Preparation and characterization of Bi26–2xMn2xMo10O69-d and Bi26.4Mn0.6Mo10–2yMe2yO69-d(Me = V, Fe) solid solutions Z. A. Mikhaylovskaya1, M. V. Morozova1, E. S. Buyanova1, S. A. Petrova2, I. V. Nikolaenko3, D. G. Kellerman3 1 Ural Federal University, 19 Mira St., Ekaterinburg, 620000, Russia 2 Institute of Metallurgy UB RAS, 101 Amundsen St., Ekaterinburg, 620016, Russia 3 Institute of Solid State Chemistry UB RAS, 91 Pervomayskaya St., Ekaterinburg, 620990, Russia Preparation and characterization of Bi 26–2x Mn 2x Mo 10 O 69-d and Bi 26.4 Mn 0.6 Mo 10–2y Me 2y O 69-d (Me = V, Fe) solid solutions ∙ ∙cm ∙ ∙cm ∙ ∙cm ∙ ∙cm Keywords: Introduction Bi2O3-based complex oxides exhibit a variety of properties includ- ing high oxide-ion conductivity and/or electronic conductivity at medium tem- peratures (573–973 K), ferroelectric and magnetic effects, as well as catalytic ac- tivity. These compounds crystallize in a number of different structure types, for example fluorite-type structures (δ-Bi2O3 -based complex oxides [1]), layered Au- rivillius type structures (the BIMEVOX family [2]) or unique columnar structures (Bi26Mo10O69 [3]). The latter is reported to contain [Bi12O14]∞ columns oriented along the y-axis, MoOn polyhedrons and «iso- lated» Bi3+ ions [4]. Bi26Mo10O69 exhibits pure one-dimensional oxide ion transport along the columnar fragments (y axis) [5]. The values of ionic conductivity are close to that observed in the BIMEVOX and stabilized zirconia oxide ion conduc- tors [6]. Hence Bi26Mo10O69-based mate- rials are being considered as alternative electrolytic materials for gas conversion membranes, separators, sensors and fuel cells. Bi26Mo10O69 is part of a small solid so- lution range in the Bi2O3-MoO3 system (2.57  ≤  Bi/Mo  ≤  2.77) [5]. A reversible monoclinic to triclinic phase transition is observed on cooling at c.a. 310  °C, re- sulting in a significant decrease in ionic conductivity and an increase in activation energy. The structure of the monoclinic form was described in [5] with the follow- ing unit cell parameters: a = 11.74 Å, b = 5.80 Å, c = 24.79 Å, β = 102.84° with P2/c space group symmetry. The monoclinic model includes bismuthate columns, «isolated» Bi3+ ions and MoO4 tetrahedra. However, the model only accounted for 68 of the 69 oxygen atoms per formula unit required to maintain electroneutra- lity. The structure of the triclinic form with one additional oxygen position was sug- gested in [7]. It includes MoO5 polyhedra as well as MoO4 tetrahedra. Solid solution formation in the Bi26Mo10O69 system can be realized by substitution of either Bi3+ or Mo6+ cati- ons. Bi26Mo10-2yMe2yO69-δ solid solutions have been reported for Me = Li, Mg, Al, Si, Ge [8]; Co [9]; V, P, W [6], while, larger cations can be substituted for bis- muth including Pb [10]; Ln [11]; Ca, Sr, Ba [6] and Mn [12]. It was shown previ- ously that the best conductivity proper- ties of Bi26-2xMn2xMo10O69-d series was re- lated to the x = 0.3 compound [12]. The present work is devoted to the following investigation of Bi26-2xMn2xMo10O69-d se- ries and the synthesis, and electrical cha- racterization of new substituted bismuth molybdates Bi26-2xMn2xMo10-2yV2yO69-d and Bi26-2xMn2xMo10-2yFe2yO69-d. Experimental Polycrystalline samples of general for- mula Bi26-2xMn2xMo10O69-d and Bi26-2xMn2x- Mo10-2yMe2y O69-d were prepared using Bi2O3 (99.9 %), MoO3 (99.5 %), MnO (99.7 %), V2O5 (99.5 %) Fe2O3 (99.5 %) as precursors. In each case, stoichiomet- ric mixtures of the starting materials were thoroughly ground in an agate mortar with ethanol as a dispersant. After drying in air, pellets were pressed and placed on a bed of unreacted powder in an alumina crucible. The pellets were heated to 823 K for 48 h, followed by quenching, regrind- ing and re-pelletizing. The samples were then heated at 1123  K for 24 h, followed by slow cooling in air to the room tem- perature. The solubility limits (x and y) and crystal structure of Bi26-2xMn2xMo10O69-d, Bi26-2xMn2xMo10-2yV2yO69-d and Bi26-2xMn2x- Mo10-2yFe2yO69-d were determined by X-ray powder diffraction (XRPD). X-ray powder diffraction data were obtained on a Bruker Advance D8 diffractometer with a VÅNTEC1 detector using the Cu- K radiation ( 1 = 1.54056 Å and 2  = =  1.54439  Å). Data were collected in flat plate / geometry and calibrated against external Si standard in the 2 range 5–70 , in steps of 0.021356 , with an effective scan time of 1 s per step. XRPD data were obtained using the equipment of the Cen- tre for Shared Use «Ural-M» (Institute of Metallurgy UB RAS, Ekaterinburg). The size of the particles of the pow- ders was studied by means of the laser diffraction method using a SALD-7101 Shimadzu analyzer. The morphology of the obtained powders and their chemi- cal composition were studied using the JEOL JSM 6390LA with a JED 2300 EDX-analyzer. Hydrostatic weighting (Archimedes method) was used for in- vestigation of density of ceramic pellets covered with a thin layer of waterproof coating. The volume porosity of ceramic samples was obtained by comparing for the experimental and theoretical (X-ray) densities of the samples. The variable tem- perature measurements of magnetic sus- ceptibility were collected on SQUID mag- netometer MPMS XL7, Quantum Design (Ural Center for Shared Use «Modern nanotechnology», Ural Federal Univer- sity, Ekaterinburg). The impedance spectroscopy method was used for electrochemical characteriza- tion of the ceramic samples of substituted Bi26Mo10O69 in the range of 523–1123  K using Elinz-3000 impedance spectro- meter. For the impedance measurements the samples were pelletized at 20 bar to yield pellets of 10 mm in diameter and ca. 2.5 mm thickness. The pressed pellets were then heated to 1123  K for 24 h, be- fore slow cooling in air to room tempera- ture. Platinum electrodes were applied to the pellets by covering them with a thin layer of NH4(PtCl6) following its decom- position at ca. 673  K. For the analysis of impedance plots the equivalent electrical circuits method was used (Zview soft- ware, Version 2.6b, Scribner Associates, Inc.). Results and discussion Characterization of ceramic and pow- der samples The solid solutions limit for Bi26-2xMn2xMo10O69-d determined by XRPD is x = 0.8. The solubility limits of Bi26-2xMn2xMo10-2yV2yO69-d and Bi26-2xMn2x- Mo10-2yFe2yO69-d are y = 0.4 and y = 0.2, re- spectively. Compositions with low dopant content crystallized in the triclinic struc- ture (y = 0.1), whereas the compositions with higher dopant content showed the monoclinic structure of Bi26Mo10O69-d [5]. The refined unit cell parameters of the Mn + Fe and Mn+V doped bismuth molyb- dates are given in Tables 1 and 2, respec- tively. They show general compression of unit cell with increase of y and a good adherence to Vegard’s law in the ranges of monoclinic modification. Decrease of homogeneity ranges as compared with Bi26Mo10-2yFe2yO69-d (y < 0.3 [13]) and Bi26Mo10-2yV2yO69-d (y < 0.7 [6]) can be ex- Table 1 Unit Cell Parameters of Bi26-2xMn2xMo10-2yV2yO69-d y Modifi-cation a±0.002, Å b±0.001, Å c±0.005, Å α±0.01, ° β±0.01, ° γ±0.01, ° V±0.01, Å3 0.1 tricl 11.751 5.858 24.561 89.80 102.19 89.20 1652.59 0.2 monocl 11.851 5.856 24.560 90 102.19 90 1666.02 0.3 monocl 11.847 5.851 24.540 90 102.27 90 1662.18 0.4 monocl 11.847 5.845 24.535 90 102.23 90 1660.39 Table 2 Unit Cell Parameters of Bi26-2xMn2xMo10-2yFe2yO69-d y Modifi-cation a±0.002, Å b±0.001, Å c±0.005, Å α±0.01, ° β±0.01, ° γ±0.01, ° V±0.01, Å3 0.1 monocl 11.731 5.796 24.772 90 102.82 90 1642.333 0.2 monocl 11.728 5.795 24.764 90 102.74 90 1641.619 plained by the distortion of MoOn polyhe- dra, caused by Mn presence. In the present work, as well as in [12], Mn ions were assumed to have an oxi- dation state of +2. Th e measurements of magnetic susceptibility vs. temperature gave a good adherence to Curie–Weiss law (Fig. 1). Th e calculated parameters of Curie–Weiss law are given in the Table 3. Calculated magnetic moment for Mn is 5.95 while the theoretical value assuming Mn+2 is 5.92. Th us manganese is a biva- lent dopant, which is typical for bismuth sublattice. Representative electron micrographs of powdered and ceramic samples are shown in Fig.  2. SEM micrographs of powdered samples showed a homoge- nous distribution of large and small grains (Fig.  2  a to d), with grain sizes in the range ~0.1–10  μm for all samples, which correlates well with the results of laser dispersion analysis. Aft er sintering of pellets, dense ceramic samples were formed (Fig.  2  a to  c). Th e majority of pores is isolated and has a spherical form. Sintered ceramic pellets’ densities of more than 97 % of the theoretical (X-ray) den- sity were confi rmed by the Archimedes method. Th e EDX analysis showed that the concentration of dopants was close to theoretical, whereas concentrations of bismuth and molybdenum couldn’t be determined separately because of the overlapping of analytical peaks. Th e con- centration of all dopants confi rms the the- oretical formula within the experimental errors. Electrical conductivity Impedance spectroscopy was used for the electrochemical characteriza- tion of the ceramic samples. Fig.  3  (a,  b) shows examples of complex plane plots of Bi26.4Mn0.6Mo9.6V0.4O69-d at diff erent tem- peratures. Th is shape of complex plane plots is typical for all studied composi- tions. Equivalent electrical circuits for every temperature gave good agreement between calculated and experimental im- pedance curves. It can be seen that the shape of the complex plane plots appears to be diff erent in diff erent temperature regions. In general, two types of complex plane plots can be distinguished: at rela- tively high temperatures and at relatively low temperatures. At high temperatures (higher than a.c. 823–873 K) the impedance plots of all in- vestigated complex oxides correspond to a broadened semicircle or two separate semicircles (Fig.  3a), the high frequency intercept is non zero. Th e equivalent elec- trical circuit used for the high temperature Table 3 Values for Curie–Weiss law χ = Ao+C/(T-θ) and μ for Bi24.4Mn1.6Mo10O69-d sample Ао cm3/mol С, cm3*K/mol θ, K μ calc μ(Mn +2)teor –0.00187 3.55 –4.7685 5.95 5.92 Fig. 1. Th e magnetic susceptibility of Bi24.4Mn1.6Mo10O69-d sample vs temperature. Points are experimental data, solid line – model curve (Curie–Weiss law) Fig. 3. Characteristic impedance plots and equivalent electrical circuits of Bi26.4Mn0.6Mo9.2V0.8O69-d at: a – high temperature region (973 K); b – low temperature region (623 K) Fig. 2. SEM-images of Bi25.4Mn0.6Mo9.6V0.4O69-d: a – ceramic pellet surface, secondary electrons imaging, scale 1:5000; b – ceramic pellet surface, backscattering electrons imaging, scale 1:6500; c – ceramic pellet cross-section, secondary electrons imaging, scale 1:5000; d – powder surface, secondary electrons imaging, scale 1:1000 region is shown in Fig.  3a. It can be de- scribed as R1 – R2(CPE1) – R3(CPE2) se- rial connection, where R2 (CPE1) and R3 (CPE2) fragments are parallel connections of resistor (R) and constant phase element (CPE). Th e R2(CPE1) and R3(CPE2) pa- rallel connections correspond to electro- chemical processes at the electrodes (the ionic migration and interfacial processes at the electrodes), what can be confi rmed by CPE1 and CPE2 capacity parameters values [14], and R1 describes bulk resist- ance of the sample. Grain boundary resist- ance wasn’t observed for all compounds. Th e similar results were presented in [15] for Bi26Mo10-xGexO69-d compounds, when only bulk resistance was observed. At low temperatures the complex plane plots of Bi26-2xMn2xMo10-2yV2yO69-d and Bi26-2xMn2xMo10-2yFe2yO69-d and their equivalent electrical circuits change (Fig. 3b). Th e impedance plot in this case consists of one separate distinct semicircle and two adjacent semi-circles. Th e high frequency intercept of the high-frequency semicircle is equal to zero. Th e equiva- lent electrical circuits can be described as R1(C1)–R2(CPE2)–R3(CPE3) serial con- nection, where Ri (CPEi or Ci) are parallel connections of resistor (R) and constant phase element (CPE) or capacitor (C). Th e R1(C1) connection describes the high frequency semi-circle, the C1 capacitance value is about 10–11 F, therefore R1 can be attributed to the total conductivity of the sample [14]. Th e R2(CPE2) element can be attributed to electrochemical processes at the electrodes (the «capacitance» value of CPE2 is about 10–6  F), R3 (CPE3) can describe complicated diff usion processes at low temperatures (the «capacitance» value of CPE2 is about 10–5 F). According to the results of the im- pedance measurements, electrical con- ductivity vs. temperature dependences were plotted for Bi26-2xMn2xMo10-2yV2yO69-d and Bi26-2xMn2xMo10-2yFe2yO69-d ce- ramic samples. Fig.  4 and 5 show Ar- rhenius plots of total conductivity of Bi26-2xMn2xMo10-2yV2yO69-d and Bi26-2xMn2x- Mo10-2yFe2yO69-d. Th e characteristic Ar- rhenius plots (log (σ)–1000/T) have lin- ear shape, and activation energy values (0.7–0.8 eV) are typical for the oxygen- ion conductors. For the parent compound Bi26Mo20O69-d, a signifi cant drop in con- ductivity and increase in activation energy (from 0.5–0.65 eV to 0.9 eV) is seen at tem- peratures below ca. 623 K due to the mon- oclinic to triclinic phase transition. Th is is not the case for the solid solutions, and it can be concluded that substitution lowers Fig. 4. Arrhenius plots of total conductivity for selected Bi26-2xMn2xMo10-2yV2yO69-d compounds Fig. 5. Arrhenius plots of total conductivity for selected Bi26-2xMn2xMo10-2yFe2yO69-d compounds the monoclinic to triclinic phase transi- tion to the temperatures below 523  K, which is the lower limit of the electrical measurements. A conductivity maximum occurs in single-phase compositions with maximum values of y = 0.2-0.4. The conductivity of Bi26.4Mn0.6Mo9.6Fe0.4O69–d was 1.2∙10–2  S∙cm–1 at 973  K and 2.2∙10–4  S∙cm–1 at 623  K, and the con- ductivity of Bi26.4Mn0.6Mo9.2V0.8O69–d was 2.2∙10–3 S∙cm–1 at 973 K and 2.2∙10–5 S∙cm–1 at 623  K, respectively. In general, the change of electroconductive properties can be explained by the increase of the electronic part of total conductivity due to the Mn doping, and by the raise of the oxygen-ion mobility due to the distortion of Mo(Fe/V)On polyhedra. Conclusions The solid solutions limit for Bi26-2xMn2xMo10O69-d is x = 0.8, and the sol- ubility limits of Bi26-2xMn2xMo10-2yV2yO69-d and Bi26-2xMn2xMo10-2yFe2yO69-d are y = 0.4 and y = 0.2, respectively. Compositions with low dopant content crystallize in the triclinic structure (y = 0.1), and composi- tions with higher dopant content – in the monoclinic structure. Mn+Fe and Mn+V doped systems were synthesized for the first time and the formation of dense ce- ramics of these systems has been demon- strated. In general, the change of electro- conductive properties depends of dopant concentration and is influenced by the metal-oxygen polyhedra distortion and the increase of the electronic part of total conductivity. Acknowledgements This work was financially supported by the Russian Foundation for Basic Research (projects № 16-33-60026, 17-53-04098). The equipment of the Ural Center for Shared Use «Modern nanotechnology» (Ural Federal University, Ekaterinburg) was used. XRPD data were obtained using the equipment of the Centre for Shared Use «Ural-M» (Institute of Metallurgy UB RAS, Ekaterinburg). References 1. Boivin JC. Structural and electrochemical features of oxide ion conductors. Int J Inorg Mat. 2001;3:1261-6. doi:10.1016/s1466-6049(01)00118-0. 2. Abraham F, Boivin JC, Mairesse G, Nowogrocki G. The bimevox series: A new fam- ily of high performances oxide ion conductors. Solid State Ionics. 1990;40-1:934-7. DOI:10.1016/0167-2738(90)90157-M. 3. Fonseca FC, Steil MC, Vannier RN, Mairesse G, Muccillo R. Grain-sized influence on the phase transition of Bi26Mo9WO69: an X-ray diffraction and impedance spectros- copy study. Solid State Ionics. 2001;140:161-71. DOI:10.1016/S0167-2738(01)00705-6. 4. Buttrey DJ, Vogt T, Yap GPA, Rheingold AL. The structure of Bi26Mo10O69. Mater Res Bull. 1997;32:947-62. DOI:10.1016/s0025-5408(97)00063-9. 5. Vannier RN, Mairesse G, Abraham F, Nowogorski G. Bi26Mo10Oδ solid solution type in the Bi2O3–MoO3–V2O5 ternary diagram. J Solid State Chem. 1996;122:394-406. DOI:10.1006/jssc.1996.0133. 6. Vannier RN, Danze S, Nowogrocki G, Huve M, Mairesse G. A new class of mo- no-dimensional bismuth-based oxide anion conductors with a structure based on [Bi12O14]∞ columns. Solid State Ionics. 2000;136-7:51-9. DOI:10.1016/S0167- 2738(00)00351-9. 7. Ling CD, Miiller W, Johnson MR, Richard D, Rols S, Madge J, Evans IR. Local structure, dynamics, and the mechanisms of oxide ionic conduction in Bi26Mo10O69. Chem Mater. 2012;24:4607-14. DOI:10.1021/cm303202r. 8. Bastide B, Enjalbert R, Salles P, Galy J. Ionic conductivity of the oxide fam- ily Bi[Bi12O14][(Mo,M)O4]5 with M=Li, Mg, Al, Si, Ge and V. Solid State Ionics. 2003;158:351-8. doi:10.1016/s0167-2738(02)00910-4. 9. Mikhailovskaya ZA, Buyanova ES, Petrova SA, Morozova MV, Zhukovskiy VM, Zakharov RG, Tarakina NV, Berger IF. Cobalt-doped Bi26Mo10O69 : crystal structure and conductivity. J Solid State Chem. 2013;204:9-15. DOI: 10.1016/j.jssc.2013.05.006. 10. Enjalbert R, Hasselmann G, Galy J. A new mixed oxide with (Bi12O14)n columns: PbBi12Mo5O34. Acta Crystallogr, Sect C: Struct Chem. 1997;53:269-72. DOI:10.1107/ s0108270196013698. 11. Galy J, Salles P, Rozier P, Castro A. Anionic conductors Ln2/3[Bi12O14](MoO4)5 with Ln=La, Nd, Gd, Ho, Yb. Synthesis–spark plasma sintering–structure–electric prop- erties. Solid State Ionics. 2006;177:2897-902. DOI:10.1016/j.ssi.2006.07.059. 12. Mikhaylovskaya ZA, Buyanova ES, Morozova MV, Petrova SA, Nikolaenko IV. Mn-doped Bi26Mo10O69-d: synthesis and characterization. Ionics. 2017;23:1107-14. DOI:10.1007/s11581-016-1917-5. 13. Mikhaylovskaya ZA, Morozova MV, Buyanova ES, Petrova SA Abrahams I. Iron- doped Bi26Mo10O69 bismuth molybdate:synthesis, properties and structure. In: Ab- stract Book of the 11th international symposium on systems with fast ionic transport. 2014 Jun 25-29; Gdańsk University of Technology, Gdańsk-Sobieszewo, Poland. 2014. P. 78. 14. Irvine JTS, Sinclair DC, West AR. Electroceramics: Characterization by impedance spectroscopy. Adv Mater. 1990;2:132-8. DOI:10.1002/adma.19900020304. Cite this article as (как цитировать эту статью) Mikhaylovskaya ZA, Morozova MV, Buyanova ES, Petrova SA, Nikolaenko IV, Kellerman DG. Preparation and characterization of Bi26-2xMn2xMo10O69-d and Bi26.4Mn0.6Mo10-2yMe2yO69-d(Me = V, Fe) solid solutions. Chimica Techno Acta. 2017;4(2):120–127. DOI:10.15826/chimtech/2017.4.2.027