Suppression of Ru (S = 1) spin dimerization in La2RuO5 by Ti substitution

La2RuO5 shows a magneto-structural phase transition at 161 K with spin dimerization and concomitant formation of a non-magnetic singlet ground state. To gain a deeper insight into the origin of this transition systematic substitution of Ru by Ti has been carried out. Polycrystalline samples have been synthesized by thermal decomposition of citrate precursors leading to La2Ru(1-y)Ti(y)O5 (0 ≤ y ≤ 0.45). The crystal structure was investigated by x-ray powder diffraction at room temperature and at 100 K. The valences of Ti and Ru were obtained from x-ray absorption near edge structure spectroscopy at the Ti-K and the Ru-LIII absorption edges, respectively. The magnetic phase transition was investigated by magnetic susceptibility measurements as a function of Ti substitution, revealing a decreasing transition temperature on increasing the level of substitution. The step-like feature in the magnetic susceptibility reflecting the Ru-Ru spin dimerization transition becomes smeared out close to y = 0.3 and completely vanishes at y = 0.45, indicating complete suppression of spin-dimer formation. Additional specific-heat measurements show a continuous decrease of the magnetic entropy peak with increasing Ti substitution mirroring the reduced number of spin dimers due to the magnetic dilution. A magnetic anomaly of the dimerization transition can hardly be detected for y ≥ 0.3. Density functional theory calculations were carried out to study changes of the electronic band structure caused by the substitution. A possibly preferred distribution of Ti and Ru and the magnetic interactions as well as the change of the density of states close to the Fermi level are investigated. Based on these experimental results a detailed (y,T) phase diagram is proposed.

Comparative Studies of Ancient Mortars From Giza, Egypt, and Nevali ÇOri, Turkey

Mortars collected from the Cheops pyramid, the Sphinx and the Chephren pyramid, all at Giza, Egypt, have been compared with recently found mortars of a cult building of the aceramic neolithic at Nevali üori (south east of Urfa, province Hilvan, Turkey), probably one of the earliest sites of civilization. As methods of investigation thermal analysis, X-ray diffraction as well as analytical scanning and transmission electron microscopy have been used. The egyptian mortars have been found to be primarily made up of gypsum, anhydrite and calcite. The comparative results with mortar of the recently detected temple site at Nevali Con, Turkey, give evidence that the burning or otherwise chemical treatment of limestone as crucial ingredient for the fabrication of mortar has been performed at a very early period (10'000 - 8000 B.C.). In order to further confirm the described findings 14C dating measurements have been carried out. The results of the described studies raise many questions concerning early civilizations and their knowledge of technologies as well as the transfer mechanisms of this knowledge.

Redox Processes in Polynary Copper Oxides and Copper Oxide / Mesoporous Silica Composites

Dynamic redox processes in different complex copper oxides and oxide / mesoporous silica composites are characterized. In the spinel-type CuxMn3.xO4 the atomic and electronic structures are described with respect to the reversible process Cu2+ + Mn3+ ⇒ Cu3+ + Mn4+, the crucial dynamic equilibrium for the catalytic oxidation of CO to CO2 at ambient temperatures. A comparison with the perovskite-type LaCu1-xMnxO3-δ is presented. In the K2NiF4-type La2-xSrxCu1-yRuyO4-δ the electronic structure is characterized with respect to the equilibrium Cu2+ + Ru5+ ⇒ Cu3+ + Ru4+. Analogous redox processes are characterized in composite compounds made up of copper oxides inserted in mesoporous MCM-type silica. Due to their structural order in the nanoscopic range, the investigation of these materials required multiple, complementary characterization methods, i.e. X-ray diffractometry, thermal analysis, scanning and transmission electron microscopy, magnetic measurements, XPS, and particularly X-ray absorption spectroscopy (EXAFS / XANES) measurements.

Centrosymmetric or Noncentrosymmetric? Case Study, Generalization and Structural Redetermination of Sr5Nb5O17

The possibility that the structure of the novel semiconducting perovskite-related material strontium niobium oxide, Sr5Nb5O17, refined by Schmalle et al. [Acta Cryst. (1995), C51, 1243–1246] in space group Pnn2, might instead belong to space group Pnnm has been investigated following an analysis of the atomic coordinates that indicated the latter space group to be more likely. All I obs were carefully remeasured, first those within a hemisphere containing c *, then all that lay within the full sphere of reflection. Refinement was undertaken, with each of two different sets of weights, in each space group. Each data set was used under three limiting intensity conditions: I obs > 4σ(I obs), I obs > 2σ(I obs) and finally with all reflections, but setting magnitudes with I obs ≤ 0 equal to 0. Fourteen separate tests based only upon the X-ray diffraction data may be used to distinguish between Pnn2 and Pnnm. Nine tests favored the latter choice, four were indeterminate and one was not used. Seven further tests may be made on the basis of physical measurement; of these, three strongly indicated Pnnm, one was indeterminate and three could not be used. The evidence clearly suggests the space group is Pnnm. The use of all reflections, including those with negative magnitude set equal to zero, is essential to avoid ambiguity in the X-ray diffraction tests and achieve the highest reliability. Refinement with weights based on variances of Type A and Type B [Schwarzenbach et al. (1995). Acta Cryst. A51, 565–569] resulted in improved reliability compared with that obtained from a popular empirical weighting scheme. The revised structure differs in several respects from that published previously.

Cu(OH)2: a New Ferroelectric

The crystal structure of Cu(OH)2, recently redetermined at room temperature by Oswald, Reller, Schmalle & Dubler [Acta Cryst. (1990), C46, 2279–2284], is shown to satisfy the structural criteria for ferroelectricity. The Cu2+ ion at 295 K is displaced 0.131 Å from the most likely paraelectric atomic arrangement, allowing the prediction to be made that the ferroelectric Curie temperature is Tc = 343 K. This prediction is in excellent agreement with an earlier report of a reversible structural transition at 325–335 K in Cu(OH)2. The presence of a λ-shaped anomaly in the heat capacity at about 335 K has been confirmed, and a dielectric anomaly is also observed at similar temperatures over a wide frequency range; the frequency dependence is dispersive, owing at least partly to ionic conductivity. The prediction is hence verified, and Cu(OH)2 is demonstrated experimentally to be a new ferroelectric.