Gd5Ni0.96Sb2.04 and Gd5Ni0.71Bi2.29: Crystal structure, magnetic properties and magnetocaloric effect. Structural transformation and magnetic properties of hexagonal Gd5Bi3

Nickel was successfully introduced into the Gd₅Sb₃ and Gd₅Bi₃ binaries to yield the Gd₅Ni_0.96(1)Sb_2.04(1) and Gd₅Ni_0.71(1)Bi_2.29(1) phases. Both Ni-substituted compounds adopt the orthorhombic Yb₅Sb₃-type structure. While the Gd₅Ni_0.71Bi_2.29 phase is thermodynamically stable at 800 °C and decomposes at lower temperatures upon annealing, it can be easily quenched to room temperature by rapid cooling from 800 °C. The Gd₅Ni_0.96Sb_2.04 phase is found to be thermodynamically stable till room temperature. Through annealing at different temperatures, Gd₅Bi₃ was proven to undergo the Mn₅Si₃-type (LT)↔Yb₅Sb₃-type (HT) transformation reversibly, whereas Gd₅Sb₃ was found to adopt only the hexagonal Mn₅Si₃-type structure. Orthorhombic Gd₅Ni_0.96Sb_2.04 and Gd₅Ni_0.71Bi_2.29 and low-temperature hexagonal Gd₅Bi₃ order ferromagnetically at 115, 162 and 112 K, respectively. In Gd₅Bi₃, the ferromagnetic ordering is followed by spin reorientation below 64 K. Magnetocaloric effect in terms of ΔS was evaluated from the magnetization data and found to reach the maximum values of −7.7 J/kgK for Gd₅Ni_0.96Sb_2.04 and −5.6 J/kgK for Gd₅Ni_0.71Bi_2.29 around their Curie temperatures.     

Dy5Ni0.66Bi2.34 and Lu5Ni0.56Sb2.44 – Substitution derivatives with the orthorhombic Yb5Sb3-type structure. Magnetocaloric effect of Dy5Ni0.66Bi2.34

Dy₅Ni_0.66Bi_2.34 and Lu₅Ni_0.56Sb_2.44 were synthesized by arc-melting and were found to adopt an orthorhombic Yb₅Sb₃-type structure. Cell parameters are a = 12.075(2), b = 9.165(2), c = 8.072(1) Å for Dy₅Ni_0.66Bi_2.34 and a = 11.6187(9), b = 8.933(1) and c = 7.8377(6) Å for Lu₅Ni_0.56Sb_2.44. Dy₅Ni_0.66Bi_2.34 undergoes a step-like ferromagnetic transition around 66 K. Magnetocaloric effect in terms of the magnetic entropy change, ΔS, reaches −3.73 J/kg K at 75 K for Dy₅Ni_0.66Bi_2.34.     

Synthesis, crystal structures, magnetic and electric transport properties of Eu11InSb9 and Yb11InSb9

Two new rare-earth metal containing Zintl phases, Eu₁₁InSb₉ and Yb₁₁InSb₉ have been synthesized by reactions of the corresponding elements in molten In metal to serve as a self-flux. Their crystal structures have been determined by single crystal X-ray diffraction—both compounds are isostructural and crystallize in the orthorhombic space group Iba2 (No. 45), Z=4 with unit cell parameters a=12.224(2) Å, b=12.874(2) Å, c=17.315(3) Å for Eu₁₁InSb₉, and a=11.7886(11) Å, b=12.4151(12) Å, c=16.6743(15) Å for Yb₁₁InSb₉, respectively (Ca₁₁InSb₉-type, Pearson's code oI84). Both structures can be rationalized using the classic Zintl rules, and are best described in terms of discrete In-centered tetrahedra of Sb, [InSb₄]⁹⁻, isolated Sb dimers, [Sb₂]⁴⁻, and isolated Sb anions, Sb³⁻. These anionic species are separated by Eu²⁺ and Yb²⁺ cations, which occupy the empty space between them and counterbalance the formal charges. Temperature-dependent magnetic susceptibility and resistivity measurements corroborate such analysis and indicate divalent Eu and Yb, as well as poorly metallic behavior for both Eu₁₁InSb₉ and Yb₁₁InSb₉. The close relationships between these structures and those of the monoclinic α-Ca₂₁Mn₄Sb₁₈ and Ca₂₁Mn₄Bi₁₈ are also discussed.     

Synthesis and solid state studies on Mb2Sb2O7 and (Mn1−xCdx)2Sb2O7 pyrochlores

Pyrochlore oxides of the type Mn₂Sb₂O₇ and (Mn_1?xCd_x)₂Sb₂O₇ have been synthesized by high-temperature solid state reactions and characterized by X-ray diffraction and chemical analysis. X-Ray diffraction studies showed that the compound Mn₂Sb₂O₇ has a rhombohedrally distorted pyrochlore structure. In the solid solutions (Mn_1?xCd_x)₂Sb₂O₇, the phases with x ≥ 0.6 are cubic. Magnetic and ¹²¹Sb Mössbauer studies indicate that all the Mn and Sb are present in the +2 and +5 state occupying A and B sites, respectively, in the pyrochlore structure. Electrical measurements indicate that the compounds are insulators or semiconductors exhibiting p-type behavior. The stoichiometry and probable cause of the rhombohedral distortion in Mn₂Sb₂O₇ and solid solutions are discussed.     

High-temperature thermoelectric studies of A11Sb10 (A=Yb, Ca)

Large samples (6-8 g) of Yb₁₁Sb₁₀ and Ca₁₁Sb₁₀ have been synthesized using a high-temperature (1275-1375 K) flux method. These compounds are isostructural to Ho₁₁Ge₁₀, crystallizing in the body-centered, tetragonal unit cell, space group I4/mmm, with Z=4. The structure consists of antimony dumbbells and squares, reminiscent of Zn₄Sb₃ and filled Skutterudite (e.g., LaFe₄Sb₁₂) structures. In addition, these structures can be considered Zintl compounds; valence precise semiconductors with ionic contributions to the bonding. Differential scanning calorimetry (DSC), thermogravimetry (TG), resistivity (ρ), Seebeck coefficient (α), thermal conductivity (κ), and thermoelectric figure of merit (zT) from room temperature to at minimum 975 K are presented for A₁₁Sb₁₀ (A=Yb, Ca). DSC/TG were measured to 1400 K and reveal the stability of these compounds to ∼1200 K. Both A₁₁Sb₁₀ (A=Yb, Ca) materials exhibit remarkably low lattice thermal conductivity (∼10 mW/cm K for both Yb₁₁Sb₁₀ and Ca₁₁Sb₁₀) that can be attributed to the complex crystal structure. Yb₁₁Sb₁₀ is a poor metal with relatively low resistivity (1.4 mΩ cm at 300 K), while Ca₁₁Sb₁₀ is a semiconductor suggesting that a gradual metal-insulator transition may be possible from a Ca_11−xYb_xSb₁₀ solid solution. The low values and the temperature dependence of the Seebeck coefficients for both compounds suggest that bipolar conduction produces a compensated Seebeck coefficient and consequently a low zT.     

Pyrochlore phases in the system ZnOBi2O3Sb2O5: II. Crystal structures of Zn2Bi3.08Sb2.92O14+δ and Zn2+xBi2.96−(x−y)Sb3.04−yO14.04+δ

Rietveld refinement of combined X-ray and neutron diffraction data has, within errors, confirmed the stoichiometries of two, cubic pyrochlore phases in the ZnOBi₂O₃Sb₂O₅ system. Neither phase has the ‘ideal’ stoichiometry, Zn₂Bi₃Sb₃O₁₄. One phase, P₁, is a Zn-rich, Bi-deficient solid solution Zn_2+xBi_{2.96−(x−y)}Sb_3.04−yO_14.04+δ. The other, P₂, is a Bi-rich line phase, stoichiometry Zn₂Bi_3.08Sb_2.92O_14+δ. Both structures have a mixture of Bi, Zn on the A-sites and Zn, Sb on the B-sites. However, Zn is displaced off-centre in the A-sites to achieve lower co-ordination number with realistic ZnO bond lengths. Additional structural complexities arise from: displacement of O(2) atoms; partial occupancies of O(1) and O(2) sites; partial occupancy of a third, interstitial oxygen site, O(3). Since the multiplicities of the off-centre sites are much higher than those of the ideal positions, there is considerable possibility for correlated short range order throughout the structures.     

Syntheses, structures, optical properties, and theoretical calculations of Cs2Bi2ZnS5, Cs2Bi2CdS5, and Cs2Bi2MnS5

Three new compounds, Cs₂Bi₂ZnS₅, Cs₂Bi₂CdS₅, and Cs₂Bi₂MnS₅, have been synthesized from the respective elements and a reactive flux Cs₂S₃ at 973 K. The compounds are isostructural and crystallize in a new structure type in space group Pnma of the orthorhombic system with four formula units in cells of dimensions at 153 K of a=15.763(3), b=4.0965(9), c=18.197(4) Å, V=1175.0(4) Å³ for Cs₂Bi₂ZnS₅; a=15.817(2), b=4.1782(6), c=18.473(3)  Å, V=1220.8(3)  ų for Cs₂Bi₂CdS₅; and a=15.830(2), b=4.1515(5), c=18.372(2) Å, V=1207.4(2) Å³ for Cs₂Bi₂MnS₅. The structure is composed of two-dimensional _∞²[Bi₂MS₅²⁻] (M=Zn, Cd, Mn) layers that stack perpendicular to the [100] axis and are separated by Cs⁺ cations. The layers consist of edge-sharing _∞¹[Bi₂S₆⁶⁻] and _∞¹[MS₃⁴⁻] chains built from BiS₆ octahedral and MS₄ tetrahedral units. Two crystallographically unique Cs atoms are coordinated to S atoms in octahedral and monocapped trigonal prismatic environments. The structure of Cs₂Bi₂MS₅, is related to that of Na₂ZrCu₂S₄ and those of the AMM′Q₃ materials (A=alkali metal, M=rare-earth or Group 4 element, M′= Group 11 or 12 element, Q=chalcogen). First-principles theoretical calculations indicate that Cs₂Bi₂ZnS₅ and Cs₂Bi₂CdS₅ are semiconductors with indirect band gaps of 1.85 and 1.75 eV, respectively. The experimental band gap for Cs₂Bi₂CdS₅ is ≈1.7 eV, as derived from its optical absorption spectrum.     

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.     

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.     

Synthesis crystal structure and ionic conductivity of Ca0.5Bi3V2O10 and Sr0.5Bi3V2O10

Two new compounds Ca_0.5Bi₃V₂O₁₀ and Sr_0.5Bi₃V₂O₁₀ have been synthesized in the ternary system: MO-Bi₂O₃-V₂O₅ system (M=M²⁺). The crystal structure of Sr_0.5Bi₃V₂O₁₀ has been determined from single crystal X-ray diffraction data, space group and Z=2, with cell parameters a=7.1453(3) Å, b=7.8921(3) Å, c=9.3297(3) Å, α=106.444(2)°, β=94.088(2)°, γ=112.445(2)°, V=456.72(4) Å³. Ca_0.5Bi₃V₂O₁₀ is isostructural with Sr_0.5Bi₃V₂O₁₀, with, a=7.0810(2) Å, b=7.8447(2) Å, c=9.3607(2) Å, α=106.202(1)°, β=94.572(1)°, γ=112.659(1)°, V=450.38(2) Å³ and its structure has been refined by Rietveld method using powder X-ray data. The crystal structure consists of infinite chains of (Bi₂O₂) along c-axis formed by linkage of BiO₈ and BiO₆ polyhedra interconnected by MO₈ polyhedra forming 2D layers in ac plane. The vanadate tetrahedra are sandwiched between these layers. Conductivity measurements give a maximum conductivity value of 4.54×10⁻⁵ and 3.63×10⁻⁵ S cm⁻¹ for Ca_0.5Bi₃V₂O₁₀ and Sr_0.5Bi₃V₂O₁₀, respectively at 725 °C.     

The luminescence of Cs3Bi2Cl9 and Cs3Sb2Cl9

The luminescence properties of Cs₃Bi₂Cl₉, α-Cs₃Sb₂Cl₉, and β-Cs₃Sb₂Cl₉ are reported and compared with those of Cs₃Bi₂Br₉. The first two compounds have comparable luminescence properties which can be described in terms of a band model. Deep center emission is observed for both compounds, whereas edge emission is observed only for Cs₃Bi₂Cl₉. The optical transitions of β-Cs₃Sb₂Cl₉ are localized on the Sb³⁺ ion. The orientation of the lone-pair orbitals of the ns² ions seems to play an important role in the formation of the cationic valence band. The α-β transformation must therefore have a considerable influence on the spectral properties of Cs₃Sb₂Cl₉.     

Preparation and crystal structure of [Bi3I(C4H8O3H2)2(C4H8O3H)5]2Bi8I30 containing the novel polynuclear [Bi8I30] anion

Preparation and crystal structure of the novel compound [Bi₃I(C₄H₈O₃H₂)₂(C₄H₈O₃H)₅]₂Bi₈I₃₀ are reported. The title compound is prepared by heating of BiI₃ and diethylene glycol at 413 K in a sealed quartz glass tube filled with argon. Deep red single crystals are grown and applied to perform X-ray powder diffraction and X-ray single-crystal diffraction measurements. The compound crystallizes triclinic with space group P-1: Z=2, a=13.217(1) Å, b=15.277(1) Å, c=22.498(1) Å, α=84.33(1), β=73.18(1), γ=67.48(1). [Bi₃I(C₄H₈O₃H₂)₂(C₄H₈O₃H)₅]₂Bi₈I₃₀ comprises the novel polynuclear [Bi₈I₃₀]⁶⁻ anion and [Bi₃I(C₄H₈O₃H₂)₂(C₄H₈O₃H)₅]³⁺ as the cation. Cation as well as the anion can be assumed to represent intermediates between solid BiI₃ and BiI₃ completely dissolved in diethylene glycol.     

Crystal structure of Ca2Ln3Sb3O14 (Ln=La, Pr, Nd and Y): A novel variant of weberite

The crystal structures of Ca₂Ln₃Sb₃O₁₄ (Ln=La, Pr, Nd and Y) and Ca₂Sb₂O₇ at room temperature were refined by the Rietveld method using combined X-ray and neutron powder diffraction data. Ca₂Sb₂O₇ adopts the weberite structure having the space group Imma. The structures of Ca₂Ln₃Sb₃O₁₄ are, however, neither the orthorhombic nor the tetragonal chiolite as has been suggested previously. They crystallize in the monoclinic space group I2/m11 belonging to a hitherto unknown type of deformation of the parent (orthorhombic) weberite structure.     

Synthesis and characterization of quaternary chalcogenides InSn2Bi3Se8 and In0.2Sn6Bi1.8Se9

Quaternary chalcogenides InSn₂Bi₃Se₈ and In_0.2Sn₆Bi_1.8Se₉ were synthesized on direct combination of their elements in stoichiometric ratios at T>800 °C under vacuum. Their structures were determined with X-ray diffraction of single crystals. InSn₂Bi₃Se₈ crystallizes in monoclinic space group C2/m (No. 12) with a=13.557(3) Å, b=4.1299(8) Å, c=15.252(3) Å, β=115.73(3)°, V=769.3(3) Å³, Z=2, and R₁/wR₂/GOF=0.0206/0.0497/1.092; In_0.2Sn₆Bi_1.8Se₉ crystallizes in orthorhombic space group Cmc2₁ (No. 36) with a=4.1810(8) Å, b=13.799(3) Å, c=31.953(6) Å, V=1843.4(6) Å³, Z=4, and R₁/wR₂/GOF=0.0966/0.2327/1.12. InSn₂Bi₃Se₈ and In_0.2Sn₆Bi_1.8Se₉ are isostructural with CuBi₅S₈ and Bi₂Pb₆S₉ phases, respectively. The structures of InSn₂Bi₃Se₈ and In_0.2Sn₆Bi_1.8Se₉ feature a three-dimensional framework containing slabs of NaCl-(311) type with varied thicknesses. Calculations of the electronic structure and measurements of electrical conductivity indicate that these materials are semiconductors with narrow band gaps. Both compounds show n-type semiconducting properties with Seebeck coefficients −270 and −230 μV/K at 300 K for InSn₂Bi₃Se₈ and In_0.2Sn₆Bi_1.8Se₉, respectively.     

Comparative high-pressure study and chemical bonding analysis of Rh3Bi14 and isostructural Rh3Bi12Br2

The binary compound Rh₃Bi₁₄ was synthesized from the elements. The compound is isostructural with Rh₃Bi₁₂Br₂, crystallizes with the orthorhombic space group Fddd (no. 70) and lattice parameters a=6.8959(15) Å, b=17.379(3) Å, c=31.758(6) Å. The crystal structure consists of a three-dimensional (3D) framework of edge-sharing cubes and square antiprisms (RhBi_8/2). It is closely related to the intermetallic compound RhBi₄, in which two Y-like frameworks of antiprisms interpenetrate. In Rh₃Bi₁₄ and Rh₃Bi₁₂Br₂, additional bismuth and bromine anions, respectively, fill the channels of the 3D polyhedral framework formed by covalently bonded rhodium and bismuth atoms. High-pressure X-ray powder diffraction data from synchrotron measurements of Rh₃Bi₁₄ and Rh₃Bi₁₂Br₂ indicate a high stability of both compounds in the investigated range from ambient pressure to ca. 30 GPa at ambient temperature.     

HNbO3 and HTaO3: New cubic perovskites prepared from LiNbO3 and LiTaO3 via ion exchange

The synthesis of HNbO₃ and HTaO₃ from LiNbO₃ and LiTaO₃ via ion exchange in hot aqueous acid solutions is reported. This reaction is accompanied by a topotactic structural transformation from the rhombohedral LiNbO₃ structure to the cubic perovskite structure; cell constants are a = 3.822(1) Å for HNbO₃ and 3.810(2) Å for HTaO₃. These new compounds have been characterized by powder X-ray diffraction, thermogravimetric analysis, and solid-state NMR. They are electronic insulators and have low ionic conductivity. Evidence of partially proton-exchanged phases Li_1?xH_xMO₃ was also seen. The possible significance of this ion exchange reaction for devices using LiNbO₃ or LiTaO₃ is discussed.     

Yb3Cu6Sn5, Yb5Cu11Sn8 and Yb3Cu8Sn4: crystal structure of three ordered compounds

Yb₃Cu₆Sn₅, Yb₅Cu₁₁Sn₈ and Yb₃Cu₈Sn₄ compounds were prepared in sealed Ta crucibles by induction melting and subsequent annealing. The crystal structures of Yb₃Cu₆Sn₅ and Yb₅Cu₁₁Sn₈ were determined from single crystal diffractometer data: Yb₃Cu₆Sn₅, isotypic with Dy₃Co₆Sn₅, orthorhombic, Immm, oI28, a=4.365(1) Å, b=9.834(3) Å, c=12.827(3) Å, Z=2, R=0.019, 490 independent reflections, 28 parameters; Yb₅Cu₁₁Sn₈ with its own structure, orthorhombic, Pmmn, oP48, a=4.4267(6) Å, b=22.657(8) Å, c=9.321(4) Å, Z=2, R=0.047, 1553 independent reflections, 78 parameters. Both compounds belong to the BaAl₄-derived defective structures, and are closely related to Ce₃Pd₆Sb₅ (oP28, Pmmn). The crystal structure of Yb₃Cu₈Sn₄, isotypic with Nd₃Co₈Sn₄, was refined from powder data by the Rietveld method: hexagonal, P6₃mc, hP30, a=9.080(1) Å, c=7.685(1) Å, Z=2, R_wp=0.040. It is an ordered substitution derivative of the BaLi₄ type (hP30, P6₃/mmc). All compounds show strong Cu-Sn bonds with a length reaching 2.553(3) Å in Yb₅Cu₁₁Sn₈.     

Synthesis and structural characterization of A3In2Ge4 and A5In3Ge6 (A=Ca, Sr, Eu, Yb)—New intermetallic compounds with complex structures, exhibiting Ge-Ge and In-In bonding

Reported are the synthesis and the structural characterization of four new polar intermetallic phases, which exist only with mixed alkaline-earth and rare-earth metal cations in narrow homogeneity ranges. (Sr_1-xCa_x)₅In₃Ge₆ and (Eu_1-xYb_x)₅In₃Ge₆ (x≈0.7) crystallize in the orthorhombic space group Pnma with two formula units per unit cell (own structure type, Pearson symbol oP56). The lattice parameters are as follows: a=13.109(3)-13.266(3) Å, b=4.4089(9)-4.4703(12) Å, and c=23.316(5)-23.557(6) Å. (Sr_1-xCa_x)₃In₂Ge₄ and (Sr_1-xYb_x)₃In₂Ge₄ (x≈0.4-0.5) adopt another novel monoclinic structure-type (space group C2/m, Z=4, Pearson symbol mS36) with lattice parameters in the range a=19.978(2)-20.202(2) Å, b=4.5287(5)-4.5664(5) Å, c=10.3295(12)-10.3447(10) Å, and β=98.214(2)-98.470(2)°, depending on the metal cations and their ratio. The polyanionic sub-structures in both cases are based on chains of InGe₄ corner-shared tetrahedra. The A₅In₃Ge₆ structure (A=Sr/Ca or Sr/Yb) also features Ge₄ tetramers, and isolated In atoms in nearly square-planar environment, while the A₃In₂Ge₄ structure (A=Sr/Ca or Eu/Yb) contains zig-zag chains of In and Ge strings with intricate topology of cis- and trans-bonds. The experimental results have been complemented by tight-binding linear muffin-tin orbital (LMTO) band structure calculations.     

Bi2n+4MonO6(n+1) with n=3, 4, 5, 6: A new series of low-temperature stable phases in the mBi2O3 - MoO3 system (1.0<m<1.7): Structural relationships and conductor properties

Four low-temperature phases with compositions Bi₁₀Mo₃O₂₄, Bi₆Mo₂O₁₅, Bi₁₄Mo₅O₃₆ and Bi₈Mo₃O₂₁ have been prepared by the n-butylamine wet synthesis method. They have been characterized by powder X-ray diffraction and transmission electron microscopy, mainly by selected area electron diffraction. The four phases present a close structural relationship and a common basic fluorite-type structure and are members of a homologous series of phases with general formula Bi_2n+4Mo_nO_6(n+1), being n=3, 4, 5 and 6, respectively. The matrices relating their superstructures and the basic fluorite type unit cell are given, as well as a general one for the whole series. The conductor behavior of these phases is characterized by impedance spectroscopy being all these materials very good ionic conductors.     

The lanthanoid polyantimonides with the ideal compositions Pr5Sb12 and Nd5Sb12 crystallizing with a new structure type and Ho2Sb5 with Dy2Sb5-type structure

The title compounds were isolated in well-crystallized form from samples with a substantial excess of antimony, annealed at temperatures slightly below the melting point of that element. Their crystal structures were determined from single-crystal diffractometer data. Pr_9-xSb_21-y and Nd_9-xSb_21-y crystallize with a new monoclinic structure type, Pearson symbol mS(62-5.4), space group Cm, Z=2 with a=2859.1(4) pm, b=426.3(1) pm, c=1356.1(2) pm, β=95.52(1)°, R=0.034 for 4351 structure factors and 188 variable parameters for Pr_9-xSb_21-y and a=2845(2) pm, b=424.7(8) pm, c=1345.9(9) pm, β=95.42(7)°, R=0.069 for 2928 F values and 188 variables for Nd_9-xSb_21-y. Of the 30 atomic sites, three show fractional occupancy corresponding to the compositions Pr_8.303(5)Sb_20.03(1) and Nd_8.30(2)Sb_19.98(9), respectively. A model for the order of occupied atomic sites with a tripled b-axis is proposed resulting in the ideal compositions Pr₅Sb₁₂ and Nd₅Sb₁₂. The holmium compound Ho₂Sb₅ has a Dy₂Sb₅-type structure: mP28, P2₁/m, a=1301.8(3) pm, b=414.9(1) pm, c=1451.1(2) pm, β=102.14(1)°, R=0.028 for 2573 F values and 86 variables. In both structure types most rare earth atoms have nine antimony neighbors forming tricapped trigonal prisms. The coordination polyhedra of the antimony atoms show a great variety, with a trigonal prism of rare earth atoms as one extreme case. The other extreme coordination of an antimony atom is a distorted octahedron formed by six antimony atoms. The differences and similarities of both structures are discussed. Chemical bonding within the antimony polyanions is analyzed on the basis of an extended Zintl-Klemm concept using bond-length-bond-strength relationships.