Crystal Structures and Properties of Eu₂GeSe₄ and Eu₂Ge₂Se₅

Abstract

Europium selenogermanate(IV) and Europium selenogermanate(III) were synthesized by direct reactions of the elements at 1023 K and their crystal structures determined by single crystal methods. α-Eu 2 GeSe 4 (P21, a = 6.964(1), b = 7.055(2), c = 8.400(2) Å , β = 108.12(2)˚, Z = 2) crystallizes as a polar variant of the monoclinic Sr 2 GeS 4 -type at room temperature. At 673 K, the structure is centrosymmetric (β-Eu 2 GeSe 4 , P2 1 /m, a = 6.969(1), b = 7.059(2), c = 8.516(2)Å , β = 107.99(2)˚, Z = 2), and a phase transition P2 1 →P2 1 /m as known from Eu 2 GeS 4 is highly probable. Eu 2 Ge 2 Se 5 (P2 1 /n, a = 8.421(4), b = 12.235(4), c = 9.127(3) Å,β = 93.67(4)˚, Z = 4) crystallizes in the Sr 2 Ge 2 Se 5 type, mainly characterized by complex [Ge 4 Se 10 ] 8- anions with homonuclear Ge-Ge bonds. Both compounds are deep red Zintl phases according to (Eu 2+ ) 2 Ge 4+ (Se 2- ) 4 and (Eu 2+ ) 2 (Ge 3+ ) 2 (Se 2- ) 5 . Magnetic susceptibility measurements show paramagnetic behavior above 20Kwith magnetic moments of 8.00(5) μB/Eu and 8.10(5) μB/Eu, respectively, indicating Eu 2+ . Eu 2 Ge 2 Se 5 orders antiferromagnetically at T N = 4.2(2) K and undergoes a metamagnetic transition at 2 K at a critical field of 0.5(3) T. The saturation moment at 2.2 K and 5.5 T is 7.00(2) μB/Eu. Eu 2 GeSe 4 is a ferromagnet with T C = 5.8(2) K and a saturation moment at 2.2 K and 5.5 T of 6.92(2) μB/Eu. 151 Eu Mössbauer spectroscopic measurements of Eu 2 GeSe 4 and Eu 2 Ge 2 Se 5 at 78 K show isomer shifts of -12.43(4) mm/s and -12.69(5) mm/s, respectively, in accordance with divalent europium.