Vol 69, No 9 (2024)

The method of modification of accordion-like complex composition Ti_0.2V_1.8CT_x MXene with tin(IV) and vanadium oxides by hydrothermal synthesis of SnO₂ in water-alcoholic medium in the presence of dispersed particles of two-dimensional vanadium-titanium carbide was developed. The Ti_0.2V_1.8CT_x-10mol.% SnO₂ composition coating was carried out by microplotter printing on a specialized substrate followed by heat treatment in air at 300°С for 1h. For the formed layer Ti_0.2V_1.8CT_x–V₂O₅–SnO₂ nanocomposite chemosensor properties were comprehensively studied for a number of analyte gases: 100 ppm CO, NH₃, NO₂, benzene, acetone, ethanol, 1000 ppm H₂, methane and 10% oxygen. Its high sensitivity and selectivity at operating temperatures of 150 and 200°С to nitrogen dioxide were shown: the responses to 100 ppm NO₂ were 281 and 873%, respectively.

Amorphous silicon dioxide samples were obtained from silicon of natural isotopic composition, highly enriched silicon–28, –29, –30 with enrichment degrees from 99.29 ± 0.01 to 99.9980 ± 0.0010 at. %, and a mixture of silicon–28 and silicon–30. The samples were characterized by IR spectroscopy. The IR spectra of the samples show a shift of the known absorption bands of valence and strain vibrations for silicon–29 and Si–30 isotopes by 0.1–30.1 cm^–1 to the low–frequency region concerning silicon–28. In silicon dioxide, a mixture of silicon–28, –30 isotopes, and natural isotopic composition, a deviation from the linear dependence of the frequency of vibration of monoisotopic patterns is observed. The positions of absorption band maxima in the model IR spectrum of silicon dioxide were calculated by the DFT method. The obtained results correlate with the experimental data with high accuracy. The values of the reduced mass and force constant of vibrating groups of atoms in silicon dioxide are determined, and the influence of these factors on the isotopic shift in the IR spectrum of silicon dioxide depending on the type of vibrations is established.

Ceramic samples of multi-component fluorides Na₃CaMg₃AlF₁₄ and NaCaMg₂F₇ doped with europium ions have been obtained by solid-phase synthesis. A comparison of the XRD patterns for the obtained compounds confirmed that a new polymorphic modification with a cubic pyrochlore structure has been obtained for Na₃CaMg₃AlF₁₄ fluoride, which differs from the polymorph of this compound with a rhombohedral structure described in the literature. The addition of NH₄(HF₂) before the last annealing provided reducing conditions for the stabilization of europium ions exclusively in the 2+ charge state. Eu²⁺ ions in the synthesized fluoride matrices emit luminescence with a band peak at about 395 nm due to 4f ⁶5d–4f ⁷ interconfiguration transitions. In these matrices the Eu²⁺ ions form predominantly one type of optical centers, which provides a fairly narrow band-width of 4f ⁶5d–4f ⁷ luminescence ~30 nm. Also, in the synthesized ceramics Eu²⁺ ions emit narrow-line luminescence with a dominant line at ~362 nm, associated with 4f⁷–4f⁷ intraconfiguration transitions from the lowest excited state ⁶P_7/2 to the ground state ⁸S_7/2. The synthesized ceramics demonstrate good temperature stability of Eu²⁺ 4f ⁶5d–4f ⁷ luminescence with thermal quenching temperature T_1/2 equal to 504 and 543 K for Na₃CaMg₃AlF₁₄:Eu²⁺ (1.0 at. %) and NaCaMg₂F₇:Eu²⁺ (0.5 at. %), respectively. This property may be of interest for the practical application of these phosphors. Additional annealing of ceramics in an argon atmosphere with the addition of NaHF₂ instead of NH₄(HF₂) led to a partial conversion of europium ions from a divalent to a trivalent state. As a result, a series of narrow luminescence lines appears in the red region of the spectrum due to intraconfiguration transitions 4f ⁶–4f ⁶ (⁵D₀ → ⁷F_J) in Eu³⁺ ions.

Solid-phase synthesis of CaYb₂Ge₄O₁₂ and CaLu₂Ge₄O₁₂ was carried out from the initial oxides of CaO, Yb₂O₃ (Lu₂O₃) and GeO₂ by firing in air at temperatures of 1223–1423 K. The crystal structure of the synthesized germanates was determined by X-ray diffraction. The high-temperature heat capacity in the temperature range of 320-1050 K was measured by differential scanning calorimetry. It has been established that the obtained data on heat capacity are well described by the Mayr-Kelly equation:

C_p(CaYb₂Ge₄O₁₂) = $(416.4\pm 0.40)+(72.67\pm 2.30)\times {10}^{-3}T-(50.13\pm 0.19)\times {10}^5{Т}^{-2}$,

C_p(CaLu₂Ge₄O₁₂) = $(450.0\pm 1.75)+(15.46\pm 1.90)\times {10}^{-3}Т-(78.67\pm 1.60)\times {10}^5{Т}^{-2}$.

Based on these results, the main thermodynamic properties of oxide compounds were calculated.

Orthogermanate Ca₃Y₂Ge₃O₁₂ has been prepared by solid-phase method from CaCO₃, Y₂O₃ and GeO₂ by firing in air at a temperature of 1773 K. Using X-ray diffraction, its crystal structure was clarified (sp. gr. Ia$\overline{3}$d, a =12.80255(14) Å, V = 2098.34(7) ų). The high-temperature heat capacity of oxide compound has been determined in the temperature range 320–1000 K by differential scanning calorimetry and the experimental data have been used to evaluate thermodynamic properties of Ca₃Y₂Ge₃O₁₂.

The compound Rb₃(SbF₃)(Zr₂F₁₁)·0.5H₂O has been synthesized, its structure and the structure of its dehydration product have been studied. The Rb₃(SbF₃)(Zr₂F₁₁)·0.5H₂O compound has a unique fluoridozirconate framework structure built from circular tetrameric fragments (Zr₄F₂₄) linked to each other at the vertices by bridging fluorine atoms. Inside the tetrameric fragments, the ZrF₈-polyhedra are linked to each other along common edges. The zirconium framework $\underset{\infty }{\overset{3}{}}$[Zr₂F₁₁]^3– is linked by fluoride bridges Zr–F–Sb from the second coordination sphere of antimony atoms. The structure of the dehydrated compound Rb₃(SbF₃)(Zr₂F₁₁) is a three-dimensional framework consisting of infinite chains of the $\underset{\infty }{\overset{1}{}}$[Zr₂F₁₁]^3– composition of ZrF₇-polyhedra bonded along the edges and vertices. The chains are linked to each other into a 3D framework by Zr–F–Sb bridges from the second coordination sphere of antimony atoms. In the temperature range 247–256 K, the compound Rb₃(SbF₃)(Zr₂F₁₁) undergoes a reversible phase transition from partially disordered high-temperature α-phase to ordered low-temperature β-phase. Taking into account fluorine atoms of the second coordination sphere and the lone electron pair coordination polyhedra are formed in structures around the antimony atoms in the form of distorted octahedra (SbEF₅) and capped octahedra (SbEF₆).

Using the relativistic linearized augmented cylindrical waves technique, the dependences of the band structure of single-walled SiC nanotubes on spin and chirality were calculated. It has been established that nanotubes are the wide-gap semiconductors with E_g equal to 2.26–3.15 eV, and the spin-orbit splittings of the valence and conduction band edges lie in the range of 0.05–3.5 meV. The energies of the spin-orbit gaps in righthanded and lefthanded enantiomers coincide, but their spin directions are opposite. Chiral nanotubes are determined that are most suitable for selective spin transport with potentially high fluxes of α- and β-electrons in opposite directions.

The temperature stages of the crystallization process of thulium titanate of pyrochlore structural type during heating of the hydroxide precursor obtained by the reverse precipitation method have been studied by DSC/TG, X-ray phase analysis and electron microscopy. The molar heat capacity of Tm₂Ti₂O₇ was measured in the temperature range 2–1870 K and on the basis of smoothed heat capacity the calculation of thermodynamic functions at 0–1900 K and the Gibbs energy of formation from oxides and from elements were performed. The contribution to the heat capacity of the Schottky anomaly at 20–320 K is highlighted.

The process of modification of Ti₂CT_x MXene multilayer by hydrothermal synthesis of bulk hierarchically organized formations of Co(CO₃)_0.5(OH)⋅0.11H₂O has been studied. It is shown that under the chosen conditions MXene is partially oxidized with the formation of aggregates of titanium dioxide nanoparticles with a diameter of ~3–10 nm on its surface. The sensing properties of the obtained composite material at room temperature and relative humidity 65±3% to a wide range of gaseous analytes (50 ppm CO, benzene, acetone, ethanol, 2500 ppm H₂, CH₄, 5% O₂ and 40 ppm NH₃, NO₂) were investigated. Increased sensitivity was found for the detection of 40 ppm NH₃ and NO₂: the responses were -91 and -63%, respectively. Some aspects of the detection mechanism are discussed. The results obtained show promising modification of multilayer MXene with semiconducting metal oxides and hierarchically formed bulk formations in order to improve its chemoresistive properties.