron which presents a remarkable distortion and so a higher Dq parameter. The mean value for the Dq and B-Racah parameter for the M(1),(2)O₆ octahedra is 685 and 850 cm⁻¹, respectively. These parameters for the most distorted M(3)O₆ polyhedron are 825 and 880 cm⁻¹, respectively. The four phases exhibit antiferromagnetic couplings as the major magnetic interactions. However, a small spin canting phenomenon is observed at low temperatures for the two phases with major content in the anisotropic-Co(II) cation.     

The phase transition behaviors of Li1−xMn0.5Fe0.5PO4 during lithium extraction studied by in situ X-ray absorption and diffraction techniques

How the structural changes take place in LiMn_yFe_1−yPO₄-type cathode materials during lithium extraction/insertion is an important issue, especially on if they go through the single-phase reaction (i.e., solid solution reaction) or the two-phase reaction regions. Here we report the studies on the phase transition behaviors of a carbon coated Li_1−xMn_0.5Fe_0.5PO₄ (CLi₁₋xMn_0.5Fe0_.5PO₄, 0.0  x  1.0) sample during the first charge using in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) techniques. The combination of in situ XAS and XRD results clearly identify two two-phase coexistence regions at two voltage plateaus of 3.6 (Fe²⁺/Fe³⁺) and 4.2 V (Mn²⁺/Mn³⁺) and a narrow intermediate region which proceeds via single-phase reaction in between two two-phase regions. In addition, simultaneous redox reactions of Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺ in the narrow single-phase region are reported and discussed for the first time.     

Crystal growth and magnetic behavior of R6T13−xAlxMy phases (R=La, Nd; T=Mn, Fe; M=main group) grown from lanthanide-rich eutectic fluxes

Binary eutectic mixtures of early lanthanide metals and late transition metals have been explored as media for crystal growth of new intermetallic phases. A large family of R₆T_13−xAl_xM_y phases (R=La or Nd; T=Fe or Mn; M=main group elements) with the La₆Co₁₁Ga₃ structure type can be crystallized from La/Ni and Nd/Fe eutectics. The tetragonal structure of these compounds features slabs of transition metal atoms capped with mixed T/Al sites and separated by layers of lanthanide ions. The growth of large crystals of the lanthanum analogs allows for the study of the anisotropic magnetic properties of the transition metal slabs. For La₆Fe_13−xAl_xM_y analogs, these order antiferromagnetically with T_N strongly dependent on the Fe/Al ratio on the mixed sites. Growth of Mn analogs is reported for the first time; the transition metal slabs in La₆(Mn/Ni)₁₀Al₃ phases order ferromagnetically with a T_C of 200 K.     

A TEM, XRD, and crystal chemical investigation of oxygen/vacancy ordering in (Ba1−xLax)2In2O5+x, 0x0.6

A careful TEM and XRD study of the (Ba_1−xLa_x)₂In₂O_5+x, 0x0.6, ‘defect-perovskite’-type solid solution has been carried out. A well-defined structural phase transition is shown to occur between x=0.1 and 0.2 from the orthorhombic brownmillerite structure type on the low x side to a multiple twinned, tetragonal 1×1×2 perovskite-related superstructure phase on the high x side at x=0.2. This phase transition correlates with an important phase transition previously observed in electrical conductivity versus temperature measurements. The existence of additional satellite reflections close to the regions of reciprocal space was found to be typical of all (Ba_1−xLa_x)₂In₂O_5+x specimens, although their intensity relative to the parent Bragg reflections systematically reduces as x increases. As x increases beyond 0.2, the -type satellite reflections initially become weaker and rather more diffuse for x=0.3 before splitting into pairs of rather weak and somewhat diffuse incommensurate satellite reflections for x=0.4 and beyond. An interpretation in terms of oxygen vacancy ordering and associated structural relaxation is given. Additional structured diffuse scattering is also observed and a tentative explanation in terms of Ba/La ordering and associated local strain distortions put forward.     

Transport and mechanical property evaluation of (AgSbTe)1−x(GeTe)x (x=0.80, 0.82, 0.85, 0.87, 0.90)

(AgSbTe₂)_1−x(GeTe)_x (known collectively by the acronym of their constituent elements as TAGS-x, where x designates the mole fraction of GeTe) materials, despite being described over 40 years ago, have only recently been studied in greater detail from a fundamental standpoint. We have prepared a series of samples with composition (AgSbTe₂)_1−x(GeTe)_x (x=0.80, 0.82, 0.85, 0.87 and 0.90). Cast ingots of the above compositions were ground and consolidated by spark plasma sintering (SPS). Sintering conditions, specifically high applied pressures of 65 MPa and slow heating rates, were identified as important variables that lead to samples with low porosity and good mechanical strength. The resulting ingots were cut for high temperature electrical, thermal transport and mechanical property evaluation. TAGS-85 was found to have the highest ZT of all samples investigated (ZT=1.36 at 700 K) as a result of its very low value of thermal conductivity. Hall effect measurements performed from 5 to 300 K found these materials to have complex multi-band transport characteristics.     

Preparation of epitaxial La0.6Ca0.4Mn1−xFexO3 (x = 0, 0.2) thin films: Variation of the oxygen content

Perovskite thin films with a nominal composition of La_0.6Ca_0.4Mn_1−xFe_xO₃ (x = 0, 0.2) were deposited by pulsed reactive crossed beam laser ablation. The film properties, such as electrical conductivity and magnetoresistance are studied as a function of the oxygen content and substrate type. The oxygen content of the thin films was determined by Rutherford Backscattering and controlled by varying the background gas pressure, pressure of the gas pulse and by using alternatively O₂ and N₂O as the gas pulse.

LaAlO₃ and SrTiO₃ were used as substrates at deposition temperature of 650 °C. The grown films were analyzed by X-ray diffraction in order to optimize the growth conditions, i.e. to obtain epitaxial thin films. Thin films doped with 20% Fe were grown under the same experimental conditions as the undoped LCMO films and the effect of the doping on the structural and transport properties of the thin films has been investigated.

The temperature of the metal–insulator transition was measured as a function of the oxygen content and substrate type.     

Conventional hydrothermal process versus microwave-assisted hydrothermal synthesis of La1−xAgxMnO3+δ (x = 0, 0.2) perovskites used in methane combustion

A study on the synthesis of La_1−xAg_xMnO_3+δ (x = 0, 0.2) using a microwave process (MWhyd) has been carried out by comparing the heating time and reaction temperature with the same factor under conventional thermal process (CHhyd). Experiments have been conducted using the hydrothermal method at medium pressure (T = 200 °C, P = 20 atm) followed by a thermal treatment of the precursor at 700 °C (10 h).Structural and physico-chemical properties of the catalysts were investigated using X-ray diffraction (XRD), BET sorption, temperature-programmed reduction or desorption, mass spectrometry (TPR-MS and TPD-MS), and X-ray photoelectron spectroscopy (XPS). While CHhyd and MWhyd powder catalysts exhibited the same XRD patterns indexed as pure perovskite structure, their surface physico-chemical properties were found to be strongly influenced by the preparation method. The effect of the nature of oxygen species, their amount and mobility, evidenced by temperature programmed experiments, on the catalytic properties in methane combustion in the presence and in the absence of hydrogen sulphide has been studied. MWhyd-La_0.8Ag_0.2MnO_3+δ catalysts were found to exhibit a much better performance in methane combustion together with higher resistance to sulphur poisoning than CHhyd catalysts.