The novel Cs4Nb6F8.5I3.5I6 octahedral niobium cluster fluoro-iodide: a step towards the Nb6F12 cluster core excision

The solid state synthesis of Cs₄Nb₆Fⁱ_8.5Iⁱ_3.5I^a₆ starting from Nb₆F₁₅ binary fluoride, as well as its crystal structure determined by X-ray single crystal diffraction, are presented in this work. This novel cluster compound is based on a Nb₆Iⁱ₃Fⁱ₆Lⁱ₃I^a₆ (L=F, I) discrete unit and crystallizes in the monoclinic system (space group C2/m; Z=4 ; a=10.4363(4) Å, b=18.1227(7) Å, c=19.5102(9) Å β=101.223(1)°, V=3619.5(3) Å³, R₁=0.057; wR₂=0.159). This halide is the first octahedral niobium cluster compound containing unshared terminal I^a ligands together with ordered μ₂-Iⁱ and μ₂-Fⁱ ligands on nine inner positions whilst the three last ones (Lⁱ) are slightly affected by a I/F random occupancy. The structural findings are discussed and compared with those of Nb₆F₁₅, Nb₆I₁₁, CsNb₆I₁₁ and the fluorochlorides and fluorobromides recently reported.     

Synthesis and structures of new niobium cluster compounds with pyridinium cations: (PyrH)2[Nb6Cl18]·EtOH (Pyr: pyridine, Et: ethyl) and the cubic modification of (PyrH)2[Nb6Cl18]

Slow crystallization of (PyrH)₂[Nb₆Cl₁₈] from hot ethanol solution affords triclinic (PyrH)₂[Nb₆Cl₁₈]·EtOH. Treatment of [Nb₆Cl₁₄(H₂O)₄]·4H₂O with pyridine in a methanol solution gives the second title compound, the cubic modification of (PyrH)₂[Nb₆Cl₁₈]. Both structures were determined by single crystal X-ray diffraction, (PyrH)₂[Nb₆Cl₁₈]·EtOH: P1¯, a=9.3475(3), b=9.3957(3), c=10.8600(3) Å, α=82.582(1)°, β=78.608(1)°, and γ=78.085(1)°, Z=1, R₁(F)/wR₂(F²)=0.0254/0.0573, cub.-(PyrH)₂[Nb₆Cl₁₈]: Fd3¯m, a=19.935(2) Å, Z=8, R₁(F)/wR₂(F²)=0.0557/0.1796. The cluster compounds contain isolated, molecular [Nb₆Clⁱ₁₂Cl^a₆]²⁻ cluster anions with an octahedron of metal atoms edge bridged by chlorido ligands with additional ones on all the six exo positions. These cluster anions are separated by the pyridinium cations and ethanol solvent molecules, respectively. For the cubic modification of (PyrH)₂[Nb₆Cl₁₈], a structural comparison is given to the known rhombohedral modification using the group-subgroup relations as expressed by a Bärnighausen tree.     

Synthesis and characterization of the novel Nb3O5F5 niobium oxyfluoride: the term n=3 of the NbnO2n−1Fn+2 series

The solid-state synthesis of the oxyfluoride Nb₃O₅F₅, its crystal structure determined from X-ray powder diffraction data as well as some physical characterizations, are reported. Nb₃O₅F₅ constitutes the term n=3 of the Nb_nO_2n−1F_n+2 series related to the Dion-Jacobson phases. It crystallizes, at room temperature, in the tetragonal system (space group I4/mmm (no. 139); Z=4; a=3.9135(1) Å, c=24.2111(2) Å, and V=370.80(3) Å³). The crystal structure appears to be an in-between of the three-dimensional network of NbO₂F and the two-dimensional packing of NbOF₃ (term n=1 of the Nb_nO_2n−1F_n+2 series). This layered structure consists of slabs made of three Nb(O,F)₆ corner-linked octahedra in thickness (n=3) shifted one from another by a ()/translation. Oxygen and fluorine atoms are randomly distributed over all the ligand sites.     

Dimensional reductions from 2-D Nb4P2S21 to 1-D ANb2PS10 (A=Na, K, Rb, Cs, Tl) and to 0-D Tl5[Nb2S4Cl8]Cl using halide molten salts

We found new synthetic routes to obtain 1-D quaternary thiophosphate compounds and a 0-D molecular complex containing a Nb₂S₄ core from a 2-D ternary thiophosphate, Nb₄P₂S₂₁. When Nb₄P₂S₂₁ was reacted with alkali metal halides (ACl; A=Na, K, Rb, Cs) or TlCl at 500-700 °C, the -S-S-S- bridges in 2-D Nb₂PS₁₀-S-S₁₀PNb₂ were excised to form a 1-D chain, and cations were inserted between the chains to form ANb₂PS₁₀ (A=Na, K, Rb, Cs, Tl). We also found that thallium chloride (TlCl) is an excellent reagent for further excision, and it substitutes chloride ligands for the sulfur ligands of 2-D Nb₄P₂S₂₁ to form the molecular complex Tl₅[Nb₂S₄Cl₈]Cl. Crystal data for TlNb₂PS₁₀: monoclinic, Pn, a=6.9452(11) Å, b=7.3761(12) Å, 12.873(2) Å, β=104.472(3)°, and Z=2. Crystal data for Tl₅[Nb₂S₄Cl₈]Cl: orthorhombic, Immm, a=7.001(5) Å, b=9.509(7) Å, c=15.546(11) Å, and Z=2.     

Synthesis and structural investigation of the compounds containing HF2 anions: Ca(HF2)2, Ba4F4(HF2)(PF6)3 and Pb2F2(HF2)(PF6)

Three new compounds Ca(HF₂)₂, Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) were obtained in the system metal(II) fluoride and anhydrous HF (aHF) acidified with excessive PF₅. The obtained polymeric solids are slightly soluble in aHF and they crystallize out of their aHF solutions. Ca(HF₂)₂ was prepared by simply dissolving CaF₂ in a neutral aHF. It represents the second known compound with homoleptic HF environment of the central atom besides Ba(H₃F₄)₂. The compounds Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) represent two additional examples of the formation of a polymeric zigzag ladder or ribbon composed of metal cation and fluoride anion (MF⁺)_n besides PbF(AsF₆), the first isolated compound with such zigzag ladder. The obtained new compounds were characterized by X-ray single crystal diffraction method and partly by Raman spectroscopy. Ba₄F₄(HF₂)(PF₆)₃ crystallizes in a triclinic space group P1¯ with a=4.5870(2) Å, b=8.8327(3) Å, c=11.2489(3) Å, α=67.758(9)°, β=84.722(12), γ=78.283(12)°, V=413.00(3) Å³ at 200 K, Z=1 and R=0.0588. Pb₂F₂(HF₂)(PF₆) at 200 K: space group P1¯, a=4.5722(19) Å, b=4.763(2) Å, c=8.818(4) Å, α=86.967(10)°, β=76.774(10)°, γ=83.230(12)°, V=185.55(14) ų, Z=1 and R=0.0937. Pb₂F₂(HF₂)(PF₆) at 293 K: space group P1¯, a=4.586(2) Å, b=4.781(3) Å, c=8.831(5) Å, α=87.106(13)°, β=76.830(13)°, γ=83.531(11)°, V=187.27(18) Å³, Z=1 and R=0.072. Ca(HF₂)₂ crystallizes in an orthorhombic Fddd space group with a=5.5709(6) Å, b=10.1111(9) Å, c=10.5945(10) Å, V=596.77(10) Å³ at 200 K, Z=8 and R=0.028.     

Orientation disorder in ammonium elpasolites: Crystal structures of (NH4)3AlF6, (NH4)3TiOF5 and (NH4)3FeF6

Crystal structures of the known (NH₄)₃AlF₆(I) and (NH₄)₃FeF₆(III) and new (NH₄)₃TiOF₅(II) elpasolites were refined by localizing anions (F⁻, O²⁻) in four acceptable positions of the cubic system Fm3m (Z=4) with a=8.9401(3), 9.1104(3), 9.110(1) Å, respectively. According to the refinement data and a rather large entropy change due to fluorine (oxygen) octahedra disordering in the above compounds and in (NH₄)₃WO₃F₃(IV) elpasolite, it was found that fluorine (oxygen) atoms are randomly distributed in two ways, in general 192l position or in mixed 24e + 96j one. Statistics in fluorine (oxygen) distribution is, probably, the result of domain structure of the crystals.     

Syntheses, crystal structures and Raman spectra of Ba(BF4)(PF6), Ba(BF4)(AsF6) and Ba2(BF4)2(AsF6)(H3F4); the first examples of metal salts containing simultaneously tetrahedral BF4 and octahedral AF6 anions

In the system BaF₂/BF₃/PF₅/anhydrous hydrogen fluoride (aHF) a compound Ba(BF₄)(PF₆) was isolated and characterized by Raman spectroscopy and X-ray diffraction on the single crystal. Ba(BF₄)(PF₆) crystallizes in a hexagonal space group with a=10.2251(4) Å, c=6.1535(4) Å, V=557.17(5) Å³ at 200 K, and Z=3. Both crystallographically independent Ba atoms possess coordination polyhedra in the shape of tri-capped trigonal prisms, which include F atoms from BF₄⁻ and PF₆⁻ anions. In the analogous system with AsF₅ instead of PF₅ the compound Ba(BF₄)(AsF₆) was isolated and characterized. It crystallizes in an orthorhombic Pnma space group with a=10.415(2) Å, b=6.325(3) Å, c=11.8297(17) Å, V=779.3(4) Å³ at 200 K, and Z=4. The coordination around Ba atom is in the shape of slightly distorted tri-capped trigonal prism which includes five F atoms from AsF₆⁻ and four F atoms from BF₄⁻ anions. When the system BaF₂/BF₃/AsF₅/aHF is made basic with an extra addition of BaF₂, the compound Ba₂(BF₄)₂(AsF₆)(H₃F₄) was obtained. It crystallizes in a hexagonal P6₃/mmc space group with a=6.8709(9) Å, c=17.327(8) Å, V=708.4(4) Å³ at 200 K, and Z=2. The barium environment in the shape of tetra-capped distorted trigonal prism involves 10 F atoms from four BF₄⁻, three AsF₆⁻ and three H₃F₄⁻ anions. All F atoms, except the central atom in H₃F₄ moiety, act as μ₂-bridges yielding a complex 3-D structural network.     

High-pressure high-temperature synthesis and crystal structure of the isotypic rare earth (RE)-thioborate-sulfides RE9[BS3]2[BS4]3S3, (RE=Dy-Lu)

Application of high-pressure high-temperature conditions (3.5 GPa at 1673 K for 5 h) to mixtures of the elements (RE:B:S=1:3:6) yielded crystalline samples of the isotypic rare earth-thioborate-sulfides RE₉[BS₃]₂[BS₄]₃S₃, (RE=Dy-Lu), which crystallize in space group P6₃ (Z=2/3) and adopt the Ce₆Al_3.33S₁₄ structure type. The crystal structures were refined from X-ray powder diffraction data by applying the Rietveld method. Dy: a=9.4044(2) Å, c=5.8855(3) Å; Ho: a=9.3703(1) Å, c=5.8826(1) Å; Er: a=9.3279(12) Å, c=5.8793(8) Å; Tm: a=9.2869(3) Å, c=5.8781(3) Å; Yb: a=9.2514(5) Å, c=5.8805(6) Å; Lu: a=9.2162(3) Å, c=5.8911(3) Å. The crystal structure is characterized by the presence of two isolated complex ions [BS₃]^3- and [BS₄]^5- as well as [□(S^2-)₃] units.     

Crystal structure and magnetic properties of Co2TeO3Cl2 and Co2TeO3Br2

The crystal structures of the two new synthetic compounds Co₂TeO₃Cl₂ and Co₂TeO₃Br₂ are described together with their magnetic properties. Co₂TeO₃Cl₂ crystallize in the monoclinic space group P2₁/m with unit cell parameters a=5.0472(6) Å, b=6.6325(9) Å, c=8.3452(10) Å, β=105.43(1)°, Z=2. Co₂TeO₃Br₂ crystallize in the orthorhombic space group Pccn with unit cell parameters a=10.5180(7) Å, b=15.8629(9) Å, c=7.7732(5) Å, Z=8. The crystal structures were solved from single crystal data, R=0.0328 and 0.0412, respectively. Both compounds are layered with only weak interactions in between the layers. The compound Co₂TeO₃Cl₂ has [CoO₄Cl₂] and [CoO₃Cl₃] octahedra while Co₂TeO₃Br₂ has [CoO₂Br₂] tetrahedra and [CoO₄Br₂] octahedra. The Te(IV) atoms are tetrahedrally [TeO₃E] coordinated in both compounds taking the 5s² lone electron pair E into account. The magnetic properties of the compounds are characterized predominantly by long-range antiferromagnetic ordering below 30 K.     

Structural relationships between new carbide La14Sn(MnC6)3 and fully ordered La11(MnC6)3

Crystals of the ternary La₁₁(MnC₆)₃ and new quaternary carbide La₁₄Sn(MnC₆)₃ phases were grown from La/Ni eutectic flux and their structures were determined by means of X-ray single crystal diffraction. La₁₁(MnC₆)₃ is a new superstructure variant of La_3.67MnC₆ (previously reported disordered subcell: P6₃/m; a₀=8.806 Å; c₀=5.329 Å, Z=2). The superstructure (R3¯; a=√3a₀=15.2649(9) Å; c=3c₀=16.013(1) Å, Z=6; R₁=0.022) is realized by complete ordering of the La chains within the columns of face-sharing carbon octahedra, with alternating La-La distances leading to R-centering and enlargement of the unit cell. The structure of the quaternary carbide La₁₄Sn(MnC₆)₃ (P6¯; a=8.756(1) Å; c=10.483(2) Å, Z=1; R₁=0.026) is closely related to that of La₁₁(MnC₆)₃ with part of the MnC₆ units replaced by Sn atoms. The structure and precise composition of La₁₄Sn(MnC₆)₃ can be derived from that of La₁₁(MnC₆)₃ by taking into account the extent of this substitution and variation in lanthanum siting in the chain of carbon octahedra. Band structure calculations indicate both phases are metallic; the La₁₁(MnC₆)₃ phase is stabilized by the ordering of La atoms which induces a pseudogap at E_F.     

Two new octahedral/pyramidal frameworks containing both cation channels and lone-pair channels: syntheses and structures of Ba2MnMn2(SeO3)6 and PbFe2(SeO3)4

The hydrothermal syntheses, single crystal structures, and some properties of Ba₂Mn^IIMn₂^III(SeO₃)₆ and PbFe₂(SeO₃)₄ are reported. These related phases contain three-dimensional frameworks of vertex (FeO₆) and vertex/edge linked (MnO₆) octahedra and SeO₃ pyramids. In each case, the MO₆/SeO₃ framework encloses two types of 8 ring channels, one of which encapsulates the extra-framework cations and one of which provides space for the Se^IV lone pairs. Crystal data: Ba₂Mn₃(SeO₃)₆, M_r=1201.22, monoclinic, P2₁/c (No. 14), a=5.4717 (3) Å, b=9.0636 (4) Å, c=17.6586 (9) Å, β=94.519 (1)°, V=873.03 (8) Å³, Z=2, R(F)=0.031, wR(F²)=0.070; PbFe₂(SeO₃)₄, M_r=826.73, triclinic, (No. 2), a=5.2318 (5) Å, b=6.7925 (6) Å, c=7.6445 (7) Å, α=94.300 (2)°, β=90.613 (2)°, γ=95.224 (2)°, V=269.73 (4) Å³, Z=1, R(F)=0.051, wR(F²)=0.131.     

Syntheses and crystal structures of [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)2: new examples of HF acting as a ligand to metal centres

[Mg(HF)₂](SbF₆)₂ and [Ca(HF)₂](SbF₆)₂ monocrystals were grown from the corresponding hexafluoroantimonates(V) dissolved in anhydrous hydrogen fluoride. [Mg(HF)₂](SbF₆)₂ crystallizes in the space group Pnma (no. 62) with a=1249.1(4) pm, b=1230.2(4) pm, c=699.1(2) pm, V=1.0742(6) nm³, Z=4. Magnesium is octahedrally coordinated by six fluorine atoms from which two belong to two HF molecules. The structure can be represented by alternating rows of magnesium and antimony atoms running parallel to the c-axis. Magnesium atoms are connected by cis bridging Sb(2)F₆ units along the a-axis and by trans bridging Sb(1)F₆ units along the b-axis. In this way a three-dimensional network is formed.[Ca(HF)₂](SbF₆)₂ crystallizes in the space group P2₁/n (no. 14) with a=935.2(3) pm, b=1088.7(3) pm, c=1104.8(3) pm, β=106.697(5)°, V=1.0774(5) nm³, Z=4. The coordination sphere around the calcium atom consists of eight fluorine atoms which define the vertices of an Archimedean antiprism. The two HF molecules directly coordinate the calcium atom and their fluorine atoms are placed in the corners of different square faces of the Archimedean antiprism. The Ca-F(HF) distances are shorter than the Ca-F(Sb) distances. The Sb(1)F₆ and Sb(2)F₆ groups have four equatorial bridging fluorine atoms, while the Sb(3)F₆ groups have only two bridging trans F ligands. The Ca atoms in the [−1,0,1] plane are connected by equatorial F ligands of Sb(1)F₆ and Sb(2)F₆ units, forming a [Ca(SbF₆)⁺]_n layer. These layers are connected by trans bridging Sb(3)F₆ groups. HF molecules occupy the space between these layers and additionally contribute to the connection between the layers by hydrogen bonding.     

Alignment of acentric MoO3F3 anions in a polar material: (Ag3MoO3F3)(Ag3MoO4)Cl

(Ag₃MoO₃F₃)(Ag₃MoO₄)Cl was synthesized by hydro(solvato)thermal methods and characterized by single-crystal X-ray diffraction (P3m1, No. 156, Z=1, a=7.4488(6)Å, c=5.9190(7) Å). The transparent colorless crystals are comprised of chains of distorted fac-MoO₃F₃³⁻ octahedra and MoO₄²⁻ tetrahedra anions, as suggested by the formulas Ag₃MoO₃F₃ and Ag₃MoO₄⁺, and are connected through Ag⁺ cations in a polar alignment along the c-axis. One Cl⁻ anion per formula unit serves as a charge balance and connects the two types of chains in a staggered fashion, offset by . In MoO₄²⁻, the Mo atom displaces towards a single oxide vertex, and in MoO₃F₃³⁻, the Mo displaces towards the three oxide ligands. The ordered oxide-fluoride ligands on the MoO₃F₃³⁻ anion is important to prevent local inversion centers, while the polar organization is directed by the Cl⁻ anion and interchain dipole-dipole interactions. The dipole moments of MoO₃F₃³⁻ and MoO₄²⁻ align in the negative c-axis direction, to give a polar structure with no cancellation of the individual moments. The direction and magnitude of the dipole moments for MoO₃F₃³⁻ and MoO₄²⁻ were calculated from bond valence analyses and are 6.1 and 1.9 debye (10⁻¹⁸ esu cm) respectively, compared to 4.4 debye for polar NbO₆ octahedra in LiNbO₃, and 4.5 debye for polar TiO₆ octahedra in KTiOPO₄ (KTP).     

Synthesis and crystal structure of the Sr2MnGa(O,F)6 oxyfluorides

The fluorine-containing derivatives of Sr₂MnGaO_5.5 were prepared by treatment with XeF₂ at temperatures ranging from 300°C to 600°C. The compounds crystallize in a tetragonal unit cell with a_t≈a_p, c_t≈2a_p (a_p—the parameter of the perovskite subcell). An increase in fluorine content is accompanied by a reduction of the Mn oxidation state due to a partial replacement of oxygen by fluorine. The crystal structure of Sr₂MnGaO_4.78F_1.22 was determined by electron diffraction and X-ray powder diffraction (a=3.85559(2) Å, c=7.78289(6) Å, S.G. P4/mmm, R_I=0.012, R_P=0.019). The structure consists of alternating (MnO₂), (SrO) and (GaO_0.78F_1.22) layers. The Ga atoms are situated in slightly elongated octahedra, the MnO₆ octahedra are characterized by two short apical Mn-O distances of 1.876(8) Å and four long equatorial ones of 1.9278(1) Å. This is interpreted as an “apically compressed” type of Jahn-Teller distortion, in contrast to the “apically elongated” one in the Sr₂MnGaO_5+δ brownmillerites with different oxygen content. Possible structural reasons for the reversed Jahn-Teller effect are discussed.     

A hybrid material based on [Mo6Br14] inorganic cluster units and [BEDO-TTF] organic monocationic radicals: Synthesis, structure and properties of (BEDO-TTF)2Mo6Br14(PhCN)4

The first charge transfer salt based on non- dimerized [BEDO-TTF]⁺ monocationic radical (BEDO-TTF=bis(ethylenedioxy)tetrathiafulvalene) associated with [Mo₆Br₁₄]²⁻ cluster anions has been synthesized by conventional electro-oxidation and characterized by single crystal X-ray diffraction, UV-VIS-NIR absorption and magnetic susceptibility measurements. (BEDO-TTF)₂Mo₆Br₁₄(PhCN)₄ crystallizes in the monoclinic system, space group P2₁/n, a=10.414(4) Å, b=21.711(7) Å, c=15.958(5) Å, β=93.65(3)°, V=3601(2) Å³, Z=2, R₁=0.0578, wR₂=0.0731. The structure of this hybrid compound is built up from a [BEDO-TTF]⁺ and PhCN (benzonitrile) organic framework in which are hosted the [Mo₆Br₁₄]²⁻ inorganic cluster units. It results in non- dimerized [BEDO-TTF]⁺ cations that exhibit a paramagnetic behavior characteristic of one unpaired electron.     

Unprecedented μ3-O face-capping ligand in a [Mo6Br6L2Br6] (L = 0.5 O + 0.5 Br) molybdenum cluster unit: crystal structure of the Cs3Mo6Br13O oxybromide

The new Cs₃Mo₆Br₁₃O oxybromide, synthesized by solid-state chemistry, crystallizes in the trigonal system (Rc space group; a = 15.5784(2) Å, c = 19.5103(5) Å, V = 4100.5(1) Å³ and Z = 6). It is based on a [Mo₆L₁₄] unit that contains an unprecedented μ₃ face-capping oxygen. The crystal structure determined by single crystal X-ray diffraction is built up from discrete face-capped [Mo₆Brⁱ₆Lⁱ₂Br^a₆]^3– (L = 0.5 O + 0.5 Br) anionic units in which two inner positions are randomly occupied by one bromine and one oxygen whereas the other ligand positions are fully occupied by bromine. The cesium cations randomly occupy two close crystallographic positions generated by the A-B-C-A′-B′-C′ close-packed stacking of the units. The cesium site occupancy is related to the random distribution of oxygen and bromine on the Lⁱ inner positions. To cite this article: K. Kirakci et al., C. R. Chimie 8 (2005).     

Synthesis and crystal structure of two tin fluoride materials: NaSnF3 (BING-12) and Sn3F3PO4

A new compound, sodium tin trifluoride (NaSnF₃, which we denote BING-12 for SUNY at Binghamton, Structure No. 12), was synthesized solvothermally from a pyridine-water solvent system. The new compound crystallized in the monoclinic space group C2/c (No. 15), with a=11.7429(12) Å, b=17.0104(18) Å, c=6.8528(7) Å, β=100.6969(2)°, V=1345.1(2) Å³ and Z=16. The layered structure consists of outer pyramidal SnF₃ units, where the fluorides surround a central layer of six- and seven-coordinate sodium atoms. The layers are stabilized by charged Na⁺ galleries that reside in the center of the layers. Tin trifluorophosphate (Sn₃F₃PO₄, Compound 2) was isolated from a related synthetic system, and crystallized in the rhombohedral space group R3 (No. 146), with a=11.8647(11) Å, c=4.6291(6) Å, V=564.34(10) Å³ and Z=3. The framework is made up of helical -Sn-F- chains, which are connected by phosphate groups. The materials were characterized by powder X-ray diffraction (PXRD), variable temperature PXRD (VT-PXRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM).     

Rb2BaNb2Se11: A new quaternary niobium polyselenide with infinite anionic chains composed of Nb2Se11 building block

The quaternary compound Rb₂BaNb₂Se₁₁ has been synthesized by reacting Nb metal with an in situ formed flux of Rb₂Se₃, BaSe and Se at 773 K. Rb₂BaNb₂Se₁₁ crystallizes in the monoclinic space group P2₁/c with four formula units and lattice parameters a=7.8438(5) Å, b=13.6959(6) Å, c=17.0677(13) Å, β=97.917(9)°. The structure consists of one-dimensional anionic chains formed by interconnection of dimeric [Nb₂Se₁₁] units. The chains are directed along the crystallographic c-axis with Rb⁺ and Ba²⁺ ions being located between the chains. The [Nb₂Se₁₁] units are formed by face sharing of two NbSe₇ bipyramids and are joined by Se₂²⁻ dianions to form infinite ¹_∞[Nb₂Se₁₁⁴⁻] chains. The compound was characterized with infrared spectroscopy in the FIR region, Raman and UV/Vis diffuse reflectance spectroscopy.     

The structural investigation of Ba4Bi3F17

The anion-excess ordered fluorite-related phase Ba₄Bi₃F₁₇ has been synthesized by a solid state reaction of BaF₂ and BiF₃ at 873 K. The crystal structure of Ba₄Bi₃F₁₇ has been studied using electron diffraction and X-ray powder diffraction (a=11.2300(2) Å, c=20.7766(5) Å, S.G. , R_I=0.020, R_P=0.036). Interstitial fluorine atoms in the Ba₄Bi₃F₁₇ structure are considered to form isolated cuboctahedral 8 : 12 : 1 clusters. The structural relationship between Ba₄Bi₃F₁₇ and similar rare-earth-based phases is discussed.