Accurate crystal-structure refinement of Ca3Ga2Ge4O14 at 295 and 100 K and analysis of the disorder in the atomic positions

X-ray natural circular dichroism in langasite crystal

“…4(b) that spectral shapes are different reflecting the dependence on the local environment of the absorbing atom. These results could be correlated with an interesting explanation of the chiral properties of Nd 3 Ga 5 SiO 14 (langasite family) that was given by Dudka & Mill (2013. The electron density fragments forming helixes along the c-axis together with splitting of atomic environments due to statistical occupation of the 2d position were suggested as a structural basis for the chirality of langasites.…”

Growth and X-ray Diffraction Study and Specific Features of Thermal Expansion of Ba3NbFe3Si2O14 Single Crystal from the Langasite Family

“…When lowering the temperature from 293 to 90.5 K, the compression coefficient α along the c axis approximately three times exceeds the compression along the a axis (Table ). At the same time, the compression ratio along the a axis in a number of other langasites is, conversely, about two-three times higher than the compression ratio along the c -axis (see, for example, refs and ). A high mobility of atoms correlates with strong piezoelectric properties of these crystals along the a axis …”

“…We conducted repeated experiments on the growth of Ba 3 NbFe 3 Si 2 O 14 crystals and prepared tens of ball-samples on which several additional X-ray diffraction experiments were performed (including many-days measurements). However, in general, the structural results for BNFS and BTFS crystals are slightly worse than for langasite crystals grown by the Czochralski method. ,, Consequently, difficulties of obtaining high-quality Ba 3 NbFe 3 Si 2 O 14 single crystals noted previously in ref are objective.…”

“…When refining the standard model of spherical atoms in the harmonic approximation, peaks with amplitudes as large as 1.1 e/Å 3 were found near atoms in the difference electron-density map. To describe this disordering of atomic sites in the BNFS structure, the anharmonic atomic displacement parameters (ADPs) were used. , The optimal anharmonic model was sought by using the “anharmonic displacement expert”, the technique of its application is described in refs and . The atomic displacements in the 3 e , 1 a , 3 f , and also O2­(6 g ) and O3­(6 g ) sites at 293 K were characterized by anharmonic tensors (model 3342234, labeling from).…”

“…Interestingly, the atoms in 3 e and 3 f sites prove to be mobile without regard to cation sites in which the replacement takes place. Such a movement of ions with a formation of electrical dipoles is the structural basis of piezoelectric properties in crystals of the langasite family. , However, the ions in 2 d sites become also to be mobile with decreasing temperature, though the crystals can be both with magnetic ions and without them. , …”