Ternary lithium stannides LixT3Sn7−x (T=Rh, Ir)

The ternary stannides Li_xRh₃Sn_7−x (x=0.45, 0.64, 0.80) and Li_xIr₃Sn_7−x (x=0.62 and 0.66) were synthesized from the elements in sealed tantalum tubes in a water-cooled sample chamber of an induction furnace. The samples were characterized by X-ray diffraction on powders and single crystals. The stannides adopt the cubic Ir₃Ge₇-type structure (space group , Z=4). In this structure type the tin atoms occupy the Wyckoff positions 12d and 16f and form two interpenetrating frameworks consisting of cubes and square antiprisms. The rhodium and iridium atoms center the square antiprisms and are arranged in pairs. With increasing lithium substitution the lattice parameter of Ir₃Sn₇ (936.7) decreases via 932.2 pm (x=0.62) to 931.2 pm (x=0.66), while the Ir-Ir distance remains almost the same (290 pm). A similar trend is observed for the rhodium compounds. The lithium atoms substitute Sn on both framework sites. However, the 16f site shows a substantially larger preference for Li occupation. This is in contrast to the isotypic magnesium based compounds.     

Synthesis and crystal structure of Bi6.4Pb0.6P2O15.2: A new polymorph in the series Bi6+xM1−xP2O15+y

Bi_6.4Pb_0.6P₂O_15.2 is a polymorph of structures with the general stoichiometry Bi_6+xM_1−xP₂O_15+y. However, unlike previously published structures that consist of layers formed by edge sharing OBi₄ tetrahedra bridged by PO₄ and TO₆ (T=transition metal) tetrahedra and octahedra the title compound's structure is more complex. It is monoclinic, C2, a=19.4698(4) Å, b=11.3692(3) Å, c=16.3809(5) Å, β=101.167(1)°, Z=10. Single-crystal X-ray diffraction data were refined by least squares on F² converging to R₁=0.0387, wR₂=0.0836 for 7023 intensities. The crystal twins by mirror reflection across (001) as the twin plane and twin component 1 equals 0.74(1). Oxygen ions are in tetrahedral coordination to four metal ions and the O(BiPb)₄ units share corners to form layers that are part of the three-dimensional framework. Eight oxygen ions form a cube around the two crystallographically independent Pb ions. Pb-O bond lengths vary from 2.265(14) to 2.869(14) Å. Pairs of such cubes share an edge to form a Pb₃O₂₀ unit. The two oxygen ions from the unshared edges are part of irregular Bi polyhedra. Other oxygen ions of Bi polyhedra are part only of O(BiPb)₄ units, and some oxygen ions of the polyhedra are also part of PO₄ tetrahedra. One, two, three and or four PO₄ moieties are connected to the Bi polyhedra. Bi-O bond lengths ?3.1 Å vary from 2.090(12) to 3.07(3) Å. The articulations of Pb cubes, Bi polyhedra and PO₄ tetrahedra link into the three-dimensional structure.     

The crystal chemistry of Bi6TP2O15+x, T=Fe, Ni, Zn: Isomorphism and polymorphism, structural relationship to Bi6TiP2O16

The crystal structures of compounds with nominal compositions Bi₆FeP₂O_15+x (I), Bi₆NiP₂O_15+x (II) and Bi₆ZnP₂O_15+x (III) were determined from single-crystal X-ray diffraction data. They are monoclinic, space group I2, Z=2. The lattice parameters for (I) are a=11.2644(7), b=5.4380(3), c=11.1440(5) Å, β=96.154(4)°; for (II) a=11.259(7), b=5.461(4), c=11.109(7) Å, β=96.65(1)°; for (III) a=19.7271(5), b=5.4376(2), c=16.9730(6) Å, β=131.932(1)°. Least squares refinements on F² converged for (I) to R1=0.0554, wR₂=0.1408; for (II) R₁=0.0647, wR₂=0.1697; for (III) R₁=0.0385, wR₂=0.1023. The crystals are complexly twinned by 2-fold rotation about , by inversion and by mirror reflection. The structures consist of edge-sharing articulations of OBi₄ tetrahedra forming layers in the a-c plane that then continue by edge-sharing parallel to the b-axis. The three-dimensional networks are bridged by Fe and Ni octahedra in (I) and (II) and by Zn trigonal bipyramids in (III) as well as by oxygen atoms of the PO₄ moieties. Bi also randomly occupies the octahedral sites. Oxygen vacancies exist in the structures of the three compounds due to required charge balances and they occur in the octahedral coordination polyhedron of the transition metal. In compound (III), no positional disorder in atomic sites is present. The Bi-O coordination polyhedra are trigonal prisms with one, two or three faces capped. Magnetic susceptibility data for compound (I) were obtained between 4.2 and 350 K. Between 4.2 and 250 K it is paramagnetic, μ_eff=6.1 μ_B; a magnetic transition occurs above 250 K.     

Ab initio structure determination and Rietveld refinement of Bi10Mo3O24 the member n=3 of the Bi2n+4MonO6(n+1) series

Bi₂O₃-MoO₃ system shows a large panoply of phases depending on Bi/Mo ratio, among them, the low temperature phases of the homologous series Bi_2(n+2)Mo_nO_6(n+1) with n=3, 4, 5 and 6. They exhibit, alike most of the phases of this system, strong fluorite sub-network. Nevertheless, a multitechnique approach has been followed in order to solve the crystal structure of the n=3 member, i.e. Bi₁₀Mo₃O₂₄. From ab initio indexing X-ray powder pattern cell parameters were derived. It belongs to the monoclinic system, space group C2, with cell parameters: a=23.7282(2) Å, b=5.64906(6) Å, c=8.68173(9) Å, β=95.8668(7)° with Z=2. The matrix relating this cell with the fluorite one is 4 0 1/0 1 0/ 0  and a cationic localization was derived. HRTEM allowed the cationic Bi and Mo order to be modified and specified, as well as to build up a full structural ab initio model on the basis of crystal chemistry considerations. Simultaneous Rietveld refinement of multipattern X-ray and neutron powder diffraction data taking advantage of the neutron scattering length for O location have been performed. The goodness of the model was ascertained by low reliability factors, weighted R_b=4.97% and R_f=3.21%. This complex Bi₁₀Mo₃O₂₄ structure, with 5Bi, 2Mo and 13O in different crystallographic positions of the asymmetric unit, shows good agreement between observed and calculated patterns within the data resolution. Moreover, the determination of this structure sets the basis for the crystallographic characterization of the complete family Bi_2(n+2)Mo_nO_6(n+1), whose guidelines are also evidenced in this paper.     

Structure and physical properties of ternary W5Si3-type antimonides and bismuthides Zr5M1-xPn2+x (M=Cr, Mn; Pn=Sb, Bi)

Four new ternary compounds Zr₅M_1-xPn_2+x (M=Cr, Mn; Pn=Sb, Bi) were synthesized by arc-melting and annealing at 800 °C. They crystallize in the tetragonal W₅Si₃-type structure. The crystal structure of Zr₅Cr_0.49(2)Sb_2.51(2) was refined from powder X-ray diffraction data by the Rietveld method (Pearson symbol tI32, tetragonal, space group I4/mcm, Z=4, a=11.1027(6) Å, c=5.5600(3) Å). Four-probe electrical resistivity measurements on sintered polycrystalline samples indicated metallic behavior. Magnetic susceptibility measurements between 2 and 300 K revealed temperature-independent Pauli paramagnetism for Zr₅Cr_1-xSb_2+x and Zr₅Cr_1-xBi_2+x, but a strong temperature dependence for Zr₅Mn_1-xSb_2+x and Zr₅Mn_1-xBi_2+x which was fit to the Curie-Weiss law for the latter with θ=-11.3 K and μ_eff=1.81(1) μ_B. Band structure calculations for Zr₅Cr_0.5Sb_2.5 support a structural model in which Cr and Sb atoms alternate within the chain of interstitial sites formed at the centers of square antiprismatic Zr₈ clusters.     

Crystal structure, magnetic and infrared spectroscopy studies of the LiCryFe1−yP2O7 solid solution

The lithium double diphosphates LiCr_yFe_1−yP₂O₇ have been investigated by X-ray diffraction, SQUID measurements and vibrational spectroscopy. The Rietveld refinements based on the XRD patterns show the existence of a continuous solid solution over the whole composition range (0?y?1.0) with a continuous evolution of the monoclinic unit cell parameters (S.G. P2₁). The transition metal ions connect the diphosphate anions forming a three-dimensional network with channels filled by Li⁺ cations expected to exhibit high mobility. All compounds order magnetically at low temperatures due the Fe-Fe interactions. The ordering temperature decreases with increasing Cr content. The slope in Curie-Weiss fits to the 1/χ vs T data in the paramagnetic domain clearly shows the existence of Fe³⁺ and Cr³⁺ in their high spin states, and a ferromagnetic component is clearly detected for y=0, 0.2 and 0.4. IR spectra have been interpreted using factor group analysis. The small shift of the frequencies is due to the influence of the chromium amount. The POP angles were estimated using the Lazarev's relationship.     

Composition-induced phase transition in Ca14Zn6−xGa10+xO35+x/2 (x=0.0 and 0.5)

Novel complex oxides Ca₁₄Zn₆Ga₁₀O₃₅ and Ca₁₄Zn_5.5Ga_10.5O_35.25 were prepared in air at 1200 °C, 72 h. Refinements of their crystal structures using X-ray powder diffraction data showed that Ca₁₄Zn₆Ga₁₀O₃₅ is ordered (S.G. F23, =0.0458, R_p=0.0485, R_wp=0.0659, χ²=1.88) and Ca₁₄Zn_5.5Ga_10.5O_35.25 disordered (S.G. F432, =0.0346, R_p=0.0601, R_wp=0.0794, χ²=2.82) variants of the crystal structure of Ca₁₄Zn₆Al₁₀O₃₅. In the crystal structure of Ca₁₄Zn₆Ga₁₀O₃₅, there are large empty voids, which could be partially occupied by additional oxygen atoms upon substitution of Zn²⁺ by Ga³⁺ as in Ca₁₄Zn_5.5Ga_10.5O_35.25. These oxygen atoms are introduced into the crystal structure of Ca₁₄Zn_5.5Ga_10.5O_35.25 only as a part of four tetrahedra (Zn, Ga)O₄ groups sharing common vertex. This creates a situation where even a minor change in the chemical composition leads to considerable anion and cation disordering resulting in a change of space group from F23 (no. 196) to F432 (no. 209).     

Crystal structure and electrical conductivity of lanthanum-calcium chromites-titanates La1−xCaxCr1−yTiyO3−δ (x=0-1, y=0-1)

Five series of perovskite-type compounds in the system La_1−xCa_xCr_1−yTi_yO₃ with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO₃-LaCr^IIIO₃-CaCr^IVO₃ quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCr^IVO₃-CaTiO₃-LaCr^IIIO₃. The solid solution La_{(1−x′−y)}Ca_(x′+y)Cr^IV_x′Cr^III_{(1−x′−y)}Ti_yO₃ exists by up to 0.6-0.7 mol fractions of CaCr^IVO₃ (x^′<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La_0.1Ca_0.9TiO₃, La_0.2Ca_0.8TiO₃, La_0.4Ca_0.6Cr_0.2Ti_0.8O₃ and La_0.3Ca_0.7Cr_0.2Ti_0.8O₃ in the system La_1−xCa_xCr_1−yTi_yO₃ were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO₂=6×10⁻¹⁶-0.21×10⁵ Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10⁻¹⁵-0.21×10⁵ Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr³⁺]/[Cr⁴⁺] and [Ti⁴⁺]/[Ti³⁺]. The maximum electrical conductivity at 900 °C and pO₂=10⁻¹⁵ Pa was found for the composition La_0.1Ca_0.9TiO₃ (near 50 S/cm) and in air at 900 °C for La_0.5Ca_0.5CrO₃ (close to 100 S/cm).     

Crystal structures of the double perovskites Ba2Sr1−xCaxWO6

Structures of the double perovskites Ba₂Sr_1−xCa_xWO₆ have been studied by the profile analysis of X-ray diffraction data. The end members, Ba₂SrWO₆ and Ba₂CaWO₆, have the space group I2/m (tilt system a⁰b⁻b⁻) and Fm3¯m (tilt system a⁰a⁰a⁰), respectively. By increasing the Ca concentration, the monoclinic structure transforms to the cubic one via the rhombohedral R3¯ phase (tilt system a⁻a⁻a⁻) instead of the tetragonal I4/m phase (tilt system a⁰a⁰c⁻). This observation supports the idea that the rhombohedral structure is favoured by increasing the covalency of the octahedral cations in Ba₂MM′O₆-type double perovskites, and disagrees with a recent proposal that the formation of the π-bonding, e.g., d⁰-ion, determines the tetragonal symmetry in preference to the rhombohedral one.     

Crystal structure of lanthanum bismuth silicate Bi2−xLaxSiO5 (x∼0.1)

A melting and glass recrystallization route was carried out to stabilize a new tetragonal form of Bi₂SiO₅ with bismuth partially substituted by lanthanum. The crystal structure of Bi_2−xLa_xSiO₅ (x∼0.1) was determined from powder X-ray and neutron diffraction data (space group I4/mmm, , c=15.227(1) Å, V=224.18 Å³, Z=2; reliability factors: R_Bragg=5.65%, R_p=14.6%, R_wp=16.8%, R_exp=8.3%, χ²=8.3 (X-ray) and R_Bragg=2.40%, R_p=8.1%, R_wp=7.5%, R_exp=4.2%, χ²=3.3 (neutrons); 11 structural parameters refined).The main effect of lanthanum substitution is to introduce, by removing randomly some bismuth 6s² lone pairs, a structural disorder in the surroundings of (Bi₂O₂)²⁺ layers, that is in the (SiO₃)²⁻ pyroxene files arrangement. It results in a symmetry increase relatively to the parent compound Bi₂SiO₅, which is orthorhombic. The two structures are compared.     

Crystal structures of the ionic conductors Bi46M8O89 (M=P, V) related to the fluorite-type structure

The crystal structures of the two oxides Bi₄₆M₈O₈₉ (M=P, V) have been solved from single crystals X-ray data at room temperature. Bi₄₆P₈O₈₉ crystallizes in the monoclinic symmetry (space group C2/m) with the cell parameters , , and β=112.14(3)°. The symmetry of Bi₄₆V₈O₈₉ is also monoclinic but the space group is P2₁/c with the unit-cell parameters: , , and β=107.27(3)°. Both structures derive from an oxygen deficient fluorite-type structure where the Bi and M cations (M=P, V) are ordered in the framework. The structures are characterised by isolated MO₄ tetrahedra (M=P, V) which contradicts the previous results. The difference between the two structures is only due to a different order of the M atoms (M=P, V) in the fluorite-type superstructure. It will be shown that some oxygen sites are partially occupied in both structures which can explain the ion conduction properties of these phases. A structural building principle will be proposed that can explain the large domain of solid solution related to the fluorite-type observed in both systems.     

Synthesis, crystal structures and ionic conductivities of Bi14P4O31 and Bi50V4O85. Two members of the series Bi18−4mM4mO27+4m (M=P, V) related to the fluorite-type structure

The two hitherto unknown compounds Bi₁₄P₄O₃₁ and Bi₅₀V₄O₈₅ were prepared by the direct solid-state reaction of Bi₂O₃ and (NH₄)H₂PO₄ or V₂O₅, respectively. Bi₁₄P₄O₃₁ crystallizes in a C-centred monoclinic symmetry (C2/c space group) with the unit-cell parameters: , , and β=93.63(1)° (Z=16). The symmetry of Bi₅₀V₄O₈₅ is also monoclinic (I2/m space group) with lattice parameters of , , and β=90.14(1)° (Z=2). Both structures correspond to a fluorite-type superstructure where the Bi and P or V atoms are ordered in the framework. An idealized structural model is proposed where the structures result of the stacking of mixed atomic layers of composition [Bi₁₄M₄O₃₁] and [Bi₁₈O₂₇] respectively. This new family can be formulated Bi_18−4mM_4mO_27+4m with M=P, V and where the parameter m (0?m?1) represents the ratio of the number of [Bi₁₄M₄O₃₁] layers to the total number of layers in the sequence. Bi₁₄P₄O₃₁ corresponds to m=1 when Bi₅₀V₈O₈₅ corresponds to m=1/3. In this last case, the structural sequence is simply one [Bi₁₄V₄O₃₁] layer to two [Bi₁₈O₂₇] layers. As predicted by the proposed structural building principle, Bi₁₄P₄O₃₁ is not a good ionic conductor. The conductivity at 650 °C is 4 orders of magnitude lower from those found in Bi₄₆M₈O₈₉ (M=P, V) (m=2/3) and Bi₅₀V₄O₈₅ (m=1/3).     

(Mn1−xPbx)Pb10+ySb12−yS26−yCl4+yO, a new oxy-chloro-sulfide with ∼2 nm-spaced (Mn,Pb)Cl4 single chains within a waffle-type crystal structure

The new oxy-chloro-sulfide (Mn_1−xPb_x)Pb_10+ySb_12−yS_26−yCl_4+yO (x ∈ [0.2-0.3]; y ∈ [0.3-1.6]) was synthesized by dry way at 500-600 °C. A single crystal ∼Mn_0.7Pb_11.0Sb_11.3S_25.3Cl_4.7O indicates a monoclinic symmetry, space group C2/m, with a = 37.480(8), b = 4.1178(8), c = 18.167(4) Å, β = 106.37(3)°, V = 2690.2(9) Å³, Z = 2. Its crystal structure was determined by X-ray single crystal diffraction, with a final R = 5.11%. Modular analysis of the crystal structure reveals a “waffle” architecture, where complex rods with lozenge section delimitate an internal channel filled by a single chain of (Mn_0.7Pb_0.3)Cl₆ octahedra connected by opposite edges. Minimal inter-chain distances are close to 18 Å. The rod wall, two-atom thick, presents, in alternation with S atoms, Pb or (Pb,Sb) cations with prismatic coordination in the internal atom layer, while the external atom layer is constituted exclusively by Sb cations with dissymmetric square pyramidal coordination. A (Pb,Sb)₂S₂ fragment connects two successive rods along (2 0 1) to form a waffle-type palissadic layer. The unique O position, half filled, presents the same environment than the isolated O positions in the oxy-sulfide Pb₁₄Sb₃₀S₅₄O₅, or oxy-chloro-sulfides Pb₁₈Sb₂₀S₄₆Cl₂O and (Cu,Ag)₂Pb₂₁Sb₂₃S₅₅ClO. This compound belongs to a pseudo-homologous series of chalcogenides with waffle structure, ordered according to the size of their lozenge shape channel. Such a complex senary compound of the oxy-chloro-sulfide type illustrates the structural competition between three cations, on one hand, and, on the other hand, three anions. This compound is of special interest regarding the 1D distribution of magnetic Mn²⁺ atoms at the ∼2 nm scale.     

Oxidation/reduction studies on ZryU1−yO2+x and delineation of a new orthorhombic phase in U-Zr-O system

In this communication, we report the oxidation and reduction behavior of fluorite type solid solutions in U-Zr-O. The maximum solubility of ZrO₂ in UO₂ lattice could be achieved with a mild oxidizing followed by reducing conditions. The role of valency state of U is more dominating in controlling the unit cell parameters than the incorporated interstitial oxygen in the fluorite lattice. The controlled oxidation studies on U-Zr-O solid solutions led to the delineation of a new distorted fluorite lattice at the U:Zr=2:1 composition. The detailed crystal structure analysis of this ordered composition Zr_0.33U_0.67O_2.33 (ZrU₂O₇) has been carried from the powder XRD data. This phase crystallizes in an orthorhombically distorted fluorite type lattice with unit cell parameters: a=5.1678(2), b=5.4848(2), c=5.5557(2) Å and V=157.47(1) Å³ (Space group: Cmcm, No. 63). The metal ions have distorted cubical polyhedra with anion similar to the fluorite structure. The excess anions are occupied in the interstitial (empty cubes) of the fluorite unit cell. The crystal structure and chemical analyses suggest approximately equal fractions of U⁴⁺ and U⁶⁺ in this compound. The details of the thermal stability as well as kinetics of formation and oxidation of ZrU₂O₇ are also studied using thermogravimetry.     

Structure and thermal conductivity of Na1−xGe3+z

Structural analyses as well as low temperature thermal conductivity is reported for the binary phase Na_1−xGe_3+z. Specimens were characterized by thermal analysis, conventional and synchrotron powder X-ray diffraction, neutron powder diffraction, ²³Na nuclear magnetic resonance spectroscopy, and electrical and thermal transport measurements. With structural characteristics qualitatively analogous to some aluminum-silicate zeolites, the crystal structure of this phase exhibits an unconventional covalently bonded tunnel-like Ge framework, accommodating Na in channels of two different sizes. Observed to be non-stochiometric, Na_1−xGe_3+z concurrently exhibits substantial structural disorder in the large channels and a low lattice thermal conductivity, of interest in the context of identifying novel low thermal conductivity intermetallics for thermoelectric applications.     

The A1−xUNbO6−x/2 compounds (x=0, A=Li, Na, K, Cs and x=0.5, A=Rb, Cs): from layered to tunneled structure

Attempts to prepare alkaline metal uranyl niobates of composition A_1−xUNbO_6−x/2 by high-temperature solid-state reactions of A₂CO₃, U₃O₈ and Nb₂O₅ led to pure compounds for x=0 and A=Li (1), Na (2), K (3), Cs (4) and for x=0.5 and A=Rb (5), Cs (6). Single crystals were grown for 1, 3, 4, 5, 6 and for the mixed Na_0.92Cs_0.08UNbO₆ (7) compound. Crystallographic data: 1, monoclinic, P2₁/c, a=10.3091(11), b=6.4414(10), c=7.5602(5) Å, β=100.65(1), Z=4, R₁=0.054 (wR₂=0.107); 3, 5 and 7 orthorhombic, Pnma, Z=8, with a=10.307(2), 10.272(4) and 10.432(3) Å, b=7.588(1), 7.628(3) and 7.681(2) Å, c=13.403(2), 13.451(5) and 13.853(4) Å, R₁=0.023, 0.046 and 0.036 (wR₂=0.058, 0.0106 and 0.088) for 3, 5 and 7, respectively; 6, orthorhombic, Cmcm, Z=8, and a=13.952(3), b=10.607(2) Å, c=7.748(2) Å, R₁=0.044 (wR₂=0.117).The crystal structure of 1 is characterized by layers of uranophane sheet anion topology parallel to the (100) plane. These layers are formed by the association by edge-sharing of chains of edge-shared UO₇ pentagonal bipyramids and chains of corner-shared NbO₅ square pyramids alternating along the [010] direction. The Li⁺ ions are located between two consecutive layers and hold them together; the Li⁺ ions and two layers constitute a neutral “sandwich” {(UNbO₆)⁻-(Li)₂²⁺-(UNbO₆)⁻}. In this unusual structure, the neutral sandwiches are stacked one above another with no formal chemical bonds between the neutral sandwiches.The homeotypic compounds 3, 5, 6, 7 have open-framework structures built from the association by edge-sharing in two directions of parallel chains of edge-shared UO₇ pentagonal bipyramids and ribbons of two edge-shared NbO₆ octahedra further linked by corners. In 3, 5 and 7, the mono-dimensional large tunnels created in the [001] direction by this arrangement can be considered as the association by rectangular faces of two columns of triangular face-shared trigonal prisms of uranyl oxygens. In 3 and 7, all the trigonal prisms are occupied by the alkaline metal, in 5, they are half-occupied. In 6, the polyhedral arrangement is more symmetric and the tunnels created in the [010] direction are built of face-sharing cubes of uranyl oxygens totally occupied by the Cs atoms. This last compound well illustrates the structure-directing effect of the conterion.     

The nature of the orthorhombic to tetragonal phase transition in Sr1−xCaxMnO3

The orthorhombic-tetragonal phase transition in the perovskite series Sr_1−xCa_xMnO₃ 0.4?x?0.6 has been studied by synchrotron X-ray powder diffraction. At room temperature the Ca rich oxides x?0.45 have the orthorhombic Pbnm superstructure whereas Sr_0.6Ca_0.4MnO₃ is two phases with both tetragonal I4/mcm and orthorhombic Pbnm. Analysis of the octahedral tilts suggest the co-existence of these two phases is a consequence of a first-order I4/mcm to Pbnm transition. The evolution of the structure of Sr_0.5Ca_0.5MnO₃ with temperature is also described and this is found to evolve from orthorhombic to tetragonal and ultimately cubic.     

Magnetic order and heavy fermion behavior in CePd1+xAl6−x: Synthesis, structure, and physical properties

The physical properties including magnetic susceptibility, specific heat, and electrical resistivity of single crystals are reported for the compound CePd_1+xAl_6−x (x=0.5) which crystallizes in the tetragonal SrAu₂Ga₅-type structure (space group P4/mmm). The compound was grown from an excess of molten Al flux from the respective elements and the crystal structure was established from single-crystal X-ray diffraction. Anomalies in the low temperature specific heat C_p(T) and electrical resistivity ρ(T) show that the compound undergoes ferromagnetic order at T_C=2.8 K. In the ordered state, CePd_1.5Al_5.5 displays heavy fermion behavior with a Sommerfeld coefficient of ca. 500 mJ/mol-K².     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.     

Synthesis, crystal structure and magnetic properties of the spin ladder compounds La2(Cu1−xZnx)2O5 and La8(Cu1−xZnx)7O19

The influence of Zn-doping on the crystal structure and magnetic properties of the spin ladder compounds La₂Cu₂O₅ (4-leg) and La₈Cu₇O₁₉ (5-leg) have been investigated. The La₂(Cu_1−xZn_x)₂O₅ and La₈(Cu_1−xZn_x)₇O₁₉ solid solutions were obtained as single phases with x=0-0.1 via the solid-state reaction method in the temperature range between 1005-1010 °C and 1015-1030 °C in oxygen and air atmospheres, respectively. The lattice parameters a and c of the monoclinic crystal structures as well as the unit cell volume V increase with increasing x, while b and β decrease for both series. The magnetic susceptibilities χ of both series show a very similar behavior on temperature as well as on Zn-doping, which is supposed to be due to the similar Cu-O coordination in both La₂Cu₂O₅ and La₈Cu₇O₁₉. For low Zn-doping (x?0.04), a spin-chain like behavior is found. This quasi-one-dimensional behavior is strongly suppressed in both series for x?0.04. Here, the maximum (characteristic for spin chains) in χ(T) disappears and χ(T) decreases monotonically with increasing temperature.