Valency changes of ytterbium in YbMnGe and in the YbMnSi1-xGex pseudo-ternary system

X-Ray diffraction on powders and single crystals has been used to study some phases in the YbMnSi1-xGex system. YbMnGe is dimorphic: the high-temperature form, stable above 1423 K, crystallizes in the hexagonal ZrNiAl-type structure, while the low-temperature form, stable below 1073 K, is orthorhombic with TiNiSi-type. The structural transition is accompanied, and probably caused, by a valence change of the ytterbium atoms, from trivalency (in the HT form) to divalency (in the LT form). The corresponding large volume increase is the cause of the observed explosive nature of the transition. The same valence transition, even if at lower temperatures, occurs also in some phases on the germanium side of the YbMnSi1-xGex system (for x=0.8, 0.9, 0.95, 0.975), but both HT and LT forms are TiNiSi-type, showing sharply different cell volumes. No transition was observed in YbMnSi and YbMnSi0.5Ge0.5, which crystallize in the TiNiSi structure and probably contain trivalent ytterbium atoms. © 2002 Elsevier Science B.V. All rights reserved.