Crystal Structure of Magnetoelectric Ba₂MnGe₂O₇ at Room and Low Temperatures by Neutron Diffraction

For a symmetry-consistent theoretical description of the ferroelectric phase of Ba₂MnGe₂O₇ melilite compound, a precise knowledge of its crystal structure is a prerequisite. Here we report results of single-crystal neutron diffraction experiments on Ba₂MnGe₂O₇ at room (300 K) and low (10 K) temperatures. The structural model based on the tetragonal space group P4̅2₁ m describes the Ba₂MnGe₂O₇ symmetry both at room and low temperatures. We found reflections forbidden in the typical P4̅2₁ m melilite-type structure. A comparison of the experimental data collected by means of both thermal and cold neutrons with simulated multiple diffraction patterns allows us to unambiguously demonstrate that forbidden peaks originate from multiple diffraction (Renninger effect) rather than from real symmetry lowering. The precise structural parameters at 300 and 10 K are presented for the first time and compared with those of other magnetoelectric melilite-type germanates.