A. Börger

A chemically driven insulator-metal transition in non-stoichiometric and amorphous gallium oxide.

Insulator-metal transitions are well known in transition-metal oxides, but inducing an insulator-metal transition in the oxide of a main group element is a major challenge. Here, we report the observation of an insulator-metal transition, with a conductivity jump of seven orders of magnitude, in highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO(1.2) at a temperature around 670 K. We demonstrate through experimental studies and density-functional-theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric Ga(2)O(3) within the metastable oxide matrix-in chemical terms by a disproportionation. This novel mechanism--an insulator-metal transition driven by a heterogeneous solid-state reaction--opens up a new route to achieve metallic behaviour in oxides that are expected to exist only as classic insulators.

Unusual optical properties of Mn-doped ZnO: the search for a new red pigment-a combined experimental and theoretical study.

A pigment of your imagination: A range of polycrystalline solid solutions of a zinc-rich Zn(x-1)Mn(x)O system (see figure) have been prepared and studied in terms of their colour, diffuse reflectance spectra, Mn valence state and electronic structure. The intense optical absorption arises from Mn(2+) doping and is thought to be due to forbidden or partially forbidden transitions between the valence and the conduction band.We report an investigation of zinc-rich polycrystalline solid solutions of the Zn(1-x)Mn(x)O system concerning the colour, the diffuse reflectance spectra, the valence state of manganese and the electronic structure. Samples were prepared by a chemical-vapour-transport-assisted route and optimized with respect to colour strength. In agreement with previous experimental results, EPR studies showed that manganese is in the divalent charge state. The nature of the very intense optical absorption, which is caused by Mn(2+) doping and determines the colour of the material, is discussed. It is argued that the Mn(2+)-induced optical absorption is due to forbidden or partially forbidden transitions between the valence and the conduction band that involve Mn admixed states. This assignment is also confirmed by quantum chemical calculations using the semiempirical molecular orbital method MSINDO.

In situ investigation of coloration processes in LiNbO3:MgO during reducing/oxidizing high-temperature treatments

The work presents experimental results of an in situ investigation of optical absorption of LiNbO(3) : MgO during reducing (95%Ar + 5%H(2)) and oxidizing (O(2)) high-temperature treatments in the temperature range from room temperature to 1073 K. The absorption spectra measured at in situ conditions at high temperatures in reducing/oxidizing atmospheres as well as the kinetics recorded at fixed wavelengths during rapid replacement of gas atmospheres have been analyzed. The origin of the changes in optical absorption caused by the reducing/oxidizing treatments is discussed in terms of hydrogen and oxygen ion diffusion and the point defect structure of the material.

High temperature optical spectroscopy of cubic holmium doped zirconia, Zr0.78Y0.21Ho0.01O1.90

The electronic absorption spectra of a Zr0.78Y0.21Ho0.01O1.90 single crystal have been measured in in situ conditions between room temperature and 1289 K. The evolution of the spectra with rising temperatures is characterised by an overall increase in intensity. Notably the intensities of the 5I8 → 5F2, 5I8 → 5S2 and 5I8 → 5G6 transitions increase by factors of two to three. The evaluation of the spectral parameters favours the conclusion that the intensity increases are due to vibronic coupling associated with effective frequencies ranging between 1 × 1013 and 5 × 1013 s−1 (330 cm−1 and 1670 cm−1). The changes in the optical spectra are reversible and are not connected with structural changes in the material.

High temperature optical spectroscopy investigations on Zr0.78Y0.18Sm0.04O1.89 and Zr0.78Y0.18Ho0.04O1.89 single crystals

Results are reported from a temperature dependent optical study of the title compounds performed at temperatures up to 1323 K. Absorption maxima and integrated absorption of rare earth f-f transitions exhibit a considerable temperature dependence, which is quantitatively described in the framework of the optical high temperature spectroscopy of d-ions. A strong, reversible band gap shift of doped and pure YSZ is observed and evaluated according to the Tauc equation. Single crystals of the title compounds were prepared by the skull melting method. Their linear expansion coefficients were determined by high-temperature X-ray diffraction.