Structural Frustration and Occupational Disorder: The Rare Earth Metal Polysulfides Tb₈S_14.8, Dy₈S_14.9, Ho₈S_14.9, and Y₈S_14.8

Dark red crystals of Y₈S_14.8, Tb₈S_14.8, Dy₈S_14.9, and Ho₈S_14.9 have been obtained following different reaction routes. The isostructural title compounds adopt the Gd₈Se₁₅ type, a 24-fold superstructure of the ZrSSi-type and can be described in space group <i>A</i>112 (non standard setting of <i>C</i>121, no. 5) with lattice parameter of <i>a</i> = 11.505(1) Å, <i>b</i> = 15.385(1) Å, <i>c</i> = 15.726(1) Å, and γ = 90.21(2)° for Y₈S_{15–<i>x</i>}; <i>a</i> = 11.660(1) Å, <i>b</i> = 15.468(2) Å, <i>c</i> = 15.844(2) Å, and γ = 90.19(2)° for Tb₈S_{15–<i>x</i>}; <i>a</i> = 11.584(1) Å, <i>b</i> = 15.340(2) Å, <i>c</i> = 15.789(2) Å, and γ = 90.34(2)° for Dy₈S_{15–<i>x</i>}; and <i>a</i> = 11.538(1) Å, <i>b</i> = 15.288(2) Å, <i>c</i> = 15.740(2) Å, and γ = 90.23(1)° for Ho₈S_{15–<i>x</i>}, respectively. The structure consists of an alternating stacking of puckered [<i>RE</i>S] (<i>RE</i>, rare-earth metals) double slabs and planar sulfur layers along [001]. The planar sulfur layers have a complex arrangement of S₂^2– dinuclear dianions, isolated S^2– ions, and vacancies. All compounds contain trivalent rare-earth metal ions, for Tb₈S_{15–<i>x</i>} and Dy₈S_{15–<i>x</i>} antiferromagnetic order was found at <i>T</i>_N = 5.4(2) K and 3.8(1) K, respectively. Short wavelength cutoff optical band gaps of 1.6 to 1.7 eV were determined

Structural Frustration and Occupational Disorder: The Rare Earth Metal Polysulfides Tb₈S_14.8, Dy₈S_14.9, Ho₈S_14.9, and Y₈S_14.8

Dark red crystals of Y₈S_14.8, Tb₈S_14.8, Dy₈S_14.9, and Ho₈S_14.9 have been obtained following different reaction routes. The isostructural title compounds adopt the Gd₈Se₁₅ type, a 24-fold superstructure of the ZrSSi-type and can be described in space group <i>A</i>112 (non standard setting of <i>C</i>121, no. 5) with lattice parameter of <i>a</i> = 11.505(1) Å, <i>b</i> = 15.385(1) Å, <i>c</i> = 15.726(1) Å, and γ = 90.21(2)° for Y₈S_{15–<i>x</i>}; <i>a</i> = 11.660(1) Å, <i>b</i> = 15.468(2) Å, <i>c</i> = 15.844(2) Å, and γ = 90.19(2)° for Tb₈S_{15–<i>x</i>}; <i>a</i> = 11.584(1) Å, <i>b</i> = 15.340(2) Å, <i>c</i> = 15.789(2) Å, and γ = 90.34(2)° for Dy₈S_{15–<i>x</i>}; and <i>a</i> = 11.538(1) Å, <i>b</i> = 15.288(2) Å, <i>c</i> = 15.740(2) Å, and γ = 90.23(1)° for Ho₈S_{15–<i>x</i>}, respectively. The structure consists of an alternating stacking of puckered [<i>RE</i>S] (<i>RE</i>, rare-earth metals) double slabs and planar sulfur layers along [001]. The planar sulfur layers have a complex arrangement of S₂^2– dinuclear dianions, isolated S^2– ions, and vacancies. All compounds contain trivalent rare-earth metal ions, for Tb₈S_{15–<i>x</i>} and Dy₈S_{15–<i>x</i>} antiferromagnetic order was found at <i>T</i>_N = 5.4(2) K and 3.8(1) K, respectively. Short wavelength cutoff optical band gaps of 1.6 to 1.7 eV were determined

Sr₂OsO₅ and Sr₇Os₄O₁₉, Two Structurally Related, Mott Insulating Osmates(VI) Exhibiting Substantially Reduced Spin Paramagnetic Response

The new osmates­(VI), Sr₂OsO₅ and Sr₇Os₄O₁₉, feature quasi-1-D polyoxo anions, consisting of corner sharing [OsO₆] octahedra. In both compounds, the magnetic moment at <i>T</i> = 300 K is significantly lower (1.2–1.3 μ_B/Os-atom) than the value expected for <i>S</i> = 1. For neither of the new osmates­(VI) is any evidence for long-range magnetic order found. For Sr₇Os₄O₁₉, magnetic susceptibility suggests an antiferromagnetic ordering at <i>T</i>_N = 43(3) K; however, no corresponding anomaly is visible in specific heat. Both compounds are semiconductors

Synthesis and Characterization of Cs_1–<i>x</i>Ti₂Te₂O (<i>x</i> ≈ 0.2): Electron Doping by Te Resulting in a Layered Metal

Reacting Cs₂O_1.3, TiTe, TiO₂, and Te under inert conditions gives powders of Cs_1–<i>x</i>Ti₂Te₂O (<i>x</i> ≈ 0.2). Small single crystals of the same phase were obtained from a CsCl salt melt in closed ampoules. This cesium dititanium ditelluride oxide (<i>P</i>4/<i>mmm</i>, <i>a</i> = 4.0934(3) Å, <i>c</i> = 8.9504(9) Å) is isostructural to CeCr₂Si₂C and contains layers of face-sharing <i>trans</i>-TiTe₄O₂ octahedra that are separated by Cs. As Ti occupies only one crystallographic site, its average oxidation state is +2.6, for the Cs deficit <i>x</i> = 0.2. The formally intermediate Ti valence state agrees well with the metallic conductivity and temperature-independent paramagnetic behavior. No superconductivity is observed down to 0.1 K in Cs_0.8Ti₂Te₂O, but the fact that this structure type can accommodate Te^2– suggests that electron doping of structurally closely related pnictide oxide superconductors, for example, BaTi₂Bi₂O, might be possible

Dumbbells of Five-Connected Ge Atoms and Superconductivity in CaGe₃

CaGe₃ has been synthesized at high-pressure, high-temperature conditions. The atomic pattern comprises intricate germanium layers of condensed moleculelike dimers. Below <i>T</i>_c = 6.8 K, type II superconductivity with moderately strong electron–phonon coupling is observed

High-Temperature Ferrimagnetism with Large Coercivity and Exchange Bias in the Partially Ordered 3<i>d</i>/5<i>d</i> Hexagonal Perovskite Ba₂Fe_1.12Os_0.88O₆

Double perovskite oxides <i>A</i>₂<i>BB</i>′O₆ combining 3<i>d</i> and 4<i>d</i> or 5<i>d</i> transition metal ions at the <i>B</i> and <i>B</i>′ sites feature a variety of magnetic and magneto-electric properties. Targeting Ba₂FeOsO₆, we synthesized powder samples of nonstoichiometric Ba₂Fe_1.12Os_0.88O₆ by solid-state reaction from the oxides. The crystal structure was investigated by using synchrotron powder X-ray and powder neutron diffraction. In contrast to Ca₂FeOsO₆ and Sr₂FeOsO₆, the compound adopts the hexagonal 6L perovskite structure (space group <i>P</i>3̅<i>m</i>1) with partial Fe–Os order at both the face-sharing <i>B</i>₂O₉ dimer and the corner sharing <i>B</i>O₆ transition metal sites. Magnetization, neutron diffraction, and ⁵⁷Fe Mössbauer spectroscopy results show that Ba₂Fe_1.12Os_0.88O₆ develops ferrimagnetic order well above room temperature at <i>T</i>_C ≈ 370 K. The nonsaturated magnetization curve at 2 K features a magnetic moment of 0.4 μ_B per formula unit at 7 T and a pronounced hysteresis with a coercive field of about 2 T. Large exchange bias effects are observed when the magnetization curves are measured after field cooling. The peculiar magnetic properties of Ba₂Fe_1.12Os_0.88O₆ are attributed to an inhomogeneous magnetic state formed as a consequence of the atomic disorder. Our results indicate that hexagonal double-perovskite-related oxides are a promising class of compounds for finding new materials with potential applications as hard magnets or in the area of spintronics

BaGe₆ and BaGe_6‑x: Incommensurately Ordered Vacancies as Electron Traps

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe₆ (<i>P</i> = 15 GPa, <i>T</i> = 1073 K) and BaGe_6–<i>x</i> (<i>P</i> = 10 GPa, <i>T</i> = 1073 K) which are metastable at ambient conditions. In BaGe_6‑<i>x</i>, partial fragmentation of the BaGe₆ network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe_6–<i>x</i> is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe_6‑<i>x</i> for <i>x</i> = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity

BaGe₆ and BaGe_6‑x: Incommensurately Ordered Vacancies as Electron Traps

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe₆ (<i>P</i> = 15 GPa, <i>T</i> = 1073 K) and BaGe_6–<i>x</i> (<i>P</i> = 10 GPa, <i>T</i> = 1073 K) which are metastable at ambient conditions. In BaGe_6‑<i>x</i>, partial fragmentation of the BaGe₆ network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe_6–<i>x</i> is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe_6‑<i>x</i> for <i>x</i> = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity

Ternary Metastable Nitrides ε‑Fe₂<i>TM</i>N (<i>TM</i> = Co, Ni): High-Pressure, High-Temperature Synthesis, Crystal Structure, Thermal Stability, and Magnetic Properties

High-pressure, high-temperature synthesis gives access to ternary metastable nitrides ε-Fe₂<i>TM</i>N (<i>TM</i> = Co, Ni) as bulk materials for the first time. Both ε-Fe₂CoN and ε-Fe₂NiN crystallize isostructural to ε-Fe₃N as evidenced by X-ray powder diffraction data. The lattice parameters of the new compounds are slightly smaller than those of ε-Fe₃N owing to the reduced atomic radii of the metal atoms. Energy-dispersive X-ray spectroscopy of metallographic samples show homogeneous metal ratios corresponding to compositions Fe_1.99(6)Co_1.01(6)N and Fe_1.97(2)Ni_1.03(2)N. Extended X-ray absorption fine spectra indicate that cobalt and nickel occupy iron positions. Thermal analysis measurements reveal decomposition of both ternary nitrides above 920 K. ε-Fe₂CoN disintegrates into N₂ and iron–cobalt alloy, while ε-Fe₂NiN decays into N₂, iron–nickel alloy as well as α-Fe. The replacement of iron by cobalt or nickel essentially lowers the saturation magnetization from roughly 6.0 μ_B/f.u. for ε-Fe₃N to nearly 4.3 μ_B/f.u. for ε-Fe₂CoN and 3.1 μ_B/f.u. for ε-Fe₂NiN. In parallel, the Curie temperature decreases from 575(3) K for ε-Fe₃N to 488(5) K for ε-Fe₂CoN and 234(3) K for ε-Fe₂NiN. Calculations of the formation enthalpies illustrate that the phases ε-Fe₂<i>TM</i>N (<i>TM</i> = Co, Ni) are thermodynamically unfavorable at ambient conditions which is consistent with our experimental observations. The substitution of one Fe by Co (Ni) yields one (two) more electrons per formula unit which reduces the magnetic interactions. First-principles analysis indicate that the replacement has a negligible influence on the electron occupation numbers and spin moments of the N and unsubstituted Fe sites, but decreases the local magnetic moments on the substituted Fe positions because the extra electrons occupy the minority-spin channel formed by states of the <i>TM</i> atoms

Cobalt-Based Single-Ion Magnets on an Apatite Lattice: Toward Patterned Arrays for Magnetic Memories

Single-ion magnets (SIMs) that can maintain magnetization direction on an individual transition metal atom represent the smallest atomic-scale units for future magnetic data storage devices and molecular electronics. Here we present a robust extended inorganic solid hosting efficient SIM centers, as an alternative to molecular SIM crystals. We show that unique dioxocobaltate­(II) ions, confined in the channels of strontium hydroxyapatite, exhibit classical SIM features with a large energy barrier for magnetization reversal (<i>U</i>_eff) of 51–59 cm^–1. The samples have been tuned such that a magnetization hysteresis opens below 8 K and <i>U</i>_eff increases by a factor of 4 and can be further enhanced to the highest values among 3d metal complexes of 275 cm^–1 when Ba is substituted for Sr. The SIM properties are preserved without any tendency toward spin ordering up to a high Co concentration. At a maximal Co content, a hypothetical regular hexagonal grid of SIMs with a 1 nm interspacing on the (001) crystal facet would allow a maximal magnetic recording density of 10⁵ Gb/cm²