THERMAL-EXPANSION OF C-14-LAVES PHASE RMN(2) (R=Y,GD,DY AND YB)

The temperature dependence of lattice spacing of metastable C14-Laves phase RMn(2) (R=Y, Gd, Dy and Yb) has been measured up to 300 K by X-r ay diffraction. Large thermal expansion was observed especially for R= Y and Dy. GdMn2 shows a structural transition from hexagonal to orthor hombic below 80 K accompanied with a volume expansion. This indicates the appearance of a local magnetic moment of Mn 3d electrons with an a ntiferromagnetic order. The results are discussed together with the da ta on other stable C14-RMn(2) and metastable C14-TbMn2. It is shown th at the appearance of local magnetic moment of itinerant Mn 3d electron s of C14-RMn(2) at low temperatures is mainly determined by the Mn-Mn distance. C14-YMn2, TbMn2 and DyMn2 are the critical substances for th e onset of Mn moment.