The pyrochlore solid solution region in the Bi2O3–CoO–Sb2O5 system is determined. A previously unknown ternary oxide Bi3Co2/3Sb7/3O11 with cubic KSbO3 structure (sp. gr. Pn-3, a = 9.5801(1) Å, wR =0.0132) is found. An isothermal section of the system is constructed in the region of CoO-Bi3SbO7-CoSb2O6-BiSbO4 at 650°C. It is shown that cobalt state in pyrochlore crystal lattice is Co2+. For pyrochlore composition Bi1.5CoSb1.5O7, the synthetic methods under hydrothermal conditions of both without microwave-assisted treatment and with it have been developed. Based on the data of X-ray diffraction analysis, local energy-dispersive X-ray analysis, scanning microscopy and IR spectroscopy, a mechanism for the pyrochlore phase formation under hydrothermal condition is proposed. The dispersed Bi1.5CoSb1.5O7 samples (SCD = 30 nm) are synthesized and the prospects of their use as catalysts for CO oxidation are shown.

The existence regions of the Y2–xMgxFeTaO7–δ (x = 0–0.15), Y2–xMgxFe1–x/2Ta1+x/2O7–δ (x = 0–0.15), Y2Fe1–xMgxTaO7–δ (x = 0–0.3), and Y2Fe1–3/2xMgxTa1+x/2O7–δ (x = 0–0.3) solid solutions were estimated. It was found that, regardless of the composition, the entry of Mg2+ into the crystal lattice of Y2FeTaO7 causes a similar distortion of the structure (space group R 3̅ → space group P3121). Based on XANES and Mössbauer spectroscopy data, it was suggested that Mg2+ ions occupied the eight-coordinated sites in the crystal lattice of solid solutions, displacing iron ions, while vacant yttrium sites in Y2–xMgxFeTaO7–δ and Y2–xMgxFe1–x/2Ta1+x/2O7–δ are occupied by tantalum ions. The XANES method confirmed the existence of Fe4+ ions in the Y2FeTaO7 solid solutions along with Fe3+. The simultaneous presence of the Ta4+ and Ta5+ ions ensures the electroneutrality of there crystal lattices. Herewith, the entry of Mg2+ does not lead to an increase in the Fe4+ concentration.

The boundaries of the existence of a solid solution with a pyrochlore structure in the Bi2O3–CoO–Sb2O5 system were found. A previously unknown ternary oxide Bi3Co2/3Sb7/3O11 was discovered, which belongs to the cubic KSbO3 structure type (space group Pn 3̅ , a = 9.5801(1) Å, wR = 0.0132). The isothermal cross section of the system in the region CoO–Bi3SbO7–CoSb2O6–BiSbO4 at 700°C was triangulated. It was shown that cobalt in the pyrochlore crystal lattice exists in the +2 oxidation state. By the example of the compound Bi1.5CoSb1.5O7, a method was developed for the hydrothermal synthesis of pyrochlore both without and with microwave irradiation. Based on the data of X-ray powder diffraction analysis, local X-ray spectral microanalysis, scanning microscopy, and IR spectroscopy, a mechanism was proposed for the formation of the pyrochlore phase under hydrothermal conditions. Fine powders samples of Bi1.5CoSb1.5O7 (coherent scattering region size 30 nm) were synthesized, and the prospects of their use as catalysts for CO oxidation were shown.

A series of solid solutions with compositions of La1-xTmxGa0.5Sb1.5O6 (x = 0.03, 0.05, 0.1) and La1-x-yzTmxTbyDyzGa0.5Sb1.5O6 (x = 0.04, 0.07, 0.1; y = 0.04, 0.07, 0.1; z = 0.04, 0.04, 0.06) has been synthesized. It is shown that all the compounds have rosiaite structure (PbSb2O6, sp.gr. P-31m). Morphology, spectral-luminescent characteristics and color coordinates of the samples were studied. It was found that luminescence spectrum of thulium-activated samples recorded upon 351 nm excitation is typical for Tm3+ ions. At the same time, under excitation by radiation with lambda = 457 nm and a power density of J = 4.7 kW/cm2, broadband thermal emission is observed, against which the bands corresponding to the 1G4 -> 3H6, 1G4 -> 3F4, 3F4 -> 3H6 & icy; 3H4 -> 3H6 transitions of Tm3+ are detected. The luminescence spectra of the samples co-activated with Tm3+, Tb3+ and Dy3+are characterized by a relatively low intensity of the Tm3+ emission, which may be due to efficient energy transfer processes between Tb3+ and Dy3+ ions. A shift from the blue to the green color regions is observed in the color coordinates of the co-activated samples upon 351 nm excitation. Radiation with lambda = 457 nm led to a shift in the emission colour to orange. Thus, it is shown that it is possible to create phosphors emitting in the blue, green and orange spectral regions by changing the wavelength of exciting radiation and activator composition for compounds with rosiaite structure.

Mg-containing solid solutions based on Y 2 FeTaO 7 and formed by various mechanisms of heterovalent substitution were synthesized and had the following compositions: Y 2 Fe 0.55 Mg 0.3 Ta 1.15 O 7 , Y 2 Fe 0.625 Mg 0.3 Ta 1.075 O 7 , Y 2 Fe 0.7 Mg 0.3 TaO 7 , Y 2 Fe 0.7 Mg 0.2 Ta 1.1 O 7 , Y 2 Fe 0.85 Mg 0.15 TaO 7 , Y 1.85 Mg 0.15 Fe 0.925 Ta 1.075 O 7 , and Y 1.85 Mg 0.15 FeTaO 7 . It was shown that all synthesized solid solutions have a pyrochlore-like layered structure (space group P 3 1 21), in which Fe 3+ ions are distributed over three structural positions. The magnetic properties of these solid solutions are due to the presence of a small ferromagnetic component in a predominantly antiferromagnetic system and characterize a ferrimagnet or a canted antiferromagnet with the Néel transition at the Néel temperature T N above room temperature. According to the data of magnetic measurements, two magnetic phase transitions to the ordered phase occur in all the studied samples. Along with the T N transition, in weak magnetic fields and below T N , there is a second transition, which is most likely due to a spin reorientation of the Morin type. The existence of magnetic ordering at room temperature in one magnetic sublattice or an internal magnetic field ( H in ) was confirmed by Mössbauer spectroscopy.

Mg-containing solid solutions based on Y2FeTaO7 and formed by various mechanisms of heterovalent substitution were synthesized and had the following compositions: Y2Fe0.55Mg0.3Ta1.15O7, Y2Fe0.625Mg0.3Ta1.075O7, Y2Fe0.7Mg0.3TaO7, Y2Fe0.7Mg0.2Ta1.1O7, Y2Fe0.85Mg0.15TaO7, Y1.85Mg0.15Fe0.925Ta1.075O7, and Y1.85Mg0.15FeTaO7. It was shown that all synthesized solid solutions have a pyrochlore-like layered structure (space group P3121), in which Fe3+ ions are distributed over three structural positions. The magnetic properties of these solid solutions are due to the presence of a small ferromagnetic component in a predominantly antiferromagnetic system and characterize a ferrimagnet or a canted antiferromagnet with the Néel transition at the Néel temperature TN above room temperature. According to the data of magnetic measurements, two magnetic phase transitions to the ordered phase occur in all the studied samples. Along with the TN transition, in weak magnetic fields and below TN, there is a second transition, which is most likely due to a spin reorientation of the Morin type. The existence of magnetic ordering at room temperature in one magnetic sublattice or an internal magnetic field (Hin) was confirmed by Mössbauer spectroscopy.

n isothermal section of the Sm 2 O 3 –Fe 2 O 3 –Ta 2 O 5 system was constructed in the subsolidus region at 1200°C. A region of the Sm 2 – x Fe 1 + x TaO 7 solid solution with the cubic pyrochlore structure (space group Fd 3̅ m ) was found. A study of the polythermal section of the system along the Sm 2 – x Fe 1 + x TaO 7 section showed that the previously known compound Sm 2 FeTaO 7 (R) with the structure of rhombohedrally distorted pyrochlore (space group R 3̅ ) is stable in the temperature range <1200°C. With increasing temperature, Sm 2 FeTaO 7 (R) becomes an intermediate phase, and a reversible transition to the cubic pyrochlore (C) phase is observed. The kinetic hindrances of the transition R 3̅ → Fd 3̅ m determine the short-term stability of the R phase at temperatures exceeding the phase transition temperature by 200°C. It was established that cubic pyrochlore exists as the Sm 2 – x Fe 1 + x TaO 7 solid solution in the ranges x = 0–0.4 at t ≥ 1200°C and x = 0.15–0.4 at lower temperatures. Two independent methods—Mössbauer spectroscopy and XANES—detected that the rhombohedral phase contains not only Fe 3+ ions but also Fe 4+ ions. The magnetic properties were studied, and it was shown that both the R phase and the C phase at T < 10 K undergo two magnetic transitions of different types: an antiferromagnetic transition observed in a strong magnetic field at 7.2 and 5.5 K, respectively, and a transition to the spin glass state in a field of 100 Oe at 8.4 and 4.8 K, respectively. It was determined that the spin glass state coexists with short-range antiferromagnetic interactions until 2.3 K. Despite the fact that the R and C phases formally have geometrically frustrated magnetic sublattices, the experimental data showed that frustration manifests itself only in the R phase.

An isothermal section of the Sm2O3–Fe2O3–Ta2O5 system was constructed in the subsolidus region at 1200°C. A region of the Sm2 – xFe1 + xTaO7 solid solution with the cubic pyrochlore structure (space group Fd 3̅ m) was found. A study of the polythermal section of the system along the Sm2 – xFe1 + xTaO7 section showed that the previously known compound Sm2FeTaO7 (R) with the structure of rhombohedrally distorted pyrochlore (space group R 3̅ ) is stable in the temperature range <1200°C. With increasing temperature, Sm2FeTaO7 (R) becomes an intermediate phase, and a reversible transition to the cubic pyrochlore (C) phase is observed. The kinetic hindrances of the transition R 3̅ → Fd 3̅ m determine the short-term stability of the R phase at temperatures exceeding the phase transition temperature by 200°C. It was established that cubic pyrochlore exists as the Sm2 – xFe1 + xTaO7 solid solution in the ranges x = 0–0.4 at t ≥ 1200°C and x = 0.15–0.4 at lower temperatures. Two independent methods—Mössbauer spectroscopy and XANES—detected that the rhombohedral phase contains not only Fe3+ ions but also Fe4+ ions. The magnetic properties were studied, and it was shown that both the R phase and the C phase at T < 10 K undergo two magnetic transitions of different types: an antiferromagnetic transition observed in a strong magnetic field at 7.2 and 5.5 K, respectively, and a transition to the spin glass state in a field of 100 Oe at 8.4 and 4.8 K, respectively. It was determined that the spin glass state coexists with short-range antiferromagnetic interactions until 2.3 K. Despite the fact that the R and C phases formally have geometrically frustrated magnetic sublattices, the experimental data showed that frustration manifests itself only in the R phase.

A new compound SmFe0.5Ta1.5O6 has been synthesized and its structure has been calculated from synchrotron powder diffraction data by the Rietveld method. It has been demonstrated that SmFe0.5Ta1.5O6 has an aeschynite structure (space group Pnma). A study of its magnetic properties has shown that the SmFe0.5Ta1.5O6 compound is paramagnetic.

The isobaric heat capacity of pyrochlore-related compounds RE2FeTaO7 (RE = Sm and Gd) was measured using an adiabatic and a differential scanning calorimetry over a wide temperature range using adiabatic and differential scanning calorimetry. An anomaly on the curves of all the compounds is clearly observed within the low-temperature range, T < 25 K, while at the high temperatures the peculiarities were not detected. The standard thermodynamic functions: heat capacity, entropy, enthalpy change and reduced Gibbs energy of studied RE2FeTaO7 were calculated. The entropy of the anomalous contribution was equal to (14.8 and 19.6) J.K-1.mol(-1) for Sm2FeTaO7 and Gd2FeTaO7, respectively. A comparative study of Y2FeTaO7 and Sm2FeTaO7 confirmed the relationship of low temperature magnetic properties with a heat capacity anomaly. The DC and AC magnetization measurements within the (300-2.3) K temperature range and in magnetic fields up to 5 kOe revealed spin glass (SG) transitions for Y2FeTaO7 and Sm2FeTaO7 at 4 K and 9 K, respectively. Against the background of developing SG in the frustrated Kagome sublattice, an antiferromagnetic transition at T similar to 8 K was detected for Sm2FeTaO7 in a large magnetic field. This transition occurs in Sm-sublattice and is responsible for the large magnetic contribution to the anomalous low temperature heat capacity in Sm2FeTaO7. (C) 2021 Elsevier Ltd.

Ln2CrTaO7 (Ln = Sm, Gd, Y) pyrochlores are synthesized by the co-precipitation with subsequent annealing. The effect of the precursor composition ((NH4)2Cr2O7, Cr(NO3)3, and CrCl3) on the precipitates reactivity and product microstructure was studied. Samples with an average particle size of 200 nm for Y2CrTaO7, 450 nm for Gd2CrTaO7 and 600 nm for Sm2CrTaO7 were used to study thermodynamic properties. The temperature dependences of heat capacity were measured by the adiabatic calorimetry (13–346.16 K) method and the ratio method using DSC measurements (330–1300 K). The thermodynamic functions of Ln2CrTaO7 compounds were calculated. The absence of polymorphic transitions up to a temperature of 1450°C for all studied compounds is shown.

Ln2CrTaO7 (Ln = Sm, Gd, Y) pyrochlores are synthesized by the co-precipitation with subsequent annealing. The effect of the precursor composition ((NH4)2Cr2O7, Cr(NO3)3, and CrCl3) on the precipitates reactivity and product microstructure was studied. Samples with an average particle size of 200 nm for Y2CrTaO7, 450 nm for Gd2CrTaO7 and 600 nm for Sm2CrTaO7 were used to study thermodynamic properties. The temperature dependences of heat capacity were measured by the adiabatic calorimetry (13–346.16 K) method and the ratio method using DSC measurements (330–1300 K). The thermodynamic functions of Ln2CrTaO7 compounds were calculated. The absence of polymorphic transitions up to a temperature of 1450°C for all studied compounds is shown.

New complex Cr-containing pyrochlores Ln(2)CrTaO(7), Ln = Y, Sm, Gd, were synthesized. The Rietveld refinement of the Ln(2)CrTaO(7) composition confirmed the pyrochlore type of the structure. The Cr3+ oxidation state is corroborated by XANES spectra. The detailed study of DC and AC magnetization of Ln(2)CrTaO(7), Ln = Y, Sm, Gd, was carried out in the temperature range 2-300 K and magnetic field up to 5 T. FM phase transitions at 143, 192, and 163 K were observed in pyrochlores with Y, Sm, and Gd, respectively. It is shown that these transitions are due to canted antiferromagnetism or weak-ferromagnetism in the Ln(2)CrTaO(7) pyrochlores. AFM interactions between Ln- and Cr-sublattices in Sm2CrTaO7 and Gd2CrTaO7 causes AFM transition at T = 42 K, after which magnetization continues increasing. Unusual magnetic properties were observed for Gd2CrTaO7 at low temperatures. Competitive FM and AFM interactions in Cr-and Gd-sublattice give rise to the occurrence of reentrant spin glass at T < 20 K. FM transition at 11 K is also detected. Spin glass state exists along with strong FM interactions. The effect of the magnetic properties on thermal expansion is also investigated for these compounds.

The existence of a wide range of pyrochlore-like (PL) Ln2FeTaO7 compounds (Ln = Pr–Yb, including Y) with a hexagonal structure (space group R$$\overline{3}$$) was shown. Two new low-temperature methods for the synthesis of Ln2FeTaO7 were developed to reduce the production temperature and time, namely, coprecipitation followed by annealing and synthesis in molten salts NaCl/KCl. The synthesis of PL phases was found to involve the formation of a nanocrystalline metastable fluorite phase, the fluorite irreversibly transforming to a PL phase as the crystallization time or temperature increases. No other transitions in Ln2FeTaO7 were recorded by DSC up to the melting point of 1320–1450°C (depending on the composition of the Ln2FeTaO7 compound). The isobaric heat capacity Cp(T) was measured and the thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy) were calculated in the temperature range 5–1300 K for Y2FeTaO7 by the way of example. In the low-temperature region of 2–25 K, an abnormal behavior of Cp(T), apparently related to the Y2FeTaO7 magnetic transition, was detected.