J. Warczewski

High temperature X-ray single crystal study of Na1/2Bi1/2TiO3

Abstract For a Na0.5B0.5TiO2 single crystal the intensity variation of the satellite reflection was measured by X-ray diffraction. No hystereses were observed for the phase transitions into both the rhombohedra1 and the tetragonal phases, however there is a coexistence of both phases in the range between ∼250°C and 300°C. By fitting the intensity vs. temperature curve for the tetragonal phase a critical exponent of 2β = 0.39(4) was obtained. The mean square displacement of the probability tensor of the high temperature cubic phase does not vary linearly with temperature which indicates that there is some disorder in this phase.

THE SYNTHESIS, CRYSTAL, MOLECULAR AND ELECTRONIC STRUCTURES OF TRIBROMO(NITROSYL) BIS(TRIPHENYLPHOSPHINE OXIDE) RHENIUM(II) AND TRIBROMO(NITROSYL) BIS(TRIPHENYLPHOSPHINE)RHENIUM(I)

Abstract Gaseous nitric oxide reacts with a benzene solution of [ReOBr3(PPh3)2] to give [ReBr3(NO) (OPPh3)2] (1). When the reaction is carried out in the presence of an excess of free triphenyl-phosphine, the product is [ReBr3(NO)(PPh3)2] (2). The latter is also isolated in the reaction of 1 with PPh3. This paper, apart from the synthetic methods, presents spectroscopic and magneto-chemical measurements, and crystal, molecular and electronic structures for 1 and 2.

THE SYNTHESIS, CRYSTAL, MOLECULAR AND ELECTRONIC STRUCTURE OF TRICHLORO(NITROSYL)BIS(TRIPHENYL-PHOSPHINE OXIDE)RHENIUM(II)

Abstract A new rhenium nitrosyl complex has been synthesized and characterized by IR, electronic and 31P NMR spectroscopy and magnetochemical measurements. Its electronic structure has been determined (Dq = 3333, Ds = -4732 and Dt = 387cm−1). The crystal and molecular structure of [ReCl3(NO)(OPPh3)2] has been solved by the heavy atom method and refined anisotropically to R = 0.466 for 3032 unique observed reflections. The title compound crystallizes in the mono-clinic space group C2/c, with a = 13.887(2), b = 13.154(2), c = 19.629(4)A, β = 96.78(2)° and Z = 4. The Re atom is of six-coordinate distorted octahedral configuration and the linear nitrosyl ligand is trans to one of triphenylphosphine oxide molecules. The OPPh3 groups of [ReCl3(NO)(OPPh3)2] are in cis positions.

New synthetic routes to some nitrosyl rhenium complexes. Crystal, molecular and electronic structure of [ReCl3(NO)(PPh3)2] and [ReCl2(CO)(NO)(PPh3)2] complexes

Abstract The reactions of the nitrosyl rhenium(II) complex [ReCl3(NO)(OPPh3)(PPh3)] with triphenylphosphine, NaBH4 in the presence of PPh3, and CO in the presence of PPh3 were examined. The rhenium nitrosyl complexes [ReCl3(NO)(PPh3)2] (1), [ReCl2(CO)(NO)(PPh3)2] (2) and [ReH2(NO)(PPh3)3]·1/2C6H6 (3) obtained were characterised by IR, UV–Vis, NMR (1H and 31P) and magnetochemical measurements. For 1 and 2, the electronic, crystal and molecular structures were determined.

New antiferromagnetic semiconductor CuCr1 5Sb0 5S4

Abstract The magnetic, electric and crystallographic properties of the new compound with the spinel structure CuCr 1.5 Sb 0.5 S 4 were studied. It appears that this compound, being a non-degenerate p-type semiconductor, has magnetic properties typical of antiferromagnets: a linear magnetic field dependence of the magnetization and a maximum of the magnetic susceptibility at temperature T N = 28 K. Also, the lattice parameter a , the anion parameter u and the temperature factors are presented. The distances between cations (both in the tetrahedral and octahedral positions) and anions show that between anions and cations in tetrahedral positions, the covalent bond dominates and between anions and cations in octahedral positions, the ionic bond dominates.

Correlations of the magnetic and electrical properties with the ionic radii of cations and anions for the series of ternary and quaternary spinel-type chromium compounds

Abstract For the compounds ACr 2 X 4 (where: A=Cd, Zn, Hg, Ga, Cu; X=O, S, Se, Te), ACr 2 x B 2−2 x X 4 (where: A=Cd, Zn; B=Ga, In; X=S, O), ZnCr 2− x In x Se 4 and Cr 2 V 4− x Mo x O 13+0.5 x correlations were observed of the magnetic and electrical properties with the ionic radii of their cations and anions, whereas for the solid solutions of the boundary compounds ACr 2 X 4 (where: A=Cd, Zn; X=S, Se, Te) and CuCr 2 X 4 (were X=S, Se, Te) not only such correlations but also the metallic conductivity and a semiconductor–metal phase transition were observed. In particular for Zn 1− x Cu x Cr 2 Se 4 this phase transition is accompanied by the magnetic AF-F phase transition, while for Cd 1− x Cu x Cr 2 Se 4 one deals with the accompanying Fi-F phase transition. In the latter spinel series there exists the ferrimagnetic ordering at low copper concentrations, whereas at high copper concentrations one deals with the ferromagnetic ordering.

Influence of the substitution of Cu ions for Cd ions on the crystallographic and magnetic properties of Cd1−xCuxCr2Se4 spinels

Abstract All the compounds under study show the spinel normal structure. The lattice parameters decrease with an increase in the Cu concentration. The samples with low Cu concentrations ( x ≤ 0.2) exhibit ferrimagnetic ordering and Curie temperatures of 150 K. The samples with high Cu concentrations ( x ≥ 0.9) are ordered ferromagnetically, and their Curie temperatures are much higher and close to 480 K. Moreover, for the four samples under investigation one can observe a significant lowering of the saturation magnetic moment in comparison with the theoretical one. This lowering seems to be connected with the high degree of defects in the crystal structure. An attempt is made to interpret this fact with the use of the vacancy parameter.

Magnetic, electrical and structural properties of some ternary and quaternary spinels with chromium

Abstract Spinel compounds with the general formulas ACr 2 X 4 and ACr 2− x B x X 4 (where A=Cd, Zn, Hg, Ga, Cu; X=S, Se, Te, O; B=Ga, In, Sb), including also substitution of two different cations in the tetrahedral subarray, exhibit different correlations of the magnetic and electrical properties. These properties are correlated with the ionic radii of cations and anions. The solid solutions of the boundary compounds reveal, e.g. correlations of metallic conductivity and a semiconductor–metal phase transition. In some spinel solid solutions the latter phase transition is accompanied by the magnetic phase transitions, i.e. either AF-F or Fi-F. Magnetic and electrical properties and their correlations in different spinel series with chromium are discussed in the frames of the phase diagram of the spinel-type compounds with chromium. Different mechanisms leading to the spin-glass state are discussed and illustrated.

Double exchange magnetic interaction and giant negative magnetoresistivity in the spin glass state of the new compound CuCr1.6Sb0.4S4

Abstract CuCr 1.6 Sb 0.4 S 4 crystallizes in the spinel structure with the lattice parameter a =9.9680(1) A and the anion parameter u =0.3820(1). The very hard magnetization of the sample, the shape of the magnetization isotherms as well as the lack of magnetic saturation even in a field of B =14 T indicate that the antiferromagnetic superexchange interaction dominates the long-range magnetic interactions between the magnetic moments localized on the chromium ions. A high positive value of the paramagnetic Curie–Weiss temperature Θ C–W (equal to 318 K) testifies that the short-range interaction is ferromagnetic and strong which means that in the corresponding microregions the double exchange interaction is present and predominates. A suggestion is made that these microregions behave like superparamagnetic clusters. As the result of the competition of the interactions mentioned above the sample under study reveals a spin glass state at low temperatures, which is confirmed by the temperature dependence of the magnetic susceptibility curves. The spin-freezing temperature is lower than 50 K. CuCr 1.6 Sb 0.4 S 4 is a semiconductor with a giant negative magnetoresistivity equal to 74% at 38 T and 3 K. The driving force of this effect is the double exchange magnetic interaction. Three values for the activation energy were observed: E 1 =0.118 eV, E 2 =0.004 eV and E 3 =0.033 eV. These are most probably connected with the energy levels generated by the crystal field.

Ferrimagnetism and metamagnetism in Cd1−xCuxCr2S4 spinels

Abstract Although the compounds CdCr2S4 and CuCr2S4 are ferromagnetic, solid solutions of the system CdCr2S4-CuCr2S4 with the spinel normal structure show a ferrimagnetic ordering of their magnetic moments with Curie temperatures of about 90 K. For all the samples under investigation one can observe a significant lowering of the saturation magnetic moment in comparison with the theoretical one. Moreover, metamagnetic thresholds were observed at different temperatures. In particular at 250 K the values of the induction of the magnetic field for these thresholds are equal to 0.1 T for x = 0.05, 0.4 T for x = 0.1, 1.1 T for x = 0.15 and 0.4 T for x = 0.2, respectively. The lowering of the saturation magnetic moment and the existence of the metamagnetic thresholds for all the samples under study seem to be connected both with the high defectiveness of the crystal structure and with the mixed valence of the chromium ions. An attempt is made to interpret this fact in terms of the RKKY, double-exchange and superexchange magnetic interactions as well as with the use of the vacancy parameter.