PREPARATION AND PROPERTIES OF FESI, ALPHA-FESI2 AND BETA-FESI2 SINGLE-CRYSTALS

FeSi and alpha-FeSi2 single crystals were prepared by the Czochraiski technique. beta-FeSi2 and alpha-FeSi2 single crystals were prepared by chemical vapour transport. FeSi single crystals 18 cm(3) in size and alpha-FeSi2 single crystals of a few cubic centimetres were obtained. beta-FeSi2 decomposes peritectically above 970 degrees C, therefore on ly needle-like crystals, each about 10 mm long, were grown from the va pour. The temperature dependence of the resistivity was measured from 4.2 K up to room temperature. alpha-FeSi2 shows a weak temperature dep endence of resistivity. The activation energy of FeSi, calculated from In p=f(1/T) is equal to 58.2 meV. the Hall coefficient and resistivit y of beta-FeSi2 single crystals was measured between 30 and 300 K. The as-grown, not intentionally doped beta-FeSi2 crystals exhibit n-type conductivity, while Cr and Al doped crystals show p-type conductivity. The Hall coefficient of n-type samples depends on the magnetic field; therefore, the transport properties of beta-FeSi2 were explained taki ng into account two types of carrier, heavy and light. At low temperat ure (32 K) heavy electrons show a mobility of mu(n) = 48 cm(2) V-1 s(- 1). An impurity band and an additional deep acceptor level are observe d in p-type crystals. The activation energies of the shallow acceptor level and deep acceptor level are equal to 55 meV and 100 meV respecti vely. The mobility of holes mu(p) in p-type crystals reaches a maximum of 1200 cm(2) V-1 s(-1) at 67 K. Magnetization and magnetic susceptib ility measurements in the temperature range 4-320 K show a small posit ive value of susceptibility and nonlinear magnetization for beta-FeSi2 but no ferromagnetic phase was detected. alpha-FeSi2 crystals show li near magnetization and susceptibility similar to beta-FeSi2. Magnetiza tion of FeSi is linear but the susceptibility passes through a minimum at about 150-200 K.