Abstract. Cubic pyrochlore of the composition Bİ2Coo.5Cro.5Nb20g+A (sp. gr. Fd-3m, a = 10.4838(8) Å) was synthesized in several stages using a solid-phase reaction from oxide precursors at a final temperature of 1050 °C. Using NEXAFS spectroscopy data, the electronic state of cobalt and chromium ions during the synthesis process was studied. It has been established that before the formation of phasepure pyrochlore, Cr(III) ions are converted to Cr(VI), and then again to Cr(III); Cobalt ions Co(III) are reduced to Co(II). NEXAFS Cr2p spectra of ceramics synthesized at 650 °C, according to the main characteristics of the spectrum, coincide with the spectrum of К2СГ2О7 and indicate the chromium content in the oxide ceramics in the form of tetrahedral CrO^- ions, and according to the nature of the Co2p spectrum, cobalt ions are in the Co(II) state and Co(III). In the composition of pyrochlore Bİ2Coo.5Cro.5Nb20g+A, synthesized at 1050 °C, cobalt and chromium appear predominantly in the form of Co(II) and Cr(III) ions. Analysis of phase transformations showed that changes in the oxidation state of transition element ions and the color of ceramics are associated with the formation of intermediate synthesis products.
Keywords: pyrochlore, bismuth niobáte, NEXAFS, cobalt.
Citation: K.A. Badanina, S.V. Nekipelov, A.M. Lebedev, N.A. Zhuk, D.S. Beznosikov, Conversion of Cr(III) and Co(III) During the Synthesis of Co/Cr Codoped Bismuth Niobáte Pyrochlore According to NEXAFS Data, J. Sib. Fed. Univ. Math. Phys., 2024, 17(5), 559-569. EDN: COUDXV.
Аннотация. Кубический пирохлор состава Bİ2Coo.5Cro.5Nb209+A (пр.гр. Fd-3m, а = 10.4838(8) А) синтезировали в несколько этапов методом твердофазной реакции из оксидных прекурсоров при финальной температуре 1050 °C. По данным NEXAFS-спектроскопии исследовано электронное состояние ионов кобальта и хрома в процессе синтеза. Установлено, что до формирования фазовочистого пирохлора ионы Сг(Ш) превращаются в Cr(VI), а затем снова в Сг(Ш); ионы кобальта Со(III) восстанавливаются до Co(II). NEXAFS Сг2р-спектры керамики, синтезированной при 650 °C, по основным характеристикам спектра совпадают со спектром К2СГ2О7 и свидетельствуют о содержании хрома в оксидной керамике в виде тетраэдрических ионов СгОд-, а по характеру Со2р-спектра ионы кобальта находятся в состоянии Со(П) и Со(Ш). В составе пирохлора Bİ2Coo.5Cro.5Nb20g+A, синтезированного при 1050 °C, кобальт и хром проявляются преимущественно в виде ионов Со(II) и Сг(Ш). Анализ фазовых превращений показал, что изменение степени окисления ионов переходных элементов и цвета керамики связано с образованием промежуточных продуктов синтеза.
Ключевые слова: пирохлор, ниобат висмута, NEXAFS, кобальт.
Introduction
Currently, pyrochlores based on bismuth niobáte are being actively studied due to their promising dielectric properties [1,21. Exhibiting low values of dielectric losses and high dielectric constant, tunable temperature coefficient of capacitance, chemical inertness with respect to lowmelting conductors, materials based on oxide pyrochlores are promising as multilayer ceramic capacitors and tunable microwave dielectric components [3]. The crystal structure of pyrochlores A2B2O6O' is formed by two interpenetrating cationic sublattices A2O' and B2O6 [4]. Octahedral positions B are occupied by relatively small cations (Nb+5), larger ions (Bi+3) are distributed in eight-coordinated positions A. A feature of bismuth-containing pyrochlores is the partial vacancy of the bismuth sublattice and the distribution of dopants - ions of transition 3d elements (Co, Cu, Zn , Mn) in both cation sublattices of bismuth and niobium, causing relaxation processes in ceramics [5,6]. New studies of pyrochlores based on bismuth niobáte doped with transition 3d ions (Cr, Mn, Fe, Co, Ni, Cu, Zn) [5-11] have shown that low-porosity ceramics with low dielectric losses and high values of dielectric constant are formed. The possibility of solid-phase synthesis of multielement pyrochlores containing various combinations of atoms of 3d elements was shown in [12]. A study of the phase formation of a representative of mixed pyrochlores Bİ2Coo.5Cro.5Nb209+A showed [13] that during the synthesis (at 650 °C) the color of the ceramics strikingly and reversibly changes from green to brown, and the synthesis of phase-pure pyrochlore occurs at a temperature not lower than 1050 °C. It has been established that the formation of the pyrochlore phase occurs through the reaction of solid-phase interaction of orthorhombic bismuth niobáte (cr-BiNbCU) with oxides of transition elements. In the presented work, based on X-ray spectroscopy data, the oxidation states of cobalt and chromium ions in ceramics are analyzed and the reason for the change in the color of ceramics is established. The phase composition of Bİ2Coo.5Cro.5Nb20g+A ceramics at intermediate stages of solid-phase synthesis was studied in detail. The data obtained contribute to a deep understanding of the processes occurring during high-temperature processing of materials.
1. Materials and methods
For the solid-phase synthesis of the Bİ2Coo.5Cro.5Nb20g+A sample, oxides of bismuth (III), niobium (V), chromium (III) and cobalt (II,III) of analytical grade taken in stoichiometric quantities were used. The stoichiometric mixture of oxides was thoroughly homogenized in an agate mortar for one hour, then pressed into disk shapes. The main stages of pyrochlore phase formation were studied using X-ray phase analysis of samples sequentially calcined at temperatures of 650, 850, 950, 1000 and 1050 °C for 15 hours at each stage of heat treatment. After each calcination step, the sample was carefully homogenized and pressed back into disk shape to ensure tight contact of the ceramic grains. X-ray data were obtained using a Shimadzu 6000 X-ray diffractometer (CuKcr radiation; 26 = 10 - 80°; scanning speed 2.0 °/min). The study of the microstructure and elemental mapping of the surface of the samples was carried out using scanning electron microscopy and energy-dispersive X-ray spectroscopy (Tescan VEGA 3LMN scanning electron microscope, INCA Energy 450 energy-dispersive spectrometer). The unit cell parameters of pyrochlores were calculated using the CSD software package [14]. Research using NEXAFS spectroscopy was carried out at the NanoFES station of the KISI synchrotron source at the Kurchatov Institute (Moscow). NEXAFS spectra were obtained by recording the total electron yield (TEY) with an energy resolution of 0.5 eV and 0.7 eV in the region of the Cr2p and Co2p absorption edges, respectively.
2. Results and discussion
As a review of the literature shows [12,13,15], the solid-phase synthesis of mixed pyrochlores based on bismuth niobáte is a multi-step process, which is associated with the low reactivity of niobium (V) oxides and some transition elements, which include CoO, NiO [12,16]. In addition, the features of the solid-phase synthesis method arc the duration of calcination and the multistage heat treatment process with intermediate remixing of the reaction mixture, which are necessary to accelerate the reaction and obtain a homogeneous synthesis product [17]. It was previously established that a sample of the composition Bi2Coo.5Cro.5Nb209+A during ceramic synthesis reversibly changes its color in the temperature range of 500-650 °C from green to brown. In order to investigate the unusual thermal behavior of the complex oxide, the charge state of transition element ions in ceramics was studied using NEXAFS spectroscopy and the phase composition of samples calcined at 650, 850, 950, 1000 and 1050 °C. According to X-ray phase analysis of a sample calcined at 650 °C, the X-ray diffraction pattern shows reflections of intermediate products of the interaction of bismuth (III) oxide with chromium (III) and niobium (V) oxides - bismuth chromate BißC^Ois (sp.gr. Ccc2\ bismuth niobates BiðNb30i5 (sp.gr. P^/mmm) and BiNb04 (sp.gr. Рппа^ monoclinic ^-Nb2Oð (sp.gr. P2/m) and pyrochlore (sp.gr. Fd-3m) [5,18-23]. Noteworthy is the formation of bismuth chromate, which has its own intense red-orange color due to electronic transitions with charge transfer [20,21]. The fact is that chromium (III) oxide does not oxidize under these synthesis conditions; this requires an oxygen atmosphere, a long duration and temperature of calcination. Meanwhile, the formation of chromium (VI) compounds can be indirectly indicated by the orange-brown color of the ceramics (Fig. 2) synthesized at 650 °C.
In order to establish or refute the presence of chromium (VI) ions in the sample, NEXAFS studies of the charge state of the ions were carried out. NEXAFS spectra of chromium ions in the composition of Bİ2Coo.5Cro.5Nb20g+A ceramics synthesized at 650 °C are presented in Fig. 1. As the figure shows, the spectra have a rich structure; in particular, absorption bands at 578, 580.5 and 589 eV can be clearly distinguished in the Cr2p3/2 and Сг2р1/2 spectra of the sample. Comparison of the spectra of the samples with the spectra of the oxides CrO3, Cr©2, Cr2©3 and potassium dichromate К2СГ2О7 [24-27] shows that the low-energy bands in the spectrum coincide in the energy position of the peaks with the spectra of CrO3 and К2СГ2О7, which indicates that chromium ions in the composition of the samples have charge state of Cr(VI) and are in a tetrahedral environment in the form of CrO^- ions, similar to К2СГ2О7, which is consistent with the results of X-ray phase analysis. It is interesting to note that the spectra of chromium change significantly with increasing temperature: in the low-temperature sample (650 °C) they practically coincide with the NEXAFS Cr2p spectrum of К2СГ2О7, and those calcined at high temperature (1050 °C) - with the spectra of СГ2О3.
Indeed, signals appear at 577 eV and 578 eV, and in the region of 586-588 eV, characteristic of Cr(III) ions in an octahedral environment [27]. This allows us to assert that the charge state of chromium ions changes with increasing temperature of heat treatment of the sample from Cr(VI) to Cr(III). As X-ray phase analysis shows, the reason for the change in the charge state of chromium ions is the formation of an intermediate synthesis product - bismuth (VI) chromate, which is stable in a given temperature range and gives the ceramic a brown color. Apparently, with an increase in the synthesis temperature, bismuth chromate thermally dissociates with the formation of oxygen and oxide compounds of chromium (III), as evidenced by the disappearance of its reflections in the X-ray diffraction pattern of the sample synthesized at a temperature of 850 °C. The dissociation products interact with precursors to form a pyrochlore phase at 1050 °C, in which chromium ions are predominantly in the form of Cr(III) ions, as evidenced by NEXAFS data. According to X-ray diffraction data, active interaction of precursors is detected at temperatures above 650 °C. Reflections of niobium (V) oxide are not detected in the X-ray diffraction patterns of samples synthesized at 850 °C and above; bismuth chromate BißC^Ois is practically not detected at 750 °C, and BiðNbsOið - at 900 °C. The pyrochlore phase appears in noticeable quantities in samples obtained at a temperature of 750 °C. At this temperature, the concentrations of and В16СГ2О15 decrease significantly. Apparently, low-temperature synthesis of pyrochlore is difficult due to the chemical inertness of Nb2Oð. It is interesting to note that the intermediate phase in the synthesis of pyrochlore is BiðNbsOið, while the analogue in the synthesis of pyrochlores based on bismuth tantalate is BisTaOy (sp.gr. Fm-3m) [28]. This is partly due to the fact that the compound BiðTasOið is unknown.
For other reasons, it can be assumed that the reactivity of chromium (III) oxide is higher than that of niobium (V), so bismuth oxide reacts with СГ2О3 first and in significant quantities. Its maximum relative content is recorded at 750 °C, then its share in the sample rapidly decreases.
BiðNbsOið is replaced by <a-BiNbO4, which, interacting with oxides of transition elements, forms a pyrochlore phase of a given composition. In the temperature range of 900-1000 °C, reflections of the pyrochlore and ct-BiNbO4 phases are clearly observed.
The precursors oxides СГ2О3 and CO3O4 do not appear on the X-ray diffraction patterns of the samples due to their low content in the initial charge and the high reactivity of chromium (III) oxide. It is interesting to note that, according to NEXAFS spectroscopy, cobalt (II,III) oxide in ceramics synthesized at 650 °C is present as an independent impurity phase. Indeed, as the NEXAFS Co2p spectra of the sample show, the spectrum of the composite in terms of the shape of the spectrum and the energy position of the spectral details is similar to the spectrum of CO3O4 oxide, in which cobalt ions are found in the form of octahedrally coordinated Co(II) and Co(III) ions [29]. Meanwhile, as the heat treatment temperature of the sample increases, the charge state of cobalt changes and cobalt ions are detected in ceramics synthesized at 1050 °C, mainly in the form of Co(II) ions. Taking into account that a temperature of 1050 °C corresponds to the production of phase-pure pyrochlore, it can be stated that cobalt ions in the composition of pyrochlore are mainly in the Co(II) state, which is confirmed in the work devoted to the study of cobalt-containing pyrochlores in the ternary system Е^Оз-Ж^Оз-СоО [30]. The change in the oxidation state of cobalt during high-temperature treatment of the sample may be associated with the process of thermal dissociation of CO3O4 at a temperature of 920 °C into CoO and oxygen [31].
As shown by elemental mapping of a sample (Fig. 4) synthesized at temperatures below 1050 °C, cobalt atoms are unevenly distributed on the surface of the sample, which indicates that cobalt atoms are not part of the pyrochlore, but represent an impurity phase, which may be CO3O4 oxide, which was subsequently subjected to thermal decomposition. Thus, cobalt enters into a high-temperature reaction with bismuth orthoniobate in the form of CoO oxide. This may be the reason that cobalt ions in pyrochlore are predominantly in the Co(II) state.
Microphotographs of the surface of the synthesized samples at temperatures of 650, 850, 950, 1000 and 1050 °C are shown in Fig. 4. A heterogeneous microstructure with heterogeneous grains and inclusions of impurity phases is characteristic of samples calcined at a temperature of 650-1000 °C. A low-porosity, dense microstructure was formed in the sample synthesized at a temperature of 1050 °C. On the surface of the ceramic, both individual small grains and partially fused grains with the formation of large agglomerates are observed, which contributes to the formation of a monolithic microstructure. The average crystallite size determined by X-ray diffraction using the Scherrer formula is ~58 nm (1050 °C), while larger crystallites in the range of 2-10 /zm (1050 °C) were determined using a scanning electron microscope (SEM). Apparently, the crystallites in the micrographs are aggregated ceramic grains of much smaller sizes. Full-profile analysis by the Rietvcld method showed that the Bİ2Coo.5Cro.5Nb20g+A sample synthesized at 1050 °C is single-phase [13].
The unit cell parameter of the В12Соо.5Сго.5no.)20д+д sample is 10.4838(8) A and exceeds the cell constant of chromium-containing pyrochlore Bi2CrNb20g+^ (a = 10.459(2)) [32], which is explained by the large radius of Co(II) ions compared to Cr(III) ions (R(Cr(III)) = 0.615 Å, R(Co(II))c.n.-6 = 0.745 Å) [33]. Since the radii of Ta(V) andNb(V) ions (R( Nb(V)/Ta(V))c.n.-6 = 0.064 nm) are equal, the lattice constants for pyrochlores based on bismuth niobáte and bismuth tantalate can be comparable. Indeed, the unit cell parameter В12СгТа20д+д (a = 10.45523(3) Å) [34] which is due to the closeness of the ionic radii (R(Mg(II)) = 0.72 Å, R(Co(II))c.n.-6 = 0.745 A). Unit cell parameter for cobalt-containing pyrochlores based on bismuth tantalate Bi1.4gCoo.8Ta1.6O7.о a = 10.54051(3) Å and for Bii.eCoo.sTai.eOyAA a = 10.5526 (2) Å significantly exceeds the parameter of chromium-cobalt-containing pyrochlore, which is associated with a significant difference in the radii of chromium (III) and cobalt (II) ions ( R(Cr(III)) = 0.615 Å, R(Co(II))c.n. 6 = 0.745 Å) [36, 37]. Local chemical analysis using energy-dispersive spectroscopy showed that the chemical composition of the synthesized Bİ2Coo.5iCro.52Nb2.o50g+A sample is close to the nominal composition. Thus, the atypical thermal behavior of the Bi2Coo.5Cro.5Nb20g+A sample is associated with the formation of an impurity phase of bismuth chromate, as shown by X-ray phase analysis and NEXAFS spectroscopy.
Conclusions
Using NEXAFS spectroscopy and X-ray phase analysis, it was determined that the change in the oxidation state of cobalt and chromium ions during the synthesis of the Bi2Coo.5Cro.5Nb20g+A sample is associated with the peculiarities of obtaining pyrochlore by the solid-phase method. It has been established that before the formation of phase-pure pyrochlore, Cr(III) ions are converted to Cr(VI) in the composition of bismuth chromate as an intermediate product of the synthesis, and then again to Cr(III) during the decomposition of bismuth chromate and the formation of pyrochlore; Cobalt ions Co (III) are reduced to Co (II) as a result of thermal dissociation of CO3O4. In the composition of pyrochlore В12Соо.5Сго.5no.>209+д, synthesized at 1050 °C, cobalt and chromium appear predominantly in the form of Co(II) and Cr(III) ions. The process of phase formation of pyrochlore is a series of sequential solid-phase reactions involving precursors. A strategically important intermediate product of the synthesis is bismuth orthoniobate <a-BiNbO4 due to the fact that doping bismuth orthoniobate with atoms of transition elements leads to the formation of the pyrochlore phase.
The NEXAFS study was carried out within the framework of the topic 122040400069-8, as well as with the financial support of the Ministry of Science and Higher Education of Russia under agreement No. 075-15-2021-1351.
Received 10.04.2024, received in revised form 29.05.2024, accepted 04.07.2024
References
[1] C.C.Khaw, K.B.Tan, C.K.Lee, High temperature dielectric properties of cubic bismuth zinc tantalate, Ceram. Intern., 35(2009), 1473-1480.
[2] G.Giampaoli, T.Siritanon, B.Day, J.Li, M.A.Subramanian, Temperature independent low loss dielectrics based on quaternary pyrochlore oxides, Prog. Solid State Chern., 50(2018), 16-23.
[3] H.Du, X.Yao, Structural trends and dielectric properties of Bi-based pyrochlores, J. Mater. Sei. Mater. Electron., 15(2004), 613-616.
[41 M.A.Subramanian, G.Aravamudan, G.V.Subba Rao, Oxide pyrochlores - a review, Prog. Solid State Chern., 15(1983), 55-143.
[5] S.Kamba, V.Porokhonskyy, A.Pashkin and ets., Anomalous broad dielectric relaxation in Bi1.5Zn1.0Nb1.5O7 pyrochlore, Phys. Rev. B., 66(2002), 054106.
[6] M.Valant, Dielectric Relaxations in Bİ2Os-Nb2O5-NiO Cubic Pyrochlores, J. Am. Ceram. Soc., 92(2009), 955-958.
[7] P.Y.Tan, K.B.Tan, C.C.Khaw, Z.Zainal, S.K.Chen, M.P.Chon, Phase equilibria and dielectric properties of BÍ3+(5/2)æMg2-xNb3_(3/2)æOi4-æ cubic pyrochlores, Ceram. Intern., 40(2014), 4237-4246.
[81 M.W.Lufaso, T.A.Vanderah, I.M.Pazos, I.Levin, R.S.Roth, J.C.Nino, V.Provenzano, P.K.Schenck, Phase formation, crystal chemistry, and properties in the system В120з-Ре20зNb2O5, Nb2O5, J. Sol. St. Chern., 179(2006), 3900-3910.
[9] T.A.Vanderah, M.W.Lufaso, A.U.Adler, I.Levin, J.C.Nino, V.Provenzano, P.K.Schenck, Subsolidus phase equilibria and properties in the system В12Оз:Мп2Оз±х:КЬ2Оз, J. Sol. St. Chern., 179(2006), 3467-347. DOI: 10.1016/j.jssc.2006.07.014.
[10] I.Levin, T.G.Amos, J.C.Nino, T.A.Vanderah, C.A.Randall, M.T.Lanagan, Structural Study of an Unusual Cubic Pyrochlore Bii.sZno.92Nbi.5O6.92-, J- Sol. St. Chern., 168(2002), 69-75.
[11] H.B.Nguyen, L.Noren, Y.Liu, R.L.Withers, X.R.Wei, M.M.Elcombe, The disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Е^Оз-МО-Ж^Оз (M = Mg, Ni) systems, J. Sol. St. Chern., 180(2007), 2558-2565.
[12] E.P.Rylchenko, B.A.Makeev, D.V.Sivkov, R.I.Korolev, N.A.Zhuk, Features of phase formation of pyrochlore-type В12Сг1/6Мп1/6Ее1/6Со1/6no.1/6Си1/6Та2Од+д, Lett. Mater., 12(2022), 486-492.
[13] N.A.Zhuk, К.A.Badanina, R.I.Korolev, В.A.Makeev, M.G.Krzhizhanovskaya, V.V.Kharton, Phase Formation of Co and Cr Co-Doped Bismuth Niobáte with Pyrochlore Structure, Inorganics, 11(2023), 288.
[14] L.G.Akselrud, Yu.N.Grin, P.Yu.Zavalij et al. CSD-universal program package for single crystal or powder structure data treatment, Thes. Rep. XII Eur. Crystallogr. Meet., 1989, 155.
[15] J.C.Nino, M.T.Lanagan, C.A.Randa, Phase formation and reactions in the Bi2O3-ZnONb2Os-Ag pyrochlore system, J. Mater. Res., 16(2001), 1460-1464.
[16] N.AZhuk., S.Yu.Kovalenko, R.I.Korolev, B.A.Makeev, M.G.Krzhizhanovskaya, D.V.Sivkov, S.V.Nekipelov, V.N.Sivkov, M.V.Yermolina, Features of Phase Formation of Pyrochloretype Ceramics BÍ2Mg(Zn)i_æNiæTa20g, ACS Omega, 8(2023), 11351-11363.
[17] K.N.Parshukova, E.P.Rylchenko, V.A.Muravyev, K.A.Badanina, B.A.Makeev, R.I.Korolev, N.A.Zhuk, Synthesis of multicomponent compounds with a pyrochlore structure, Glass and ceramics, 95(2022), 34-39.
[18] J.A.Da Cruz, E.A.Volnistem, R.F.Ferreira, F D.B.reitas, A.J.M.Sales, L.C.Costa, M.P.F.Graca, Structural characterization of Brazilian niobium pentoxide and treatment to obtain the single phase (Н-Ж^Об), Thermal Sei. Engineering Progr., 25(2021), 101015. DOI: 10.1016/j.tsep.2021.101015
[19] J.Gopalakrishnan, R A.amanan, C.N.R.Rao, D.A.Jefferson, J.Smith David, A homologous series of recurrent intergrowth structures of the type Bİ4Am+n_2Bm+nO3(m+n)+6 formed by oxides of the aurivillius family, Sol. St. Chern., 55(1984), 101-105.
[20] J.Grins, S.Esmaeilzadeh, S.Hull, Structure and Ionic Conductivity of BigC^Oið, a New Structure Type Containing (Bi^O^sn+n Columns and Cr()| Tetrahedra, Journal of Solid State Chemistry, 163(2002), 144-150.
[21] Y.H.Liu, J.B.Li, J.K.Lianga, J.Luo, L.N.Ji, J.Y.Zhang, G.H.Rao, Phase diagram of the В120з-Сг20з system, Mater. Chem. Phys., 112(2008), 239-243.
[22] R.S.Roth, J.L.Waring, Synthesis and stability of bismutotantalite, stibiotantalite and chemically similar ABO4 compounds, Am. Mineral., 48 (1963), 1348-1356.
[23] N.AZhuk., M.G.Krzhizhanovskaya, V.A.Belyy, B.A.Makeev, High-Temperature Crystal Chemistry of a-, ß-, and y-BiNbO4 Polymorphs, Inorg. Chern., 58(2019), 1518-1526. DOL 10.1021/acs.inorgchem.8b03024
[24] S.-Y.Jeong, J.-B.Lee, H.Na, T.-Y.Seong, Epitaxial growth of СГ2О3 thin film on AI2O3 (0001) substrate by radio frequency magnetron sputtering combined with rapid-thermal annealing, Thin Solid Films, 518(2010), 4813-4816.
[25] H.A.Bullen, S.J.Garrett, СЮ2 by XPS: Comparison of CrO2 Powder to CrO2 Films on T1O2 (110) Single Crystal Surfaces, Surf. Sei. Spectra, 8(2001), 225-233.
[26] C.Theil, J.Van Elp, F.Folkmann, Ligand field parameters obtained from and chemical shifts observed at the Cr L2,3 edges, Physical Review B, 59(1999), no. 12, 7931-7936.
[27] Y.Ito, T.Tochio, A.M.Vlaicu and ets., The contribution of the ligands around Cr to the resonant inelastic L X-ray emission spectra, J. Electron Spectroscopy and Related Phenomena, 101-103(1999), 851-858.
[28] M.Struzik, X.Liu, I.Abrahams, F.Krok, M.Malys, J.R.Dygas, Defect structure and electrical conductivity in the pseudo-binary system BigTaOy-BigNbOy, Solid State Ionics, 218(2012), 25-30.
[29] T.J.Regan, H.Ohldag, C.Stamm, F.Nolting, J.Luning, J.Stöhr, W R.L.hite, Chemical effects at metal/oxide interfaces studied by x-ray-absorption spectroscopy, Phys. Rev. B, 64(2001), 214422.
[30] T.A.Vanderah, T.Siegrist, M.W.Lufaso, M.C.Yeager, R.S.Roth, J.C.Nino, S.Yates, Phase Formation and Properties in the System Bi2O3:2CoOi+æ:Nb2O5, Eur. J. Inorgan. Chem., 2006(2006), 4908-4914.
[31] В.Bingyang, A.Hamidreza, L.Junhuaö Comparison of the performance for oxidation of formaldehyde on папо-СозО4, 2D-CO3O4, and 3D-CO3O4 catalysts, Applied Catalysis B: Environmental, 142-143(2013), 677-683.
[32] I.V.Piir, D.A.Prikhodko, S.V.Ignatchenko, A.V.Schukariov, Preparation and structural investigations of the mixed bismuth niobates, containing transition metals, Sol. St. Ion., 101103(1997), 1141-1146.
[33] R.D.Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A, 32(1976), 751-767.
[34] N.A.Zhuk, N.A.Sekushin, M.G.Krzhizhanovskaya and ets., Cr-doped bismuth tantalate pyrochlore: electrical and thermal properties, crystal structure and ESR, NEXAFS, XPS spectroscopy, Mater. Res. Bull., 158(2023), 112067.
[35] N.A.Zhuk, M.G.Krzhizhanovskaya, A.V.Koroleva and ets., Cr and Mg codoped bismuth tantalate pyrochlores: Thermal expansion and stability, crystal structure, electrical and optical properties, NEXAFS and XPS study, J. Sol. St. Chern., 323(2023), 124074. DOI: 10.1016/j.jssc.2023.124074
[36] N.A.Zhuk, K M.G.rzhizhanovskaya, A.V.Koroleva and ets., Spectroscopic characterization of cobalt doped bismuth tantalate pyrochlore, Sol. St. Sei., 125(2022), 106820.
[37] N.A.Zhuk, M.G. Krzhizhanovskaya, N.A.Sekushin, D.V.Sivkov, I. E Abdurakhmanov, Crystal structure, dielectric and thermal properties of cobalt doped bismuth tantalate pyrochlore, J. Mater. Res. Technoi., 22(2023), 1791-1799. DOL 10.1016/j.jmrt.2022.12.059
You have requested "on-the-fly" machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Show full disclaimer
Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer
© 2024. This work is published under http://journal.sfu-kras.ru/en/series/mathematics_physics (the “License”). Notwithstanding the ProQuest Terms and Conditions, you may use this content in accordance with the terms of the License.
Abstract
Аннотация. Кубический пирохлор состава Bİ2Coo.5Cro.5Nb209+A (пр.гр. Fd-3m, а = 10.4838(8) А) синтезировали в несколько этапов методом твердофазной реакции из оксидных прекурсоров при финальной температуре 1050 °C. По данным NEXAFS-спектроскопии исследовано электронное состояние ионов кобальта и хрома в процессе синтеза. Установлено, что до формирования фазовочистого пирохлора ионы Сг(Ш) превращаются в Cr(VI), а затем снова в Сг(Ш); ионы кобальта Со(III) восстанавливаются до Co(II). NEXAFS Сг2р-спектры керамики, синтезированной при 650 °C, по основным характеристикам спектра совпадают со спектром К2СГ2О7 и свидетельствуют о содержании хрома в оксидной керамике в виде тетраэдрических ионов СгОд-, а по характеру Со2р-спектра ионы кобальта находятся в состоянии Со(П) и Со(Ш). В составе пирохлора Bİ2Coo.5Cro.5Nb20g+A, синтезированного при 1050 °C, кобальт и хром проявляются преимущественно в виде ионов Со(II) и Сг(Ш). Анализ фазовых превращений показал, что изменение степени окисления ионов переходных элементов и цвета керамики связано с образованием промежуточных продуктов синтеза.