IONIZATION OF INTERSTITIAL IRON ATOMS IN BETA-RHOMBOHEDRAL BORON

Mossbauer spectra of beta-rhombohedral boron with interstitially distr ibuted iron atoms up to 3.33 at% are presented. Taking into account th at the six D sites in the unit cell are saturated by two Fe atoms, the distribution of the Fe atoms on A and D sites proved to be simply sta tistical. At lower Fe contents single iron atoms on A sites are ionize d to Fe3+, while towards higher Fe contents a rapidly increasing share of Fe2+ ions is found. On D sites the ionization of single Fe atoms i s 2+, but it changes to 3+, when two Fe atoms are accommodated in the D sites of one unit cell. The isomer shift of the Mossbauer doublets y ields an estimation of the electron transfer to the boron framework. T ransition from p- to n-type takes place, when six electrons per unit c ell are transferred from the iron ions to the boron framework. This nu mber corresponds to the sum of the unoccupied sites in the upper valen ce band and the possible electron traps generated by an electron-phono n interaction in the B-12 icosahedra.