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=2Na_XK_1-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.

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), Na_0,5 K_0,5WSbO₆ (b), Na_0,6K_0,4WSbO₆ (c), Na_0,8K_0,2WSbO₆ (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 Na_xK_1-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 Na_xK_1-xWSbO₆ (x = 0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6).

?- 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_?K_1-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.

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