Symmetry and structure of multiferroic Ba2CoGe2O7

Crystal structure of Ba2CoGe2O7 at room temperature (RT) and 90 K has been probed by single-crystal diffraction of x-ray synchrotron radiation. The space group (SG) found at both temperatures is well approximated by P-42(1)m, in agreement with expectation for melilite-like compounds. The real structure of Ba2CoGe2O7 is more distorted and has a lower symmetry, as follows from observation of a set of superstructure reflections violating 2(1) symmetry. Symmetry analysis based on the observed average-structure SG P-42(1)m revealed few possible candidates for the true structure that imposes different constraints on magnetic and polar properties.