New compounds of the ThCr2Si2-type and the electronic structure of CaM2Ge2 (M: Mn-Zn)

Two new compounds were synthesized by beating mixtures of the elements at 975-1025 K and characterized by single-crystal X-ray methods. CaZn2Si2.(a = 4.173(2) Angstrom, c = 10.576(5) Angstrom) and EuZn2Ge2 (a = 4.348(2) Angstrom, c = 10.589(9) Angstrom) crystallize in the ThCr2Si2-type structure (space group 14/mmm; Z = 2). Magnetic susceptibility measurements of EuZn2Ge2 show Curie-Weiss behavior with a magnetic moment of 7.85(5) mu(B)/Eu and a paramagnetic Curie temperature of 10(1) K. EuZn2Ge2 orders antiferromagnetically at T-N = 10.0(5) K and undergoes a meta-magnetic transition at a low critical field of about 0.3(2) T. The saturation magnetization at 2 K and 5.5 T is 6.60(5) mu(B)/Eu. Eu-151 Mossbauer spectroscopic experiments show one signal at 78 K at an isomer shift of -11.4(1) mm/s and a line width of 2.7(1) mm/s compatible with divalent europium. At 4.2 K full magnetic hyperfine field splitting with a field of 26.4(4) T is detected. The already known compounds CaM2Ge2 (M: Mn-Zn) also crystallize in the ThCr2Si2-type structure. Their MGe4 tetrahedra are strongly distorted with M=Ni and nearly undistorted with M = Mn or Zn. According to LMTO electronic band structure calculations, the distortion is driven by a charge transfer from M-Ge antibonding to bonding levels. (C) 2002 Elsevier Science (USA).