No abstract

The transition metals of the iron group a r e frequently subject of X-ray photoelectron spectroscopy (XPS) /1 to 3/. To study the influence of alloying and/or phase transitions some iron alloys were chosen: Fe Ni Fe Cr All samples have been carefully homogenized in a hydrogen atmosphere at high temperatures. The change in the lattice structure at phase transition from b. c . c . to f . c . c . Torr. Oxygen andThe measured spectra of the 2p core electrons are shown in Fig. l a to d, without any correction for spectrometer resolution, which i s in our case 0.5 eV FWHM. Table 1 gives the binding energy of the split lines, the spin-orbit splitting AE, and an asymmetry parameter r (2/3) (full width at 2/3 of the peak maximum). Turning to our spectra we see that the binding energy i s almost unchanged under alloying and phase transition. This leads to the conclusion that for FeNi alloys of the invar type changes in the coordination of the surrounding atoms do 8 physica (b)

For a number of PdAg alloys the photoelectron spectra of the core electrons are studied in a systematical way. The role of the so-called extrinsic and intrinsic losses, satellite structures, and lineshapes is to be discussed for 3p and 3d lines. Existing theories are discussed for the given cases. A multi-electron mechanism for explaining satellite structures is proposed. Asymmetry of the Pd 3d lines is studied and a strong dependence on the density of states a t the Fermi energy is found.Fur eine Anzahl von PdAg-Legierungen werden die Photoelektronenspektren systematisch untersucht. Die Rolle der sogenannten extrinsischen und intrinsischen Verluste, der Satellitenstrukturen und der Linienform wird fur die 3p-und die 3d-Linien diskutiert. Einige der vorhandenen Theorien werden mit den experimentellen Ergebnissen verglichen. Ein Vielelektronen-Anregungsmechanismus wird zur Erklarung der Satellitenstruktur bei Pd vorgeschlagen. Die Asymmetrie der Pd 3d-Linien wird studiert und eine starke Abhangigkeit von der Znstandsdichte an der Fermienergie gefunden. John et al. [a]have extended the scattering concept to Bloch electrons being scattered by the core hole potential. The muffin-tin method has been used to calculate the phase shifts. Their concept works well for transition metals.Concerning theoretical work dealing with the occurrence of line asymmetry, satellites, and structure a t the high-energy side of the photoelectron lines the papers of Kaga, Kotani, and Tojozawa [5,6] and the series of papers of Grebennikov et al. [7] should be mentioned. Multi-electron excitations as shake-up processes have been discussed, among others, by Nemoshkalenko and Aleshin [S]. For the discussion and application of the "sudden approximation" to the core hole creation process the paper of Meldner and Perez [9] is recommended. l ) DDR 8027 Dresden, DDR. 2, DDR 402 Halle (Saale), DDR.

Yeasurements of positron lifetime, Doppler broadening and angular correlation have been taken from n-type sintered ceramics on the basis of BaTiO,. Polycrystalline n-doped BaTiO, shows a pronounced positive temperature coefficient (PTC) of electrical resistance, e.g. an increase of resistivity by several orders of magnitude within a temperature interval of a few degrees in the vicinity of the ferroelectric phase transition a t 130 "C centigrade (eg. HEYWANG). This little-understood effect usually is discussed in terms of potential barriers resulting from an inhomogeneous distribution of Ba vacancies near grain boundaries (a review of models is given by LEWIS et al.).The present investigations performed by positron annihilation at technical PTC ceramics in as-received state and after special ageing cycles give evidence of existence of vacancy-type defects becoming mobile above 700 ... 800 " centigrade. High-temperature ageing as well as long sintering intervals obviously lead t o a homogeneous distribution of the defects that are thought to be Ba vacancies. Neither lifetime spectra nor angular correlation measurements with magnetic field gave an indication of positronium formation in the materials studied. Just as little the ferroelectric phase transition came t o light in the parameters of angular correlation curves measured below and above the Curie point. Experimental resultsPositron lifetime measurements have been taken from technical sintered ceramic samples of (BaCaPb)TiO, with varying La and Nb dopant concentration a t room temperature in as-received state (Fig. 1). Clearly, two lifetime components can be resolved, z, = 160 ... 165 ps, z2 x 280 ps.The intensity of the z, component increases with Nb concentration whereas in the case of La doped material there seems t o be only a very slight variation of la. In the perovskite lattice La and Nb are known t o occupy A (Ba) sites and B (Ti) sites, respectively.

The results of measurements of Compton profiles, photoelectron spectra, positron annihilation curves, and lifetime spectra of several NiAl alloys are discussed in terms of bonding and stability in transition metal aluminides.The interpretation of experimental Compton profiles suggests a transfer of nearly-free electrons from A1 sites to more localized d-like states on Ni-sites. The transfer increases with A1 contents up to 50 atomic percent. From X-ray photoelectron spectra in the oase of Al-rich alloys a different behaviour follows. The results support a model established by WENGER. Following this in the composition range above 50 percent A1 no further charge is transferred to Ni cells. The Ni d states seem t o be occupied in the Al-rich alloys. A preferential annihilation of positrons in Ni cells is though to have its origin in the enhanced negative charge of these cells caused by charge transfer.