Agrellite, NaCa₂Si₄O₁₀F, is a tubular silicate mineral which crystal structure is characterized by extended [Si₈O₂₀]^8– tubes and has a two-dimensional channel system. The mineral is a representative of a complex silicate family which contains some structural voids but cannot be considered as microporous because of small channel widths. However, the channel system of such minerals is able to host single guest atoms, molecules or radicals which can affect their physical properties. Presently, the exact mechanism of such hosting is undetermined. However, such information could be quite useful for materials’ application as zeolites as well as for a better understanding of their formation mechanisms. In this work we couple X-ray diffraction, infrared (IR) spectroscopy and ab initio calculations to identify structural features in agrellite from Malyy Murun massif (Russia) caused by incorporation of either H₂O or OH⁻ into the channel system. We construct structural models of water-containing NaCa₂Si₄O₁₀F and identified H₂O positions. The derivation of H₂O sites is based on simulation of IR-spectra. Infrared spectroscopy in combination with the ab initio calculation has proven to be an effective tool for the identification of the structural positions of hydroxyl anions (OH⁻) and neutral water groups (H₂O) in minerals.

Solid solution perovskite Pb(Zr_1−xTi_x)O₃ (PZT) is an industrially important material. Despite the long history of experimental and theoretical studies, the structure of this material is still under intensive discussion. In this work, we have applied structure searching coupled with density functional theory methods to provide a multiphase description of this material at x = 0.4. We demonstrate that the permutational freedom of B-site cations leads to the stabilisation of a variety of local phases reflecting a relatively flat energy landscape of PZT. Using a set of predicted local phases we reproduce the experimental pair distribution function (PDF) profile with high accuracy. We introduce a complex multiphase picture of the structure of PZT and show that additional monoclinic and rhombohedral phases account for a better description of the experimental PDF profile. We propose that such a multiphase picture reflects the entropy reached in the sample during the preparation process.

The single crystals of CsI-Yb²⁺ were grown, and their spectroscopic studies were conducted. The observed luminescence in CsI-Yb²⁺ is due to 5d–4f transitions in Yb²⁺ ions. Using time-resolved spectroscopy, spin-allowed and spin-forbidden radiative transitions of ytterbium ions at room temperature were found. The excitation spectra of Yb²⁺ luminescence bands were obtained in the range of 3–45 eV. The mechanism of charge compensation of Yb²⁺ ions in a CsI crystal was also studied, the spectrum of the thermally stimulated depolarization current was measured, and the activation energies of the two observed peaks were calculated. These peaks belong to impurity–vacancy complexes in two different positions. The charge compensation of Yb²⁺ occurs via cation vacancies in the nearest-neighbor and next-nearest-neighbor positions.The Yb²⁺ ions are promising dopants for CsI scintillators and X-ray phosphors in combination with SiPM photodetectors.

The minerals of carletonite group, fluorcarletonite, KNa₄Ca₄[Si₈O₁₈](CO₃)₄(F,OH)·H₂O and carletonite, Na₄Ca₄[Si₈O₁₈](CO₃)₄(OH,F)·H₂O, were investigated using a multi-method approach. A detailed comparative chemical study of the minerals was carried out using electron probe microanalysis and Fourier transform infrared spectroscopy. Using X-ray techniques and the results obtained, geometrical and distortion characteristics of the mineral structures are calculated and the successful crystal-structure refinement of these two natural compounds are given. Using spectroscopic and luminescence methods and ab initio calculations, it is shown that hole defects (CO₃)^•– are responsible for the colouration of the samples studied. Luminescence due to 5d–4f transition in Ce³⁺ ions is observed in both investigated compounds. Moreover, luminescence attributed to intrinsic luminescence, corresponding to the decay of electronic excitations of (CO₃)^2– complexes in the carletonite sample, is registered for the first time in phyllosilicates. An analysis of the optical absorption spectra and g-tensor values suggests that (CO₃)^•– defects in the crystal structure are localised in the C1 positions. Identification of these specific properties for these sheet silicates, with a two-dimensional infinite tetrahedral polymerisation, indicates that carletonites could be prospective materials for novel phosphors and luminophores.

The article describes the results of a comprehensive study of the extra-framework components of scapolites using quantum–chemical calculations, electronic and vibrational spectroscopy, and single-crystal X-ray diffraction and crystal structure refinement. The ab initio calculations were performed using an embedded-cluster approach of extra-framework components in various cation surroundings. As a result, through comparing the experimental and ab initio calculation results, the energies of the electronic and vibrational transitions of various extra-framework components (CO₃)²⁻, (CO₃)^$·-$, ${\mathrm{S}}_3^{·-}$, ${\mathrm{S}}_2^{·-}$—as well as the role of these components in the process of the lowering of the symmetry—were determined for scapolites belonging to the marialite–meionite solid–solution series. The nature of the various colors of the scapolites has also been established. Colors from purple to blue are a result of the presence of radiation-induced pairs of defects: carbonate radical anions (CO₃)^$·-$ and F-centers. However, polysulfide ${\mathrm{S}}_3^{·-}$ radical anions are found in some violet scapolites.