Structural data on the Cs 4 (HSO 4 ) 3 (H 2 PO 4 ) single crystals have been obtained by neutron diffraction methods at the MOND experimental station of the National Research Centre “Kurchatov Institute.” The structural model of the crystals has been refined, hydrogen atoms have been localized with high accuracy, and the existence of hydrogen bonds of three types in the structure has been shown.

The defect structure of crystals of as-grown nonstoichiometric tysonite (LaF3 type) La1 − y Sr y F3 − y phase (y = 0, 0.05, 0.07, and 0.15) has been investigated by X-ray diffraction and neutron diffraction. A concentration-driven structural transition from the β-La1 − y Sr y F3 − y modification to the α-La1 − y Sr y F3 − y modification in the range y = 0.05–0.10 has been observed for the first time. This transition correlates with the maximum of fluorine-ion conductivity (y = 0.05 ± 0.02) and the maximum in the melting curve (y = 0.07 ± 0.03). β-LaF3 and β-La0.95Sr0.05F2.95 crystals belong to the trigonal system (sp. gr. \(P\bar 3c1\), Z = 6) and are twinned according to the merohedral twin law. Fine changes in the fluorine sublattice of β-La0.95Sr0.05F2.95 increase conductivity σ293K to 3 × 10−4 S/cm; hence, these crystals can be used as fluorine-conducting solid electrolytes (FCSEs) in solid-state ionics devices. Additional possibilities of ion transport over twin joints are discussed. In α-La0.85Sr0.15F2.85 crystal the symmetry group changes to P63/mmc, Z = 2; it is not twinned and its fluorine-ion conductivity is reduced.

The neutron irradiation of ferroelectrics is efficiently used to form structural states that cannot be obtained by conventional technologies. To date, the effect of neutron irradiation on the structure and properties of BaTiO 3 has been studied for only ceramic materials. We have considered the influence of fast-neutron irradiation ( F = 1 × 10 17 cm −2 ) on the structure and properties of BaTiO 3 single crystals for the first time. The structural changes occurring in irradiated BaTiO 3 and their correlation with the behavior of dielectric and nonlinear optical characteristics are analyzed with the aid of a specially developed method for taking into account the experimental correction to diffuse scattering. Neutron irradiation to the aforementioned dose retains the polar structure of the material and only slightly changes atomic displacements. The radiationinduced structural changes occur according to the high-temperature type to form a structure similar to the cubic modification of unirradiated BaTiO 3 crystal.