Pyrochlore Type Complex Oxide the Formation of Phases in the Example of Compounds and the Distribution of Ions in Them

Bozorov XN and Lupitskaya YA

Published on: 2022-02-24


The processes of phase formation were studied on the example of the complex oxide (1-x) K2CO3 - xNa2CO3 - Sb2O3 - WO3 system of the pyrochlore-type Me2CO3-Sb2O3-WO3 system, and thermo gravimetric and radiological analyzes of these phases were performed.  The distribution of ions in the obtained phases was studied.


Complex oxides of antimony; Pyrochlore type structure; Phase formation; Space group Fd3m; Solid phase reaction


Solid electrolytes with ionic conductivity for potassium cations are of interest for use in various energy systems. However, unlike sodium solid electrolytes, relatively little is known about solid potassium conductors; therefore, the search for new potassium super ion conductors remains relevant. The synthesis of new materials with high ion permeability allows the creation of various electrochemical devices based on them, including sensors, current sources, fuel cells and more. Compounds based on pyrochlore - type complex driving oxides are of particular importance because they are good ion conductors and ion exchangers [1-6]. These compounds can be obtained by solid phase synthesis in a system containing Me2CO3-Sb2O3-WO3 (where Me = K, Na) [7-14]. However, the formation and stability of the phases obtained in this system have not yet been studied, and the concentration interval for the synthesis of pyrochlore - type phases has not been determined. In this regard, the purpose of this work was to study the processes of phase formation in the system Me2CO3-Sb2O3-WO3 (where Me = K, Na), to determine the composition and structure of the formed phases. Trivalent antimony and hexavalent tungsten powder oxides, chemically pure potassium and sodium carbonates were obtained as starting reagents. Sample synthesis was performed using a well-known method [15-18]. The mixtures were prepared with different ratios of (1-x)K2CO3-?Na2CO3-Sb2O3-WO3 reagents, and the molar ratio of the components of the mixture x varied by 0.1 steps in the concentration range 0 ≤ x ≤ 1.0 (Table 1). After the samples were thoroughly ground with a thief, they underwent two-stage curing in air at a temperature of 673 K and 1170 K.  At a temperature of 1170 K, the samples are stored for 13 hour until a continuous sample mass is obtained. The gross composition of the final products of solid phase synthesis was calculated by the following reaction:

(1-x)K2CO3+?Na2CO3+Sb2O3+2WO3+0,5?2=2NaXK1-xWSbO6+2CO2    (1), where 0 ≤ x ≤ 1.0 (Table 1).

It has been hypothesized that solid phase synthesis will result in the complete decomposition of potassium and sodium carbonates and the oxidation of the trivalent lead to the pentavalent state.

Table 1: The mass of the initial reagents and the gross composition of the final product, x reagents of the initial mixture were obtained as a result of solid phase synthesis at different molar ratios.










Brutto content












Na0,2K 0,8WSbO6






Na 0,4K 0,6WSbO6






Na0,6K 0,4WSbO6






Na0,8 K0,2WSbO6







Thermogravimetric studies show that the formation of pyrochlore phases proceeds in several stages and is characterized by low temperature (temperature range 297–623 K) and high temperature (temperature range 653–1123 K) areas. Thus, the DTG curves of the initial mixture of (1-?)K2CO3-?Na2CO3-Sb2O3-WO3 content record the degradation processes, the decomposition of potassium carbonate, and the maxima showing the oxidation of the trivalent driving ions to the pentavalent state (Figure1, DTG). In this case, the TG curves show places where the mass of the samples does not change with increasing temperature (Figure1, TG). This indicates that the phases of a particular composition are formed in these temperature ranges.

Figure 1: Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of thermolysis of the initial mixture containing

The phase composition was observed in filtered CuKα1- irradiation using an X-ray analysis method performed on a D8 ADVANCE diffractometer (Brooker, Germany). Parameter a of the elementary cell was determined by reflex 10.6.2. The error in determining parameter a was ± 0.003 Å. X-rays of the obtained samples are a complex set of diffraction maxima characterized by two concentration fields (Figure 2abcd). In the first field (0 ≤ x ≤ 0.6) the diffraction pattern does not change significantly. A certain set of diffraction maxima is observed, the number of which does not change within a certain angle of inclination (Figure 2abc). In the second field (0.6 ≤ x ≤ 1.0) this group of reflexes disappears and maxima appear at other diffraction angles (Figure 2, d). X-rays of 0 ≤ x ≤ 0.6 (first field) of the purified mixtures contain a certain set of diffraction maxima satisfactorily described for cubic syngonium crystals, and the loss of reflexes indicates that the phases differ in composition in this concentration range with the pyrochlore - type structure Fd3m spatial symmetry groups are formed (Table 1).The observation of additional reflexes in the second area (0.6 ≤ x ≤ 1.0) is associated with the formation of phases with a different type of symmetry (Figure 1d).

Figure 2: Diffract grams of samples obtained after curing the initial mixture of 1170 K and having the following composition: KWSbO6 (a), Na0,5 K0,5WSbO6 (b), Na0,6K0,4WSbO6 (c), Na0,8K0,2WSbO6 (d).

The phases with a pyrochlore - type structure differ from each other by the value of the element cell parameter a (Figure 3a). With an increase in sodium ions in the system, radiographs show a redistribution of reflex intensity with even and odd indices. In particular, the intensity of the < 311 > reflex decreases monotonically with increasing x relative to the < 222 > reflex (Figure 3b). At the same time, the element cell parameter rises from 10,234 Å to 10,263 Å (Figure 3a).

Figure 3: Changes in the elemental cell parameter and the relative intensity of the reflex I311/I222 (b) depend on the sodium ions in the phases containing NaxK1-xWSbO6 at x at 0 ≤ x ≤ 0.6.

Such a decrease in the intensity of reflexes with odd indices in the obtained phases is associated with the filling of 16d - positions with sodium ions [19]. Thus, in the KWSbO6 composition phase (x = 0), ions of close radius (r (Sb (V)) = 0.062 nm and r (V (VI))= 0.065 nm [20]) prefer six coordinates and are statistically located in the center of the octahedron (positions 16c). Oxygen anions are located at the top of the octahedron (48f - positions), forming strong covalent bonds with antimony and tungsten cations, and thus form the anionic base of a pyrochlore - type structure. The structure formed only at the edges of the [W(VI)O3]0 [Sb(V)O3]1- octahedron has an excess negative charge [21]. The presence of positively charged ions is necessary to maintain the electronic neutrality of the system, in which case they are potassium ions. Partial replacement of tungsten ions with lead ions vacates positions 8b (Table 2), where potassium ions are located, the radius of which (r(K+)=0,133 nm, r(Na+)=0,098 nm) is greater than that of sodium ions. [20]. This is indicated by close values of the intensity of the double and odd index reflexes (Figure 3b). As the number of sodium ions in the phases increases, they fill the empty 16d-positions, while the potassium ions are removed from the 8b - positions (Table 2). This leads to an increase in the elemental cell parameter (Figure 3a) and a decrease in the intensity of reflexes with odd indices (Figure 3b). Thus, the following model of filling the positions of the pyrochlore - type structure in the obtained phases can be proposed. Antimony and tungsten ions are statistically located in position 16c, oxygen anions in position 48f, and potassium and sodium ions in positions 8b- and 16d, respectively (Table 2). 

Table 2: Distribution of ions in a regular system of Pyrochlore type structure points in the phases NaxK1-xWSbO6 (x = 0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6).


x, Relative
















































?- vacancies in positions 16d- and 8b- of the pyrochlore - type structure When the (1-?) K2CO3-?Na2CO3-Sb2O3-WO3 mixture is heated, phases with a pyrochlore - type structure with Na?K1-xWSbO6 (?=0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6) content are formed in the (0 ≤ x ≤ 1) field. In the obtained phases, the following distribution of ions is carried out in the point system of the group Fd3m spatial symmetry: antimony and tungsten ions are statistically located at 16c- positions, oxygen anions - at 48f- positions, potassium and sodium ions - at 8b- and 16d- positions, respectively.




  1. Palguev SF. Polymer membranes, polyantimony acids. Hardness of electrolyte with proton conductivity. Journal of Applied Chemistry. 1996; 69: 3 - 11.
  2. Yaroslavtsev AB. Proton conductivity of inorganic hydrates. Advances in Chemistry. 1994; 5: 449-455.
  3. Ukshe Hardness of electrolytes. 1977; 175.
  4. Tretyakov YD. Development of the chemistry of solid-phase materials with high ionic conductivity. THE USSR. Inorgan. materials. 1979; 15: 1014-1018.
  5. Wuensch BJ, Eberman KW, Heremans C, Ku EM, Onnerud P, Yeo EME, et al. Connection between proton conductivity of pyrochlore fuel-cell materials and structural change with composition and temperature. Solid State Ionics. 2000; 129: 111 - 133.
  6. Belinskaya FA, Militsina EA. Inorganic ion-exchange materials based on sparingly soluble surmi (B) compounds Uspekhi khimii. 1980; 49: 1904 - 1936.
  7. Burmistrov VA, Zakharyevich DA. Formation of ion-conducting phases with the structure of defective pyrochlore in the K2O- Sb2O3-WO3 system. Inorganic Materials. 2003; 34: 77-81.
  8. Burmistrov VA, Ryabishev VY, Ryabishev YM, Neryakhina SS. Formation of sodium antimonates during solid-phase interaction Sb2O3–Na2CO3. Journal of Inorganic Chemistry. 1997; 42: 1905-1907.
  1. Belinskaya FA, Militsina EA. Inorganic ion-exchange materials based on sparingly soluble compounds of antimony (B). Advances in Chemistry. 1980; 49: 1904-1936.
  2. Stewart DI, Knop SO. Pyrochlores VI. Preparative chemistry of sodium and silver antimonates and related compounds. Can J 1970; 48: 1323-1332.
  3. Trofimov VG, Trofimov VG, Sheinkman AI, Goldstein LM, Kleshev GV. On the pyrochlore-type phase in the Na-Sb-O system. Crystallography. 1971; 16: 438-440.
  4. Burmistrov VA, Lupitskaya YA. Formation of ion-conducting phases with the structure of defective pyrochlore in the system ??2CO3-?Sb2?3-2(2-?) WO3 (0<?<2.0) during heating. Abstracts of the 12th International Symposium "Order, Disorder and Properties of Oxides. Rostov-on-Don. 2009; 123-126.
  5. Stevart DI, Knop O, Woodhams FWD, Ayasse C. Pyrochlores VII. The oxides of antimony: an X-ray and mosabauer study. Can J Chem. 1972; 50: 690-701.
  6. Klestchov DG, Burmistrov VA, Sheinkman AI, Pletnev RN. Composition and structures of phases formed in the process of hydrated antimony pentaoxide thermolysis. J Solid State Chem. 1991; 94: 220-226.
  7. Burmakin EI. Principles of Mudflow Directed Synthesis of Highly Conductive Solid Electrolytes Based on Complex Oxides Tez. Report VI All-Union Conference on Electrochemistry. Chernivtsi. 1988; 3: 285 - 286.
  8. Mirkin LI. Handbook of X-ray diffraction analysis of polycrystals. M GIF-ML. 1961; 863.
  9. Balicheva TG, Roy NI. Study of the structure of products of polycondensation of oxycompounds C6(B) by IR spectroscopy and thermal analysis. In the book: Problems of modern chemistry of coordination compounds. Leningrad. LGU. 1974; 231-265.
  10. Wendlandt U, Mir Thermal methods of analysis. 1978; 276.
  11. Burmistrov VA, Ryabishev VYu, Ryabishev YM, Neryakhina SS. Formation of sodium antimonates during solid-phase interaction of Sb2O3 - Na2CO3. Journal of Inorganic Chemistry. 1997; 42: 1905 -
  12. Efimov AI, Belorukova LP, Vasilkova IV, Chechev VP. Properties of inorganic compounds. Directory. L Chemistry. 1983; 392.
  13. Serezhkin VN, Pushkin DV, Buslaev YA. Stereochemical features of oxygen compounds of antimony. Journal of inorgan. Chemistry. 1999; 44: 76 - 80.