Nouveaux echangeurs cationiques avec une structure a tunnels entrecroises: Les oxydes A12M33O90 et A12M33O90, 12H2O

New oxides with A₁₂M₃₃O₉₀ formula (A = Rb, Cs, Tl) have been synthesized. They crystallize in the trigonal system and can be described by pyrochlore and A₂M₇O₁₈ phases intergrowth. Cationic ionexchange properties of these compounds are brought out in aqueous solutions and in solid state. So, new hydrated oxides are prepared and their thermal decomposition has been studied. Relations between ionexchange properties and structure are discussed.     

Synthesis and characterizations of two anhydrous metal borophosphates: M2BP3O12 (M=Fe, In)

Two members of M^III₂BP₃O₁₂ borophosphates, namely Fe₂BP₃O₁₂ and In₂BP₃O₁₂, were synthesized by the solid-state method and characterized by the X-ray single crystal diffraction, the powder diffraction and the electron microscopy. They both crystallize in the hexagonal system, space group P6(3)/m (no. 176) and feature 3D architectures, build up of the M₂O₉ units and B(PO₄)₃ groups via sharing the corners; however, they are not isomorphic for the different crystallographically distinct atomic positions. Optical property measurements of both compounds and magnetic susceptibility measurements of Fe₂BP₃O₁₂ also have been performed. Moreover, in order to gain further insights into the relationship between physical properties and band structure of the M^III₂BP₃O₁₂ borophosphates, theoretical calculations based on density functional theory (DFT) were performed using the total-energy code CASTEP.     

Synthesis and characterization of phosphates in molten systems Cs2O-P2O5-CaO-M2O3 (M—Al, Fe, Cr)

The crystallization of complex phosphates from the melts of Cs₂O-P₂O₅-CaO-M^III₂O₃ (M^III—Al, Fe, Cr) systems have been investigated at fixed value Cs/P molar ratios equal to 0.7, 1.0 and 1.3 and Са/Р=0.2 and Ca/М^III=1. The fields of crystallization of CsCaP₃O₉, β-Ca₂P₂O₇, Cs₂CaP₂O₇, Cs₃CaFe(P₂O₇)₂, Ca₉M^III(PO₄)₇ (M^III—Fe, Cr), Cs_0.63Ca_9.63Fe_0.37(PO₄)₇ and CsCa₁₀(PO₄)₇ were determined. Obtained phosphates were investigated using powder X-ray diffraction and FTIR spectroscopy. Novel whitlockite-related phases CsCa₁₀(PO₄)₇ and Cs_0.63Ca_9.63Fe_0.37(PO₄)₇ have been characterized by single crystal X-ray diffraction: space group R3c, a=10.5536(5) and 10.5221(4) Å, с=37.2283(19) and 37.2405(17) Å, respectively.     

Phase formation features in the systems M2MoO4-Fe2(MoO4)3 (M=Rb, Cs) and crystal structures of new double polymolybdates M3FeMo4O15

The systems M₂MoO₄-Fe₂(MoO₄)₃ (M=Rb, Cs) were shown to be non-quasibinary joins of the systems M₂O-Fe₂O₃-MoO₃. New compounds M₃FeMo₄O₁₅ were revealed along with the known MFe(MoO₄)₂ and M₅Fe(MoO₄)₄. The unit cell parameters of the new compounds are a=11.6192(2), b=13.6801(3), c=9.7773(2) Å, β=92.964(1)°, space group P2₁/c, Z=4 (M=Rb) and a=11.5500(9), b=9.9929(7), c=14.513(1) Å, β=90.676(2)°, space group P2₁/n, Z=4 (M=Cs). In the structures of M₃FeMo₄O₁₅ (M=Rb, Cs), a half of the FeO₆ octahedra share two opposite edges with two MoO₆ octahedra linked to other FeO₆ octahedra through the bridged MoO₄ tetrahedra by means of the common oxygen vertices to form the chains along the a axis. The difference between the structures is caused by diverse mutual arrangements of the adjacent polyhedral chains.     

Novel KTP-like complex phosphates KM0.33Nb0.67PO5 (M—Mn, Co)

Single crystals of KTP-related phosphates of general formula KM^II_0.33Nb_0.67PO₅ (M^II—Co, Mn) have been obtained in the pseudo-binary system K₂Mo₂O₇-KM^II_0.33Nb_0.67PO₅ by means of the flux technique. The compounds have been studied by single crystal and powder X-ray diffraction, FTIR and UV-VIS spectroscopy. Both complex phosphates belong to the tetragonal system and crystallize in the enantiomorphous polar space groups P4₁ and P4₃. The structure contains cis-like helical chains which are connected by corner sharing Nb(M^II)O₆ octahedra and further linked through the PO₄ tetrahedra. The potassium cations are located in the channels running along [001] direction of the anionic framework [M^II_0.33Nb_0.67PO₅]⁻ and are irregularly coordinated by eight oxygen atoms. Potassium atom site is split into two equally occupied positions with q=0.5.     

Nouveaux échangeurs cationiques avec une structure à tunnels entrecroisés: les niobates et tantalates A10M29.2O78 et A10M29.2O78·1OH2O

New oxides with formula A₁₀M_29.2O₇₈ (A = Rb, Cs; M = Ta, Nb) have been synthesized. They crystallize in the hexagonal system with cell parameters: a = 7.5 Å, c = 36.4Å. Structural study on powders shows that the framework can be described by hexagonal tungsten bronze and A₂M₇O₁₈ phases intergrowth. Cationic ion exchange properties of these compounds are shown in aqueous solution. Thus, new hydrated oxides have been prepared.     

Etude à haute pression des tétramétaphosphates du type M2P4O12 (M = Ni,Mg, Cu,Co, Fe,Mn, Cd). Données cristallographiques sur tous les composés M2P4O12

Fe₂P₄O₁₂ has been prepared and identified as an isotype of the other M^II₂P₄O₁₂ tetrametaphosphates (M^II = Ni, Mg, Cu, Co, Mn, Cd). Its monoclinic unit cell:

$\text{a=11.952,b=8.359,c=9.932}\text{A}\text{?}$

$\text{β=118°76}$

contains 4 formula units. The space group is $\text{C2}\text{c}$. For tetrametaphosphates with M^II = Ni, Mg, Cu, Co, and Mn we found a new denser phase induced at 80 kbar and 1000°C. In the case of Fe₂P₄O₁₂ the unit cell of this new form is

$\text{a=9.777,b=8.994,c=4.968}\text{A}\text{?}$

$\text{β=107°22}$

with Z = 2 and two possible space groups Cc or $\text{2C}\text{c}$. This dense phase exists at ordinary pressure for the zinc salt.     

Intergrowth of hexagonal tungsten bronze and perovskite-like structures: The oxides ACu3M7O21 (A = K,Rb, Cs,TI; M = Nb,Ta)

Seven oxides ACu₃M₇O₂₁ have been isolated with A = K, Rb, Tl, Cs for M = Ta and A = K, Rb, Cs for M = Nb. These phases are orthorhombic: $\text{a ? 28}$ Å, $\text{b ? 7.50}$ Å, and $\text{c ? 7.55}$ Å, probable space group Cmmm. Their structure has been established from an X-ray diffraction study and from high-resolution microscopy observations. The structure consists of an intergrowth of single hexagonal tungsten bronze AM₃O₉ slices and double distorted perovskite Cu₃M₄O₁₂ slabs (M = Nb, Ta) in which copper has a square coordination. The host lattice of these compounds can be considered as the member “n = 1; n′ = 2” of a series of intergrowths corresponding to the formulation |M₃O₉|^H_n|M₂O₆|^P_n′.     

Synthesis and structure of a new family of phases, A2MGe5O12: A = Rb,Cs; M = Be,Mg, Co,Zn

A new family of eight germanate phases, A₂MGe₅O₁₂: A = Rb, Cs; M = Be, Mg, Co, Zn, has been synthesized. They are cubic with a in the range 13.7 to 14.0 Å, Z = 8, and space group $\text{I}\text{4}\text{3d}$. These phases, named the β phases, are isostructural with KBSi₂O₆ which has a structure related to that of pollucite, CsAlSi₂O₆. The structure of one, Rb₂ZnGe₅O₁₂, has been refined to an R value of 0.079 using X-ray powder diffraction data. Several of the new phases are polymorphic. Cs₂ZnGe₅O₁₂, Cs₂CoGe₅O₁₂, and Rb₂MgGe₅O₁₂ form low-temperature, δ polymorphs which have primitive cubic unit cells. Rb₂ZnGe₅O₁₂ forms a low-temperature, ε polymorph which is probably a tetragonal distortion of the β structure.     

Structure cristalline et propriétés physiques électriques et magnétiques des phases M0.50NbSe2 (M = Ti,V, Cr)

The structure of M_0.50NbSe₂ (M = Ti, V, Cr) phases is reported. A detailed crystal structure analysis has been performed on Cr_0.50NbSe₂. Large single crystals were grown by chemical transport reaction with bromine as the transport agent. Electrical and magnetic properties have been measured in the 4.2–300°K range. Susceptibilities of both Cr_0.50NbSe₂ and Ti_0.50NbSe₂ follow the Curie-Weiss law. At low temperature (T < 53°K) an antiferromagnetic ordering is observed for Cr_0.50NbSe₂. V_0.50NbSe₂ exhibits a temperature-independent paramagnetism. Transport properties of M_0.50NbSe₂ phases are consistent with their metallic behavior and show several transitions at low temperature. The physical properties are discussed along with the reported crystal structure.     

Magnetic properties and structural transitions of orthorhombic fluorite-related compounds Ln3MO7 (Ln=rare earths, M=transition metals)

Magnetic properties and structural transitions of ternary rare-earth transition-metal oxides Ln₃MO₇ (Ln=rare earths, M=transition metals) were investigated. In this study, we prepared a series of molybdates Ln₃MoO₇ (Ln=La-Gd). They crystallize in an orthorhombic superstructure of cubic fluorite with space group P2₁2₁2₁, in which Ln³⁺ ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated sites). All of these compounds show a phase transition from the space group P2₁2₁2₁ to Pnma in the temperature range between 370 and 710 K. Their magnetic properties were characterized by magnetic susceptibility measurements from 1.8 to 400 K and specific heat measurements from 0.4 to 400 K. Gd₃MoO₇ shows an antiferromagnetic transition at 1.9 K. Measurements of the specific heat for Sm₃MoO₇ and the analysis of the magnetic specific heat indicate a “two-step” antiferromagnetic transition due to the ordering of Sm magnetic moments in different crystallographic sites, i.e., with decreasing temperature, the antiferromagnetic ordering of the 7-coordinated Sm ions occur at 2.5 K, and then the 8-coordinated Sm ions order at 0.8 K. The results of Ln₃MoO₇ were compared with the magnetic properties and structural transitions of Ln₃MO₇ (M=Nb, Ru, Sb, Ta, Re, Os, or Ir).     

Synthesis and photoluminescence properties of the high-brightness Eu-doped M2Gd4(MoO4)7 (M=Li, Na) red phosphors

A series of red-emitting phosphors Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) have been successfully synthesized at 850 °C by solid state reaction. The excitation spectra of the two phosphors reveal two strong excitation bands at 396 nm and 466 nm, respectively, which match well with the two popular emissions from near-UV and blue light-emitting diode chips. The intensity of the emission from ⁵D₀ to ⁷F₂ of M₂(Gd_1−xEu_x)₄(MoO₄)₇ phosphors with the optimal compositions of x=0.85 for Li or x=0.70 for Na is about five times higher than that of Y₂O₃:Eu³⁺. The quantum efficiencies of the entitled phosphors excited under 396 nm and 466 nm are also investigated and compared with commercial phosphors Sr₂Si₅N₈:Eu²⁺ and Y₃A₅O₁₂:Ce³⁺. The experimental results indicate that the Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) phosphors are promising red-emitting phosphors pumped by near-UV and blue light.     

Two isotypic diphosphates LiM2H3(P2O7)2 (M=Ni, Co) containing ferromagnetic zigzag MO6 chains

Two new isotypic phosphates LiNi₂H₃(P₂O₇)₂ (1) and LiCo₂H₃(P₂O₇)₂ (2) have been hydrothermally synthesized and structurally characterized by the single-crystal X-ray diffraction technique. They crystallize in the monoclinic space group C2/c with the lattice: a=10.925(2) Å, b=12.774(3) Å, c=8.8833(18) Å, β=123.20(3)° for 1 and a=10.999(2) Å, b=12.863(3) Å, c=8.9419(18) Å, β=123.00(3)° for 2. The transition metal atoms are octahedrally coordinated, whereas the lithium and phosphorus atoms are all tetrahedrally coordinated. As the lithium-induced derivatives of MH₂P₂O₇ (M=Ni, Co), 1 and 2 possess the same structure with MH₂P₂O₇ in terms of topology, comprising the MO₆ zigzag chains and P₂O₇ as the interchain groups. The magnetisms of 1 and 2 could be interpreted by adopting a quasi-one-dimensional (1D) zigzag chain model as that in their parent compounds: both 1 and 2 have ferromagnetic (FM) NiO₆/CoO₆ chains; 1 shows a FM cluster glass behavior at low temperatures, which is originated from the possible antiferromagnetic (AFM) next-nearest-neighbour intrachain interactions; 2 shows a AFM ordering at T_N=2.6 K and a metamagnetic transition at H_C=4.2 kOe at 1.8 K.     

Magnetic study of Cr ion in M2CrV3O11−x (MZn, Mg) compounds

Electron paramagnetic resonance (EPR) and magnetic susceptibility measurements on the recently synthesized vanadates M₂CrV₃O_11−x (M=Zn, Mg) have been analyzed. Two absorption lines with g≈2.0 (type I) and g≈1.98 (type II) were recorded in the EPR spectra, which can be attributed to V⁴⁺ ions and Cr³⁺ ion clusters (pairs), respectively. The exchange constant J between Cr³⁺ ions has been calculated, using both EPR and magnetic susceptibility data. Fitting of the EPR and magnetic susceptibility data has been carried out. The sign of J is a negative one for all samples and indicates antiferromagnetic interactions. Some difference in the J constant value among samples has been obtained. Volumetric titration confirms distinctly the presence of vanadium V⁴⁺ ions in the investigated compounds.     

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 first quaternary lanthanide(III) nitride iodides: NaM4N2I7 (M=La-Nd)

In attempts to synthesize lanthanide(III) nitride iodides with the formula M₂NI₃ (M=La-Nd), moisture-sensitive single crystals of the first quaternary sodium lanthanide(III) nitride iodides NaM₄N₂I₇ (orthorhombic, Pna2₁; Z=4; a=1391-1401, b=1086-1094, c=1186-1211 pm) could be obtained. The dominating structural features are chains of trans-edge linked [NM₄]⁹⁺ tetrahedra, which run parallel to the polar 2₁-axis [001]. Between the chains, direct bonding via special iodide anions generates cages, in which isolated [NaI₆]^5- octahedra are embedded. The IR spectrum of NaLa₄N₂I₇ recorded from 100 to 1000 cm^-1 shows main bands at υ=337, 373 and 489 cm^-1. With decreasing radii of the lanthanide trications these bands, which can be assigned as an influence of the vibrations of the condensed [NM₄]⁹⁺ tetrahedra, are shifted toward higher frequencies for the NaM₄N₂I₇ series (M=La-Nd), following the lanthanide contraction.     

Preparation and photoluminescence properties of Mn-activated M2Si5N8 (M=Ca, Sr, Ba) phosphors

Mn²⁺-doped M₂Si₅N₈ (M=Ca, Sr, Ba) phosphors have been prepared by a solid-state reaction method at high temperature and their photoluminescence properties were investigated. The Mn²⁺-activated M₂Si₅N₈ phosphors exhibit narrow emission bands in the wavelength range of 500-700 nm with peak center at about 599, 606 and 567 nm for M=Ca, Sr, Ba, respectively, due to the ⁴T₁(⁴G)→⁶A₁(⁶S) transition of Mn²⁺. The long-wavelength emission of Mn²⁺ ion in the host of M₂Si₅N₈ is attributed to the effect of a strong crystal-field of Mn²⁺ in the nitrogen coordination environment. Also it is observed that there exists energy transfer between M₂Si₅N₈ host lattice and activator (Mn²⁺). The potential applications of these phosphors have been pointed out.     

The non-centrosymmetric borate oxides, MBi2B2O7 (M=Ca, Sr)

Two novel noncentrosymmetric borates oxides, MBi₂B₂O₇ or MBi₂O(BO₃)₂ (MCa, Sr), have been synthesized by solid-state reactions in air at temperatures in the 600-700 °C range. Their crystal structures have been determined ab initio and refined using powder neutron diffraction data. CaBi₂B₂O₇ crystallizes in the orthorhombic Pna2₁ space group with a=8.9371(5) Å, b=5.4771(3) Å, c=12.5912(7) Å, Z=4, R_wp=0.118, χ²=2.30. SrBi₂B₂O₇ crystallizes in the hexagonal P6₃ space group with a=9.1404(4) Å, c=13.0808(6) Å, Z=6, R_wp=0.115, χ²=4.15. Large displacement parameters suggest the presence of disorder in SrBi₂B₂O₇ as also revealed by diffuse 2×a superstructure reflections in electron diffraction patterns. Both structures are built of identical (001) neutral layers of corner-sharing BO₃ triangles and MO₆ trigonal prisms forming six-membered rings in which Bi₂O groups are located. Adjacent layers are stacked in a staggered configuration and connected through weak Bi-O bonds. A moderate efficiency for second harmonic generation (SHG) has been measured for a powder sample of CaBi₂B₂O₇ (d_eff=2d_eff(KDP)).     

Application de la spectrometrie vibrationnelle à un probleme de non-stoechiométrie par insertion dans les tunnels pentagonaux des réseaux (M6X4O26) (M = Nb,Ta; X = Si,Ge)

The ir spectra of A₃M₆Si₄O₂₆ (A = Ba, Sr; M = Nb, Ta) and K₆M₆Si₄O₂₆ oxides, whose structure contains linear Si₂O₇ groups, are discussed with particular emphasis on the peculiar behavior of the antisymmetric stretching frequency of the linear SiOSi bridge. In accord with previous data, this frequency is the highest of the spectrum (near 1200 cm^?1), but it is significantly lowered (by about 75 cm^?1) when passing from the A₃M₆Si₂₄O₂₆ to the K₆M₆Si₄O₂₆ compounds. This is readily explained by the peculiar structure of the K₆ compounds, in which three (out of the six) K⁺ cations are located near the bridge oxygen (A₂ sites), these sites remaining empty in the A₃M₆Si₄O₂₆ compounds. The resulting KO bonding weakens the SiO bond, thus leading to a lowering of the corresponding bridge frequency. The same type of explanation holds for the presence of a new band at an intermediate frequency (about 1150 cm^?1) in phases of intermediate composition K_6?2xBa_xM₆Si₄O₂₆, this new band being correlated with a partial occupancy of the A₂ sites. This has been applied to, and is a sensitive means of, detecting nonstoichiometry in the A₂ sites of other compounds with (M₆X₄O₂₆) layers (X = Si, Ge) such as Ba_6+xNb₁₄Si₄O₄₇, K₈M₁₄Si₄O₄₇, and K₁₀M₂₂X₄O₆₈ (M = Nb, Ta).