Nontrivial Recurrent Intergrowth Structure and Unusual Magnetic Behavior of Intermetallic Compound Fe_32+δGe₃₃As₂

A new phase Fe_32+δGe₃₃As₂ (δ ≤ 0.136) was obtained by two-step synthesis from the elements. Fe_32+δGe₃₃As₂ crystallizes in its own structure type (space group P6/mmm, Z = 1, a = 11.919(3) Å, c = 7.558(4) Å) that can be described as a recurrent two-dimensional intergrowth of two intermetallic structure types, MgFe₆Ge₆ and Co₂Al₅. Their blocks are represented by infinite columns in the structure. No visible structural changes were observed in the temperature range from 10 to 300 K. At 125 K, Fe_32+δGe₃₃As₂ undergoes an antiferromagnetic-like transition, while above 150 K it shows a typical Curie-Weiss paramagnetic behavior. Below the transition temperature, a peculiar field-dependent magnetic susceptibility, that shows a significant increase of the susceptibility upon increasing the magnetic field, and a change in transport properties have been observed. Above 140 K, Fe_32+δGe₃₃As₂ reveals a metallic behavior, in agreement with electronic structure calculation, while below this point the resistivity nonmonotonically increases upon cooling. The Seebeck coefficient is positive, indicating that holes are the major charge carriers, and shows a broad maximum around 57 K.