The first rubidium rare-earth(III) thiophosphates: Rb3M3[PS4]4 (M=Pr, Er)

The two non-isotypical rubidium rare-earth(III) thiophosphates Rb₃M₃[PS₄]₄ of praseodymium and erbium can easily be obtained by the stoichiometric reaction of the respective rare-earth metal, red phosphorus and sulfur with an excess of rubidium bromide (RbBr) as flux and rubidium source at 950°C for 14 days in evacuated silica tubes. The pale green platelet-shaped single crystals of Rb₃Pr₃[PS₄]₄ as well as the pink rods of Rb₃Er₃[PS₄]₄ are moisture sensitive. Rb₃Pr₃[PS₄]₄ crystallizes triclinically in the space group (, , , α=84.329(4)°, β=88.008(4)°, γ=80.704(4)°; Z=2), Rb₃Er₃[PS₄]₄ monoclinically in the space group P2₁/n (, , , β=95.601(6)°; Z=4). In both structures, there are three crystallographically different rare-earth cations present. (M1)³⁺ is eightfold coordinated in the shape of a square antiprism, (M2)³⁺ and (M3)³⁺ are both surrounded by eight sulfur atoms as bicapped trigonal prisms each with a coordination number of eight as well as for the praseodymium, but better described as CN=7+1 in the case of the erbium compound. These [MS₈]¹³⁻ polyhedra form a layer according to by sharing edges with the isolated [PS₄]³⁻ tetrahedra (d(P-S)=200-209 pm, ?(S-P-S)=102-116°). These layers are stacked with a repetition period of three in the case of the praseodymium compound, but of only two for the erbium analog. The rubidium cation (Rb1)⁺ is located in cavities of these layers and tenfold coordinated in the shape of a tetracapped trigonal antiprism. The also tenfold but more irregularly coordinated rubidium cations (Rb2)⁺ and (Rb3)⁺ reside between the layers.     

Synthesis and crystal structure of Ln2MGe4O12, Ln=rare-earth element or Y; M=Ca, Mn, Zn

The crystal structure of the promising optical materials Ln₂M²⁺Ge₄O₁₂, where Ln=rare-earth element or Y; M=Ca, Mn, Zn and their solid solutions has been studied in detail. The tendency of rare-earth elements to occupy six- or eight-coordinated sites upon iso- and heterovalent substitution has been studied for the Y_2−xEr_xCaGe₄O₁₂ (x=0-2), Y_2−2xCe_xCa_1+xGe₄O₁₂ (x=0-1), Y₂Ca_1−xMn_xGe₄O₁₂ (x=0-1) and Y_2−xPr_xMnGe₄O₁₂ (x=0-0.5) solid solutions. A complex heterovalent state of Eu and Mn in Eu₂MnGe₄O₁₂ has been found.     

Phase formation in the systems Ag2MoO4-MO-MoO3 (M=Ca, Sr, Ba, Pb, Cd, Ni, Co, Mn) and crystal structures of Ag2M2(MoO4)3 (M=Co, Mn)

Phase equilibria in the systems Ag₂MoO₄-MMoO₄ (M=Ca, Sr, Ba, Pb, Ni, Co, Mn) and subsolidus phase relations in the systems Ag₂MoO₄-MO-MoO₃ (M=Ca, Pb, Cd, Mn, Co, Ni) were investigated using XRD and thermal analysis. The systems Ag₂MoO₄-MMoO₄ (M=Ca, Sr, Ba, Pb, Ni) belong to the simple eutectic type whereas in the systems Ag₂MoO₄-MMoO₄ (M=Co, Mn) incongruently melting Ag₂M₂(MoO₄)₃ (M=Co, Mn) were formed. In the ternary oxide systems studied no other compounds were found. Low-temperature LT-Ag₂Mn₂(MoO₄)₃ reversibly converts into the high-temperature form of a similar structure at 450-500°C. The single crystals of Ag₂Co₂(MoO₄)₃ and LT-Ag₂Mn₂(MoO₄)₃ were grown and their structures determined (space group , Z=2; lattice parameters are a=6.989(1) Å, b=8.738(2) Å, c=10.295(2) Å, α=107.67(2)°, β=105.28(2)°, γ=103.87(2)° and a=7.093(1) Å, b=8.878(2) Å, c=10.415(2) Å, α=106.86(2)°, β=105.84(2)°, γ=103.77(2)°, respectively) and refined to R(F)=0.0313 and 0.0368, respectively. The both compounds are isotypical to Ag₂Zn₂(MoO₄)₃ and contain mixed frameworks of MoO₄ tetrahedra and pairs of M²⁺O₆ octahedra sharing common edges. The Ag⁺ ions are disordered and located in the voids forming infinite channels running along the a direction. The peculiarities of the silver disorder in the structures of Ag₂M₂(MoO₄)₃ (M=Zn, Mg, Co, Mn) are discussed as well as their relations with analogous sodium-containing compounds of the structural family of Na₂Mg₅(MoO₄)₆. The phase transitions in Ag₂M₂(MoO₄)₃ (M=Mg, Mn) of distortive or order-disorder type are suggested to have superionic character.     

Ternary silicides M2Cr4Si5 (M=Ti, Zr, Hf): filled variants of the Ta4SiTe4 structure type

The isostructural ternary silicides M₂Cr₄Si₅ (M=Ti, Zr, Hf) were prepared by arc-melting of the elemental components. The single-crystal structure of Zr₂Cr₄Si₅ was determined by X-ray diffraction (Pearson symbol oI44, orthorhombic, space group Ibam, Z=4, a=7.6354(12) Å, b=16.125(3) Å, c=5.0008(8) Å). Zr₂Cr₄Si₅ adopts the Nb₂Cr₄Si₅-type structure, an ordered variant of the V₆Si₅-type structure. It consists of square antiprisms that have Zr and Cr atoms at the corners and Si atoms at the centers; they share opposite faces to form one-dimensional chains _∞¹[Zr_4/2Cr_4/2Si] surrounded by additional Si atoms and extending along the c direction. In a new interpretation of the structure, additional Cr atoms occupy interstitial octahedral sites between these chains, clarifying the relation between this structure and that of Ta₄SiTe₄. The formation of short Si-Si bonds in Zr₂Cr₄Si₅ is contrasted with the absence of Te-Te bonds in Ta₄SiTe₄. The compounds M₂Cr₄Si₅ (M=Ti, Zr, Hf) exhibit metallic behavior and essentially temperature-independent paramagnetism. Bonding interactions were analyzed by band structure calculations, which confirm the importance of Si-Si bonding in these metal-rich compounds.     

New Ba5M5−xPtxClO13 (M=Fe, Co) oxychlorides with layered perovskite-related structure

Crystals of Ba₅Fe_5−xPt_xClO₁₃ and Ba₅Co_5−yPt_yClO₁₃ were prepared for x=1.31, 1.51, 1.57, 1.59 and y=0, 0.97 and 1.08 in a BaCl₂ flux and investigated by X-ray diffraction methods. These compounds adopt a 10H perovskite structure built from the stacking of BaO₃ and BaOCl layers in the sequence (BaO₃)₄(BaOCl) with space group P6₃/mmc. The cation sites within the trimeric unit of face-sharing octahedra are occupied by Co or Fe and Pt ions, while the tetrahedral sites formed between BaO₃ and BaOCl layers are only occupied by Fe or Co. Moreover, oxygen stoichiometry indicates an average oxidation state for Co and Fe higher than +III, indicating the stabilization of Co⁴⁺ and Fe⁴⁺.     

Double (n=2) and triple (n=3) [M4Bi2n−2O2n] polycationic ribbons in the new Bi∼3Cd∼3.72M∼1.28O5(PO4)3 oxyphosphate (M=Co, Cu, Zn)

The crystal structure of the new Bi_∼3Cd_∼3.72Co_∼1.28O₅(PO₄)₃ has been refined from single crystal XRD data, R₁=5.37%, space group Abmm, a=11.5322(28) Å, b=5.4760(13) Å, c=23.2446(56) Å, Z=4. Compared to Bi_∼1.2M_∼1.2O_1.5(PO₄) and Bi_∼6.2Cu_∼6.2O₈(PO₄)₅, this compound is an additional example of disordered Bi³⁺/M²⁺ oxyphosphate and is well described from the arrangement of double [Bi₄Cd₄O₆]⁸⁺ (=D) and triple [Bi₂Cd_3.44Co_0.56O₄]⁶⁺ (=T) polycationic ribbons formed of edge-sharing O(Bi,M)₄ tetrahedra surrounded by PO₄ groups. According to the nomenclature defined in this work, the sequence is TT/DtDt, where t stands for the tunnels created by PO₄ between two subsequent double ribbons and occupied by Co²⁺. The HREM study allows a clear visualization of the announced sequence by comparison with the refined crystal structure. The Bi³⁺/M²⁺ statistic disorder at the edges of T and D entities is responsible for the PO₄ multi-configuration disorder around a central P atom. Infrared spectroscopy and neutron diffraction of similar compounds (without the highly absorbing Cadmium) even suggests the long range ordering loss for phosphates. Therefore, electron diffraction shows the existence of a modulation vector q^*=1/2a^*+(1/3+ε)b^* which pictures cationic ordering in the (001) plane, at the crystallite scale. This ordering is largely lost at the single crystal scale. The existence of mixed Bi³⁺/M²⁺ positions also enables a partial filling of the tunnels by Co²⁺ and yields a composition range checked by solid state reaction. The title compound can be prepared as a single phase and also the M=Zn²⁺ term can be obtained in a biphasic mixture. For M=Cu²⁺, a monoclinic distortion has been evidenced from XRD and HREM patterns but surprisingly, the orthorhombic ideal form can also be obtained in similar conditions.     

On the structural and magnetic properties of the ternary silicides Ce6M1.67Si3 (M=Co, Ni) and Ce5Ni1.85Si3

Contrary to that reported previously, the ternary silicide “Ce₆Ni₂Si₃” does not exist. The melting of this alloy, followed or not by annealing, leads to the existence of the two new ternary compounds, Ce₆Ni_1.67Si₃ and Ce₅Ni_1.85Si₃. The investigation of these ternary silicides based on nickel and Ce₆Co_1.67Si₃ by X-ray diffraction on single crystal reveals an ordered distribution between Ni (or Co) and Si atoms. The nickel or cobalt positions in the chains of face-shared octahedra of cerium are not fully occupied with a strong delocalisation of their electron density. The structural investigations of these compounds confirm that the “Ce₆Ni₂Si₃” and “Ce₅Ni₂Si₃” structural type have to be rewritten as Ce₆Ni_2−xSi₃ and Ce₅Ni_2−xSi₃. Magnetisation and specific heat measurements evidence a magnetic ordering at 3.8(2) K for Ce₆Ni_1.67Si₃ and a heavy fermion behaviour for Ce₆Co_1.67Si₃.     

Syntheses, structure and magnetic properties of pillared layered diphosphonates: M2(O3PC6H4PO3)(H2O)2 (M=Co, Ni)

This paper describes the hydrothermal syntheses of two isostructural metal bisphosphonates: M₂(O₃PC₆H₄PO₃)(H₂O)₂ [M=Co^II (1), Ni^II (2)]. Single-crystal structure determination of compound 1 revealed a pillared layered structure in which the phenyl groups connect the inorganic layers of cobalt phosphonate. Crystal data for 1: orthorhombic, space group Pnnm, a=19.306(5), b=4.8293(12), c=5.6390(14) Å, V=525.7(2) Å³, Z=2. Magnetic susceptibility data indicate that antiferromagnetic interactions are mediated in both cases.     

Structural investigation of R2Mo4O15 (R=La, Nd, Sm), and polymorphs of the R2Mo4O15 (R=rare earth) family

Pyrolysis of rare earth (R) polyoxomolybdate, [R₂(H₂O)₁₂Mo₈O₂₇]·xH₂O (R=La, Nd and Sm), at 750°C for 2-8 h results in crystallization of R₂Mo₄O₁₅ compounds. β-La₂Mo₄O₁₅ crystallizes together with an α-form in monoclinic P2₁/a (No. 14), a=13.8893(5), b=13.0757(4), c=20.0927(8) Å, β=95.199(2)°, V=3634.1(2) Å³, Z=12, R₁(I>2σ(I))=0.048, R_w (all data)=0.116. The structure is built up with {LaO_n} (n=9, 10) and {MoO_n′} (n′=4-6) polyhedral units. The {LaO_n} units are polymerized into a linear {La₆O₃₉}_∞ chain, while the {MoO_n} are connected together to form {Mo₄O₁₅} and {Mo₇O₂₆} groups. The structure can be related to the α-form by partial rearrangement of O atoms and small shifts of La and Mo atoms. The R₂Mo₄O₁₅ (R=Nd and Sm) compounds are isomorphous with the previously reported R=Eu and Gd analogs, crystallizing in triclinic, (No. 2), a=9.4989(5) and 9.4076(7), b=11.0088(7) and 10.9583(8), c=11.5665(6) and 11.5234(8) Å, α=104.141(3) and 104.225(3), β=109.838(3) and 109.603(3), γ=108.912(3) and 108.999(3)°, V=987.3(1) and 970.5(1) Å³, Z=3, R₁(I>2σ(I))=0.028 and 0.030, R_w (all data)= 0.079 and 0.094, respectively. The crystal structure is composed of {RO₈} and {MoO_n′} (n′=4-6) polyhedral units. The molybdate units are condensed to give a corrugated {Mo₄O₁₇}_∞ chain. The square-antiprismatic {RO₈} units share their trigonal and square faces, forming {R₂O₁₃} and {R₂O₁₂} groups, respectively. A very short R?R distance (3.557(6) Å for R=Nd; 3.4956(6) Å for R=Sm) is achieved in the latter unusual {R₂O₁₂} group. A common cationic arrangement was found in all the structures in the R₂Mo₄O₁₅ family: a R-R pair with the shortest separation and surrounding 12 Mo atoms. The symmetry of the cationic arrangement was reduced with an increase of atomic number of R, viz. La>Ce, Pr>Nd-Gd≈Tb, Ho.     

On the crystal chemistry of Tm2Ni1.896(4)In, Tm2.22(2)Ni1.81(1)In0.78(2), Tm4.83(3)Ni2In1.17(3), and Er5Ni2In

The rare earth-nickel-indides Tm₂Ni_1.896(4)In, Tm_2.22(2)Ni_1.81(1)In_0.78(2), Tm_4.83(3)Ni₂In_1.17(3), and Er₅Ni₂In were synthesized from the elements by arc-melting and subsequent annealing for the latter three compounds. Three indides were investigated by X-ray powder and single crystal diffraction: Mo₂FeB₂ type, P4/mbm, Z=2, a=731.08(4), c=358.80(3) pm, wR₂=0.0201, 178 F² values, 13 variables for Tm₂Ni_1.896(4)In, a=734.37(7), c=358.6(1) pm, wR₂=0.0539, 262 F² values, 14 variables for Tm_2.22(2)Ni_1.81(1)In_0.78(2), and Mo₅SiB₂ type, I4/mcm, a=751.0(2), c=1317.1(3) pm, wR₂=0.0751, 317 F² values, 17 variables for Tm_4.83(3)Ni₂In_1.17(3). X-ray powder data for Er₅Ni₂In revealed a=754.6(2) and c=1323.3(5) pm. The Mo₂FeB₂ type structures of Tm₂Ni_1.896(4)In and Tm_2.22(2)Ni_1.81(1)In_0.78(2) are intergrowths of slightly distorted CsCl and AlB₂ related slabs, however, with different crystal chemical features. The nickel sites within the AlB₂ slabs are not fully occupied in both indides. Additionally In/Tm mixing is possible at the center of the CsCl slab, as is evident from the structure refinement of Tm_2.22(2)Ni_1.81(1)In_0.78(2). The Mo₅SiB₂ type structures of Tm_4.83(3)Ni₂In_1.17(3) and Er₅Ni₂In can be considered as an intergrowth of distorted CuAl₂ and U₃Si₂ related slabs in an ABA′B′ stacking sequence along the c-axis. Again, one thulium site shows Tm/In mixing. The U₃Si₂ related slab has great structural similarities with the Mo₂FeB₂ type structure of Tm₂Ni_1.896(4)In and Tm_2.22(2)Ni_1.81(1)In_0.78(2). The crystal chemical peculiarities and chemical bonding in these intermetallics are briefly discussed.     

Syntheses, structure, and magnetic properties of several LnYbQ3 chalcogenides, Q=S, Se

The six LnYbQ₃ compounds β-LaYbS₃, LaYbSe₃, CeYbSe₃, PrYbSe₃, NdYbSe₃, and SmYbSe₃ have been synthesized from high-temperature solid-state reactions of the constituent elements at 1223 K. The compounds are isostructural to UFeS₃ and crystallize in the space group Cmcm of the orthorhombic system with four formula units in a cell. Cell constants (Å) at 153 K are: β-LaYbS₃, 3.9238(8), 12.632(3), 9.514(2); LaYbSe₃, 4.0616(8), 13.094(3), 9.932(2); CeYbSe₃, 4.0234(5), 13.065(2), 9.885(1); PrYbSe₃, 4.0152(5), 13.053(2), 9.868(1); NdYbSe₃, 4.0015(6), 13.047(2), 9.859(1); SmYbSe₃, 3.9780(9), 13.040(3), 9.860(2). The structure is composed of layers of YbQ₆ (Q=S or Se) octahedra that alternate with layers of LnQ₈ bicapped trigonal prisms along the b-axis. Because there are no Q-Q bonds in the structure the formal oxidation states of Ln/Yb/Q are 3+/3+/2−. Magnetic susceptibility measurements indicate that CeYbSe₃ and SmYbSe₃ are Curie-Weiss paramagnets over the temperature range 5-300 K.     

The metal-rich palladium chalcogenides Pd2MCh2 (M=Fe, Co, Ni; Ch=Se, Te): Crystal structure and topology of the electron density

Crystals of Pd₂MCh₂ (M=Fe, Co, Ni; Ch=Se, Te) were synthesized by heating the elements at 823-1323 K in silica ampoules under argon atmosphere. Their structures were determined by single-crystal X-ray diffraction at room temperature. The metallic compounds crystallize in a variant of the K₂ZnO₂ type (Ibam, Z=4, Pd₂CoSe₂: a=5.993(1), b=10.493(2), c=5.003(1) Å; Pd₂FeSe₂: a=5.960(1), b=10.576(2), c=5.078(1) Å; Pd₂CoTe₂: a=6.305(1), b=11.100(2), c=5.234(1) Å; Pd₂NiTe₂: a=6.286(1), b=11.194(2), c=5.157(1) Å). One-dimensional tetrahedra chains with remarkably short M—M bonds run along [001], separated by [Pd₂] dumbbells with palladium in fivefold coordination of selenium or tellurium atoms. The structure may also be described as a filled variant of the SiS₂ type. M atoms occupy of the tetrahedral voids and the Pd atoms fill all octahedral voids in a distorted ccp motif of chalcogen atoms. Even though the Pd₂MCh₂ compounds are isotypic to K₂ZnO₂ from the crystallographic viewpoint, we find a different bonding situation with additional homo- and heteronuclear metal-metal bonds between the Pd and Co atoms. The electronic structures and topologies of the electron densities of Pd₂CoSe₂ and isotypic Na₂CoSe₂ are analyzed and compared by using Bader's AIM theory. Different values of topological charge transfer and electron density flatness indices uncover striking quantitative differences in the nature of chemical bonding between the metallic compound Pd₂CoSe₂ and nonmetallic Na₂CoSe₂.     

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.     

Two metal chalcogenides, Hg2Te2X2 (XBr, I): 3-D framework constructed from novel left-handed helices

Two isostructural metal chalcogenides, Hg₂Te₂Br₂ (1) and Hg₂Te₂I₂ (2), were obtained by solid-state reactions and structurally characterized. Compounds 1 and 2 crystallize in the acentric space group P4₃2₁2 of the tetragonal system with eight formula units in a cell: a=10.2388(9), c=14.480(2) Å, V=1518.0(3) Å³, R₁/wR₂=0.0670/0.1328 for 1 and a=10.711(3), c=15.025(8) Å, V=1724(1) Å³, R₁/wR₂=0.0637/0.1233 for 2. Both compounds are characterized by a three-dimensional (3-D) framework structure, which is composed by interconnected left-handed helices formed by both tetrahedral and trigonal Hg atoms. Optical absorption spectra of 1 and 2 reveal the presence of sharp optical gaps of 2.06 and 1.85 eV, respectively, suggesting that both materials are semiconductors. TG-DTA measurements show that both compounds are thermally stable up to 200 °C. The composition of both compounds is well confirmed by the semiquantitative microscope analyses.     

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.     

Syntheses and structures of CsHo3Te5 and Cs3Tm11Te18 and the electronic structure of CsHo3Te5

Single crystals of CsHo₃Te₅ and Cs₃Tm₁₁Te₁₈ have been grown as byproducts in the synthesis of CsLnZnTe₃ (Ln=Ho or Tm) through the reaction of Ln, Zn, and Te with a CsCl flux at 850 °C. The crystal structures have been determined from single-crystal X-ray diffraction data. CsHo₃Te₅ crystallizes in space group Pnma of the orthorhombic system whereas Cs₃Tm₁₁Te₁₈ crystallizes in the space group C2/m of the monoclinic system. Each of the compounds adopts a three-dimensional structure; each possesses tunnels built from LnTe₆ octahedra that are filled with Cs atoms. The pseudo-rectangular tunnel in CsHo₃Te₅ is large enough in cross-section to accommodate two symmetrically equivalent Cs atoms. In the Cs₃Tm₁₁Te₁₈ structure there are two different sized tunnels: the smaller one is only large enough to host one Cs atom per unit cell whereas the larger one can accommodate two Cs atoms. The electronic structure of CsHo₃Te₅ was calculated. The band gap is estimated to be about 1.2 eV, consistent with the black color of the crystals.     

New ternary rare-earth metal boride carbides R15B4C14 (R=Y, Gd-Lu) containing BC2 units: Crystal and electronic structures, magnetic properties

The ternary rare-earth boride carbides R₁₅B₄C₁₄ (R=Y, Gd-Lu) were prepared from the elements by arc-melting followed by annealing in silica tubes at 1270 K for 1 month. The crystal structures of Tb₁₅B₄C₁₄ and Er₁₅B₄C₁₄ were determined from single crystal X-ray diffraction data. They crystallize in a new structure type in space group P4/mnc (Tb₁₅B₄C₁₄: a=8.1251(5) Å, c=15.861(1) Å, Z=2, R₁=0.041 (wR₂=0.088) for 1023 reflections with I_o>2σ(I_o); Er₁₅B₄C₁₄: a=7.932(1) Å, c=15.685(2) Å, Z=2, R₁=0.037 (wR₂=0.094) for 1022 reflections with I_o>2σ(I_o)). The crystal structure contains discrete carbon atoms and bent CBC units in octahedra and distorted bicapped square antiprisms, respectively. In both structures the same type of disorder exists. One R atom position needs to be refined as split atom position with a ratio 9:1 indicative of a 10% substitution of the neighboring C⁴⁻ by C₂⁴⁻. The actual composition has then to be described as R₁₅B₄C_14.2. The isoelectronic substitution does not change the electron partition of R₁₅B₄C₁₄ which can be written as (R³⁺)₁₅(C⁴⁻)₆(CBC⁵⁻)₄•e⁻. The electronic structure was studied with the extended Hückel method. The investigated compounds Tb₁₅B₄C₁₄, Dy₁₅B₄C₁₄ and Er₁₅B₄C₁₄ are hard ferromagnets with Curie temperatures T_C=145, 120 and 50 K, respectively. The coercive field B_C=3.15 T for Dy₁₅B₄C₁₄ is quite remarkable.     

Crystal structures and chemistry of double perovskites Ba2M(II)M′(VI)O6 (M=Ca, Sr, M′=Te, W, U)

Structures of the double perovskites Ba₂M(II)M ′(VI)O₆ (M=Ca, Sr, M′=Te, W, U) at room temperature have been investigated by the Rietveld method using X-ray and neutron powder diffraction data. For double perovskites with M=Sr, the observed space groups are I2/m (M′ =W) and (M′=Te), respectively. In the case of M=Ca, the space groups are either monoclinic P2₁/n (M′=U) or cubic (M′=W and Te). The tetragonal and orthorhombic symmetry reported earlier for Ba₂SrTeO₆ and Ba₂CaUO₆, respectively, were not observed. In addition, non-ambient X-ray diffraction data were collected and analyzed for Ba₂SrWO₆ and Ba₂CaWO₆ in the temperature range between 80 and 723 K. It was found that the rhombohedral structure exists in Ba₂SrWO₆ above room temperature between the monoclinic and the cubic structure, whereas the cubic Ba₂CaWO₆ undergoes a structural phase transition at low temperature to the tetragonal I4/m structure.     

Three-dimensional five-connected coordination polymer [M2(C3H2O4)2(H2O)2(μ2-hmt)]n with 46 topologies (M=Zn, Cu; hmt=hexamethylenetetramine)

Two novel three-dimensional five-connected coordination polymers [M₂(C₃H₂O₄)₂(H₂O)₂(μ₂-hmt)]_n with 4⁴6⁶ topologies (M=Zn, Cu; hmt=hexamethylenetetramine) were synthesized and characterized by elemental analysis, crystal structure, IR, thermal gravimetric analyses. Both [Zn₂(C₃H₂O₄)₂(H₂O)₂(μ₂-hmt)]_n and [Cu₂(C₃H₂O₄)₂(H₂O)₂(μ₂-hmt)]_n all crystallize in the orthorhombic system, space group Imm2, and with Z=2. Metal ions have all octahedral geometry coordinated by four oxygen atoms from three malonates, one oxygen atom from a water molecule and one nitrogen atom of hmt ligand. Each malonate binds a metal ion with its two oxygen atoms in a chelating mode and connects to adjacent two metal ions with another two oxygen atoms to form an infinite wavy layer. The layers are bridged by μ₂-hmt molecules to form a three-dimensional framework with channels. The magnetic susceptibility data show there is a weak antiferromagnetic exchange interaction in the complex [Cu₂(C₃H₂O₄)₂(H₂O)₂(μ₂-hmt)]_n.