Origin of forbidden reflections in multiferroic Ba$_2$CoGe$_2$O$_7$ by neutron diffraction: Symmetry lowering or Renninger effect?

TL;DR

This study uses neutron diffraction to determine whether forbidden reflections in Ba$_2$CoGe$_2$O$_7$ are due to actual symmetry lowering or multiple diffraction effects, concluding the latter and confirming the tetragonal structure.

Contribution

The paper clarifies the origin of forbidden reflections in Ba$_2$CoGe$_2$O$_7$, demonstrating they result from multiple diffraction rather than symmetry lowering, refining the structural model at room temperature.

Findings

Forbidden reflections are due to multiple diffraction, not symmetry lowering.

The tetragonal space group $P\bar{4}2_1m$ accurately describes the structure.

Neutron diffraction confirms previous X-ray structural parameters.

Abstract

For a symmetry consistent theoretical description of the multiferroic phase of Ba$_2$CoGe$_2$O$_7$ a precise knowledge of its crystal structure is a prerequisite. In our previous synchrotron X-ray diffraction experiment on multiferroic Ba$_2$CoGe$_2$O$_7$ at room temperature we found forbidden reflections that favour the tetragonal-to-orthorhombic symmetry lowering of the titled compound. Here, we report the results of room-temperature single-crystal diffraction studies with both hot and cold neutrons to differentiate between the real symmetry lowering and multiple diffraction (the Renninger effect). A comparison of the experimental multiple diffraction patterns with simulated ones rules out the symmetry lowering. Thus, the structural model based on the tetragonal space group $P\bar{4}2_1m$ was selected to describe the Ba$_2$CoGe$_2$O$_7$ symmetry at room temperature. The precise structural parameters from neutron diffraction at 300K are presented and compared with the previous X-ray diffraction results.