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We have investigated several mixed-valence vanadium oxides, single-crystals by X-ray photoelectron spectroscopy (XPS). Many studies have been performed on V 2 O 5 , VO 2 and V 2 O 3 . Less investigated are the mixed-valence compounds, as the V 3 O 5 , V 4 O 7 and V 6 O 13 systems, which are discussed here. They exhibit as a function of temperature metal–insulator transitions (MIT). A detailed analysis for these systems, above and below the MIT temperature is given. The core-level spectra for the mixed-valence compounds are compared with those of the related systems, V 2 O 5 , VO 2 and V 2 O 3 having V 5+ , V 4+ and V 3+ ions, respectively. The electronic structures for all investigated oxides are characterized by strong V 3d–O 2p hybridization and they are discussed in relation with similar studies.

A VO2 single-crystal has been subject of a combined investigation by high resolution XPS, x-ray emission spectroscopy with electron and energy-selective x-ray excitation (V L-alpha, V K-beta-5- and O K-alpha emission) and x-ray absorption spectroscopy (O1s). We performed first principles tight-binding LMTO band structure calculations of VO2 in both monoclinic and tetragonal rutile structures and compare the densities of states with the experimental data. From this we conclude that the electronic structure of VO2 is more bandlike than correlated.

The ternary compounds Ca 2 CuO 3 , Sr 2 CuO 3 , and their solid solution Sr 2− x Ca x CuO 3 have been prepared by the decomposition of coprecipitated oxalates and/or by conventional solid state reaction. Both reaction methods are compared. The obtained compounds crystallize in the orthorhombic system. The lattice constants were determined, and the formation of solid solution crystals over the whole range of composition was obtained. The compounds were characterized concerning their thermal stability and their decomposition in moist air. Thermogravimetric, X ray, and vapor pressure measurement show that the compounds are of a very slight variation in oxygen stoichiometry. First measurements of the specific heat and IR spectra are discussed.

In order to optimize the synthesis of BaCuO2, different modes were developed and estimated. Wet chemical powders produced by coprecipitation of oxalates or hydroxycarbonates were characterized and their reactivity is compared with the conventional ceramic process of firing a mixture of BaCO3 and CuO by DTA, XRD and IR-investigations. This becomes possible, because the decomposition of these chemical precursors to BaCO3 and CuO is finished up to 600 °C. Hence, the same reaction for the formation of BaCuO2 is monitored. Zur Optimierung der Synthese von BaCuO2 wurden verschiedene Methoden der Praparation entwickelt und getestet. Durch Mischfallung hergestellte Pulver (Oxalate und Hydroxycarbonate) werden charakterisiert und ihre Reaktivitat mit der einer Mischung aus Bariumcarbonat und Kupferoxid verglichen. Das ist moglich, da sich die genannten Mischfallprodukte in einer ersten Stufe bei Temperaturen bis 600 °C zu Bariumcarbonat und Kupferoxid zersetzen.

Etude du comportement magnetique des monocristaux de Mn 2 Mo 3 O 8 en presence de champs magnetiques faibles dans l'intervalle de temperatures allant de 4.3 a 180 K. Mesure de la variation de la susceptibilite magnetique statique χ avec la temperature. Dependance thermique du moment magnetique. Observation d'une transition de phase magnetique

We present resonant inelastic x-ray scattering (RIXS) results on insulating monoclinic $\mathrm{V}{\mathrm{O}}_{2}$ with excitation across the ${L}_{2,3}$ edges of vanadium. The spectra measured near threshold in the ${L}_{3}$ region and in the ${L}_{2}$ region show a Raman behavior with a clear peak around $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ transferred energy. The analysis of the data allows the neutral excitation spectrum of the system up to about $2--2.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to be determined. These excitations are due to $dd$ transitions, whose main contributions are discussed. The possibility of ${{d}^{1};{d}^{1}\ensuremath{\rightarrow}{d}^{2};{d}^{0}}$ charge fluctuations within a dimer in RIXS is also discussed. The results are compared with available information on optical constants and on electron energy loss spectroscopy. Due to the low symmetry of the system, there is a similarity between the Raman spectrum obtained with RIXS and the optical absorption spectrum.

We present a detailed study of the valence and conduction bands of VO2 across the metal-insulator transition using bulk-sensitive photoelectron and O K x-ray absorption spectroscopies. We observe a giant transfer of spectral weight with distinct features that require an explanation which goes beyond the Peierls transition model as well as the standard single-band Hubbard model. Analysis of the symmetry and energies of the bands reveals the decisive role of the V 3d orbital degrees of freedom. Comparison to recent realistic many body calculations shows that much of the k dependence of the self-energy correction can be cast within a dimer model.

We found direct experimental evidence for an orbital switching in the V $3d$ states across the metal-insulator transition in ${\mathrm{VO}}_{2}$. We have used soft-x-ray absorption spectroscopy at the V ${L}_{2,3}$ edges as a sensitive local probe and have determined quantitatively the orbital polarizations. These results strongly suggest that, in going from the metallic to the insulating state, the orbital occupation changes in a manner that charge fluctuations and effective bandwidths are reduced, that the system becomes more one dimensional and more susceptible to a Peierls-like transition, and that the required massive orbital switching can only be made if the system is close to a Mott insulating regime.

High-temperature superconducting samples of YBa2Cu3Ox with a defined oxygen stoichiometric amount have been prepared using a special equilibrium method. Results of structural investigations as well as thermal, electrical and magnetic measurements are discussed trying to understand the mechanism of the superconductivity and to make available standard values.

Das V3O5 gehort zu den Magneli-Phasen VnO2n−1 und besitzt einen Leitfahigkeitsubergang bei 154 °C. Der Charakter des Uberganges, die Anderungen der elektrischen, thermoelektrischen, magnetischen, strukturellen u. a. Eigenschaften und der Einflus hydrostatischen Druckes und ternarer Komponenten (Dotierungen) auf den Ubergang werden beschrieben. Das Verhalten des V3O5 ordnet sich gut in das Eigenschaftsbild der anderen Magneli-Phasen ein. Der Ubergang ist als Ladungsordnungs-Ladungsunordnungs-Ubergang zu charakterisieren bei wesentlicher Wirkung der Elektronenkorrelation. V3O5 belonging to the Magneli phases VnO2n−1 shows a conductivity transition at 154 °C. The character of the transition, the changes of the electric, thermoelectric, magnetic, structural and other properties and the influence of hydrostatic pressure and ternary components (dopings) on the transition are described. The properties of V3O5 settle down well in the behaviour of the other Magneli phases. The transition is characterized as a charge order-disorder transition with important effect of electron correlation.

Under the influence of hydrostatic pressure a reduction of the phase transition temperature (Tt) having an initial inclination of 2.1 K/kbar is observed within the range of 0 and 12 kbar. This value conforms very well with the 2.4 K/kbar calculated according to Clausius-Clapeyron. Contrary to the other magneli phases of vanadium, V3O5 shows no coupling of the vanadium atoms which could contribute to the energy gap. The main cause for the phase transition could be probably the Colombian interactions between the vanadium and oxygen ions. Unter dem Einflus von hydrostatischem Druck wird im Gebiet zwischen 0 und 12 kbar eine Erniedrigung der Phasenumwandlungstemperatur (Tt) mit einer Anfangsneigung von 2.1 K/kbar beobachtet. Dieser Wert stimmt gut mit dem nach Clausius-Clapeyron berechneten Wert von 2,4 K/kbar uberein. Im Gegensatz zu den anderen Magneliphasen des Vanadins, zeigt das V3O5 keine Kopplung der Vanadinatorne, die zur Energielucke beitragen konnte. Als Hauptursache fur den Phasenubergang konnen wahrscheinlich die coulombschen Wechselwirkungen zwischen den Vanadin- und Sauerstoffionen angesehen werden.

The electrical resistivity (semiconductor–metal transition), the thermoelectric power, and the magnetic susceptibility of well-characterized VO2 monocrystals and powdered samples in the range of homogeneity were measured. The physical properties gave no hint of an extension of the range of homogeneity towards oxygen excess. An oxygen deficit (VO1.994) strongly influence the semiconductive properties of VO2 below the phase transition temperature, but hardly influences the metallic properties above the transition temperature. This behaviour may be explained by the formation of vanadium interstitial atoms and a rise in the concentration of mobile electrons connected with it. These results explain discrepancies in the data of the physical properties in literature, and show how the properties of VO2 can be purposefully changed. Es wurden der elektrische Widerstand (Halbleiter–Metall-Ubergang), die differentielle Thermospannung und die magnetische Suszeptibilitat von gut charakterisierten VO2-Einkristallen und Pulverproben im Homogenitatsbereich gemessen. Die physikalischen Eigenschaften gaben keinen Hinweis auf die Ausdehnung des Homogenitatsbereiches zu Sauerstoffuberschus hin. Sauerstoffmangel (VO1,994) beeinflust die Halbleitereigenschaften des VO2 unterhalb der Phasenumwandlungstemperatur stark, die metallischen Eigenschaften oberhalb der Umwandlungstemperatur kaum. Dieses Verhalten la6t sich durch Bildung von Vanadiumzwischengitteratomen und einer damit verbundenen Erhohung der Konzentration beweglicher Elektronen erklaren. Die Ergebnisse klaren Diskrepanzen in den Angaben physikalischer Eigenschaften in der Literatur und zeigen, wie man die Eigenschaften des VO2 gezielt verandern kann.

The coexistence of two phases at metal—insulator transition leads to peculiarities of the physical properties. This work reports on such peculiarities of the behaviour of the thermoelectric power and resistivity of vanadium oxides. An immediate proof for the coexistence of two phases is given by the behaviour of the X-ray reflexes. Die Koexistenz von zwei Phasen wahrend des Metall—Halbleiter-Phasenuberganges fuhrt zu Besonderheiten in den physikalischen Eigenschaften. In dieser Arbeit wird uber derartige Besonderheiten im Verhalten der Thermokraft und des Widerstandes an Vanadiumoxiden berichtet. Einen direkten Beweis fur die Koexistenz der zwei Phasen geben die Rontgenuntersuchungen.

Details are given of the change of the electrical transport properties as well as of the magnetic and thermodynamic properties at phase transition of monocrystalline V3O5. The phase transition above room temperature is investigated. V3O5 monocrystals are produced by chemical transport with TeCl4. The influences of the range of homogeneity on the mentioned properties at phase transition are studied and the results discussed. Es werden Angaben uber die Anderung der elektrischen Transporteigenschaften, der magnetischen und thermodynamischen Eigenschaften beim Phasenubergang von einkristallinem V3O5V gemacht. Der Phasenubergang oberhalb Raumtemperatur wird unter-sucht. Die V3O5-Einkristalle werden durch chemischen Transport mit TeCl4 hergestellt. Der Einflus des Homogenitatsbereiches auf die genannten Eigenschaften beim Phasenubergang wird untersucht und die erhaltenen Ergebnisse werden diskutiert.

Results of experiments concerning the shift of the transition temperature of the high- and low-temperature phase transition in (V0.99Cr0.01)2O3 due to substituting 16O by 18O are presented. Whereas for the low-temperature phase transition there is an increase of transition temperature, a decrease is observed for the high-temperature phase transition. In accordance to previous considerations this can be explained by considering dynamic effects of electron-phonon interaction in the metallic phase. The expected ratio of the shifts of transition temperature agrees nearly quantitatively with experiment. Es werden die Ergebnisse zur Verschiebung der Ubergangstemperatur des Hoch- und Tieftemperaturphasenubergangs in (V0,99Cr0,01)2O3 infolge einer Substitution von 16O durch 18O mitgeteilt. Wahrend fur den Tieftemperaturphasenubergang ein Anstieg der Ubergangstemperatur beobachtet wird, erfolgt ein Abfall fur den Hochtemperaturphasenubergang. In Ubereinstimmung mit fruheren Betrachtungen kann dies durch Berucksichtigung dynamischer Effekte der Elektron-Phonon-Wechselwirkung in der metallischen Phase erklart werden. Das erwartete Verhaltnis der Verschiebung der Ubergangstemperatur stimmt nahezu quantitativ mit dem Experiment uberein.

Vanadium dioxide is prepared by sintering, chemical transport, and deposition in an open system and the optimal preparation conditions are given. The products, prepared by these methods, are characterized by crystallographical and chemical analysis. The upper and lower phase boundary of vanadium dioxide are determined to VO2.002±0.002 and VO1.995±0.002, respectively. Vanadiumdioxid wurde durch Sintern, chemischen Transport und Abscheidung im offenen System hergestellt. Die optimalen Darstellungsbedingungen werden angegeben. Die nach den drei Methoden hergestellten Produkte werden kristallographisch und chemisch charakterisiert. Die obere Phasengrenze des Vanadiumdioxids wurde zu VO2,002=0.002 und die untere zu VO1,995±0,002 bestimmt.

In the vanadium oxides V2O3, V3O5, and VO2 oxygen is partially replaced by the stable isotope O18. The concentration of O18 in the oxides is determined by IR spectroscopy. The resulting increase of the temperature Tc of the semiconductor—metal transition, found by the measurement of the electrical resistance, is discussed on the basis of different models of phase transition. The results can be qualitatively explained by a concept, where the transition is caused by electron—electron interaction, which is strongly modified by electron—phonon coupling. In den Vanadinoxiden V2O3, V3O5 und VO2 wird ein Teil des Sauerstoffs durch das stabile Isotop O18 ersetzt. Die Konzentration des O18 in den Oxiden wird durch IR-Spektroskopie bestimmt. Die resultierende Erhohung der Phasenumwandlungstemperatur Tc des Halbleiter—Metall-Phasenuberganges, die durch Messung des elektrischen Widerstandes ermittelt wurde, wird an Hand verschiedener Modelle fur den Phasenubergang diskutiert. Dieses Ergebnis kann qualitativ in einem Modell verstanden werden, in dem der Halbleiter—Metall-Phasenubergang durch die Elektron—Elektron-Wechselwirkungen verursacht wird, die durch die Elektron—Phonon-Kopplung stark modifiziert ist.

Nachdem in einer vorangegangen Arbeit zum Transportsystem VOTeCl allgemein und zum Transportverhalten des VO2 berichtet wurde, wird in diesem Teil der Transport der Magneliphasen und des V6O13 und V2O5 behandelt. Aus der Gasphasenzusammensetzung uber den festen Phasen beim chemischen Transport mit TeCl4 werden die Transportfolge, die Transportrichtung und Transportraten berechnet. Die Ergebnisse der Rechnung werden mit den experimentellen Befunden verglichen. Die einzelnen Oxide werden phasenrein abgeschieden, ihre Phasengrenzen ermittelt und die Temperaturabhangigkeit der oberen Phasengrenzzusammensetzung von V2O3 bestimmt. It was reported on the transport system VOTeCl and on the transport behaviour of VO2 in a previous paper. This paper deals with the chemical transport of Magneli phases VnO2n–1, and of V6O13, and of V2O5. Resulting from the composition of the gas phase over the solid phases in the presence of TeCl4 the chronological sequence of the transported phases as well as the direction and rate of transport are calculated. These results are compared with experiments. All of the oxides are obtained without deposition of any other phases. Their phase boundaries are determined. The dependence on temperature is investigated for the upper phase boundary of V2O3.

Electrical resistance and DTA measurements on single crystals of Fe3O4, in which the oxygen is partially replaced by the stable isotope 18O, show a positive isotope shift in the Verwey transition temperature TV of δTV = (6.1 ± 0.05) K for the 18O concentration of 43%. This result can be qualitatively explained by a theory which is based on the concept of the condensation of a coupled charge density–phonon mode at TV and takes into account both the electron–phonon and the electron–electron interactions. Elektrische Widerstands- und DTA-Messungen an Magnetiteinkristallen, in denen der Sauerstoff teilweise durch das stabile Isotop 18O ersetzt ist, zeigen eine positive Isotopieverschiebung der Verwey-Ubergangstemperatur TV von δTV = (6,1 ± 0,05) K fur eine 18O-Konzentration von 43%. Dieses Ergebnis kann qualitativ mit Hilfe einer Theorie erklart werden, die auf dem Konzept der Kondensation einer gekoppelten Ladungsdichte–Phonon-Mode bei TV beruht und sowohl die Elektron–Phonon-Wechselwirkung als auch die Elektron–Elektron-Wechselwirkung berucksichtigt.

Das thermodynamische Verhalten des Systems VOTeCl wird beschrieben. Von besonderer Bedeutung fur das Transportverhalten ist der O2-Koexistenzgleichgewichtsdruck. Fur die Beschreibung des Transportes unterhalb 700 °C ist die Existenz von TeOCl2 in der Gasphase Voraussetzung. Aus dem Zusammenwirken des O2-Koexistenzgleichgewichtsdruckes mit den diminierenden Gasphasenspezies konnen die Transportrichtung, die Transportrate und die Sauerstoffan- und -abreicherung der kristallisierenden Phase berechnet und vorausgesagt werden. VO2-Einkristalle unterschiedlicher Phasenzusammensetzung werden hergestellt. The chemical transport behaviour of the vanadium oxides from V2O3 to V2O5 with TeCl4 is described. The O2 coexistence equilibrium pressure is of especial importance for the transport behaviour. The existence of TeOCl2 in the gas phase is the premise for the description of the transport behaviour below 700 °C. The transport direction, the transport rate and the increase or decrease of the oxygen content in the deposited phase, respectively, are calculated from the cooperation of the O2 coexistence equilibrium pressure with the composition in the gas phase. Single crystals of Vo2 are obtained with different phase compositions.

The chemical transport of VO2 with TeCl4 was investigated in the temperature range of about 1000° and 600°. In the first case there is TeO2 existing in the gaseous phase, in the second case the TeO2 has no noticeable vapour pressure. The transport takes place from the hot to the cool zone. In both cases we obtained high transport rates of 10 to 20 mg h-1. The composition of the gaseous phase is calculated and the transport mechanisms are discussed. It is possible to obtain VO2 with the upper and lower phase boundary by variation of the initial compositions. Starting with compositions lower than VO1.99 we obtained near 1000° in the deposition zone V8O15 first and after a time VO2 with the lower phase boundary. Compositions higher than VO2.00 give vanadium dioxide with the upper phase boundary. In the temperature range of about 600° however we obtained the phase with the higher oxygen content first. The results are compared with those, which have been obtained by the transport of VO2 with HCl and Cl2 – which takes place from the cool to the hot zone.