Synthesis, structure and magnetic properties of Ba₃M₂Ge₄O₁₄ (M = Mn and Fe): Quasi-one-dimensional zigzag chain compounds

A series of trivalent transition metal germanates with a general formula of A₃M₂Ge₄O₁₄ (A ​= ​Sr, Ba, M ​= ​Mn and Fe) was synthesized using a high-temperature (580 ​°C) hydrothermal technique. Compound I, Ba₃Mn₂Ge₄O₁₄, is a new structure type that is characterized by one-dimensional Mn–O–Mn chains made from edge-sharing highly distorted Mn³⁺O₆ units, and exhibits antiferromagnetic behavior below 15 ​K with field induced metamagnetic transitions. The A₃Fe₂Ge₄O₁₄ (A ​= ​Sr and Ba, II and III, respectively) series belongs to the well-known langasite structure type, crystallizing in space group P321. In A₃Fe₂Ge₄O₁₄, there is one octahedral Fe³⁺ site and one mixed-occupancy Fe³⁺/Ge⁴⁺ site that form the one-dimensional substructure. The magnetic susceptibility data of Ba₃Fe₂Ge₄O₁₄ reveals a much simpler magnetic structure with an antiferromagnetic transition near 5.5 ​K. The Jahn-Teller distortion of Mn³⁺ appears to be influential in the phase formation, leading to significantly different structures for these Mn³⁺ and Fe³⁺ compounds with similar chemical formulas.