The theory of l a t t i c e dynamics for crystals with defects goes back to the work of LIFSHITS /1.1/ in the years 1943 and 1944. Due to the development of new experimenta l methods such as the M~ssbauer ef fect , neutron scatter ing, and infrared absorpt ion th is f i e l d expanded very quickly from about 1960 on and the theory has been worked out in de ta i l , especial ly by Maradudin, Ludwig and others. Al l essential parts of the theory were developed more than ten years ago. Several review ar t i c les by LUDWIG /1 .2 ,3 / , MARADUDIN /1 .4 / , MARADUDIN et a l . /1 .5 / , ELLIOTT /1 .6 / , KLEIN /1 .7 / , and LIFSHITS and KOSEVICH /1.8/ appeared at that time as well as a conference report /1 .9 / . In the las t ten to f i f t een years our knowledge of point defects has increased considerably. Compared to the s i tuat ion about ten years ago when one understood the pr incipal effects of point defects on the dynamics of l a t t i ces , such as the occurrence of local ized and resonant modes, we have now a much better knowledge about the d i f fe ren t types of defects and the i r v ibrat ional character ist ics. Not only have a very large number of subst i tut ional defects been studied, but we begin also to learn about such important defects l i ke vacancies or s e l f i n t e r s t i t i a l s . Also theore t ica l ly we can now make quite sophisticated model calculat ions which is mainly due to the development of numerical programs for the l a t t i c e Green's function. The aim of th is a r t i c l e is to review our present knowledge about point defects. Since this f i e l d has expanded so tremendously, several res t r i c t ions have to be made:

Dynamical properties of point defects in metals

Mössbauer studies at hhigh pressure

We report the results of Gorsky effect, neutron scattering and Mossbauer effect experiments in which we studied various correlated transport properties (diffusion coefficients) of hydrogen in tantalum at high hydrogen concentrations (up to a hydrogen-to-tantalum atomic ratio c of 0.75). In the Gorsky effect measurements we determined the decay rate (described by the chemical or Fick diffusion coefficient) and the amplitude of the hydrogen density fluctuations which extended spatially over large (i.e. sample) dimensions. Secondly we used neutron spectroscopy to measure the self-diffusion (tracer diffusion) coefficient of hydrogen which describes the diffusive motion of a single (tagged) hydrogen interstitial. Finally we used Mossbauer relaxation experiments employing the high resolution 6.2 keV γ resonance of 181Ta to study the amplitude and decay rate of hydrogen density fluctuations with dimensions of the order of a lattice parameter.

Self-diffusion and collective diffusion of hydrogen in TaHx☆

A constant-velocity electromechanical Mossbauer drive with a velocity range from -15 to +15 mm s-1 and with a velocity resolution of 0.005 mm s-1 is described. The negative and positive velocities are preset separately on two 14 bit BCD selectors. Two feedback loops are used for the control; one with a derivative (velocity) pickup, the other with a proportional (displacement) transducer. The design is original, and the equipment has been in use faultlessly since 1985.

http://iopscience.iop.org/article/10.1088/0022-3735/21/7/002/pdf

A constant velocity Mossbauer drive

The observation of the Mossbauer resonance of the 6.2 keV gamma rays of tantalum 181 in tantalum metal over the temperature range of 300 to 2400 K is reported here. This is the highest temperature range for any Mossbauer study up to now. Slight deviation of the linear dependence of the isomer shift is noted at higher temperatures. The increase and following decrease of the line-width with temperature due to absorption, diffusion and degassing of residual gases (oxygen and nitrogen) is considered.

High Temperature Behavior of the 6.2 keV Mössbauer Transition of Tantalum-181 in Tantalum Metal

In attempting to improve the resolution of the Mossbauer method considerable attention has been devoted to the few potential Mossbauer resonances with lifetimes in the microsecond region [1–4]. It has been shown recently, that in particular the 6.2-keV gamma transltion of Ta-181 (T1/2 = 6.8 μs) warrants great promise for high-resolution Mossbauer studies of hyperfine interactions. This is true for magnetic-dipole and electric-quadrupole hyperfine interactions as well as for isomer shifts [5–12].

/pdf/mossbauer-isomer-shifts-of-the-6-2-kev-gamma-rays-of-7mgsfwe2b5.pdf

D. Salomon

Papers

Mössbauer Isomer Shifts of the 6.2-keV Gamma Rays of Tantalum-181