Rare-earth manganese germanides RE2+MnGe2+ (RE=La, Ce) built from four-membered rings and stellae quadrangulae of Mn-centred tetrahedra

Abstract

Reactions of the elements through arc-melting and annealing at 800 °C have led to the ternary rare-earth manganese germanides RE2+xMnGe2+y (RE=La, Ce). The approximate composition is RE2.1MnGe2.2, as determined by single-crystal X-ray diffraction studies on the La member (space group P4/nmm, a=16.0491(4) Å, c=8.1587(2) Å, Z=16). Although this composition is close to other germanides RE2MGe2 (Sc2CoSi2-type), the structure is completely unrelated. Rather, La2.1MnGe2.2 contains layers built up of Mn-centred tetrahedra in two types of arrangements, four-membered rings and stellae quadrangulae, with La and additional Ge atoms in the intervening spaces. Disorder of La atoms, isolated Ge atoms, and Ge2 dimers takes place within tunnels in the structure. Electrical and magnetic measurements on La2.1MnGe2.2 indicate metallic behaviour and likely ferromagnetic ordering with a transition temperature above 300 K.