Structure and physical properties of rare-earth zinc antimonides REZn1-xSb2 (RE=La, Ce, Pr, Nd, Sm, Gd, Tb)

The ternary rare-earth zinc antimonides REZn_1-xSb₂ (RE=La, Ce, Pr, Nd, Sm, Gd, Tb) were prepared by heating at 1050 °C followed by annealing at 600 °C. For all members, single-crystal X-ray diffraction studies indicated that the Zn deficiency is essentially fixed, corresponding to the formula REZn_0.6Sb₂, with no appreciable homogeneity range. These compounds adopt the HfCuSi₂-type structure (Pearson symbol tP8, space group P4/nmm, Z=2). Single-crystal electrical resistivity measurements confirmed the occurrence of an abrupt resistivity decrease near 4 K for RE=Ce, and a less pronounced one for RE=La, Pr, and Gd. Except for the ferromagnetic Ce (T_c=2.5 K) and antiferromagnetic Tb (T_N=10 K) members, all remaining compounds exhibit no long-range magnetic ordering down to 2 K, instead showing temperature-independent (RE=La), van Vleck (RE=Sm), or Curie-Weiss paramagnetism (RE=Pr, Nd, Gd).     

Ternary rare-earth zinc arsenides REZn1-xAs2 (RE=La-Nd, Sm)

The ternary rare-earth zinc arsenides REZn_1−xAs₂ (RE=La-Nd, Sm) were prepared by reaction of the elements at 800 °C. Single-crystal and powder X-ray diffraction analysis revealed a defect SrZnBi₂-type average structure for the La member (Pearson symbol tI16, space group I4/mmm, Z=4; a=4.0770(9) Å, c=20.533(5) Å), in contrast to defect HfCuSi₂-type average structures for the remaining RE members (Pearson symbol tP8, space group P4/nmm, Z=2; a=4.0298(5)-3.9520(4) Å, c=10.222(1)-10.099(1) Å in the progression from Ce to Sm). The homogeneity range is not appreciable (estimated to be narrower than 0.6<1−x<0.7 in SmZn_1−xAs₂) and the formula REZn_0.67As₂ likely represents the Zn-rich phase boundary. The Ce-Nd members are Curie-Weiss paramagnets. LaZn_0.67As₂ shows activated behavior in its electrical resistivity, whereas SmZn_0.67As₂ exhibits anomalies in its temperature dependence of the electrical resistivity.     

Structure and magnetic properties of rare-earth chromium germanides RECrxGe2 (RE=Sm, Gd-Er)

The ternary rare-earth chromium germanides RECr_xGe₂ (RE=Sm, Gd-Er) have been obtained by reactions of the elements, either in the presence of tin or indium flux, or through arc-melting followed by annealing at 800 °C. The homogeneity range is limited to 0.25?x?0.50 for DyCr_xGe₂. Single-crystal and powder X-ray diffraction studies on the RECr_0.3Ge₂ members revealed that they adopt the CeNiSi₂-type structure (space group Cmcm, Z=4, a=4.1939(5)-4.016(2) Å, b=16.291(2)-15.6579(6) Å, c=4.0598(5)-3.9876(2) Å in the progression for RE=Sm to Er), which can be considered to be built up by stuffing transition-metal atoms into the square pyramidal sites of a “REGe₂” host with the ZrSi₂-type structure. (The existence of YbCr_0.3Ge₂ is also implicated.) Only the average structure was determined here, because unusually short Cr-Ge distances imply the development of a superstructure involving distortions of the square Ge net. Magnetic measurements on RECr_0.3Ge₂ (RE=Gd-Er) indicated that antiferromagnetic ordering sets in below T_N (ranging from 3 to 17 K), with additional transitions observed at lower temperatures for the Tb and Dy members.     

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.     

New members of ternary rare-earth metal boride carbides containing finite boron-carbon chains: RE25B14C26 (RE=Pr, Nd) and Nd25B12C28

New ternary rare-earth metal boride carbides RE₂₅B₁₄C₂₆ (RE=Pr, Nd) and Nd₂₅B₁₂C₂₈ were synthesized by co-melting the elements. Nd₂₅B₁₂C₂₈ is stable up to 1440 K. RE₂₅B₁₄C₂₆ (RE=Pr, Nd) exist above 1270 K. The crystal structures were investigated by means of single-crystal X-ray diffraction. Nd₂₅B₁₂C₂₈: space group P, a=8.3209(7) Å, b=8.3231(6) Å, c=29.888(2) Å, α=83.730(9)°, β=83.294(9)°, γ=89.764(9)°. Pr₂₅B₁₄C₂₆: space group P2₁/c, a=8.4243(5) Å, b=8.4095(6) Å, c=30.828(1) Å, β=105.879(4)°, V=2100.6(2) Å³, (R1=0.048 (wR2=0.088) from 2961 reflections with I_o>2σ(I_o)); for Nd₂₅B₁₄C₂₆ space group P2₁/c, Z=2, a=8.3404(6) Å, b=8.3096(6) Å, c=30.599(2) Å, β=106.065(1)°. Their structures consist of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with cumulene-like molecules [B₂C₄]⁶⁻ and [B₃C₃]⁷⁻, nearly linear [BC₂]⁵⁻ and bent [BC₂]⁷⁻ units and isolated carbon atoms. Structural and theoretical analysis suggests the ionic formulation for RE₂₅B₁₄C₂₆: (RE³⁺)₂₅[B₂C₄]⁶⁻([B₃C₃]⁷⁻)₂([BC₂]⁵⁻)₄([BC₂]⁷⁻)₂(C⁴⁻)₄·5e⁻ and for Nd₂₅B₁₂C₂₈: (Nd³⁺)₂₅([B₂C₄]⁶⁻)₃([BC₂]⁵⁻)₄([BC₂]⁷⁻)₂(C⁴⁻)₄·7e⁻. Accordingly, extended Hückel tight-binding calculations indicate that the compounds are metallic in character.     

Syntheses and crystal structures of RE3MnSn5−x (RE=Tm, Lu) with 3D Mn-Sn framework

Four new isostructural rare earth manganese stannides, namely RE₃MnSn_5−x (x=0.16(6), 0.29(1) for RE=Tm, x=0.05(8), 0.21(3) for RE=Lu), have been obtained by reacting the mixture of corresponding pure elements at high temperature. Single-crystal X-ray diffraction studies revealed that they crystallized in the orthorhombic space group Pnma (No. 62) with cell parameters of a=18.384(9)-18.495(6) Å, b=6.003(3)-6.062(2) Å, c=14.898(8)-14.976(4) Å, V=1644.3(14)-1679.0(9) Å³ and Z=8. Their structures belong to the Hf₃Cr₂Si₄ type and feature a 3D framework composed of 1D [Mn₂Sn₇] chains interconnected by [Sn₃] double chains via Sn-Sn bonds, forming 1D large channels based on [Mn₄Sn₁₆] 20-membered rings along the b-axis, which are occupied by the rare earth atoms. Electronic structure calculations based on density functional theory (DFT) for idealized “RE₃MnSn₅” model indicate that these compounds are metallic, which are in accordance with the results from temperature-dependent resistivity measurements.     

Epitaxial stabilization of (110)-layered perovskites of the RE2Ti2O7 (RE=La, Nd, Sm, Gd) family

Thin films of RE₂Ti₂O₇ (RE=La, Nd, Sm, Gd) were deposited on single crystal SrTiO₃ (110) substrates at 900 °C using pulsed laser deposition. X-ray diffraction (XRD) results showed sharp (00k) peaks (in θ-2θ scans) with narrow rocking curves (ω-scan peak widths of 0.4-0.9°), indicating that all compositions adopted the (110)-layered perovskite structure. While this is the stable structure for RE=La and Nd, it is metastable for RE=Sm and Gd. The metastable compounds are formed directly through epitaxial stabilization at these high temperatures and are shown to be isostructural to monoclinic La₂Ti₂O₇. The a, b, and c lattice parameters decreased monotonically with decreasing size of the RE cation, while the monoclinic angle remained fairly constant. The epitaxial relationship between the (110)-layered RE₂Ti₂O₇ films and the SrTiO₃(110) substrate was found by XRD and transmission electron microscopy to be . The single-phase, metastable, epitaxial, 100 nm thick films maintained the layered perovskite structure even after annealing at 900 °C for two hours in 200 Torr of oxygen.     

New examples of ternary rare-earth metal boride carbides containing finite boron-carbon chains: The crystal and electronic structure of RE15B6C20 (RE=Pr, Nd)

The ternary rare-earth metal boride carbides RE₁₅B₆C₂₀ (RE=Pr, Nd) were synthesized by co-melting the elements. They exist above 1270 K. Their crystal structures were determined from single-crystal X-ray diffraction data. Both crystallize in the space group P1¯, Z=1, a=8.3431(8) Å, b=9.2492(9) Å, c=8.3581(8) Å, α=84.72(1)°, β=89.68(1)°, γ =84.23(1)° (R1=0.041 (wR2=0.10) for 3291 reflections with I_o>2σ(I_o)) for Pr₁₅B₆C₂₀, and a=8.284(1) Å, b=9.228(1) Å, c=8.309(1) Å, α=84.74(1)°, β=89.68(1)°, γ=84.17(2)° (R1=0.033 (wR2=0.049) for 2970 reflections with I_o>2σ(I_o)) for Nd₁₅B₆C₂₀. Their structure consists of a three-dimensional framework of rare-earth metal atoms resulting from the stacking of slightly corrugated and distorted square nets, leading to cavities filled with unprecedented B₂C₄ finite chains, disordered C₃ entities and isolated carbon atoms, respectively. Structural and theoretical analyses suggest the ionic formulation (RE³⁺)₁₅([B₂C₄]⁶⁻)₃([C₃]⁴⁻)₂(C⁴⁻)₂·11ē. Accordingly, density functional theory calculations indicate that the compounds are metallic. Both structural arguments as well as energy calculations on different boron vs. carbon distributions in the B₂C₄ chains support the presence of a CBCCBC unit. Pr₁₅B₆C₁₈ exhibits antiferromagnetic order at T_N=7.9 K, followed by a meta-magnetic transition above a critical external field B>0.03 T. On the other hand, Nd₁₅B₆C₁₈ is a ferromagnet below T_C≈40 K.     

Crystal structures of two new oxysulfides La5Ti2MS5O7 (M=Cu, Ag): evidence of anionic segregation

Two new compounds, La₅Ti₂MS₅O₇ (M=Cu, Ag) were synthesized and their structures solved from single crystal X-ray data. Both compounds are isotypic. They crystallize in the orthorhombic system (space group Pnma, Z=4) with lattice constants a=19.423(1) Å, b=3.9793(2) Å, c=18.1191(9) Å for La₅Ti₂CuS₅O₇, and a=19.593(2) Å, b=3.9963(1) Å, and c=18.2973(15) Å for La₅Ti₂AgS₅O₇. The structure of these compounds is built from fragments of the rock-salt, perovskite and fluorite types and a clear anionic segregation of the anions appears in the structure. La₅Ti₂CuS₅O₇ and La₅Ti₂AgS₅O₇ exhibit an orange-yellow color and measurement of their optical band gap gave 2.02 and 2.17 eV, respectively.     

RE3Ga9Ge (RE=Y, Ce, Sm, Gd and Yb): compounds with an open three-dimensional polygallide framework synthesized from liquid gallium

The RE₃Ga₉Ge compounds (RE=Y, Ce, Sm, Gd and Yb) were synthesized at 850°C in quantitative yield from reactions containing excess liquid Ga. The orthorhombic crystal structure is characterized by a unique three-dimensional open Ga framework with parallel straight tunnels. In the tunnels, inserted are arrays of the RE atoms together with interpenetrated monoatomic RE-Ga-Ge planes. A complex disordered arrangement of the RE and Ga atoms is observed in the monoatomic plane. Depending on the extent of disorder, the crystal structure could be presented either in a sub-cell (no ordering) or in a super-cell (partial ordering). Single-crystal X-ray data for Ce₃Ga₉Ge sub-structure: space group Immm, Z=2, cell parameters a=4.3400(12) Å; b=10.836(3) Å; and c=11.545(3) Å; super-structure: space group Cmma, Z=8, cell parameters a=8.680(3) Å; b=23.090(7) Å; and c=10.836(3) Å. The refinement based on the full-matrix least squares on F_o²[I>2σ(I)] converged to final residuals R₁/wR₂=0.0226/0.0528 and 0.0729/0.1569 for the sub- and super-structures, respectively. The relationship between the disordered sub-structure and partially ordered super-structure is discussed. Magnetic susceptibility measurements show Curie-Weiss behavior at the temperatures above 30 K with the negative Weiss constants Θ=−49(1) and−7.7 K for Gd and Ce analogs, respectively. An antiferromagnetic transition is observed in the Gd analog at T_N=26.1 K. The μ_eff obtained for both analogs is close to the RE³⁺ free-ion value.     

Structure and physical properties of nonstoichiometric rare-earth cadmium antimonides, RECd1−xSb2 (RE=La, Ce, Pr, Nd, Sm)

The ternary rare-earth cadmium antimonides RECd_1−xSb₂ (RE=La, Ce, Pr, Nd, Sm) were prepared by reaction of the elements at 1000 °C. The presence of Cd defects, previously found for LaCd_0.700(5)Sb₂ and CeCd_0.660(4)Sb₂, has been confirmed by single-crystal X-ray diffraction studies for the isotypic compounds PrCd_0.665(3)Sb₂, ), NdCd_0.659(3)Sb₂, ), and SmCd_0.648(3)Sb₂, ). These compounds adopt the HfCuSi₂-type structure (Pearson symbol tP8, space group P4/nmm, Z=2). The electrical and magnetic properties of samples with nominal composition RECd_0.7Sb₂ were investigated. All exhibit metallic behaviour, but CeCd_0.7Sb₂ undergoes an abrupt drop in its electrical resistivity below 3 K. LaCd_0.7Sb₂ exhibits temperature-independent Pauli paramagnetism and SmCd_0.7Sb₂ displays van Vleck paramagnetism. The remaining compounds obey the modified Curie-Weiss law at high temperatures. CeCd_0.7Sb₂ undergoes ferromagnetic ordering below 3 K, reaching a saturation magnetization of ∼1.0 μ_B, whereas PrCd_0.7Sb₂ and NdCd_0.7Sb₂ remain paramagnetic down to 2 K.     

Crystal growth, structure determination, and optical properties of new potassium-rare-earth silicates K3RESi2O7 (RE=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)

Single crystals of K₃RESi₂O₇ (RE=Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were grown from a potassium fluoride flux. Two different structure types were found for this series. Silicates containing the larger rare earths, RE=Gd, Tb, Dy, Ho, Er, Tm, Yb crystallize in a structure K₃RESi₂O₇ that contains the rare-earth cation in both a slightly distorted octahedral and an ideal trigonal prismatic coordination environment, while in K₃LuSi₂O₇, containing the smallest of the rare earths, lutetium is found solely in an octahedral coordination environment. The structure of K₃LuSi₂O₇ crystallizes in space group P6₃/mmc with a=5.71160(10) Å and c=13.8883(6) Å. The structures containing the remaining rare earths crystallize in the space group P6₃/mcm with the lattice parameters of a=9.9359(2) Å, c=14.4295(4) Å, (K₃GdSi₂O₇); a=9.88730(10) Å, c=14.3856(3) Å, (K₃TbSi₂O₇); a=9.8673(2) Å, c=14.3572(4) Å, (K₃DySi₂O₇); a=9.8408(3) Å, c=14.3206(6) Å, (K₃HoSi₂O₇); a=9.82120(10) Å, c=14.2986(2) Å, (K₃ErSi₂O₇); a=9.80200(10) Å, c=14.2863(4) Å, (K₃TmSi₂O₇); a=9.78190(10) Å, c=14.2401(3) Å, (K₃YbSi₂O₇). The optical properties of the silicates were investigated and K₃TbSi₂O₇ was found to fluoresce in the visible.     

Crystal structures of lazulite-type oxidephosphates TiTi3O3(PO4)3 and M4Ti27O24(PO4)24 (M=Ti, Cr, Fe)

Single crystals of the oxidephosphates Ti^IIITi^IV₃O₃(PO₄)₃ (black), Cr^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (red-brown, transparent), and Fe^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (brown) with edge-lengths up to 0.3 mm were grown by chemical vapour transport. The crystal structures of these orthorhombic members (space group F2dd ) of the lazulite/lipscombite structure family were refined from single-crystal data [Ti^IIITi^IV₃O₃(PO₄)₃: Z=24, a=7.3261(9) Å, b=22.166(5) Å, c=39.239(8) Å, R₁=0.029, wR₂=0.084, 6055 independent reflections, 301 variables; Cr^III₄Ti^IV₂₇O₂₄(PO₄)₂₄: Z=1, a=7.419(3) Å, b=21.640(5) Å, c=13.057(4) Å, R₁=0.037, wR₂=0.097, 1524 independent reflections, 111 variables; Fe^III₄Ti^IV₂₇O₂₄(PO₄)₂₄: Z=1, a=7.4001(9) Å, b=21.7503(2) Å, c=12.775(3) Å, R₁=0.049, wR₂=0.140, 1240 independent reflections, 112 variables). For Ti^IIITi^IVO₃(PO₄)₃ a well-ordered structure built from dimers [Ti^III,IV₂O₉] and [Ti^IV,IV₂O₉] and phosphate tetrahedra is found. The metal sites in the crystal structures of Cr₄Ti₂₇O₂₄(PO₄)₂₄ and Fe₄Ti₂₇O₂₄(PO₄)₂₄, consisting of dimers [M^IIITi^IVO₉] and [Ti^IV,IV₂O₉], monomeric [Ti^IVO₆] octahedra, and phosphate tetrahedra, are heavily disordered. Site disorder, leading to partial occupancy of all octahedral voids of the parent lipscombite/lazulite structure, as well as splitting of the metal positions is observed. According to Guinier photographs Ti^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (a=7.418(2) Å, b=21.933(6) Å, c=12.948(7) Å) is isotypic to the oxidephosphates M^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (M^III: Cr, Fe). The UV/vis spectrum of Cr₄Ti₂₇O₂₄(PO₄)₂₄ reveals a rather small ligand-field splitting Δ_o=14,370 cm⁻¹ and a very low nephelauxetic ratio β=0.72 for the chromophores [Cr^IIIO₆] within the dimers [Cr^IIITi^IVO₉].     

Crystallochemistry of the novel two-layer RECuMg4 (RE=La, Tb) ternary compounds

The crystal structures of the new ternary compounds LaCuMg₄ and TbCuMg₄ were studied by X-ray powder diffraction and single-crystal methods, respectively. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDXS) was used for examining microstructure and phase composition. LaCuMg₄ crystallizes in the UCoAl₄ structure type (space group P6¯2m, Pearson code hP18, a=1.03911(1), c=0.45126(1) nm, Z=3, R_F=0.0654), while TbCuMg₄ exhibits a new structure (space group Cmmm, Pearson code oS48, a=1.35797(6), b=2.03333(9), c=0.39149(2) nm, Z=8, wR₂=0.0426). Both structures represent a family of two-layer compounds. All interatomic distances indicate metallic type bonding. The structural peculiarities of these compounds and their relations are discussed.     

Magnetic and Transport Properties of RESnxSb2 (RE=La, Ce, Pr, Nd, Sm; x=0.5, 0.7)

The nonstoichiometric rare-earth tin antimonides RESn_xSb₂ (RE=La, Ce, Pr, Nd, Sm) were characterized by ¹¹⁹Sn Mössbauer spectroscopy and their transport and magnetic properties were measured. The presence of nearly zero-valent Sn is suggested by the similarity of the ¹¹⁹Sn Mössbauer parameters in LaSn_xSb₂ (0.1≤x≤0.7) to those of elemental β-Sn. All RESn_0.7Sb₂ compounds exhibit metallic behavior. CeSn_0.7Sb₂ and NdSn_0.7Sb₂ show drops in resistivity below 8 K; this is attributed to a transition to a magnetically ordered state. At 25 K, CeSn_0.7Sb₂ also displays a resistivity minimum characteristic of ordered Kondo lattices. Magnetic studies indicate that, below 4 K, CeSn_xSb₂ (x=0.5, 0.7) orders ferromagnetically, whereas NdSn_xSb₂ (x=0.5, 0.7) orders antiferromagnetically and undergoes a metamagnetic transition at H_C=5.5 T and 2 K. Neither PrSn_xSb₂ nor SmSn_xSb₂ (x=0.5, 0.7) displays long-range magnetic ordering above 2 K.     

Syntheses, crystal structures and optical spectroscopy of Ln2(SO4)3·8H2O (Ln=Ho, Tm) and Pr2(SO4)3·4H2O

The lanthanide sulphate octahydrates Ln₂(SO₄)₃·8H₂O (Ln=Ho, Tm) and the respective tetrahydrate Pr₂(SO₄)₃·4H₂O were obtained by evaporation of aqueous reaction mixtures of trivalent rare earth oxides and sulphuric acid at 300 K. Ln₂(SO₄)₃·8H₂O (Ln=Ho, Tm) crystallise in space group C2/c (Z=4, a_Ho=13.4421(4) Å, b_Ho=6.6745(2) Å, c_Ho=18.1642(5) Å, β_Ho=102.006(1) Å³ and a_Tm=13.4118(14) Å, b_Tm=6.6402(6) Å, c_Tm=18.1040(16) Å, β_Tm=101.980(8) Å³), Pr₂(SO₄)₃·4H₂O adopts space group P2₁/n (a=13.051(3) Å, b=7.2047(14) Å, c=13.316(3) Å, β=92.55(3) Å³). The vibrational and optical spectra of Ho₂(SO₄)₃·8H₂O and Pr₂(SO₄)₃·4H₂O are also reported.     

Structure and magnetic properties of RE2CuIn3 (RE=Ce, Pr, Nd, Sm and Gd)

The ternary copper indides RE₂CuIn₃≡RECu_0.5In_1.5 (RE=Ce, Pr, Nd, Sm and Gd) were synthesized from the elements in sealed tantalum tubes in an induction furnace. They crystallize with the CaIn₂-type structure, space group P6₃/mmc, with a statistical occupancy of copper and indium on the tetrahedral substructure. These indides show homogeneity ranges RECu_xIn_2−x. Single crystal structure refinements were performed for five crystals: CeCu_0.66In_1.34 (a=479.90(7) pm, c=768.12(15) pm), PrCu_0.52In_1.48 (a=480.23(7) pm, c=759.23(15) pm), NdCu_0.53In_1.47 (a=477.51(7) pm, c=756.37(15) pm), SmCu_0.46In_1.54 (a=475.31(7) pm, c=744.77(15) pm), and GdCu_0.33In_1.67 (a=474.19(7), c=737.67(15) pm). Temperature-dependent susceptibility measurements show antiferromagnetic ordering at T_N=4.7 K for Pr₂CuIn₃ and Nd₂CuIn₃ and 15 K for Sm₂CuIn₃. Fitting of the susceptibility data of the samarium compound revealed an energy gap ΔE=39.7(7) K between the ground and the first excited levels.     

Crystal structure and properties of the new vanadyl(IV)phosphates Na2MVO(PO4)2, M=Ca and Sr

Two new complex vanadyl(IV)phosphates Na₂MVO(PO₄)₂ (M=Ca, Sr) were synthesized in evacuated quartz ampoules and investigated by means of X-ray diffraction, electron microscopy, DTA, ESR and magnetic susceptibility measurements. The crystal structure of Na₂SrVO(PO₄)₂ was solved ab initio from X-ray powder diffraction data. Both compounds are isostructural: a=10.5233(3) Å, b=6.5578(2) Å, c=10.0536(3) Å and a=10.6476(3) Å, b=6.6224(2) Å, c=10.2537(3) Å for Ca and Sr, respectively; S.G. Pnma, Z=4. The compounds have a three-dimensional structure consisting of V⁴⁺O₆ octahedra connected by PO₄ tetrahedra via five of the six vertexes forming a framework with cross-like channels. The strontium and sodium atoms are located in the channels in an ordered manner. Electron diffraction as well as high-resolution electron microscopy confirmed the structure solution. The new vanadylphosphates are Curie-Weiss paramagnets in a wide temperature range down to 2 K with θ=12 and 5 K for Ca and Sr phases, respectively.     

Quaternary selenostannates Na2−xGa2−xSn1+xSe6 and AGaSnSe4 (A=K, Rb, and Cs) through rapid cooling of melts. Kinetics versus thermodynamics in the polymorphism of AGaSnSe4

The quaternary alkali-metal gallium selenostannates, Na_2−xGa_2−xSn_1+xSe₆ and AGaSnSe₄ (A=K, Rb, and Cs), were synthesized by reacting alkali-metal selenide, Ga, Sn, and Se with a flame melting-rapid cooling method. Na_2−xGa_2−xSn_1+xSe₆ crystallizes in the non-centrosymmetric space group C2 with cell constants a=13.308(3) Å, b=7.594(2) Å, c=13.842(3) Å, β=118.730(4)°, V=1226.7(5) Å³. α-KGaSnSe₄ crystallizes in the tetragonal space group I4/mcm with a=8.186(5) Å and c=6.403(5) Å, V=429.1(5) Å³. β-KGaSnSe₄ crystallizes in the space group P2₁/c with cell constants a=7.490(2) Å, b=12.578(3) Å, c=18.306(5) Å, β=98.653(5)°, V=1705.0(8) Å³. The unit cell of isostructural RbGaSnSe₄ is a=7.567(2) Å, b=12.656(3) Å, c=18.277(4) Å, β=95.924(4)°, V=1741.1(7) Å³. CsGaSnSe₄ crystallizes in the orthorhombic space group Pmcn with a=7.679(2) Å, b=12.655(3) Å, c=18.278(5) Å, V=1776.1(8) Å³. The structure of Na_2−xGa_2−xSn_1+xSe₆ consists of a polar three-dimensional network of trimeric (Sn,Ga)₃Se₉ units with Na atoms located in tunnels. The AGaSnSe₄ possess layered structures. The compounds show nearly the same Raman spectral features, except for Na_2−xGa_2−xSn_1+xSe₆. Optical band gaps, determined from UV-Vis spectroscopy, range from 1.50 eV in Na_2−xGa_2−xSn_1+xSe₆ to 1.97 eV in CsGaSnSe₄. Cooling of the melts of KGaSnSe₄ and RbGaSnSe₄ produces only kinetically stable products. The thermodynamically stable product is accessible under extended annealing, which leads to the so-called γ-form (BaGa₂S₄-type) of these compounds.     

Hydrothermal synthesis, thermal, structural, spectroscopic and magnetic studies of the Mn5−xCox(HPO4)2(PO4)2(H2O)4 (x=1.25, 2, 2.5 and 3) finite solid solution

The Mn_5−xCo_x(HPO₄)₂(PO₄)₂(H₂O)₄ (x=1.25, 2, 2.5, 3) finite solid solution has been synthesized by mild hydrothermal conditions under autogeneous pressure. The phases crystallize in the C2/c space group with Z=4, belonging to the monoclinic system. The unit-cell parameters obtained from single crystal X-ray diffraction are: a=17.525(1), b=9.0535(6), c=9.4517(7) Å, β=96.633(5) ° being R1=0.0436, wR2=0.0454 for Mn75Co25; a=17.444(2), b=9.0093(9), c=9.400(1) Å, β=96.76(1) ° being R1=0.0381, wR2=0.0490 for Mn60Co40; a=17.433(2), b=8.9989(9), c=9.405(1) Å, β=96.662(9) ° being R1=0.0438, wR2=0.0515 for Mn50Co50 and a=17.4257(9), b=8.9869(5), c=9.3935(5) Å, β=96.685(4) ° being R1=0.0296, wR2=0.0460 for Mn40Co60. The structure consists of a three dimensional network formed by octahedral pentameric entities (Mn,Co)₅O₁₆(H₂O)₆ sharing vertices with the (PO₄)³⁻ and (HPO₄)²⁻ tetrahedra. The limit of thermal stability of these compounds is, approximately, 165 °C, near to this mean temperature the phases loose their water content in two successive steps. IR spectra show the characteristic bands of the water molecules and the phosphate and hydrogen-phosphate oxoanions. The diffuse reflectance spectra are consistent with the presence of MO₆ octahedra environments in slightly distorted octahedral geometry, except for the M(3)O₆ octahedron which presents a remarkable distortion and so a higher Dq parameter. The mean value for the Dq and B-Racah parameter for the M(1),(2)O₆ octahedra is 685 and 850 cm⁻¹, respectively. These parameters for the most distorted M(3)O₆ polyhedron are 825 and 880 cm⁻¹, respectively. The four phases exhibit antiferromagnetic couplings as the major magnetic interactions. However, a small spin canting phenomenon is observed at low temperatures for the two phases with major content in the anisotropic-Co(II) cation.