Features of hydrogen-induced amorphization of the Er(Co, Mn) 2 pseudo-binary Laves compound

Abstract

Hydrogen-induced amorphization behavior of Er(Co 1− x Mn x ) 2 pseudo-binary Laves compound is investigated by means of X-ray diffractometry and transmission electron microscopy. The substitution of Mn for Co results in a homogenous single phase within the whole composition range. Up to x = 0.8, C15 cubic phase is obtained, but C14 structure is found at higher Mn composition. At all compositions except for the binary ErMn 2, the amorphization phenomena are observed. Thus it is suggested that amorphization of the Laves phases by the hydrogen absorption is not concerned with the crystal structure but the chemical bonding between the unlike atoms causing the kinetic constraints of the motion of the metal atoms. By the Mn addition into ErCo 2, the amorphization temperature, which is defined as the temperature amenable to the full amorphization reaction in a fixed times of 24 h, gradually decreases from 350°C for ErCo 2 to 230°C for Er(Co 0.2Mn 0.8) 2 within the C15 region. This behavior is explained by the variation of the formation enthalpy ( ΔH f) and the bulk modulus of the compound with the Mn content. A compound with a high negative ΔH f and high modulus is amorphized at higher temperature range due to the larger activation energy barrier for the motion of the metal atoms than other compounds with relatively lower heat of mixing and modulus values.