Syntheses and crystal structures of three new borates templated by transition-metal complexes in situ

Three new cobalt borate compounds, [Co(DIEN)₂][B₅O₆(OH)₄]₂ (DIEN=diethylenetriamine) (1), [B₅O₇(OH)₃Co(TREN)] (TREN=tris(2-aminoethyl)amine) (2), and [Co₂(TETA)₃][B₅O₆(OH)₄]₄ (TETA=triethylenetetramine) (3) have been hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction, IR, elemental analysis and thermogravimetry. The structures exhibit interesting 3D supramolecular hydrogen-bonded architectures, involving the similar borate polyanion [B₅O_6+n(OH)_4−n]⁽ⁿ⁺¹⁾⁻(n=0 for 1 and 3, and n=1 for 2) and the templating transition metal complexes which are generated in situ under mild solvothermal conditions. Crystal data: 1, monoclinic, space group C2/m (No. 12), , , , β=93.601(4)°, , Z=2; 2, monoclinic, P2₁/c (No. 14), , , , β=99.926(4)°, , Z=4; 3, triclinic, space group P-1 (No. 2), , , , α=77.009(5)°, β=80.095(2)°, γ=82.334(3)°, , Z=2.     

Crystal growth, characterization and physical properties of PrNiSb3, NdNiSb3 and SmNiSb3

The crystal structures of three new intermetallic ternary compounds in the LnNiSb₃ (Ln=Pr, Nd and Sm) family have been characterized by single crystal X-ray diffraction. PrNiSb₃, NdNiSb₃ and SmNiSb₃ all crystallize in an orthorhombic space group, Pbcm (No. 57), Z=12, with , , , and ; , , , and ; and , , , and , for Ln=Pr, Nd and Sm, respectively. These compounds consist of rare-earth atoms located above and below layers of nearly square, buckled Sb nets, along with layers of highly distorted edge- and face-sharing NiSb₆ octahedra. Resistivity data indicate metallic behavior for all three compounds. Magnetization measurements show antiferromagnetic behavior with (PrNiSb₃), 4.6 K (NdNiSb₃), and 2.9 K (SmNiSb₃). Effective moments of 3.62 μ_B, 3.90 μ_B and 0.80 μ_B are found for PrNiSb₃, NdNiSb₃ and SmNiSb₃, respectively, and are consistent with Pr³⁺ (f ²), Nd³⁺ (f ³), and Sm³⁺ (f ⁴).     

Synthesis and characterizations of well-defined dendrimer-like copolymers with the second and third generation based on polystyrene and poly(l-lactide)

Dendrimer-like copolymers with two and three generations, (polystyrene)₃-b-(poly(l-lactide))₆ (PSt₃-b-PLLA₆) and PSt₃-b-PLLA₆-b-PSt₁₂ have been successfully prepared using core-first method. The first step of this synthesis is the preparation of three-armed PSt by atom transfer radical polymerization (ATRP) of St using 1,1,1-tri(methylene-α-bromoisobutyryl)propane as initiator. Terminal divergence of the polymers obtained was achieved by the reaction of terminal bromines with branching agent, 2,2-dimethyl-1,3-dioxolane-4-methanol. After deprotection, the polymer with six terminal hydroxyl groups was used in the ring-opening polymerization of LLA. The dendrimer-like copolymer with PLLA as a second generation diverged continuously by the reaction of 6 terminal hydroxyl groups with branching agent, 2,2-bis(methylene-α-bromoisobutyryl)propionyl chloride. The resultant polymer with 12 terminal bromines was used as macroinitiator in the ATRP of St to produce the target dendrimer-like copolymer, PSt₃-b-PLLA₆-b-PSt₁₂. The structures of polymers obtained from each step were confirmed by their ¹H NMR spectra and GPC measurements. DSC results show one for the three-armed PSt, , and for the dendrimer-like copolymer with two generations, C(PSt(PLLA)₂)₃, and , and for the copolymer with three generations, C(PSt(PLLA(PSt)₂)₂)₃.     

Synthesis, structural determination and magnetic properties of layered hybrid organic-inorganic, iron (II) propylphosphonate, Fe[(CH3(CH2)2PO3)(H2O)], and iron (II) octadecylphosphonate, Fe[(CH3(CH2)17PO3)(H2O)]

Fe[(CH₃(CH₂)₂PO₃)(H₂O)] (1) and Fe[(CH₃(CH₂)₁₇PO₃)(H₂O)] (2) were synthesized by reaction of FeCl₂·6H₂O and the relevant phosphonic acid in water in presence of urea and under inert atmosphere. The compounds were characterized by elemental and thermogravimetric analyses, UV-visible and IR spectroscopy. The crystal structure of (1) was determined from X-ray single crystal diffraction studies at room temperature: monoclinic symmetry, space group P2₁, , , , and β=98.62(3)°. The compound is lamellar and the structure is hybrid, made of alternating inorganic and organic layers along the c direction. The inorganic layers consist of Fe(II) ions octahedrally coordinated by five phosphonate oxygen atoms and one from the water molecule, separated by bi-layers of propyl groups. A preliminary structure characterization of compound (2) suggests a similar layered structure, but with an interlayer spacing of 40.3 Å. The magnetic properties of the compounds were both studied by a dc and ac SQUID magnetometer. Fe[(CH₃(CH₂)₂PO₃)(H₂O)] (1) obeys the Curie-Weiss law at temperatures above 50 K (, ), indicating a Fe +II oxidation state, a high-spin d⁶ (S=2) electronic configuration and an antiferromagnetic exchange couplings between the near-neighbouring Fe(II) ions. Below , Fe[(CH₃(CH₂)₂PO₃)(H₂O)] exhibits a weak ferromagnetism. The critical temperature of has been determined by ac magnetic susceptibility measurements. Compound (2) shows the same paramagnetic behaviour of the iron (II) propyl derivative. The values of C and θ were found to be and −44 K, respectively, thus suggesting the presence of Fe +II ion in the S=2 spin state and antiferromagnetic interactions between Fe(II) ions at low temperatures. Zero-field and field cooled magnetic susceptibility vs. T plots do not overlap below , suggesting the presence of an ordered magnetic state. The critical temperature, T_N, has been located by the peaks at from the ac susceptibility (χ′and χ″) vs. T plots. Below T_N hysteresis loops recorded in the temperature region show an S-shape, while below 15 K assume an ellipsoid form. They reveal that compound (2) is a weak ferromagnet. The critical temperature T_N in these layered Fe(II) alkylphosphonates is independent of the distance between the inorganic layers.     

Structural homologies in benzylamino-N,N-bis methylphosphonic acid and its layered zirconium derivative

A new layered zirconium diphosphonate fluoride, ZrF(O₃PCH₂)₂NHCH₂C₆H₅ has been prepared and its structure determined ab initio by X-ray powder data and refined with the Rietveld method (orthorhombic, , , , space group Pbca, , Z=8, R_wp=0.080). Both phosphonic groups of each diphosphonate building block are bonded to zirconium atoms on the same side of each layer. Benzyl groups from adjacent layers are interdigitated in the interlayer region, with probable π-π stacking interactions. The structure of the free benzylamino-N,N-bis methylphosphonic acid has been determined by single crystal X-ray data (monoclinic, space group P2₁, , , , β=92.930(3)°, , Z=2, R₁=0.072, wR₂=0.150). As in the zirconium derivative, benzyl groups from adjacent layers are interdigitated and create a regular alternation of polar and non-polar regions.     

Structural characterization and ferroelectric ordering in (C3N2H5)5Sb2Br11

A ferroelectric crystal (C₃N₂H₅)₅Sb₂Br₁₁ has been synthesized. The single crystal X-ray diffraction studies (at 300, 155, 138 and 121 K) show that it is built up of discrete corner-sharing bioctahedra and highly disordered imidazolium cations. The room temperature crystal structure has been determined as monoclinic, space group, P2₁/n with: , and and β=96.19^°. The crystal undergoes three solid-solid phase transitions: ) discontinuous, continuous and discontinuous. The dielectric and pyroelectric measurements allow us to characterize the low temperature phases III and IV as ferroelectric with the Curie point at 145 K and the saturated spontaneous polarization value of the order of along the a-axis (135 K). The ferroelectric phase transition mechanism at 145 K is due to the dynamics of imidazolium cations.     

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.     

Thermoelectric properties and antiferromagnetism of the new ternary transition metal telluride CrAuTe4

The new ternary transition metal telluride CrAuTe₄ has been discovered through solid-state reaction of the elements. The crystal structure is solved in the monoclinic space group P2/m (No. 10) with lattice parameters , , , and β=90.604(10)°. The structure is related to that of the binary compound AuTe₂, and a derivation of the structure of CrAuTe₄ from AuTe₂ is shown. Measurements of the thermopower, thermal conductivity, electrical resistivity, and magnetic susceptibility are presented. The compound undergoes a paramagnetic to antiferromagnetic transition at .     

Phase transitions in Ca3(BN2)2 and Sr3(BN2)2

α-Ca₃(BN₂)₂ crystallizes in the cubic system (space group: ) with one type of calcium ions disordered over of equivalent (8c) positions. An ordered low-temperature phase (β-Ca₃(BN₂)₂) was prepared and found to crystallize in the orthorhombic system (space group: Cmca) with lattice parameters: , , and . Structure refinements on the basis of X-ray powder data have revealed that orthorhombic β-Ca₃(BN₂)₂ corresponds to an ordered super-structure of cubic α-Ca₃(BN₂)₂. The space group Cmca assigned for β-Ca₃(BN₂)₂ is derived from by a group-subgroup relationship.DSC measurements and temperature-dependent in situ X-ray powder diffraction studies showed reversible phase transitions between β- and α-Ca₃(BN₂)₂ with transition temperatures between 215 and 240 °C.The structure Sr₃(BN₂)₂ was reported isotypic with α-Ca₃(BN₂)₂ () with one type of strontium ions being disordered over of equivalent (2c) positions. In addition, a primitive () structure has been reported for Sr₃(BN₂)₂. Phase stability studies on Sr₃(BN₂)₂ revealed a phase transition between a primitive and a body-centred lattice around 820 °C. The experiments showed that both previously published structures are correct and can be assigned as α-Sr₃(BN₂)₂ (, high-temperature phase), and β-Sr₃(BN₂)₂ (, low-temperature phase).A comparison of Ca₃(BN₂)₂ and Sr₃(BN₂)₂ phases reveals that the different types of cation disordering present in both of the cubic α-phases () have a directing influence on the formation of two distinct (orthorhombic and cubic) low-temperature phases.     

Extension of the La7Mo7O30 structural type with La7Nb3W4O30 and La7Ta3W4O30 compounds

Two compounds of formula La₇A₃W₄O₃₀ (with A=Nb and Ta) were prepared by solid-state reaction at 1450 and 1490 °C. They crystallize in the rhombohedric space group R-3 (No. 148), with the hexagonal parameters: , and , . The structure of the materials was analyzed from X-ray, neutron and electronic diffraction. These oxides are isostructural of the reduced molybdenum compound La₇Mo₇O₃₀, which are formed of perovskite rod along [111]. An order between (Nb, Ta) and W is observed.     

Synthesis of rare-earth selenate and selenite materials under “sol-gel” hydrothermal conditions: crystal structures and characterizations of La(HSeO3)(SeO4) and KNd(SeO4)2

Two rare-earth compounds containing selenium atoms, La(HSeO₃)(SeO₄) with a new open framework structure and KNd(SeO₄)₂ with a layered structure, have been synthesized under “sol-gel” hydrothermal conditions for the first time. Single-crystals of La(HSeO₃)(SeO₄) crystallize in the monoclinic system (P2₁, , , , β=104.91(3)°, Z=2, R_All=0.032). The structure contains puckered polyhedral layers made of LaO_x (x=9,10) and SeO₄ groups, which are connected via SeO₃-uints to the 3D structure. The crytal structure of KNd(SeO₄)₂ (monoclinc, P2₁/c, , , , β=91.38(3)°, Z=4, R_All=0.051) contains honeycomb-like six-ring NdO₉ polyhedra forming layers which are further decorated with SeO₄ tetrahedra. The K⁺ ions occupy the interspaces of these layers and provide the charge balance.     

Systems R-Fe-O (R=Ho, Er): Thermodynamic properties of ternary oxides using differential scanning calorimetry and solid-state electrochemical cells

The thermodynamic properties of three different types of ternary oxides RFeO₃(s), R₃Fe₅O₁₂(s) and RFe₂O₄(s) (where R=Ho and Er) have been determined by calorimetric and solid-state galvanic cell methods. Heat capacities of RFeO₃(s) and R₃Fe₅O₁₂(s) have been determined by differential scanning calorimetry from 130 to 860 K. Heat capacity measurements from 130 to 860 K revealed λ-type anomalies for RFeO₃(s) and R₃Fe₅O₁₂(s) compounds which are assigned due to magnetic order-disorder transitions. The oxygen chemical potentials corresponding to the three-phase equilibria involving these ternary oxides have been determined by using solid-state electrochemical cells. The standard molar Gibbs energies of formation of RFeO₃(s), R₃Fe₅O₁₂(s) and RFe₂O₄(s) have been computed from the oxygen potential data. Based on the thermodynamic information, oxygen potential diagrams have been computed for the systems R-Fe-O (R=Ho and Er) at two different temperatures: T=1250 and 1450 K. Thermodynamic functions like , , H^o, G^o, , , , , and have been generated for the compounds RFeO₃(s) and R₃Fe₅O₁₂(s) based on the experimental data obtained in this study and the available data in the literature.     

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.     

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.     

Synthesis, crystal structure and magnetic properties of an Os-containing pillared perovskite La5Os3MnO16

A new Os-containing, pillared perovskite, La₅Os₃MnO₁₆, has been synthesized by solid state reaction in sealed quartz tubes. This extends the crystal chemistry of these materials which had been known only for Mo and Re, previously. The crystal structure has been characterized by X-ray and neutron powder diffraction and is described in space group C-1 with parameters a=7.9648(9) Å; b=8.062(1) Å; c=10.156(2) Å, α=90.25(1)°, β=95.5(1)°; γ=89.95(2)°, for La₅Os₃MnO₁₆. The compound is isostructural with the corresponding La₅Re₃MnO₁₆ phase. A very short Os-Os distance of 2.50(1) Å was found in the dimeric pillaring unit, Os₂O₁₀, suggestive of a triple bond as demanded by electron counting. Nearly spin only values for the effective moment for Os⁵⁺ () and Mn²⁺ () were derived from magnetic susceptibility data. Evidence for magnetic transitions was seen near ∼180 and 80 K. Neutron diffraction data indicate that T_c is 170(5) K. The magnetic structure of La₅Os₃MnO₁₆ at 7 K was solved revealing that Os⁵⁺ and Mn²⁺ form ferrimagnetically coupled layers with antiferromagnetic interlayer ordering. The ordered moments are for Mn²⁺ and for Os⁵⁺, which are reduced from the respective spin only values of 5.0 and . The observation of net ferrimagnetic (antiparallel) intraplanar coupling between Os⁵⁺(t_2g³) and Mn²⁺(t_2g³e_g²) is interesting as it appears to contradict the Goodenough-Kanamori rules for 180° superexchange.     

Dimorphism in iron(II) methylphosphonate: Low-temperature crystal structure and temperature-dependent Mössbauer studies of a new form of the layered weak ferromagnet Fe[(CH3PO3)(H2O)]

A second form of the literature-known layered weak ferromagnet Fe[(CH₃PO₃)(H₂O)] has been isolated. The crystal structure determination of this new form (2) has been carried out at T=300, 200 and 130 K. It crystallizes in the orthorhombic space group Pmn2₁: a=5.7177(11), b=8.8093(18), , while form (1) crystallizes in the space group Pna2₁: a=17.58(2), b=4.814(1), . Mössbauer spectroscopy on form (2) has been performed in the temperature range 4-300 K; and, at , a drastic change in the quadrupole splitting (ΔE) and a broadening of the doublet components is noticed. But surprisingly, on cooling the crystal, no structural change is observed, which could account for the increase in ΔE. Below , ⁵⁷Fe spectra transform into hyperfine splitting patterns which reveal a magnetically ordered state in agreement with the results of earlier magnetic susceptibility studies.     

Combined powder neutron and X-ray diffraction study of charge and orbital order in Bi0.75Sr0.25MnO3

The room temperature structure of Bi_0.75Sr_0.25MnO₃ has been fitted to high-resolution synchrotron X-ray and time-of-flight neutron powder diffraction data. Constrained structural models were refined using a Pn11 supercell (, , , and α=89.894(1)°) of the underlying Pnma perovskite structure. The best-fit model evidences a 3:1 Mn³⁺/Mn⁴⁺charge ordering with only 30% of the ideal separation of bond valence sums. An ordered intergrowth of antiferro-orbitally ordered (LaMnO₃ type) and charge and ferro-orbitally ordered (YBaMn₂O₆ type) blocks is observed. Off-centre Bi/Sr displacements are ferroelectrically ordered in this model.     

Effect of initial monomer concentration on the equilibrium swelling and elasticity of hydrogels

The linear swelling ratio α and the effective network chain length N of a series of poly(N,N-dimethylacrylamide) (PDMAAm) hydrogels were investigated as a function of the gel preparation concentration . PDMAAm hydrogels were prepared at a fixed cross-linker ratio but at various initial monomer concentrations. It was found that α is not a monotonic function of . As is increased, α first decreases up to about and remains constant in a narrow range of , but then it increases continuously. The -dependence of α is due to the variation of the network chain length N depending on the gel preparation concentration. In the range of below 0.1, N follows the scaling relationship , while at higher concentrations, N varies only slightly with . The increase of α with N obeys the relation , as predicted by the Flory-Rehner theory.