Wolfgang Jeitschko

Die Kristallstruktur von Ti3SiC2—ein neuer Komplexcarbid-Typ

Die Struktur von Ti3SiC2 wird aus Einkristallaufnahmen bestimmt. Die Gitterparameter der hexagonalen Zelle sind:a=3,068,c=17,669 A undc/a=5,759. Die Titan-Atome besetzen die Punktlagen 2a) und 4f) (zTi=0,135), die Silicium-Atome die Punktlage 2b) und die Kohlenstoff-Atome die Punktlage 4f) (zC=0,5675) in der Raumgruppe D 6h 4 −P63/mmc. Die Struktur gehort zu den Komplexcarbiden mit oktaedrischen Bauelementen [T 6C].

Kohlenstoffhaltige ternäre Verbindungen (H-Phase)

In einer Notiz wurde auf das Bestehen einer neuen ternaren Kristallart in Systemen vom Typus M (Ubergangsmetall)-Me (Metametall)-X (Kohlenstoff) hingewiesen1. Diese Kristallart, deren Zusammensetzung in der Nahe der Kohlenstoffstabilisierten β-Manganphase, z.B. Mo3Al2C2, angenommen wurde, bezeichneten wir mit H-Phase. In der Folge gelang es, diese Phase in einer Reihe von weiteren Dreistoffen aufzufinden und mit Hilfe von Einkristallen im Cr−Al−C-System deren Zusammensetzung und Kristall-Struktur zu ermitteln.

Preparation and structural investigations of antimonides with the LaFe4P12 structure

Abstract Fourteen compounds LnT4Sb12 (Ln  La, Ce, Pr, Nd, Sm, Eu; T  Fe, Ru, Os) were prepared for the first time. Their isotypy with LaFe4P12 (filled CoAs3 structure) is shown by their X-ray powder patterns. LaFe4Sb12 and CeFe4Sb12 show metallic conductivity. The crystal structure of LaFe4Sb12 was refined from single-crystal counter data to a residual value of 0.046 for 389 independent structure factors. A comparison of the structural data for LaFe4Sb12 with those for the previously determined LaFe4P12 and LaFe4As12 structures shows systematic trends in the positional and thermal parameters. The thermal amplitudes of the lanthanum atoms are highest in LaFe4Sb12, indicating that the polymeric [Fe4Sb12]n “framework” is somewhat too large for the lanthanoids. This can also be concluded from an almost non-existent lanthanoid contraction. Systematic trends of the cell volumes suggest that cerium and europium have mixed valencies in most compounds with this structure.

Quaternary rare earth transition metal arsenide oxides RTAsO (T=Fe, Ru, Co) with ZrCuSiAs type structure

Abstract The equiatomic quaternary compounds R FeAsO ( R =La–Nd, Sm, Gd), R RuAsO ( R =La–Nd, Sm, Gd–Dy) and R CoAsO ( R =La–Nd) crystallize with the tetragonal ZrCuSiAs type structure, which was refined from single-crystal X-ray diffractometer data of PrFeAsO: P 4/ nmm , Z =2, a =398.5(1) pm, c =859.5(3) pm, R =0.058 for 167 structure factors and 12 variable parameters. All atomic positions are fully occupied. Chemical bonding in PrFeAsO can be rationalized with oxidation numbers corresponding to the formula Pr +3 Fe +2 As −3 O −2 . An electron count of 18 for the iron atoms can only be achieved if Fe–Fe interactions of 281.8 pm are considered as bonding.

Magnetic properties of alkaline earth and lanthanoid iron antimonides AFe4Sb12 (A = Ca, Sr, Ba, LaNd, Sm, Eu) with the LaFe4P12 structure

Abstract The magnetic properties of the nine title compounds were studied by magnetic susceptibility measurements with a SQUID magnetometer between 2 and 300 K. At temperatures above 100 K, all compounds with the exception of the cerium and samarium compounds show Curie-Weiss behaviour. The effective magnetic moments of the alkaline earth compounds vary between 3.7 ± 0.2 μB and 4.0 ± 0.2 μB per formula unit. For LaFe4Sb12 the moment is smaller (3.0 ± 0.2 μB) indicating a higher degree of electronic saturation within the Fe4Sb12 polyanion. At lower temperatures, the magnetic susceptibilities of these compounds deviate from the Curie-Weiss law, however, they remain field independent. A mixed valent behaviour is observed for the cerium compound and the samarium compound reflects the Van Vleck paramagnetism of the Sm3+ ions. The magnetic susceptibilities of the other lanthanoid compounds correspond to the sums of the susceptibilities of the Fe4Sb12 polyanion and the Ln3+ and Eu2+ ions, respectively. These antimonides order ferromagnetically with Curie temperatures varying between 5(1) K for PrFe4Sb12 and 82(2) K for EuFe4Sb12. The magnetic properties of these compounds are discussed within the framework of a rigid band structure.

The rate earth transition metal phosphide oxides LnFePO, LnRuPO and LnCoPO with ZrCuSiAs type structure

Abstract The compounds LnFePO (Ln = La−Nd, Sm, Gd), LnRuPO (Ln = La−Nd, Sm, Gd) and (Ln = La−Nd, Sm) crystallize with the tetragonal) ZrCuSiAs type structure (P4/nmm, Z = 2), which was refined from single-crystal X-ray data of PrFePO (a = 391.13(6) pm, c = 834.5(2) pm, R = 0.026) and CeRuPo (a = 402.6(1) pm, c = 825.6(2) pm, R = 0.018). The refinement of the occupancy parameters showed the oxygen position to be fully occupied in both compounds. The oxygen content of the samples was also proven by EDAX analyses. The structures of the compounds SmFePO and LaCoPO were refined by Rietveld analyses of X-ray powder data.

Ternary arsenides with LaFe4P12-type structure

Ten new ternary arsenides with composition LnT4As12 (Ln = rare earth element, T=Fe, Ru, or Os) were synthesized by reaction of the elemental components in silica ampoules. Their X-ray powder patterns show them to crystallize with the LaFe4P12-type structure. Their lattice constants are reported. The crystal structure of LaFe4As12 was refined from single-crystal X-ray counter data to a conventional R value of 0.039 for 312 independent reflections. While the La-As and Fe-As distances are considered normal, the As-As distances of 2.57 and 2.58are found too be unusually large, as compared to distances of other two-electron As-As bonds. It is suggested that this is due to filling of As-As antibonding states.

Some physical properties of LaFe4P12 type compounds

Abstract Several pnictides with the filled skutterudite (LaFe 4 P 12 type) structure were investigated by X-ray diffraction, magnetic susceptibility and electrical conductivity measurements and Mossbauer spectroscopy. The structure refinement of CeFe 4 P 12 from single crystal X-ray data resulted in a conventional residual value of 0.022 for 11 variable parameters and 317 reflections. LaFe 4 P 12 and CeFe 4 P 12 are paramagnetic at room temperature. EuFe 4 P 12 is a ferromagnet with a Curie temperature of 99 K. CeFe 4 P 12 and CeFe 4 As 12 are semiconducting. LaFe 4 Pn 12 ( Pn = P , As or Sb ) and CeFe 4 Sb 12 show metallic conductivity. The room temperature 57 Fe Mossbauer spectra of LaFe 4 P 12 , CeFe 4 P 12 and EuFe 4 P 12 are symmetrical doublets. The isomer shifts are compatible with low spin or metallic Fe. At 1.5 K the 57 Fe Mossbauer spectrum of EuFe 4 P 12 under an external field shows that Fe does not carry any magnetic moment. 151 Eu Mossbauer spectra of EuFe 4 P 12 at 4.2 and 300 K show unusual hyperfine parameters.

Ternary lanthanoid-transition metal pnictides with ThCr2Si2-type structure

Abstract Fifteen new compounds AB 2 X 2 were synthesized where A is a lanthanoid, B = Fe, Co, or Ni, and X = P, As, or Sb. They crystallize with the ThCr 2 Si 2 (BaAl 4 , CeAl 2 Ga 2 ) type of structure. Their lattice constants are reported. Single crystals of the ternary phosphides were obtained using tin as a flux. The crystal structure of EuCo 2 P 2 (ThCr 2 Si 2 -type) was refined from single-crystal counter data to a conventional R value of 0.047 for 210 unique structure factors. While in most silicides with ThCr 2 Si 2 -type structure, the Si atoms form pairs, the P atoms in EuCo 2 P 2 are isolated from each other. Thus in an extreme ionic formulation, the compound can be rationalized as Eu 2+ Co 2+ 2 P 3− 2 . Bonding in these and the Zintl compounds is briefly discussed.

ZrCuSiAs: A “filled” PbFCl type

ZrCuSiAs is tetragonal, space group P4nmm, a = 3.6736, c = 9.5712A, Z = 2. The structure is a “filled” PbFCl type, or it may alternatively be regarded as a tetragonally distorted MnCu2Al (Heusler) structure. HfCuSiAs is iso-structural. Bonding in PbFCl-related structures is discussed.