Phase relations in NaxCrxTi8−xO16 at 1350 °C and crystal structure of hollandite-like Na2Cr2Ti6O16

Phase relations of rutile, freudenbergite, and hollandite structures were examined in the pseudobinary system NaCrO₂-TiO₂ (i.e., Na_xCr_xTi_8−xO₁₆) at 1350 °C. The hollandite structure was obtained in the composition range 1.7?x?2.0. The symmetry of the samples at room temperature was tetragonal for x=1.7 and 1.75, and monoclinic for x=1.8 and above. Single crystals of monoclinic hollandite Na₂Cr₂Ti₆O₁₆ were grown and the structure refinement has been carried out using an X-ray diffraction technique. The space group was I2/m and cell parameters were a=10.2385(11), b=2.9559(9), c=9.9097(11)Å, and β=90.545(9)° with Z=1. The Na ion distribution in the tunnel was markedly deformed from that in the tetragonal form. It was suggested that Cr/Ti ratios were different between the two framework metal sites.     

Structural characterization of the hollandite host lattice for the confinement of radioactive cesium: Quantification of the amorphous phase taking into account the incommensurate modulated character of the crystallized part

X-ray patterns of the Ba₁Cs_0.28Fe_0.82Al_1.46Ti_5.72O₁₆ compound evidence two wide peaks at low angle in addition to the well defined peaks of the I4/m hollandite structure type. Two hypotheses have been explored to account for these features: the coexistence of the hollandite phase with an amorphous phase and the appearance of a commensurate or incommensurate modulated structure associated with a cationic ordering, as proposed in the literature. Actually, even if the amorphous phase quantification by the Rietveld method reveals about 15 wt% of non-crystalline phase in some of the powdered sample, the origin of the two wide peaks was found to stem from the incommensurate modulated character of the hollandite structure type (super space group I4/m(0 0 γ)0 0) with a distribution of the modulation wavevectors presumably related to slight chemical composition changes.     

Electrochemical immobilization of Cs in single-crystalline SYNROC

The development of a disposal technique for the radiotoxic ¹³⁷Cs in nuclear wastes is one of the most urgent issues in nuclear fuel technology. An effective disposal method of ¹³⁷Cs is to immobilize it in a synthetic rock (SYNROC) material: cesium titanate hollandite, ¹³⁷Cs_xTi₈O₁₆ (I4/m, , ). Practical applications of ¹³⁷Cs_xTi₈O₁₆ have been restricted so far because the conventional synthetic method requires strong chemical reducers and reaction temperatures higher than 1250 °C. In this report, we present a milder preparation method of Cs_xTi₈O₁₆ by electrolysis of a mixture of Cs₂MoO₄ and TiO₂ in ambient atmosphere at 900 °C. The Cs content in the resultant single-crystalline Cs_1.35Ti₈O₁₆ is competitive with the highest value in polycrystalline Cs_1.36±0.03Ti₈O₁₆ prepared by the conventional synthetic method. The electrochemical preparation of Cs_1.35Ti₈O₁₆ is a promising way to immobilize a high quantity of ¹³⁷Cs ions in a stable form of single-crystalline SYNROC.     

The role of orbital ordering in the tetragonal-to-cubic phase transition in CuCr2O4

Copper(II) chromite (CuCr₂O₄) undergoes a first-order structural transition from a tetragonal distorted spinel structure in space group (I4₁/amd) to a cubic spinel structure in Fd3¯m, near 600 °C. The transition has been followed using synchrotron X-ray powder diffraction between room temperature and 750 °C. The structure changes as a consequence of a transition from an orbitally ordered to orbital disordered state associated with a Jahn-Teller-type distortion of the CuO₄ tetrahedron. The orbital melting results in a small increase in cell volume.     

The structural chemistry of Bi14MO24 (M=Cr, Mo, W) phases: bismuth oxides containing discrete MO4 tetrahedra

The phases Bi₁₄MO₂₄ (M=Cr, Mo, W) have been studied using differential scanning calorimetry, variable temperature X-ray powder diffraction and neutron powder diffraction. All three compounds were found to undergo a phase change, on cooling, from the previously reported tetragonal symmetry (I4/m) to monoclinic symmetry (C2/m). Transition temperatures were determined to be ∼306 K (M=W) and ∼295 K (M=Mo), whereas a gradual transition between 275 and 200 K was observed for M=Cr. The high and low temperature structures are very similar, as indicated by the relationship between the monoclinic and tetragonal unit cell parameters: a_m=√2a_t, b_m=c_t, c_m=a_t, β∼135°. High-resolution neutron powder diffraction data, collected at 400 and 4 K, were used to establish the nature of the transition, which was found to involve a reduction in the statistical possibilities for orientation of the MO₄ tetrahedra. However, in both tetragonal and monoclinic variants, a degree of orientational disorder of the tetrahedra occurs to give partially occupied sites in the average unit cell.     

Crystal structures and magnetic properties of CeAu4Si2 and CeAu2Si2

Single crystals of CeAu₄Si₂ and CeAu₂Si₂ have been grown out of ternary fluxes rich in Au, and the former, also by sintering the stoichiometric composition at 750 °C. The single-crystal X-ray refinement result for CeAu₄Si₂ is orthorhombic, Cmmm (No. 65, Z=2), different from a tetragonal result found from an X-ray powder diffraction refinement [H. Nakashima, et al., J. Alloys Compds. 424 (2006) 7]. For CeAu₂Si₂, this is the first report of the stoichiometric crystalline phase, in the known tetragonal I4/mmm structure. The anisotropic field- and temperature-dependent magnetizations, as well as specific heat and resistivity data are compared. Although both compounds have related structural packing, they present unique magnetic features. CeAu₂Si₂ is a typical antiferromagnet with T_N=8.8(1) K and CeAu₄Si₂ features a ferromagnetic component below T_c=3.3(1) K. Both phases have effective moments close in value to that of free Ce³⁺.     

Crystal growth and structural investigation of A2BReO6 (A=Sr, Ba; B=Li, Na)

Single crystals of the double perovskite rhenates A₂BReO₆ (A=Sr, Ba; B=Li, Na) were grown out of molten hydroxide fluxes. Single crystals of orange/yellow Ba₂LiReO₆, Ba₂NaReO₆ and Sr₂LiReO₆ were solved in the cubic, Fm-3m space group with a=8.1214(11) Å, 8.2975(3) Å, and 7.9071(15) Å, respectively, while Sr₂NaReO₆ was determined to be monoclinic P2₁/n with a=5.6737(6) Å, b=5.7988(6) Å, c=8.0431(8) Å, and β=90.02(6) °. The cubic structure consists of a rock salt lattice of corner-shared ReO₆ and MO₆ (M=Li, Na) octahedra which, in the monoclinic structure, are both tilted and rotated. A discrepancy exists between the symmetry of Sr₂LiReO₆ indicated by the single-crystal refinement of flux-grown crystals (cubic, Fm-3m) and the symmetry indicated by the powder diffraction data collected on polycrystalline samples prepared by the ceramic method (tetragonal, I4/m). It is possible that the cubic crystals are a kinetic product that forms in small quantities at low temperatures, while the powder represents the more stable polymorph that forms at higher reaction temperature.     

Synthesis and characterization of three-dimensionally ordered macroporous ternary oxide

A three-dimensionally ordered macroporous (3DOM) ternary oxide, CsAlTiO₄, with a framework related to ‘stuffed-tridymite’ has been synthesized at temperatures 500-700 °C using a sol-gel precursor solution and templating with polystyrene spheres. The 3DOM material displayed pore diameters of 0.5-0.8 μm with the walls composed of anhedral and acicular CsAlTiO₄ crystals whose dimensions ranged from 16 to 25 nm. Microanalysis confirmed near-stoichiometric proportions (1:1:1) of Cs, Al and Ti. The effect of sintering temperature on the macroporous structure and on the CsAlTiO₄ walls was studied. As the sintering temperature increased from 500 to 600 °C the unit cell parameters varied through dilation (a and b) and contraction (c-axis), followed by a reversal of these trends from 700 to 900 °C. This behaviour in non-equilibrated CsAlTiO₄ can be attributed to distortion of the (Al, Ti)O₄ tetrahedral framework, however at the highest temperature the cell constants stabilized close to those reported for single crystal CsAlTiO₄. X-ray amorphous content was significant in all materials varying from 73 wt% after 500^oC and reducing to 44 wt% at 900 °C.     

The effect of disorder in Ba2YTaO6 on the tetragonal to cubic phase transition

Synchrotron X-ray diffraction and Raman spectroscopy have been used to study the structure of the complex perovskite Ba₂YTaO₆, at temperatures down to 100 K. Where the Ta and Y cations exhibit long-range rock-salt like ordering, Ba₂YTaO₆ displays a continuous phase transition from a high temperature cubic structure, described in space group Fm3?m, to a tetragonal, I4/m, structure near 260 K. This transition is inhibited if extensive disorder and/or vacancies are/is present in the sample.     

Structural evolution of (Ca0.35Sr0.65)TiO3 perovskite at high pressures

Lattice parameters of a synthetic powder sample of Ca_0.35Sr_0.65TiO₃ perovskite have been determined by the method of Le Bail refinement, using synchrotron X-ray diffraction patterns collected at pressures up to 15.5 GPa with a membrane-driven diamond anvil cell. At ambient conditions, diffraction data were consistent with the I4/mcm structure reported previously in the literature for the same composition. Diffraction data collected at high pressures were consistent with tetragonal (or, at least, pseudo-tetragonal) lattice geometry, and no evidence was found for the development of any of the orthorhombic structures identified in other studies of (Ca, Sr)TiO₃ perovskites. Additional weak reflections, which could not be accounted for by the normal I4/mcm perovskite structure, were detected in diffraction patterns collected at pressures of 0.9-2.5 GPa, and above ∼13.5 GPa, however. Small anomalies in the evolution of unit cell volume and tetragonal strain were observed near 3 GPa, coinciding approximately with breaks in slope with increasing pressure of bulk and shear moduli for a sample with the same composition which had previously been reported. The anomalies could be due either to new tetragonal↔tetragonal/pseudo-tetragonal phase transitions or to subtle changes in compression mechanism of the tetragonal perovskite structure.     

B-site disordering in Ba3Ln2MoO9 (Ln=Ho, Er) perovskites: A neutron diffraction study

We describe the preparation, structure determination and magnetic properties of two Ba perovskites containing rare-earth cations at the B-sublattice. Ba₃Ln₂MoO₉ (Ln=Ho³⁺ and Er³⁺) were synthesized by ceramic procedures. Joint X-ray (XRPD) and neutron (NPD) powder diffraction refinements were carried out to analyse the crystal structure. At room temperature, both phases are tetragonal, space group I4/mcm, Z=4. Ln and Mo atoms are found to be distributed at random over the octahedral sites of the perovskites. Magnetic measurements at 0.1 T show that both samples are paramagnetic between 3 and 300 K, following a Curie-Weiss law. M vs. H curves show a region of paramagnetic behaviour and above 2.5 T a magnetic saturated system is observed. Finally, the temperature evolution of the NPD patterns of Ba₃Ho₂MoO₉ reveals the absence of long-range magnetic ordering down to 2 K.     

Hydrothermal synthesis and characteristics of 3-D hydrated bismuth oxalate coordination polymers with open-channel structure

Two new 3-D porous bismuth coordination polymers, (C₅NH₆)₂[Bi₂(H₂O)₂(C₂O₄)₄]·2H₂O 1 and (NH₄)[Bi(C₂O₄)₂]·3H₂O 2, have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction. Compound 1 crystallizes in the monoclinic symmetry, P2₁/c space group with a=10.378(2) Å, b=17.285(3) Å, c=16.563(5) Å, α=90°, β=119.66(2)°, γ=90°, V=2581.8(10) Å³, Z=4, R₁=0.0355 and wR₂=0.0658 for unique 4713 reflections I >2σ(I). Compound 2 crystallizes in the tetragonal symmetry, I4₁/amd space group with a=11.7026(17) Å, b=11.7026(17) Å, c=9.2233(18) Å, α=90°, β=90°, γ=90°, V=1263.1(4) Å³, Z=32, R₁=0.0208 and wR₂=0.0518 for unique 359 reflections I> 2σ(I). Compounds 1 and 2 are 3-D open-framework structures with a 6⁶ uniform net, which consist of honeycomb-like layers connected to each other by oxalate units. While different guest molecules fill in their cavities of honeycomb-like layers, study of ultrasonic treatment on 2 indicates the replacement of NH₄⁺ by K⁺ on potassium ion exchange. Thermogravimetric analysis indicates that the open-channel frameworks are thermally stable up to 200 °C, and other characterizations are also described by elemental analysis, IR and ultraviolet-visible diffuse reflectionintegral spectrum (UV-Vis DRIS).     

Synthesis and crystal structure of Mg2B24C, a new boron-rich boride related to “tetragonal boron I”

Single crystals of Mg₂B₂₄C, a new boron-rich boridecarbide of magnesium, were synthesized as black needles and columns by reaction of the elements in Ta ampoules and BN crucibles at 1300 °C. The crystal structure was determined by X-ray diffraction (P-4n2, a=8.9391(13)Å, c=5.0745(10)Å, Z=2, 713 reflections, 64 variables, R₁(F)=0.0235, wR₂(I)=0.0591). It is closely related to “tetragonal boron I” and can be described as a tetragonal rod packing of corner-linked B₁₂ icosahedra with C and Mg atoms in the voids. Each B₁₂ icosahedron has 2 B-C bonds and 10 exohedral bonds to other icosahedra, 2 within the rod and 4×2 to neighbouring rods. The isolated C atoms are 4-fold coordinated forming distorted tetrahedra. Mg is placed on two crystallographically independent positions within the three-dimensional B₁₂C network. Mg₂B₂₄C is the first example for a compound related to “tetragonal boron I” with a stoichiometric composition.     

Synthesis, crystal structure and physico-chemical properties of the new quaternary oxide Sr5BiNi2O9.6

A new black quaternary oxide Sr₅BiNi₂O_9.6 was synthesized by solid state reaction at 1200 °C. Its structure was solved by electron crystallography and X-ray powder refinement, yielding a tetragonal structure with space group I4/mmm, a=5.3637 (2) Å, c=17.5541(5) Å, Z=4. The structure can be described as a stacking of (Bi,Sr)-O rocksalt slabs and SrNiO_3−δ perovskite slabs. The initial nickel valence is close to +3.1. Thermogravimetry and high-temperature oxygen coulometry showed that this compound has variable oxygen content as a function of temperature and oxygen pressure, and ultimately decomposes when heated in low oxygen pressure above 800 °C. It is a metallic conductor with n-type conduction. Its thermoelectric power was determined and found to be −20 and −38 μV/K at 300 and 650 °C, respectively. Magnetic measurements confirm the nickel valence close to +3 and show evidence of magnetic ordering at 20 K.     

Structure and conductivity of strontium-doped cerium orthovanadates Ce1−xSrxVO4 (0≤x≤0.175)

A-site substituted cerium orthovanadates, Ce_1−xSr_xVO₄, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce_1−xSr_xVO₄ is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I4₁/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce_0.825Sr_0.175VO₄. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H₂/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm⁻¹ in air to 30-45 mS cm⁻¹ in reduced atmosphere. Sample Ce_0.825Sr_0.175VO₄ is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells.     

Phase relations in a barium-rich high-temperature region (25-45 mol% CuO, 900-1100 °C) of the BaO-CuOx system

The phase relations have been studied in the BaO-CuO_x system in the range of 25.0-45.0 mol% CuO at 900-1100 °C at P(O₂)=21 kPa (air) by visual polythermal analysis (VPA), powder X-ray diffraction (XRD), electron diffraction (ED) with simultaneous energy-dispersive X-ray (EDX) elemental analysis in a transmission electron microscope (TEM), and iodometric chemical analysis. The discrete deviations 2.02 (101:50), 2.04 (102:50), 2.10 (105:50) of Ba/Cu (Ba:Cu) composition from the stoichiometric ratio 2:1 have been found for the known Ba₂CuO_3+δ oxides in the subsolidus region 900-970 °C. Unit cell parameters of the 101:50 orthorhombic oxide, 102:50 tetragonal one, 105:50 orthorhombic one are, respectively, a=4.049, b=3.899, c=13.034 Å; a=3.985, c=12.968 Å; a=4.087, b=3.897 and c=12.950 Å. ED patterns of the 101:50, 102:50, 105:50 oxides show characteristic supercell reflections with the respective vector 1/60[5 4 0], ≈2/11〈1 1 0〉 and 1/6[2 0 0]. Oxides of the 2:1, 7:4, 5:3 and 23:20 compositions have been found in the crystallization region 970-1050 °C. Unit cell parameters of the 2:1 orthorhombic oxide are a=4.095, b=3.795, c=13.165 Å. Interplanar spacings and X-ray characteristic peak intensities of the 7:4, 5:3 and 23:20 oxides are given. Oxides 2:1 and 7:4 melt pseudocongruently at 1020 and 1005 °C, oxides 5:3 and 23:20 melt incongruently at 995 and 980 °C, respectively. A diagram of the phase relations in the studied region of the BaO-CuO_x system has been constructed, whose structure is considered as the total projection of phase states of the system existing for different x.     

Ba14Zn5−xAl22+x: a new polar intermetallic compound with a novel 2D network

A new ternary, intermetallic compound, Ba₁₄Zn_5−xAl_22+x, was synthesized by heating the pure elements at 900°C. This compound crystallizes in the monoclinic space group I2/m, Z=2, with a=10.474(2) Å, b=6.0834(14) Å, c=34.697(8) Å and β=90.814(4)°. The crystal structure of Ba₁₄Zn_5−xAl_22+x consists of [Zn_5−xAl_22+x] slabs that are built with a novel, two-dimensional (2D) network of Zn and Al atoms involving eight-membered rings sandwiched between two layers of trigonal bipyramids interconnected by three-center bonding. Tight-binding, linear muffin-tin orbital (TB-LMTO-ASA) calculations have been performed to understand the relationship between composition and orbital interactions in the electronegative element framework. This new structure is closely related to the high-pressure, cubic Laves-type structure of BaAl₂ as well as the ambient pressure binary compound, Ba₇Al₁₃. The degree of valence electron charge transfer from the electropositive Ba atoms is related to the Al:Ba molar ratio in the Ba-Zn-Al system.     

Two non-centrosymmetric cubic seleno-germanates related to CsCl-type structure: Synthesis, structure, magnetic and optical properties

Two related non-centrosymmetric, cubic, quaternary chalcogenides, containing europium have been synthesized employing the molten flux method and by the reaction of europium halides with the ternary seleno-germanate, Na₆Ge₂Se₆. The reactions of Eu, Ge and Se were accomplished in a molten Na₂Se₂ flux at 800 °C for 150 h in an evacuated fused silica ampoule and yielded Na₂EuGeSe₄ (I). Similarly, Na_0.75Eu_1.625GeSe₄ (II), could be synthesized at slightly lower temperature (750 °C) with a different starting ratio of Eu, Ge, Se and Na₂Se₂. A reaction between EuCl₃ and Na₆Ge₂Se₆ in 1:2 ratio at 650 °C for 96 h in an evacuated fused silica ampoule yielded Na₂EuGeSe₄ (I), while the reaction between EuI₂ and Na₆Ge₂Se₆ in 1:1 ratio under similar conditions yielded Na_0.75Eu_1.625GeSe₄ (II). Crystal data for these compounds are as follows: I, cubic, space group I4¯3m (no. 217), a=7.3466(3), Z=2; II, cubic, space group I4¯3d (no. 220), a=14.7065(8), Z=16. The crystal structures of I and II are closely related and can be compared to a CsCl-type and its ordered superstructure, respectively. These compounds are semiconductors with optical band gaps around 2 eV. The temperature dependence of the magnetic susceptibility indicated that both compounds are paramagnetic with μ_eff.=7.6 and 7.73 μ_B, for I and II, respectively, close to the theoretical value of 7.94 μ_B for Eu²⁺. Raman spectroscopic characterization of the compounds is also reported.     

Structural phase transition and magnetic properties of double perovskites Ba2CaMO6 (M=W, Re, Os)

Structures and magnetic properties for double perovskites Ba₂CaMO₆ (M=W, Re, Os) were investigated. Both Ba₂CaReO₆ and Ba₂CaWO₆ show structural phase transitions at low temperatures. For Ba₂CaReO₆, the second order transition from cubic to tetragonal I4/m has been observed near 120 K. For Ba₂CaWO₆, the space group of the crystal structure is I4/m at 295 K and the transition to monoclinic I2/m has been observed between 220 K. Magnetic susceptibility measurements show that Ba₂CaReO₆ (S=1/2) and Ba₂CaOsO₆ (S=1) transform to an antiferromagnetic state below 15.4 and 51 K, respectively. Anomalies corresponding to their structural phase transition and magnetic transition have been also observed through specific heat measurements.