High-entropy solid solutions xNa1/2Bi1/2TiO3 - (1-x)(Ba1/3Sr1/3Ca1/3)TiO3 (x = 0.2, 0.3, 0.4, 0.5, 0.6) were synthesized by the solid-phase method. The obtained solid solutions possess a cubic perovskite structure and show strong relaxor behaviour. The properties of the ceramics are characterized by the dielectric spectroscopy and Nyquist analysis.

Phase relations in the La2O3-Eu2O3-MoO3 ternary system were studied using polycrystalline samples obtained in air at 1250 degrees C via solid-state reaction of metal oxides. The system contains a broad region of a cubic fluorite-like phase of lanthanum-europium molybdates, named LEMO, which is isostructural with Nd5Mo3O16+delta. However, the homogeneity range of this phase is quite limited and occurs at a lower MoO3 content compared to neodymium molybdate. The citrate method was applied for the synthesis of LEMO ceramic samples with a relative density of 98 %. The crystal structure of La2.5Eu2.5Mo2.625O15.75 was analyzed by the Rietveld refinement using powder XRD data. The analysis revealed structural features of this europium-containing fluorite-like molybdate, specifically the splitting of oxygen atomic positions in the octahedral voids within the fluorite structure. The fluorite-like molybdate samples exhibited high electrical conductivity, with the grain bulk contribution at about 10-2 S/cm at 900 degrees C, and intense red luminescence under near-UV and blue radiation.

The solid-state and sol-gel synthesis of Nd5Mo3O16+δ neodymium molybdate has been investigated. The sequence of chemical transformations was studied using differential thermal analysis, X-ray diffraction analysis, and infrared spectroscopy. It has been demonstrated that the solid-state synthesis occurs stepwise through the formation of Nd2(MoO4)3 and phases with monoclinic structures. The using of the sol-gel method allows obtaining a single-phase neodymium molybdate directly after decomposition of the organic precursor and decreasing of the synthesis temperature by 200 K. The investigation of the crystal structure was carried out by high resolution neutron diffraction, which revealed non-uniform changes in the neodymium-oxygen interatomic distances with increasing temperature.

Samples in the La2MoO6 - Sm2MoO6 - MoO3 system were obtained using the solid-state synthesis method. The phase formation in the given compositional section was firstly studied, and the homogeneity region of the fluorite-like lanthanum-samarium molybdate was determined, as confirmed by XRD analysis including Rietveld refinement and SEM with energy-dispersive microanalysis. The fluorite-like phase contains less molybdenum than the isostructural neodymium molybdate Nd5Mo3O16+delta. The homogeneity region of fluorite-like solid solutions has been determined, which corresponds to the La5-xSmxMo2.75O15.75 section in the composition range 2.5 <= x <= 3.5. The crystal structure is characterized by the splitting of cation positions and the presence of excess oxygen in the octahedral voids of the structure. Oxygen atoms in the octahedral positions are displaced from the center of the voids and enter into the molybdenum coordination sphere. The density of single-phase ceramic samples increases with the increase in samarium content. The conductive properties of single-phase ceramics were investigated in an air atmosphere using impedance spectroscopy. The total conductivity was similar to 10(-2) S cm(-1) for La2.5Sm2.5Mo2.75O15.75 at 800 degrees C. The grain bulk conductivity exhibits a break at a temperature of approximately 600 degrees C, which may be explained by the depletion of charge carriers in the structure.

Polyoxotungstosilicates with the general formulas Cat4[SiW12O40] · mH2O and Cat6[SiW11O39Ni(H2O)] · nH2O were synthesized, where Cat = Rb+, Cs+, (CH3)4N+. By means of IR spectroscopy and x-ray diffraction analysis, it was shown that the compounds have the Keggin anion structure. The thermolysis of the obtained compounds within the temperature range of 600 – 800°C resulted in the formation of previously unknown pyrochlore-structure phases Rb12/13Si2/13W22/13Ni2/13O6 and Cs12/13Si2/13W22/13Ni2/13O6 with the with the unit cell parameters a of 10.284 and 10.309 Å, as well as phases with tungsten bronze structure Rb12/20Si3/20W36/20O6 and Si3/38W36/38O3. The synthesis of tungsten silicates with pyrochlore and tungsten bronze structure through thermolysis of polyoxotungstosilicates reduces the temperature of their preparation to 600 – 650°C and heating time to 1 h, thus extending the ranges of chemical compositions and their morphological diversity.

Novel lanthanum–samarium molybdates with a fluorite-like structure were synthesized from metal oxides using solidstate synthesis. The total conductivity of these phases reached 10–2 S cm–1 at 700°C.

Neodymium molybdate with a cubic fluorite-like structure is obtained by solid state reactions from metal oxides. The formation of the final product occurs through the formation at 700°C of a monoclinic Ln2MoO6-type structure (space group C2/c) probably containing vacancies in the neodymium and oxygen lattices. Neodymium molybdate obtained at 900°C crystallizes in the space group Pn $$\bar {3}$$ n with the cell parameter a ≈ 11.039 Å. The crystal structure of neodymium molybdate obtained at 700 and 900°C is studied by neutron diffraction and atomistic modeling using the GULP program in the pressure range 0–5.9 GPa, which demonstrates stability of the cubic structure at elevated pressures.

Samples of the Nd5−xTbxMo3O16+δ series were obtained by solid-state synthesis from metal oxides at 1050°C. The formation of solid solutions based on cubic and monoclinic phases and a two-phase region between them was observed in the Nd5−xTbxMo3O16+δ compositions. Increasing the terbium content in the system leads to a decrease in the unit cell parameters of the cubic and monoclinic phases within their homogeneity regions, which confirms the formation of solid solutions. It has been established that the terbium oxidation state in the Nd5Mo3O16+δ crystal structure is +3. The predominant placement of terbium atoms in the 8c position is observed by crystal structure refinement and confirmed by the results of the atomistic simulation. The introduction of terbium into the crystal structure of neodymium molybdate leads to a decrease in the Ln1–O1 and Ln2–O2 interatomic distances. The atomistic simulation was performed by the GULP program using the fit potential of the terbium ion. Terbium molybdate with Tb5Mo3O16+δ composition is a subtraction solid solution based on Tb2MoO6. Increasing the unit cell parameters of Tb5Mo3O16+δ monoclinic phase compared to Tb2MoO6 was confirmed by structure refinement and atomistic simulation.

Atomic ordering in many perovskites can be controlled by suitable synthesis conditions or annealing at elevated temperatures and is yet another phenomenon that can be used to manipulate their properties. In this work we have synthesized low impurity high quality medium-entropy ceramics of PbSc1/4In1/4Nb1/4Ta1/4O3 (PSINT), which can be formally represented as a solid solution of PbSc1/2Nb1/2O3 (PSN), PbSc1/2Ta1/2O3 (PST), PbIn1/2Nb1/2O3 (PIN), and PbIn1/2Ta1/2O3 (PIT) – the perovskites exhibiting atomic order–disorder phenomena. PSINT ceramics reveals pronounced dielectric maximum and a relaxor behavior. Estimation of the atomic order–disorder phase transition temperature by Monte-Carlo calculations shows that for PSINT it should fall within the same range as for PSN, PST, PIN, and PIT. Prolonged annealing of PSINT at the corresponding temperatures indeed results in clear changes in the dielectric behavior but does not lead to noticeable differences in the X-ray powder diffraction that could be interpreted as appearance of atomic ordering.

Neodymium molybdate with a cubic fluorite-like structure was obtained by solid state reactions from metal oxides. The formation of the final product occurs through the formation of a monoclinic structure of Ln2MoO6 type (space group C2/c) at 700°C, which probably contains vacancies in neodymium and oxygen lattices. Neodymium molybdate obtained at 900°C crystallizes in the space group Pn\(\bar {3}\)n with the cell parameter a ≈ 11.039 Å. The crystal structure of neodymium molybdate obtained at 700 and 900°C was studied by neutron diffraction and atomistic modeling using the GULP program in the pressure range 0–5.9 GPa, which demonstrated the stability of the cubic structure at elevated pressure.

The polycrystalline samples of the x/6Pr6O11 – MoO3 (2 ≤ x ≤ 6) series were prepared by conventional solid-state route. The phase formation was studied by XRD analysis in the temperature range of 1000 – 1200°C. After calcination at 1100°С, the formation of praseodymium molybdate with Pr4MoO9+δ composition is observed, which decomposes into a mixture of Pr2MoO6 and Pr6MoO12+δ phases at 1200°С. It was first established that Pr4MoO9+δ praseodymium molybdate has a monoclinic lattice with parameters a = 16.7211(9) Å, b = 11.9894(7) Å, c = 9.5588(5) Å, β = 109.6149(7) °, sp. gr. C 2/m. Crystal structure was refined by high resolution neutron diffraction data. The homogeneity regions of Pr4MoO9+δ and Pr6MoO12+δ were determined to be 4 < x < 4.2 and 4.57 < x < 6, respectively. It was shown that the conductivity of this molybdates increases with praseodymium oxide content in the Pr5Mo3O16+δ – Pr4MoO9+δ – Pr6MoO12+δ series.

Atomic ordering in many perovskites can be controlled by suitable synthesis conditions or annealing at elevated temperatures and is yet another phenomenon that can be used to manipulate their properties. In this work we have synthesized low impurity high quality medium-entropy ceramics of PbSc1/4In1/4Nb1/4Ta1/4O3 (PSINT), which can be formally represented as a solid solution of PbSc1/2Nb1/2O3 (PSN), PbSc1/2Ta1/2O3 (PST), PbIn1/2Nb1/2O3 (PIN), and PbIn1/2Ta1/2O3 (PIT) - the perovskites exhibiting atomic order-disorder phenomena. PSINT ceramics reveals pronounced dielectric maximum and a relaxor behavior. Estimation of the atomic order-disorder phase transition temperature by Monte-Carlo calculations shows that for PSINT it should fall within the same range as for PSN, PST, PIN, and PIT. Prolonged annealing of PSINT at the corresponding temperatures indeed results in clear changes in the dielectric behavior but does not lead to noticeable differences in the X-ray powder diffraction that could be interpreted as appearance of atomic ordering.

— Isomorphous bismuth substitution for neodymium in the Nd 5 Mo 3 O 16 + δ compound has been studied by X-ray diffraction. The crystal structure of a single-phase solid solution with the composition Nd 4.95 Bi 0.05 Mo 3 O 16 + δ has been refined by the Rietveld method, and the electrical conductivity of the modified molybdate has been measured in the temperature range 300–700°C.

Tungstophosphatozincates with the Keggin anion structure Kt5[PW11O39Zn(H2O)]∙nH2O, Kt = Rb+, Cs+, (CH3)4N+; (C2H5)4N+ were synthesized. Their thermolysis process was studied by differential scanning calorimetry, thermogravimetry, infrared spectroscopy, X-ray diffraction analysis and electron microscopy. The products of their thermal decomposition, phases with the pyrochlore structure and tungsten bronzes, were identified.

The article deals with the synthesis, study of thermal decomposition and identification of the thermolysis products of cesium tungstophosphates that are promising compounds in the field of materials science, catalysis and other fields of science and technology. Compounds with the Keggin anion structure are synthesized from aqueous solutions: Cs3[PW12O40] ∙ 9H2O; Cs5Na2[PW11O39(H2O)] ∙ 5H2O and Cs5[PW11O39Ni0,5Cu0,5(H2O)] ∙ 4H2O. The processes of their thermal decomposition are investigated and some regularities of their thermolysis are established. Thermolysis products are identified: Cs3PW12O40, phases with the structure of pyrochlore and hexagonal tungsten bronze of the composition Cs10/13Na4/13P2/13W22/13O6 and Cs10/13P2/13Ni1/13Cu1/13W22/13O6. The unit cell parameters of the phase with the pyrochlore structure are determined. Research results confirm that phosphorus, nickel and copper ions are included in the structure of pyrochlore and hexagonal tungsten bronze. Phases similar to this chemical composition are not previously known in the literature. The studied tungstophosphates and their thermolysis products are promising compounds for obtaining heterogeneous catalysts for the oxidation of organic compounds and selective sorbents. The research results can be useful for predicting the thermal properties and phase composition of thermolysis products of similar polyoxometallates in order to obtain new compounds with the structure of pyrochlore and hexagonal tungsten bronze, as well as composite materials based on them.

This paper reports the synthesis of compounds with the pyrochlore and hexagonal tungsten bronze structures via thermal decomposition of heteropolyoxometalates. Using aqueous solutions, we have synthesized tungstophosphatometalates with the Keggin structure and the general formula Ct5[PW11O39(H2O)Z]⋅nH2O, where Ct = Rb+ or Cs+ and Z = Co2+, Ni2+, or Cu2+. We have studied the thermal decomposition of these compounds and identified their thermolysis products: phases with the pyrochlore and hexagonal tungsten bronze structures. Our results confirm that phosphorus, cobalt, nickel, and copper ions become incorporated into the pyrochlore and hexagonal tungsten bronze structures of the CtnxPxZxW2–2xO6 compounds. No phases with similar chemical compositions have been reported previously. Their synthesis temperature has been lowered by 200°C and the calcination time has been reduced by a factor of 2 in comparison with conventional synthesis methods. The proposed schemes of thermolysis of rubidium and cesium tungstophosphatometalates will be useful for predicting the thermal properties and phase composition of thermolysis products of analogous heteropolyoxometalates in designing new inorganic materials based on them.

This paper presents the investigation of the heterovalent substitution of cadmium for lanthanum in the La2-xCdxMoO6-x/2 system. The samples were synthesized by the solid state reaction method at 1000°C. The samples were characterized by X-ray powder diffraction with Rietveld refinements, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy methods. The study results revealed that cadmium incorporation in the lanthanum molybdate leads to the transformation of the tetragonal structure of La2MoO6 to a cubic fluorite-like one. The content of the cubic phase reaches 94% in the Lа1.4Cd0.6MoO5.7 sample. The unit cell parameter of fluorite-like-phase decreases with cadmium content rising. The preferred location of cadmium ions in the cubic structure was established by the Rietveld refinement method. The heterovalent substitution cadmium for lanthanide in tetragonal La2MoO6 molybdate leads to the cubic fluorite phase stabilization in a similar way as it occurs in the process of reduction.

The purpose of the work is influence investigation of modifying Nd5Mo3O16+δ oxygen-conducting fluorite-related compound by lead at the crystal structure and conductivity. The substitution of lead for neodymium was studied by XRD (with structure refinement), scanning electron microscopy, FTIR-spectroscopy and conductivity measurements. The compositions Nd5-xPbxMo3O16+δ (x = 0 – 1.6) were obtained by a solid state reaction from the oxides. It was determined that single-phase solid solution Nd5-xPbxMo3O16+δ is formed up to x ≈ 0.82. The Rietveld structure refinement shows that lead is statistically located in the Ln1 and Ln2 positions. The introduction of lead does not significantly affect the nature and values of conductivity.