Physical properties and lattice dynamics of bixbyite-type V2O3

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Some time ago, we reported the synthesis of bixbyite-type V2O3, a new metastable polymorph of vanadium sesquioxide. Since, a number of investigations followed, dealing with different aspects like electronic and magnetic properties of the material, the deviation from ideal stoichiometry or the preparation of nanocrystals as oxygen storage material. However, most of the physical properties were only evaluated on a theoretical basis. Here, we report the lattice dynamics and physical properties of bixbyite-type V2O3 bulk material, which we acquired from physical property measurements and neutron diffraction experiments over a wide temperature range. Besides attributing different possible orientations of the magnetic moments for V1 and V2 to the identified antiferromagnetic (AFM) ground state with a Néel temperature of 38.1(5) K, we use a first order Grüneisen approximation to determine lattice-dependent parameters for the relatively stiff cubic lattice, and, amongst others identify the Debye temperature to be as low as 350 ± 65 K.DFG, 73789094, SPP 1415: Kristalline Nichtgleichgewichtsphasen - Präparation, Charakterisierung und in situ-Untersuchung der Bildungsmechanisme

Mechanochemical Synthesis of Cu2MgSn3S8 and Ag2MgSn3S8

Two new thiospinels of the type AI2BIICIV3SVI8 were successfully synthesized via a mechanochemical route using binary sulfides and sulfur. Cu2MgSn3S8 and Ag2MgSn3S8 are the first AI2BIICIV3SVI8 compounds with magnesium as divalent cation. The crystal structures of Cu2MgSn3S8 and Ag2MgSn3S8 were refined in the cubic space group Fd3m using X‐ray powder diffraction. According to UV/Vis measurements, a direct optical bandgap of ca. 1.65 eV was determined for both Cu2MgSn3S8 and Ag2MgSn3S8. Temperature‐dependent magnetic susceptibility measurements of the Cu2MgSn3S8 sample indicate diamagnetism. A 119Sn Mössbauer spectrum confirms the tetravalent state of tin, underlining the electron‐precise description.TU Berlin, Open-Access-Mittel - 202

Laves phases: superstructures induced by coloring and distortions

The structural chemistry of Laves phases, especially with respect to their superstructures induced by coloring and distortions is discussed. Starting from the three classical Laves phases MgCu2, MgZn2 and MgNi2, the more complex Komura phases are derived. Different possibilities of their description are summarized. In the second part, the superstructures are discussed based on their respective prototypes. The crystal chemical relationships are illustrated based on group-subgroup descriptions using the Bärnighausen formalism

Mixed Valence Europium Nitridosilicate Eu2SiN3

The mixed valence europium nitridosilicate Eu2SiN3 has been synthesized at 900°C in welded tantalum ampules starting from europium and silicon diimide Si(NH)2 in a lithium flux. The structure of the black material has been determined by single-crystal X-ray diffraction analysis (Cmca (no. 64), a=542.3(11) pm, b=1061.0(2) pm, c=1162.9(2) pm, Z=8, 767 independent reflections, 37 parameters, R1=0.017, wR2=0.032). Eu2SiN3 is a chain-type silicate comprising one-dimensional infinite nonbranched zweier chains of corner-sharing SiN4 tetrahedra running parallel [100] with a maximum stretching factor fs=1.0. The compound is isostructural with Ca2PN3 and Rb2TiO3, and it represents the first example of a nonbranched chain silicate in the class of nitridosilicates. There are two crystallographically distinct europium sites (at two different Wyckoff positions 8f) being occupied with Eu2+ and Eu3+, respectively. 151Eu Mössbauer spectroscopy of Eu2SiN3 differentiates unequivocally these two europium atoms and confirms their equiatomic multiplicity, showing static mixed valence with a constant ratio of the Eu2+ and Eu3+ signals over the whole temperature range. The Eu2+ site shows magnetic hyperfine field splitting at 4.2 K. Magnetic susceptibility measurements exhibit Curie-Weiss behavior above 24 K with an effective magnetic moment of 7.5 μB/f.u. and a small contribution of Eu3+, in accordance with Eu2+ and Eu3+ in equiatomic ratio. Ferromagnetic ordering at unusually high temperature is detected at TC=24 K. DFT calculations of Eu2SiN3 reveal a band gap of ∼0.2 eV, which is in agreement with the black color of the compound. Both DFT calculations and lattice energetic calculations (MAPLE) corroborate the assignment of two crystallographically independent Eu sites to Eu2+ and Eu3+

High-temperature synthesis, crystal structure, optical properties, and magnetism of the carbidonitridosilicates Ho2[Si4N6C] and Tb2[Si4N6C]

The novel carbidonitridosilicates Ho2[Si4N6C] and Tb2[Si4N6C] were obtained by the reaction of the respective lanthanoid metal with carbon and Si(NH)2 in a radiofrequency furnace at the temperature of 1700°C. According to the single-crystal structure analysis of Ho2[Si4N6C] (P21/c, Z = 4, a = 593.14(1), b = 989.74(1), c = 1188.59(2) pm, β = 119.58(4)°, R1 = 0.0355, wR2 = 0.0879, 2187 F2 values, 119 parameters) the compound contains a condensed network of corner-sharing star like [C(SiN3)4] units. The holmium ions are situated in channels along [100]. The UV–VIS absorption spectrum of Ho2[Si4N6C] shows the typical Ho3+ absorption bands. The spectroscopic results show that the 4f states remain almost unaffected by the coordination sphere and thus it is impossible to distinguish between the two crystallographic sites of Ho3+ in the UV–VIS spectrum. Magnetic susceptibility measurements of Tb2[Si4N6C] and Ho2[Si4N6C] show Curie–Weiss behaviour above 150 K with experimental magnetic moments of 9.57(6) μB/Tb and 10.27(4) μB/Ho. The Weiss constants are −15(1) K and −11(1) K for the terbium and holmium compounds, respectively. Down to 2 K no magnetic ordering could be detected. The magnetization curves at 2 K show an increase of the magnetization with increasing flux density, indicating partial parallel spin alignments. At 5 T the magnetizations reach values of 4.15(5) μB/Tb and 4.75(5) μB/Ho, respectively

Study of the structural transition and hydrogenation of CeTiGe

International audienceThere remains some disagreement in the literature on CeTiGe over the presence of a structural transition from the low temperature CeFeSi-type form to the high temperature CeScSi-type structure. We present a detailed study of the effect of temperature on this structural transition. Furthermore, the same hydride is obtained after hydrogenation of both forms. Using neutron powder diffraction we find that the structure of CeTiGeH1.5 corresponds to a stuffed variant of the CeScSi-type structure with space group I4/mmm, a = 4.0785(1) Å and c = 17.1060(8) Å. The H atoms occupy both the Ce4 tetrahedral sites and the Ti4Ce square based pyramidal sites for a total hydrogen occupancy of 1.5 H f.u.−1. Preliminary examinations of the magnetic properties after hydrogenation reveal the onset of low temperature magnetic order around 3.5 K, suggesting for the first time a hydrogen induced magnetic order for an intermetallic with CeScSi-type structure

Ferromagnetic Ordering in the Thallide EuPdTl 2

The new thallide EuPdTl 2 , synthesized from the elements in a sealed tantalum tube in a highfrequency furnace, was investigated by X-ray diffraction on powders and single crystals: MgCuAl 2 type, Cmcm, Z = 4, a = 446.6(1), b = 1076.7(2), c = 812.0(2) pm, wR2 = 0.0632, 336 F 2 values, 16 variables. The structure can be considered as an orthorhombically distorted, palladium-filled variant of the binary Zintl phase EuTl 2 . The palladium and thallium atoms build up a three-dimensional [PdTl 2 ] polyanion with significant Pd-Tl (286 -287 pm) and Tl-Tl (323 -329 pm) interactions. The europium atoms fill distorted hexagonal channels of the [PdTl 2 ] polyanion. Susceptibility measurements show a magnetic moment of 7.46(5) µ B /Eu atom, indicative of divalent europium. EuPdTl 2 is a soft ferromagnet with a Curie temperature of T C = 12.5(5) K