The influence of Fe alloying on the formation of thermal vacancies in Ni75Al25−xFex has been specifically studied by high-temperature, positron lifetime spectroscopy (PLS). The results are consistent with a previously reported decrease of the activation energy for the vacancy formation upon alloying of Ni75Al25 with Fe derived from residual isochronal resistometry (REST): an indirect method for vacancy studies.

The nanocrystallization mechanism of an amorphous alloy is discussed based on the kinetics of open nanospaces in Fe78B13Si9. There already exists a high concentration of Fe-enriched fluctuated sites with open nanospaces in the amorphous matrix. The structural and compositional fluctuation helps transient short-range Fe diffusion in the metastable amorphous matrix with an increase of temperature, triggering highly concentrated α-Fe nucleation. Along with the growth of α-Fe nucleus, Fe atoms are transferred from the intergranular amorphous phase to Fe-based nanocrystallites. The nanocrystallization of α-Fe is achieved through nucleation by short-range Fe diffusion and its growth by nanovoid-mediated long-range Fe diffusion.

Local atomic environment of vacancies in nonstoichiometric titanium monoxide ranging in composition from TiO 0.74 to TiO 1.26 was studied by electron-positron annihilation. Analysis of the Doppler broadening spectra of the annihilation gamma line for titanium and liquid oxygen showed that positrons in titanium monoxide are trapped by titanium vacancies. Experiments revealed that the lifetime of positrons in ordered and disordered titanium monoxide TiO y increases with increasing oxygen content y and varies from 184 to 210 ps. Data on the valence electron density permitted the prediction that the lifetime of free positrons in stoichiometric titanium monoxide is about 140 ps and the lifetime of positrons localized in an oxygen vacancy is about 170 ps. The method used to analyze the gamma-line Doppler broadening spectra makes it possible to determine the type and number of atoms around a vacancy and to investigate order-disorder phase transformations in nonstoichiometric compounds.

positron annihilation techniques in the composition range from TiO0.74 to TiO1.26. For nonstoichiometric titanium monoxide, a high concentration of vacancies has been suggested on both the titanium and the oxygen sublattice. From the analysis of the core electron momentum distribution of the atoms surrounding the vacancies in disordered as well as in ordered titanium monoxide, oxygen atoms are identified to form the local vacancy environment, indicating that positrons are trapped by titanium and not by oxygen vacancies. Positron lifetime measurements have shown that the decrease of the oxygen content, y, and the process of disordering in nonstoichiometric titanium monoxide TiOy, are accompanied by an increase of the valence electron density on the titanium vacancy.

The nanocrystallization-induced structural evolution of the intergranular amorphous phase in a Fe78B13Si9 alloy was investigated by X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM), and positron annihilation spectroscopy. Crystallization occurs at 773 K, where nanocrystallites of α-Fe with an average grain size of a few tens of nanometers are formed in an amorphous matrix. With increasing annealing temperature up to 973 K, the average grain size increases up to ∼80 nm. In the as-prepared sample corresponding to an amorphous precursor, more than 90% of the positrons are localized at vacancy-sized free volumes dominantly surrounded by Fe atoms and other positrons are trapped by microvoids. Along with the appearance of nanocrystallites and their growth due to annealing, the concentration of microvoids is increased in the intergranular amorphous phase.

We report on the crystallization behavior of initially amorphous precursor-derived Si3B1C4.3N2 ceramics, making use of macroscopic and atomic scale investigation techniques. As derived from our kinetic studies upon heating, the following temperature sequence of solid state processes is observed with approximately the same kinetic time constants: after an initial densification in the amorphous state (1673 K), crystallization occurs (1873 K) with subsequent strain relaxation and disappearance of nanovoids (2073 K). This behavior indicates an increasing activation enthalpy of the atomic mechanisms giving rise to these processes when the annealing temperature is increased. The nanovoids may be located in the turbostratic BNCx layers as derived from positron annihilation studies. For grain growth at high temperatures an activation energy of H-G = 3.9 eV is derived.

For the identification of vacant lattice sites in the high-temperature intermetallic compound Ru46Al54, positron lifetime measurements as well as coincident measurements of the two Doppler-broadened positron-electron annihilation photons have been employed. These data demonstrate that thermal vacancies are formed on the Ru metal sublattice similar to FeAl and NiAl where the thermal vacancies are also formed on the transition metal sublattice.

The Nd self-diffusivity has been studied in nanocrystalline Nd-rich Nd2Fe14B. From the analysis of the diffusion profiles grain-boundary diffusion coefficients DGBNd are derived similar to the values observed recently for Fe59 diffusion in this material. Above the intergranular melting transition, a second diffusion path indicates rapid diffusion in the liquid intergranular phase.

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.

In the present paper we succeeded in studying structural phase transitions from an atomistic point of view by positron annihilation Doppler broadening. This differs and is complementary to conventionally used diffraction experiments with large coherence lengths. In the exemplary case of the 1140 K order-disorder transition in decagonal Al71.5Ni14Co14.5 quasicrystals the importance of this atomistic approach and its wide scope of application is demonstrated.

Formation of thermal vacancies in icosahedral Zn65Mg25Er10 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.

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 H-V(F)=(1.6 +/- 0.1) eV was determined and a low migration enthalpy H-V(M) 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.

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 Al-71,Ni-5(14),Co-5(14) quasicrystals.

Positron annihilation spectroscopy on nanocrystalline Pd85Zr15 shows the segregation of Zr at the interfaces and a decrease of the fraction of nanovoids the size of 10 to 15 missing atoms upon isochronal annealing up to Ta=1100 K by which the grain size increases up to d=185 nm. In this state, the positron trapping at interfacial free volumes reversibly changes contingent upon the temperature whereas the interfacial composition is unchanged when the measuring temperature varies.

Atomic free volumes of O.2[XNa 2 O.(1-X)Rb 2 O].0.BB 2 O 3 glasses and of amorphous-B 2 O 3 have been studied by positron annihilation spectroscopy. Free volumes of the size of about 0.270 nm 3 are present in amorphous B 2 O 3 . By adding Na 2 O and Rb 2 O to pure B 2 O 3 these free volumes are reduced in size to 0.034 nm and 0.073 nm 3 , respectively, concomitant with a densification of the glass matrix of the alkali borate glass. The densification slightly increases with increasing Na content.

For a detailed understanding of high-temperature processes in complex solids the identification of the sublattice on which thermal defects are formed is of basic interest. Theoretical studies in intermetallic compounds favor a particular sublattice for thermal vacancy formation. In the present study we detect in ordered MoSi2 thermal vacancies with a low formation enthalpy of H(F)(V)=(1.6+/-0.1) eV, and we succeed in showing by experimental and theoretical efforts that they are preferentially formed on the Si sublattice. By these data self-diffusion in MoSi2 can be understood.