HYDROGEN-INDUCED PHASE-TRANSITIONS IN YMN2

X-ray and magnetic investigations of the YMn2Hx hydrides at temperatur es ranging from room temperature down to 4.2 K for the concentration r ange x less than or equal to 1.2 are presented and analyzed. It is sho wn, that in the concentration range 0.5<x<0.8 the paramagnetic cubic p hase of the hydrides transforms at 245 K into a tetragonally distorted (c/a <1) and magnetically ordered phase. This transition temperature is more than two times higher than for pure YMn2. Below similar to 100 K a second transition into a phase also tetragonally distorted and ma gnetically ordered is observed, but with c/a>1, The observed phase tra nsitions are interpreted as of martensitic type. In the border ranges for x<0.5 and x>0.8 a complex behaviour with several two phase ranges as a function of temperature is observed. For x<0.35 a spinodal decomp osition is detected in the temperature range 100-130 K. For x greater than or equal to 1.2 no structural distortion is visible when crossing the temperature of the magnetic transition. On the basis of the prese nted data a phase diagram is proposed. The observed phase transitions are discussed and interpreted in terms of Mn-Mn magnetic interactions in dependence of the lattice expansion caused by hydrogen, taking into account the possible ordering of the hydrogen atoms. (C) 1998 Elsevie r Science S.A.