A brief review focuses on studies into the structural reasons for the nonlinear optical properties of crystals of the potassium titanyl phosphate family, performed at the Shubnikov Institute of Crystallography. Accurate X-ray diffraction data are discussed, providing evidence that the optical susceptibility of crystals is related not only to the alternation of long and short Ti–O bonds in the chains of TiO₆ octahedra, but to the geometry of tetrahedral anions and the alkaline cation arrangement in the structure channels, as well. The contribution of each of the three structural components depends on the crystal composition.

The structure of the cubic metastable β_ms phase of La₂Mo₂O₉ single crystal has been precisely investigated by X-ray diffraction at 33 K for the first time. The measurement of the unit-cell parameter of this crystal in the range from room temperature to 33 K showed that the unit-cell parameter and volume change continuously in this range. The crystal has a similar structure at T = 33 K and at room temperature. A local lowering of the symmetry for La and Mo atoms, caused by their displacement, is confirmed, and a similar displacement (which was not observed at room temperature) is revealed for O(1) atoms. The thermal parameters for O(2) and O(3) atoms do not change with a decrease in temperature, in contrast to the thermal parameters of Mo, La, and O(1) atoms. This fact indicates that the O(2) and O3 atoms in this crystal are statically disordered.

The multiferroic RFe₃(BO₃)₄ family is characterized by diverse magnetic, magnetoelectric, and magnetoelastic properties, the fundamental aspects of which are essential for modern electronics. The present research, using single-crystal X-ray diffraction (XRD) and Mössbauer spectroscopy (MS) in the temperature range of 25–500 K, aimed to analyze the influence of local atomic coordination on magnetoelectric properties and exchange and super-exchange interactions in RFe₃(BO₃)₄. Low-temperature, single-crystal XRD data of the magnetically ordered phase of RFe₃(BO₃)₄ at 25 K, which were obtained for the first time, were supplemented with data obtained at higher temperatures, making it possible to draw conclusions about the mechanism of the structural dynamics. It was shown that, in structures with R = Gd, Ho, and Y (low-temperature space group P3₁21), a shift in oxygen atoms (O2, second coordination sphere of R atoms) was accompanied by rotation of the B2O₃ triangle toward R atoms at low temperatures, and by different rearrangements in iron chains of two types, in contrast to Nd and Sm iron borates (space group R32). These rearrangements in the structures of space group P3₁21 affected the exchange and super-exchange paths at low temperatures. The MS results confirm the influence of the distant environment of atoms on the magnetoelectric properties of rare-earth iron borates at low temperatures.