Structural, magnetic, and thermal characteristics of the phase transitions in Gd5GaxGe4−x magnetocaloric materials

Temperature-dependent, single crystal and powder X-ray diffraction studies as well as magnetization, and heat capacity measurements were carried out on two phases of the Gd₅Ga_xGe_4−x system: for x=0.7 and 1.0. Gd₅Ga_0.7Ge_3.3 shows three structure types as a function of temperature: (i) from 165 K to room temperature, the orthorhombic Sm₅Ge₄-type structure exists; (ii) below 150 K, it transforms to a orthorhombic Gd₅Si₄-type structure; and (iii) a monoclinic Gd₅Si₂Ge₂-type component is observed for the intermediate temperature range of 150 K≤T≤165 K. This is the first time that all these three structure types have been observed for the same composition. For Gd₅Ga_1.0Ge_3.0, the room temperature phase belongs to the orthorhombic Pu₅Rh₄-type structure with interslab contacts between main group atoms of 2.837(4) Å. Upon heating above 523 K, it transforms to a Gd₅Si₄-type structure with this distance decreasing to 2.521(7) Å before decomposing above 573 K.     

Structure and Magnetism of Pr1−xSrxFeO3−δ

Crystal structure, redox, and magnetic properties for the Pr_1−xSr_xFeO_3−δ solid-solution phase have been studied. Oxidized samples (prepared in air at 900°C) crystallize in the GdFeO₃-type structure for 0≤x≤0.80, and probably in the Sr₈Fe₈O₂₃-type (unpublished) structure for x=0.90. Reduced samples (containing virtually only Fe³⁺) crystallize as the perovskite aristotype for x=0.50 and 0.67 with randomly distributed vacancies. The Fe⁴⁺ content increases linearly in the oxidized samples up to x≈0.70, whereupon it stabilizes at around 55%. Antiferromagnetic ordering of the G type is observed for oxidized samples (0≤x≤0.90) which show decreasing Néel temperature and ordered magnetic moment with increasing x, while the Néel temperature is nearly constant at 700 K for reduced samples. Electronic transitions for iron from an average-valence state via charge-separated to disproportionated states are proposed from anomalies in magnetic susceptibility curves in the temperature ranges 500–600 K and 150–185 K.     

Synthesis of ternary and quaternary CuInxGa1−xSe2 (0 ≤ x ≤ 1) semiconductor nanocrystals

Semiconducting CuIn_xGa_1−xSe₂ nanocrystals (20–30 nm) have been synthesized over the whole composition range using a facile solution-based method. Depending on the synthesis conditions, the nanocrystals exhibit a cubic or spherical morphology with a narrow size distribution. The band gap increases with increasing Ga concentration and the values are close to those observed in the bulk.     

A TEM investigation of the (Bi1−xSrx)FeO3−x/2, 0.2≤x≤0.67, solid solution and a suggested superspace structural description thereof

A careful transmission electron microscopy (TEM) investigation of an incommensurately modulated member of the (Bi_1−xSr_x)Fe³⁺O_3−x/2□_x/2, 0.2≤x≤0.67, solid solution has been carried out. High resolution (HR) TEM imaging is used to show the presence of at least 6-fold twinning on a rather fine (5 nm) scale. The (3+1)-d superspace group symmetry is suggested to be or one of the non-centrosymmetric sub-groups thereof, namely , , and . A superspace construction is then used to propose the nature of the local compositional ordering and, hence, of the oxygen-deficient slab that intergrows with the perovskite slab to produce the observed solid solution phase. The proposed compositional superspace atomic surfaces can be used to produce model structures at any composition within the solid solution range.     

Structures and energies of CnS (1 ≤ n ≤ 20) sulphur carbide clusters

C_nS (1 ≤ n ≤ 20) clusters have been investigated by means of the density functional theory. As a general rule, when 1 ≤ n ≤ 17 the energetically most favorable isomers are found to be the linear arrangement of nuclei (C_∞v) with the sulphur atom at the very end of the carbon chain. The electronic ground state is alternately predicted to be ¹∑⁺ for odd n or ³∑⁻ for even n with a conspicuous odd–even effect in the stability of these clusters. The C₁₈S cluster is predicted to have a S-capped monocyclic structure (¹A₁), but with a low barrier to linearity. On the other hand, C₁₉S and C₂₀S are unambiguously linear in the ¹∑⁺ and ³∑⁻ electronic ground states, respectively.     

Crystal chemistry, modulated structure, and electrical conductivity in the oxygen excess scheelite-based compounds La1−xThxNbO4+x/2 and LaNb1−xWxO4+x/2

For La_1−xTh_xNbO_4+x/2, three phases with broad homogeneity regions occur, for 0.075 ≤ x ≤ 0.37, 0.41 < x < 0.61, and 0.65 ≤ x ≤ 0.74. All are related to the scheelite structure type, with at least the first exhibiting an incommensurate structural modulation. An analogous structurally modulated phase was found for LaNb_1−xW_xO_4+x/2 for 0.11 ≤ x ≤ 0.22. Additional phases occur at La_0.2Th_0.8NbO_4.4 and LaNb_0.4W_0.6O_4.3. The electrical conductivity and the direction and wavelength of the structural modulation have been characterized for the La_1−xTh_xNbO_4+x/2 phase with 0.075 ≤ x ≤ 0.37.     

Preparation and electrical properties of new oxide ion conductors Ce6−xDyxMoO15−δ (0.0 ≤ x ≤ 1.8)

Ce_6−xDy_xMoO_15−δ (0.0 ≤ x ≤ 1.8) were synthesized by modified sol–gel method. Structural and electrical properties were investigated by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The XRD patterns showed that the materials were single phase with a cubic fluorite structure. Impedance spectroscopy measurement in the temperature range between 350 °C and 800 °C indicated a sharp increase in conductivity for the system containing small amount of Dy₂O₃. The Ce_5.6Dy_0.4MoO_15−δ detected to be the best conducting phase with the highest conductivity (σ_t = 8.93 × 10⁻³ S cm⁻¹) is higher than that of Ce_5.6Sm_0.4MoO_15−δ (σ_t = 2.93 × 10⁻³ S cm⁻¹) at 800 °C, and the corresponding activation energy of Ce_5.6Dy_0.4MoO_15−δ (0.994 eV) is lower than that of Ce_5.6Sm_0.4MoO_15−δ (1.002 eV).     

Analysis of thermo-magnetic fluctuations above the glass-transition temperature in Bi1.6Pb0.5Sr2−xEuxCa1.1Cu2.1O8+δ (0.000 ≤ x ≤ 0.180) system

The resistivity of Bi_1.6Pb_0.5Sr_2−xEu_xCa_1.1Cu_2.1O_8+δ (0.000 ≤ x ≤ 0.180) superconductor has been measured as a function of temperature and magnetic field. The resistivity shows a glassy behavior even at higher temperatures and magnetic fields for the Eu-doped samples as compared with the Eu free sample. The values of glass-transition temperature [T_g], magnetic field dependent activation energy [U₀(B)] and the temperature and magnetic field dependent activation energy [U₀(B,T)] are found to be maximum for optimal doping levels (x = 0.135) which shows that the flux lines are effectively pinned in this sample. Also for temperatures below the superconducting transition temperature (T_C), a scaling of measured resistivity curves in magnetic field (B = 0.4 and 0.8 T) is obtained and this scaling is quite useful for better understanding of the behavior of the flux vortices in high temperature superconductors.     

Structural chemistry and magnetic behavior of ternary gallides REAuxGa4−x, RE = La, Ce, Pr, Nd, and Sm

Two new series of ternary gallides with the chemical formulas (La, Ce, Pr, Nd, Sm)Au_xGa_4−x and (La, Ce, Pr, Nd, Sm)Au_1.5Ga_2.5 were synthesized from the elements by arc melting. From X-ray powder diffraction analysis the REAu_xGa_4−x series of compounds was found to be isotypic and crystallize with BaAl₄ type of structure; the homogeneity range at 600°C of each of the REAu_xGa_4−x phases was established, revealing remarkable deviations from Vegard's rule. At 600°C the REAu_xGa_4−x phases of the BaAl₄ type were observed to be in thermodynamic equilibrium with a structure variant crystallizing at the composition REAu_1.5Ga_2.5 and with a narrow homogeneous range. In case of PrAu_1.5Ga_2.5 the structure type was refined from X-ray single-crystal counter data (CaBe₂Ge₂ type, space group P4/nmm, R_w = 0.046). Gold and gallium atoms were generally found on separate crystallographic sites; however, a statistical distribution of 51% Au + 49% Ga was derived for the 2b sites. ¹⁹⁷Au Mössbauer spectroscopy confirmed the occupational mode of the gold atoms in CeAu_1.5Ga_2.5. For the BaAl₄-type phases, X-ray powder and Mössbauer data revealed preferential occupation of the 4e sites of I4/mmm by gold atoms; practically no Au was observed on the 4d sites. Magnetic susceptibilities were determined over a temperature range extending from 2 to 1100 K. Above liquid nitrogen temperatures the paramagnetic behavior of the (Ce, Pr, Nd)Au_xGa_4−x and the (Ce, Pr, Nd)Au_1.5Ga_2.5 compounds is characterized by magnetic moments close to the ideal trivalent rare earth values. Lanthanum compounds are diamagnetic, whereas SmAu_xGa_4−x alloys are characterized by a typical Van Vleck-type paramagnetism of closely spaced multiplets. At very low temperatures onset of ferromagnetic ordering is observed for the BaAl₄-type series of compounds. No superconductivity was encountered down to 2 K.     

The defect solid solution Na7/8(Fe7/8+xTi9/8−2xSbx)O4 (0 ≤ x ≤ 1/3): Evidence of Na mobility in the tunnels of a quadruple rutile-chain structure

A new defect solid solution, the series Na_7/8(Fe^III_7/8+xTi^IV_9/8−2xSb^V_x)O₄, was synthesized. Its homogeneity range is rather wide: 0 <- x ≤ 0.33. The incorporation of Sb^V gives rise to a progressive increase of the parameters of the orthorhombic unit cell. X-ray powder structure calculations point to a partial occupancy of the large double tunnels in a quadruple rutile-chain structure. A significant ordering of cations over the octahedral framework is observed, owing to a Ti^IV---Sb^V segregation. Electrical measurements emphasize a cationic conductivity, mainly related to a 1D motion of Na^I cations. A transition from a low activation energy process—E_A ≤ 0.20 eV—to a high activation energy one—E_A ≈ 0.75 eV—systematically occurs at T ≈ 440°C, independent of the Sb^V concentration. A possible skew motion from a half tunnel to another one is proposed as a tentative explanation of the high-temperature conductivity mechanism.     

Li3−xTi2(PO4)3 (0 ≤ x ≤ 1): A new mixed valent titanium(III/IV) phosphate with a NASICON-type structure

A new NASICON-related structure of lithium titanium phosphate Li_2.72Ti₂(PO₄)₃ has been determined. This compound crystallizes in an orthorhombic system, Pbcn, with a = 12.064 (3) Å, b = 8.663 (3) Å, c = 8.711 (4) Å, V = 910.4 (8) ų, and Z = 4. The single crystal structure of this novel mixed valent titanium(III/IV) phosphate reveals one titanium atom per asymmetric unit. Two lithium sites are characterized by a pair of distorted polyhedra, Li(1)O₄ and Li(2)O₅, which share a common edge resulting in a short Li(1) … Li(2) distance, i.e., 2.29 (5) Å. Magnetic susceptibility and microprobe analysis confirmed the structural composition. The room temperature ionic conductivity is comparable with that of the known Li_1+xTi^IV_2−xIn^III_x(PO₄)₃, which suggests possible fast ionic conductivity.     

Orthorhombic–orthorhombic phase transitions in Nd2NiO4+δ (0.067≤δ≤0.224)

Variation of the phases of Nd₂NiO_4+δ with the excess oxygen concentration δ has been examined at room temperature in the range 0.067≤δ≤0.224 using the X-ray powder diffraction technique. The phases observed at room temperature are orthorhombic-I (0.21<δ≤0.224), orthorhombic-IV (0.175<δ≤0.21), orthorhombic-II (0.15<δ≤0.175), orthorhombic-II+quasi-tetragonal-I (0.10<δ≤0.15), and quasi-tetragonal-I (0.067<δ≤0.10).     

Preparation and characterization of lithium ion-conducting glass-ceramics in the Li1 + xCrxGe2 − x(PO4)3 system

Li₂O–Cr₂O₃–GeO₂–P₂O₅ based glasses were synthesized by a conventional melt-quenching method and successfully converted into glass-ceramics through heat treatment. Experimental results of DTA, XRD, ac impedance techniques and FESEM indicated that Li_1.4Cr_0.4Ge_1.6(PO₄)₃ glass-ceramics treated at 900 °C for 12 h in the Li_1 + xCr_xGe_2 − x(PO₄)₃ (x = 0–0.8) system exhibited the best glass stability against crystallization and the highest ambient conductivity value of 6.81 × 10⁻⁴ S/cm with an activation energy as low as 26.9 kJ/mol. In addition, the Li_1.4Cr_0.4Ge_1.6(PO₄)₃ glass-ceramics displayed good chemical stability against lithium metal at room temperature. The good thermal and chemical stability, excellent conducting property, easy preparation and low cost make it promising to be used as solid-state electrolytes for all-solid-state lithium batteries.     

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.     

Structur–Composition Relations and Fractional Site Occupancy of New M5Ge4 Compounds in the System Ge–Ta–Zr

The new ternary phases Zr_4−xTa_1+xGe₄ (0.1<x<0.4) and Zr_2+xTa_3−xGe₄ (0.1<x<1.1) were prepared from the elements by arc melting and subsequent induction heating at 1400–1450°C. Single-crystal X-ray diffraction was used to determine their structures and to refine mixed site occupancies. Zr_4−xTa_1+xGe₄ was found to crystallize in the monoclinic space group P2₁/c (structure type: U₂Mo₃Si₄) and the compound Zr_2−xTa_3−xGe₄ shows orthorhombic symmetry (space group Pnma, structure type: Sm₅Ge₄). The close structural relationship between the two structures is discussed. Both phases exhibit pronounced differential fractional site occupancy of Ta and Zr on the metal sites and considerable composition ranges. Extended Hückel calculations were performed for various site occupancy models and Mulliken overlap populations for the different lattice sites of each structure were calculated for these models. The correlation of the cumulated Mulliken overlap populations and the atomic orbital populations with the actual site occupancies is discussed.     

The oxycarbonates Sr4(Fe2−xMnx)1+y(CO3)1−3yO6(1+y): nano, micro and average structural approach

The possibility to synthesize layered oxycarbonates, with nominal composition Sr₄Fe_2−xMn_xO₆CO₃ involving trivalent manganese, with 0≤x≤1.5, is reported for the first time. The structural study of Sr₄FeMnO₆CO₃ using NPD, HREM, Mössbauer and XANES, shows that this phase is closely related to n=3 member of the Ruddlesden–Popper family. It derives from the latter by replacing the middle layer of transition metal octahedra by triangular CO₃ groups, with two different “flag” and “coat hanger” configurations. The magnetic order is antiferromagnetic and fundamentally different from the magnetic behavior of Sr₄Fe₂O₆CO₃.     

Magnetic structure of the UCuxSi2−x system

Neutron powder diffraction experiments were performed on selected compositions of the UCu_xSi_2−x system exhibiting an interesting magnetic phase diagram towards the composition: spin fluctuation behaviour for x<0.49, ferromagnetism for 0.49≤x<0.80, spin glass state for 0.80<x<0.92 and finally antiferromagnetism for 0.92<x≤0.96. At 1.5 K, the compounds UCu_0.49Si_1.51 (hexagonal AlB₂ modification) and UCu_0.65Si_1.35 show a collinear ferromagnetic structure where the uranium magnetic moments equal to 1.1(1) and 2.5(1)μ_B, respectively, are aligned in the basal plane of the [U₆] trigonal prisms. On the contrary, UCu_0.96Si_1.04 adopts a non-collinear antiferromagnetic structure similar to that observed for UCuSn. Moreover, the study confirms the absence of long range magnetic order for UCu_0.90Si_1.10.     

Insulator–metal transition in Nd0.8Na0.2Mn(1−x)CoxO3 perovskites

The electric and magnetic properties of the perovskites Nd_0.8Na_0.2Mn_(1−x)Co_xO₃ (0x0.2) prepared by the usual ceramic procedure were investigated. The insulator-to-metal-like (IM) transition, closely related to a ferromagnetic arrangement, was revealed for the composition of x=0.04 and a similar tendency was detected for x=0. The insulating behavior persists down to low temperatures for higher contents of cobalt ions in spite of the transition to the bulk ferromagnetism. The properties are interpreted in terms of the steric distortion, tilting of the Mn(Co)O₆ octahedra and the double-exchange interactions of the type Mn³⁺–O²⁻–Mn⁴⁺and Mn^3.5+δ–O²⁻–Co²⁺, respectively. Presence of antiferromagnetic domains in the ferromagnetic matrix for the most of cobalt-substituted samples is supposed.     

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.     

Metallic behavior of the Zintl phase EuGe2: combined structural studies, property measurements, and electronic structure calculations

The Zintl compound EuGe₂ crystallizes in the trigonal space group (No. 164) with the CeCd₂-structure type. Its structure can be formally derived from the hexagonal AlB₂-structure type by a strong puckering of the hexagonal layers. The chemical bonding in EuGe₂ can be rationalized according to the Zintl concept as (Eu²⁺)(Ge¹⁻)₂, since the europium atoms are divalent and each germanium atom receives one additional valence electron. In that sense, EuGe₂ is expected to be a closed-shell compound with semiconducting behavior. However, temperature dependent resistivity measurements show EuGe₂ to be metallic. Subsequently, detailed crystallographic studies revealed the structure and the composition of EuGe₂ to be free of defects and impurities, which, along with the confirmed divalent oxidation state of the europium atoms by means of magnetic measurements, make EuGe₂ another example of a metallic Zintl phase. These results are in good agreement with the results of electronic structure calculations such as TB-LMTO-ASA (LDA) and FLAPW (GGA), which reveal non-zero DOS at the Fermi level.