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13Condensed Matter and Interphases (Kondensirovannye sredy i mezhfaznye granitsy)
Original articles
DOI: https://doi.org/10.17308/kcmf.2020.22/2966 ISSN 1606-867X
Received 31 July 2020 eISSN 2687-0711
Accepted 15 August 2020 Published online 30 September 2020
Obtaining Iron (III) - Containing Triple Molybdate K5FeZr(MoO4)6 by Sol-Gel Technology
© 2020 A. V. LogvinovaH a, B.G. Bazarovab, J. G. Bazarova
aBaikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, 6 ul. Sakhyanova, Ulan-Ude 670047, Republic of Buryatia, Russian Federation
bBuryat State University,
24a ul. Smolina, Ulan-Ude 670000, Republic of Buryatia, Russian Federation Abstract
Oxide compounds, as the basis of promising materials, are used in various fields of modern technologies due to their electrical and optical properties. Some of them, possessing a combination of ferroelectric, scintillation, electrical, and optical properties, are being studied as promising materials for electronics. In this case, their dispersion plays an important role.
Traditionally, the synthesis of oxide compounds is carried out by ceramic technology. More promising for the synthesis of fine powders are the methods of "soft" chemistry, among which we have identified and applied the sol-gel method. In this method, "mixing" occurs at the molecular level, which contributes to an increase in the reaction rates and a decrease in the synthesis temperature. The method involves the use of inorganic salts as precursors in combination with complexing agents (citric acid). The use of such precursors allows one to achieve high uniformity at relatively low temperatures. A feature of this approach is the use of fewer organic compounds: an aqueous solution of citric acid is used as a chelating agent. The aim of this work was to obtain triple molybdate by sol-gel technology (SGT) based on the example of iron-containing potassium zirconium molybdate.
The iron-containing triple potassium zirconium molybdate was obtained using the of citrate sol-gel technology and solidphase synthesis (SPS) methods. The triple molybdate obtained by two methods was characterized by X-ray phase analysis, DSC, and impedance spectroscopy.
The developed sol-gel synthesis technique allowed lowering the synthesis temperature, to obtain triple molybdate with high values of homogeneity, dispersion, and electrical conductivity. This technique can be used to obtain double and triple zirconium (hafnium) molybdates containing a trivalent cation.
Keywords: iron-containing, triple molybdate, zirconium, potassium series, sol-gel synthesis.
Funding: The study was conducted within the framework of the state order by the Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences, and partially funded by the Russian Foundation for Basic Research (project 18-03-00557a).
For citation: Logvinova A. V., Bazarov B.G., Bazarova Zh. G. Obtaining iron (III) - containing ternary molybdate K5FeZr (MoO4)6 by sol-gel technology. Kondensirovannye sredy i mezhfaznye granitsy = Condensed Matter and Interphases. 2020;22(3): 353-359. DOI: https://doi.org/10.17308/kcmf.2020.22/2966
El Alexandra V. Logvinova, e - mail: logvinova_alexsandra@bk.ru
S)__®_J The content is available under Creative Commons Attribution 4.0 License.
1. Introduction
Molybdates of different valence elements are of interest due to their ion-conducting [1-7] and luminescent properties [8-15]. The methods developed to date for obtaining new promising molybdates are very diverse [16-18]. The most common method for the preparation of triple molybdates is solid-phase synthesis, which is characterized by the simplicity of the experiment. The main disadvantage of this method is the high temperature and duration of the synthesis. An alternative methods are the so-called "soft chemistry" or solution methods: sol-gel, precipitation from aqueous solutions, etc. Solution methods allow obtaining nanoscale objects and significantly reduce both the temperature and the duration of synthesis.
Now, special attention is paid to finely dispersed and nanodispersed materials. The reduction of the particle size of the compounds allows to obtain materials with unique properties (optical, ion-conducting, etc.).
The aim of this work was to obtain triple molybdate by solid-phase synthesis (SPS) and sol-gel technology (SGT) using the example of iron-containing potassium zirconium molybdate.
2. Experimental
For the synthesis of triple molybdate K5FeZr (MoO4)6 we have developed a sol-gel method based on a citrate gel. Potassium nitrate KNO3, iron nitrate Fe(NO3)3-9H2O (CP) , zirconyl nitrate
ZrO(NO3)2-2H2O and ammonium paramolybdate (NH4) 6Mo7O24-4H2O were used as starting materials, citric acid was used as a complexing agent. The sol-gel synthesis scheme is shown in Fig. 1. Stoichiometric amounts of the source components were dissolved in distilled water.
Prepared solutions of potassium and iron nitrates, zirconium and ammonium para-molybdate were added to the citric acid solution with the formation of metal-citrate complex. The resulting solution was evaporated at a temperature of 70-80 °C until the solution passed into a sol and then into a gel. The resulting gel, which was a translucent mass, was first dried in the oven at a temperature of ~100-150 °C until the water was completely removed, and then it was dried in the furnace at a temperature of 200 °C for the conversion into a xerogel. Xerogel was an amorphous mass that was easily ground into powder in a mortar. Molybdate powders K5FeZr(MoO4)6 (no.1) were obtained after annealing in the furnace at a temperature of 500-550 ° C.
Along with the synthesis of K5FeZr(MoO4)6 using sol-gel technology, this molybdate was obtained using ceramic technology.
For solid-phase synthesis, commercial reagents K2MoO4 (CP), Fe(NO3)3-9H2O (CP), MoO3 (P.A.) and ZrO(NO3)2-2H2O (P.A.) were used the starting materials. Molybdate Fe2(MoO4)3 was obtained by solid-phase synthesis from molybdenum trioxide and iron nitrate by annealing at 400-800 °C for
500 - 550 C (10 h)
_J_
Triple molybdate powder Fig. 1. Sol-gel synthesis scheme
150 h and intermediate homogenization of the samples. Synthesis of Zr(MoO4)2 was carried out according to the solid-phase method by stepwise annealing of a stoichiometric mixture of zirconyl nitrate and molybdenum trioxide in the temperature range of 450-750 °C for 100150 h. Triple molybdate K5FeZr(MoO4)6 (sample no. 2) was obtained using ceramic technology by annealing stoichiometric amounts of reaction mixtures K2Moo4, Fe2(MoO4)3 and Zr (MoO4)2 in the temperature range 400-550 °C with a stepwise increase in temperature with a step of 50 °C and homogenization before each change in the heat treatment mode.
The powders of samples 1 and 2 obtained after annealing were studied by X-ray diffraction analysis (XRD) on a D8 Advance diffractometer (Bruker) using CuKa radiation.
The diffractogram of the synthesized molybdate sample no. 2 (solid-phase synthesis) coincides with the diffractogram of the powder of sample no. 1 (sol-gel method).
Thermal analysis was performed on a NETZSCH STA 449 F1 Jupiter device. The recording was carried out in an argon atmosphere in platinum crucibles.
Electron microscopic studies of the samples were performed using a scanning electron microscope "Hitachi-3400N". The accelerating voltage was 20 keV, the operating distance was 10 mm.
180 000 160 000 140 000 f 120 000 g 100 000 ° 80 000 60 000 40 000 20 000
0
3. Results and discussion
The diffractogram (no. 2) obtained during X-ray phase analysis K5FeZr(MoO4)6 is shown in Fig. 2.
According to XRD data, synthesized molybdate K5FeZr(MoO4)6 (sample no. 2) was isostructural to triple molybdate Rb5FeHf(MoO4)6 [19] and crystallizes in the hexagonal system P63, Z = 2. The identification of parameters of the unit cells of the obtained phases was performed using single crystal data of the isostructural compound. The calculation was carried out according to the uniquely indexed lines of powder XRD patterns of triple molybdate using the TOPAS 4.2 software package. The parameters of unit cells are presented in the Table.
The thermal characteristics of the obtained compounds were studied by differential scanning calorimetry (DSC) in the temperature range 25750 °C using NETZSCH STA 449 F1 Jupiter device. On the DSC curves of molybdate (sample no. 2) (Fig. 3), two endothermic effects were recorded. The first endothermic effect should be attributed to the presence of a polymorphic transition in the sample. The second endothermic effect on the DSC curves corresponded to the melting point. The compound melted incongruently. The same effects were recorded for molybdate (sample no. 1).
The morphology of the products was studied using scanning electron microscopy, which allowed estimating the particle size.
hkl_Phase 0.00 %
■ . I _■ \ .1. ■ y . ■ ■ I l1 . 1 V .'" f .'.'". '."J1 'l'y l"1 VT "M^ 'F" l'.'.""l'.'V "ri'^'I'/T1 T1 'f'" T'T T""' t T1.""
10 15 20 25 30 35 40
45 50 55 60 2Th Degrees
65 70 75 80 85 90 95 100
Fig. 2. The results of processing X-ray diffraction pattern of sample no. 2 of the composition K5FeZr (MoO4)6 using the TOPAS 4.2 software package: blue line - experimental data; red line - calculated profile; strokes correspond to interplanar distances; the curve below is the difference between the experimental and calculated values
Table. Crystallographic and thermal characteristics of K5FeZr(MoO4)6 (sample no. 2)
Compound Unit Cell Parameters T phase T melting
a, Â c, Â V, Â3 change
K5FeZr(MoO4)6 10.088(1) 15.089(1) 1330.0(3) 554 668
5
1.5-
Fig. 3. DSC curve for K5FeZr(Mo04)6 sample (no. 2)
Electron microscopic examination of samples K5FeZr(MoO4)6 obtained by solid-phase synthesis showed that the compounds are spherical particles with sizes from ~ 1 to ~ 6 ^m. Samples of K5FeZr(MoO4)6 obtained by sol-gel technology, consist of spherical particles with sizes from ~ 0.2. up to ~ 4 mm as can be seen on microphotographs (Fig. 4).
Temperature and frequency conductivity dependencies of K5FeZr(MoO4)6 (sample no. 1) was investigated in the temperature range 473-863 K using a "Z-1500J" impedance meter in heating and cooling modes (2 K/min) at a frequency in the range of 1 Hz - 1 MHz.
Molybdate K5FeZr(MoO4)6 was obtained by two methods in the form of a powder was compressed under pressure into tablets in the form of disks with the diameter of 10 mm and thickness of 1-2 mm. These tablets were annealed at 550600 °C for 10 h. Electrodes were applied to the surface of the disks by firing with platinum paste before measurements. The temperature dependence of the conductivity K5FeZr(MoO4)6 (sample no. 1) in Arrhenius coordinates is shown in Fig. 5. When the sample is heated, an abrupt increase in conductivity occurred in the phase transition region (880-900 K), reaching a value of 10-25 S/cm, which is an order of magnitude higher than the conductivity of K5FeZr(MoO4)6 (sample no. 2), obtained by solid-phase synthesis (Fig. 6).
2017.09.12 L D3.6 x2.0k 30 um
c
2017 09.12 d
L D3.5 x8 0k 10 um
Fig. 4. Microphotographs of K5FeZr(MoO4)6 samples obtained solid-phase synthesis (a, b) and sol-gel technology (c, d)
Fig. 5. Temperature dependence of conductivity K5FeZr(MoO4)6 (sample no. 1) obtained by sol-gel technology
Fig. 6. A fragment of the temperature-frequency dependence of the conductivity K5FeZr(MoO4)6 (sample no. 2) obtained by solid-phase synthesis
4. Conclusions
A sol-gel method for the synthesis of triple molybdates M5RZr(MoO4)6 (R = trivalent metals) based on the for example of K5FeZr(MoO4)6 was developed. Iron-containing triple molybdate was obtained by two methods: ceramic and sol-gel technology.
Studies of the obtained molybdates by X-ray diffraction analysis, DSC, electron microscopy, and impedance spectroscopy showed that the compounds crystallize in the hexagonal syngony, in the space group P63, Z = 2, undergoing a phase transition at 554 °C and melt incongruently at 668 °C. The molybdate obtained by the sol-gel
technology consists of particles with sizes from ~ 0.2 to ~ 4 pm. The developed technique for the synthesis of sol-gel technology, in comparison with ceramic technology, allows not only to reduce the synthesis temperature, but also to obtain compounds in a nanodispersed state with high electrical conductivity. The electrical conductivity of this molybdate is an order of magnitude higher than the conductivity of molybdate obtained by ceramic technology.
Acknowledgements
The studies were carried out using scientific equipment of the Laboratory of Oxide Systems of Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences and Collective use centre of the Buryat scientific centre of the Siberian branch of the Russian Academy of Sciences.
Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.
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Information about the authors
Aleksandra V. Logvinova, PhD student, Laboratory of Oxide Systems, Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences (BINM SB RAS), Ulan-Ude, Russian Federation; e-mail: Logvinova_Alexsandra@bk.ru. ORCID iD: https://orcid.org/0000-0001-9850-2719.
Bair G. Bazarov, DSc in Physics and Mathematics, Leading Researcher, Laboratory of Oxide Systems, Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences (BINM SB RAS), Associate Professor at the Department of Inorganic and Organic chemistry, Banzarov Buryat State University, Ulan-Ude, Russian Federation; e-mail: bazbg@rambler.ru. ORCID iD: https://orcid. org/0000-0003-1712-6964.
Jibzema G. Bazarova, DSc in Chemistry, Chief Scientist, Laboratory of Oxide Systems, Baikal Institute of Nature Management, Siberian Branch of the Russian Academy of Sciences (BINM SB RAS), Ulan-Ude, Russian Federation; e-mail: jbaz@binm.ru. ORCID iD: https://orcid.org/0000-0002-1231-0116....
All authors have read and approved the final manuscript.
Translated by Valentina Mittova
Edited and proofread by Simon Cox
