W. Sprengel

Identification and study of vacancies in titanium monoxide by means of positron annihilation techniques

Structural vacancies are of interest for an understanding of the crystal structure and the properties of metal oxides. Here we report on the identification and study of structural vacancies in titanium monoxide TiOy with a B1 (NaCl) structure. Structural vacancies were identified by coincident Doppler broadening studies of the positron–electron annihilation radiation. The analysis shows that the nearest-neighbor atoms of vacancies are oxygen atoms. Taking into account the B1 structure of titanium monoxide this means that vacancies are located on the titanium sublattice. Positron trapping in titanium vacancies allows the study of the electronic structure of these vacancies by positron lifetime spectroscopy which shows that the valence electron density in the titanium vacancy increases with decreasing oxygen content y in TiOy. The increase of the valence electron density is accompanied by a decrease of the mean number of nearest-neighbor oxygen atoms in the vicinity of the titanium vacancy from 6 to 5 atoms.

High-Accuracy Lattice Constant Measurements of Electron-Irradiated 6H-SiC Single Crystals

6H-SiC single crystals have been irradiated with 3 MeV elect rons at a temperature of 200 K creating mainly vacancies on the carbon and the silicon sublattice s. Positron annihilation experiments show that only well defined point defects have appeared. Th influence of electron irradiation and of subsequent annealing on the lattice constants was studied by x-ray diffraction. An increase of the lattice constants by a relative value of 6 ⋅10 due to the irradiation with a fluence of 1.8⋅10cm was observed. Annealing at 1000 °C in Ar atmosphere leads to a regai n of the preirradiation values. Furthermore, the spread of lattice paramete rs pr sent in the as-grown samples is reduced after irradiation and annealing. Higher annealing tempera tures up to 1850 °C do not lead to further changes. Experimental Sample treatment. For the present studies, 6H-SiC single crystal substrates of a total thickness of 300 μm and with a 5μm thick epitaxial 6H-SiC layer (1.4 ⋅10cm n-type nitrogen doping) from Cree Inc., USA, were used which show a bulk positron lifetime of 146 ps [1]. For the positron annihilation experiments pieces of approx. 1 ×1 cm were cut from the as-grown wafer. Small cubes with an edge length of about 300 μm were prepared for the present high-accuracy lattice constant measurements. The crystals used were chosen randomly from a la rger number prepared from a 1 ×1 cm piece of a wafer. The electron irradiation for generation of vacancies and interstit ial was performed at the Stuttgart Dynamitron accelerator [2]. The maximum specimen temperature during irr adiation was 220 K. The applied electron energy was 3 MeV with a radiation dose of φ = 1.8⋅10cm. Annealing was performed with a high temperature furnace at the University of Erlangen. The temperature was 1000 °C applied for 10 min and the specimens were held in Ar atmosphere [3]. All samples were subject to the same cycle: measurement of lattice parameters – annealing – measurement of lattice parameters – irradiation – measurement of lat ice parameters – annealing – measurement of lattice parameters. Positron annihilation. Coincidence measurements of the Doppler broadening of the positronelectron annihilation γ spectrum for background suppression were performed with a collinear set-up of two high-purity Ge detectors. The full-width at half-maximum ( FWHM) of the energy resolution function of the detectors was equal to about 1.2 keV at 511 keV. The posit ron lifetime was measured by means of a fast-slow γγ spectrometer with a time resolution of 205 ps (FWHM). For both techniques a NaCl positron emitter stacked between two identical SiC specimen plates was used. X-ray measurements. The x-ray line profiles were taken on a Huber 4-circle diffra ctometer with a graphite monochromator and Mo tube, optimized for a good resolution function. The lattic parameter measurements were performed with a modified Philips PW1100 4-circle diffractometer again equipped with a graphite monochromator and a Mo tube. Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 289-292 doi:10.4028/www.scientific.net/MSF.433-436.289

8 – Diffusion in Nanocrystalline Materials

Publisher Summary nThis chapter discusses the diffusion in nanocrystalline materials, which are polycrystals with an ultrafine grain size and a high volume fraction of atoms located in interfaces. Nanocrystalline materials have gained considerable interest due to high prospects for their attractive potential applications arising from improved mechanical and magnetic properties compared to their coarse-grained counterparts. With respect to both the structure and the physical properties of nanocrystalline solids, the understanding of the atomic transport properties in these materials represents a key research issue. In general, the atomic transport in nanocrystalline materials differs substantially from that in coarse-grained or single-crystalline materials. This is due to the fact that, in nanocrystalline solids, the crystallite interfaces provide paths of high diffusivity, whereas in more coarse-grained crystals, volume self-diffusion or substitutional diffusion dominates at least at temperatures higher than approximately half of the melting temperature. Interface diffusion, in combination with a high fraction of atoms in interfaces, gives rise to modified physical properties of nanocrystalline solids.

Identification of Lattice Vacancies and Structural Phase Transitions in Solids by Positron Annihilation Spectroscopy

Atomic defects as, e.g., vacancies and structural phase transitions play an important role in solid state physics. In the present review paper we first demonstrate that vacancies in the compound semiconductor SiC can be selectively introduced on the C or the Si sublattices and specifically detected by employing positron lifetime spectroscopy and coincident measurements of the Doppler broadening of the positron-electron annihilation radiation. In addition these techniques are shown to be most useful for studying structural phase transitions in complex solids on an atomic level as demonstrated in the case of decagonal Al71,5Ni14,5Co14 quasicrystals.

Defects Distribution of Pr2Fe14B Hard Magnetic Magnet from Amorphous to Nanostructures Characterized by Positron Annihilation Spectroscopy

The defect distributions have been investigated using positron lifetime spectroscopy on amorphous and nanocrystalline Pr2Fe14B samples, produced by melt-spinning and nanocrystallization route. The main two components can be concluded that were ascribed to vacancy-like defects in the intergranular layers or the interfaces, and microvoids or large free volumes with size compared to several missing atoms at the interactions of the atomic aggregates or the crystallites. The remarkable changes in the positron lifetimes from the amorphous structure to the nanocrystalline with varied sizes can be interpreted, indicating that the structural transformation and the grain growth induce the defect distribution changes occurring at the interfaces with different shape and size.

Vacancy Studies in Silicon-Rich Intermetallic Compounds: MoSi2

Thermal vacancy formation was studied in MoSi2 by the temperature dependence of the mean positron life time and of the W parameter. A low vacancy formation enthalpy of F V H =(1.6±0.1) eV was determined and a low migration enthalpy M V H was estimated. Coincident measurement of the Doppler broadening of the positron-electron annihilation radiation at high electron momenta indicate thermal vacancy formation predominantly on the Si-sublattice. A high thermal vacancy concentration and a high vacancy mobility on the Si-sublattice explains directly the observation that Si diffusion in MoSi2 is substantially faster than Mo diffusion.

Complex Intermetallic Compounds: Defects, Disordering, Details

A short overview will be given on the thermodynamics of the formation of thermal defects in intermetallic aluminides. We focus on thermal vacancies studied by the specific techniques of positron annihilation and time-differential dilatometry and discuss the results together with self-diffusion data. We then demonstrate that these techniques can be employed for studying vacancies in compound semiconductors specifically. Furthermore, structural order-disorder phase transitions can be investigated from an atomistic point of view by making use of positron annihilation as shown in the exemplary case of decagonal Al-Ni-Co quasicrystals.

High temperature vacancy studies of icosahedral Zn65Mg25Er10 quasicrystal

Formation of thermal vacancies in icosahedral Zn 65 Mg 25 Er 10 quasicrystals has been specifically studied from room temperature to about 720 K by positron annihilation spectroscopy employing two-detector coincident Doppler broadening techniques. Significant vacancy formation was observed for temperatures higher than 0.6T m . An apparent vacancy formation enthalpy of 1.2 eV was determined. The results are discussed in comparison with high temperature vacancy processes in quasicrystals as well as in other complex solids.