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New Rb3PO4-based ceramic materials with high rubidium-cation conductivity in the Rb3–2x Pb x PO4 system have been synthesized and studied. Introduction of Pb2+ cations leads to a sharp increase in the conductivity of rubidium orthophosphate due to formation of cation vacancies and, at temperatures 350–550°C, also due to the stabilization of high-temperature cubic modification Rb3PO4. At high temperatures, the electrolytes prepared have very high ion conductivity higher than 10–1 S cm–1 at 700°C, which is higher than the values previously obtained in similar systems with additions of tin and cadmium ions. The factors influencing the transport properties of the materials under study are discussed.

New materials of the K3 − 2xPbxPO4 system with high potassium-cation conductivity have been synthesized and studied. It has been found that the introduction of Pb2+ cations substantially increases the conductivity of K3PO4 due to the formation of potassium vacancies and the stabilization of the high-temperature cubic structure of the orthophosphate. At low temperatures, the maximum conductivity has been observed in the composition range x = 0.15–0.20 and varies from ∼10−2 S cm−1 at 400°C to ∼10−1 S cm−1 at 700°C. The factors influencing the transport properties of the materials under study have been discussed.

New potassium-conducting solid electrolytes of the K3 − 2xCdxPO4 system are synthesized and studied. A wide range of solid solutions reaching x ≈ 0.35 with the structure of high-temperature modification of potassium orthophosphate forms in the system. An addition of cadmium ions leads to an abrupt increase in the K3PO4 conductivity due to the formation of potassium vacancies. The highest conductivity is approximately 10−2 S cm−1 at 300°C and above 10−1 S cm−1 at 700°C.

The lithium-conducting solid electrolytes in the Li4 − 2xCdxGeO4 (0 ≤ x ≤ 0.6) system are synthesized. Their crystal structure and temperature and concentration dependences of conductivity are studied. The specimens with the highest conductivity have a γ-Li3PO4-derivative structure. The solid solutions with x = 0.15–0.25 are stable at the room temperature, whereas the specimens with x ≥ 0.3 decompose yielding Li2CdGeO4 below 310 ± 10°C. Li3.6Cd0.2GeO4 solid solution exhibits the highest conductivity (5.25 × 10−2 S cm−1 at 300°C). The factors, which affect the conductivity of synthesized solid electrolytes, are considered.

Solid solutions based on rubidium monogallate RbGaO2 with a general formula Rb2−2xGa2−xAxO4 (A = P, V, Nb, and Ta) are synthesized. Their crystal structure and temperature and concentration dependences of conductivity are studied. The highest rubidium-cationic conductivity is (1.8–3.9) × 10−3 S cm−1 at 400°C and (1.4–2.1) × 10−2 S cm−1 at 700°C. These results are compared with the data for rubidium monogallate doped with four-charged cations and solid solutions based on RbAlO2.

Solid solutions based on RbFeO2 in Rb2 − 2xFe2 − xNb(Ta)xO4 (0 ≤ x ≤ 0.2) systems were synthesized. Their crystal structure and the temperature and concentration dependences of the total and ion conductivities were studied. The ranges of composition and temperature with dominant rubidium-cation conductivity were determined. For the Rb1.9Fe1.95Ta0.05O4 solid solution, the rubidium-cation conductivity was found to be 2.3 × 10−2 S/cm at 200°C, which exceeds the electric conductivity of other known polycrystalline rubidium-cation solid electrolytes.

Solid solutions based on CsFeO2 in Cs2 − 2xFe2 − xAxO4 systems (A = P, V) were synthesized. Their crystal structure and the temperature and concentration dependences of the total conductivity and its electronic component were studied. The ranges of temperature and composition in which cesium-cation conductivity was dominant were determined. The obtained data were compared with the results of studies of other cesium-conducting solid electrolytes.

The rubidium monoferrite RbFeO2-based solid solutions with the composition Rb2 − 2xFe2 − xPxO4 have been synthesized, and their crystal structure and the temperature and concentration dependences of the total and electron conductivities have been studied. The introduction of P5+ ions has been found to sharply decrease the electron conductivity that prevails in pure rubidium monoferrite and, at the same time, to increase the ionic conductivity. The latter becomes dominant as the phosphorus concentration increases. The maximum rubidium-cation conductivity of the materials under study is ∼3 × 10−2 S/cm at 300°C and ∼3 × 10−1 S/cm at 700°C. The results have been compared with the previously obtained data for similar solid solutions based on rubidium monogallate and monoaluminate.

New solid electrolytes with a high conductivity by K+ ions in the K1 − 2xSrxGaO2 system are synthesized and studied. It is found that the introduction of Sr2+ ions into potassium monogallate leads to the formation of solid solutions with KGaO2 structure in a wide range of additive concentration. These solid solutions exhibit a high conductivity; the conductivity increases monotonically with increasing concentration of strontium within the single-phase range. The electrical characteristics are related to the electrolyte structure. The results are compared with the earlier data for the gallate solid electrolytes with the additives of four-charged cations and the systems based on potassium monoferrite and monoaluminate.

Solid electrolytes with potassium-cation conductivity in the K1 − 2xPbxGaO2 system were synthesized and studied. It was found that solid solutions based on potassium monogallate are formed in a wide range of compositions. They contain vacancies in the potassium sublattice that provide for high conductivity of electrolytes. The relationship is considered between electric characteristics of solid electrolytes and the composition and structure of solid solutions. The results are compared to the earlier obtained data for similar solid electrolytes based on potassium monoaluminate and monoferrite.

Rb2 − 2x Al2 − x A x O4 (A = Nb, Ta) solid solutions have been synthesized, and their conductivity has been measured as a function of temperature and composition. The highest rubidium ion conductivity in the Rb2 − 2x Al2 − x A x O4 solid solutions is 3.16 × 10−3 S/cm at 300°C and ∼ 2 × 10−2 S/cm at 700°C. The high rubidium ion conductivity of the synthesized solid electrolytes is mainly due to the formation of rubidium vacancies when Nb5+ or Ta5+ substitutes for Al3+ and to the specific features of the crystal structure of RbAlO2.

New potassium ion conducting solid electrolytes based on potassium monoferrite have been prepared through partial substitution of the divalent cation Cd2+ on the potassium site, and their properties have been investigated. The introduction of cadmium cations sharply increases the electrical conductivity of KFeO2 over the entire temperature range studied. In addition to the maximum in conductivity at the boundary of the K1 − 2xCdxFeO2 solid solution, there is a maximum at a higher cadmium content (x = 0.30–0.35). The possible origins of this maximum are discussed.

Solid solutions based on cesium monogallate CsGaO2 are synthesized in the Ga2O3-TiO2-Cs2O system. Their crystalline structure and also temperature and concentration conductivity dependences are studied. The cesium cation character of conductivity is confirmed. The most conducting samples contain an excess of cesium oxide and have the structure of high-temperature γ-modification of KAlO2. Their specific conductivity is (5.0–6.7) × 10−3 S cm−1 at 400 °C, (2.5–5.0) × 10−2 S cm−1 at 700°C at the activation energy of 33–35 kJ/mol−1.

RbAlO2-based solid solutions are synthesized in the systems Rb2 − xAl2 − xAxO4 (A = P, V). Temperature and concentration dependences of their conductance are studied; the rubidium-cationic character of the conductance is corroborated. The high ionic conductivity of the synthesized phases (∼5 × 10−3 S cm−1 at 300°C, ∼2 × 10−2 S cm−1 at 700°C) is caused by (a) formation of rubidium vacancies as a result of the replacing of Al3+ ions by fivefold-charged cations of phosphorus or vanadium and (b) disordered crystal structure of γ-KAlO2 type. The obtained data are compared with the results of the studying of similar systems.

Cs3 − 3xMxPO4 (M = Sc, Y, La, Sm, Nd) solid electrolytes have been synthesized, their phase composition has been determined, and their electrical conductivity has been measured as a function of temperature. In all of the systems, we have identified cesium orthophosphate based solid solutions. Above ∼550°C, the solid solutions are isostructural with the high-temperature, cubic phase of Cs3PO4. They offer high cesium ion conductivity owing to the formation of cesium vacancies via 3Cs+ → M3+ substitutions and the decrease in phase transition temperature. The conductivity of the synthesized solid solutions, (4.8−5.6) × 10−3 S/cm at 300°C and (1.6−1.9) × 10−1 S/cm at 800°C, is at the level of earlier studied Cs3 − 2xMxIIPO4 solid electrolytes.

Solid solutions on the basis of cesium orthophosphate in the Cs3−x P1−x E x O4 (E = S, Cr, Mo, W) systems are synthesized. The temperature and concentration dependences of the systems’ electroconductivity are studied. The cesium-cation character of conduction in the systems is confirmed. Factors that bear upon the transport properties of the synthesized phases are analyzed.

AlN-Mg3N2 solid solutions are synthesized, and their electrical conductivity and ionic transference number are measured. In the range 500–1150°C, the conductivity of the solid solutions increases with Mg3N2 concentration. The ionic transference number is close to unity, and the ionic transport is mainly due to aluminum ions.

Solid electrolytes in the systems K3 – 3xMexPO4 (Me = Sc, Y, In, La, Nd, Gd, Tb) are synthesized. Their phase composition and the temperature and concentration dependences of their electroconductivity are studied. In all the systems there form solid solutions based on K3PO4, which have a high potassium cation conductance. The latter is due to the formation of potassium vacancies at substitutions 3K+→ Me3+ and, at lower temperatures, to stabilization of a high-temperature β-modification of potassium orthophosphate. The electroconductivity of synthesized solid solutions, which equals (4–7) × 10–3 and ∼10–1 S cm–1 at 300 and 700°C, is similar to that of solid electrolytes K3 – xP1 – xExVIO4 and K3 – 4xEx′IVPO4.

In the sodium-orthophosphate-based solid solutions in Na3 – 2x M x PO4 systems (M = Cd, Pb), the electroconduction is maximum near the upper concentration boundaries of the single-phase regions: x ≈ 0.4 for M = Cd and x ≈ 0.25 for M = Pb. The conductivity values at 300°C are 6.25 × 10–3 and 2.5 × 10–3 S/cm, respectively. The conduction of synthesized solid electrolytes has a co-cation nature. Their electric characteristics, inferior to those of the Na3PO4-based solid solutions obtained via heterovalent substitutions of another type, may be a manifestation of an effect similar to the polyalkali effect.

Linear thermal expansion coefficients (K) for co-cation solid electrolytes of three types are measured. The electrolytes include solid solutions (K1 – xRb x )3.8Me0.1P2O7 (Me = Ca, Ba) with the structure of γ-K4P2O7 (I); fcc-solid solutions (A1 – xA" x )MO2–EO2 (A, A" = K, Rb, Cs; M = Al, Ga, Fe; E = Si, Ge, Ti) of γ-KFeO2 type (II); and tetragonal solid solutions 0.8(Li1 – xNa x AlO2) · 0.2TiO2 with the LiAlO2 structure (III). Dependences of K on the ratio of alkali cation amounts in I and II have a maximum, i.e. the polyalkali effect (PAE) of K is observed, while in III this dependence is practically linear (PAE of K is absent). The correlation is found between PAE of K and the electroconductivity of the electrolytes.

Solid solutions based on K3PO4 in systems K3 – 4xE"xPO4 (E" = Si, Ti, Ge, Zr, Sn, Hf, Ce) are synthesized. The crystal structure, thermal behavior, and electroconduction of the synthesized solutions is studied. The narrowest single-phase regions take place in the systems where E" = Si, Ge (x ≤ 0.025), and the widest, in the system with Zr (x ≤ 0.125, at 700°C). Introducing Ti or Sn additives (x ≥ 0.05) and minimum quantities of Zr, Hf, or Ce (x = 0.025) into potassium orthophosphate leads to stabilization of highly-conductive β-modification of K3PO4 at room temperature. Maximum values of potassium-cation conduction in all the systems studied correspond to regions of single-phase solid solutions based on K3PO4. The maximum electroconductivity (0.005 S cm–1 at 300°C, ∼0.1 S cm–1 at 700°C) and the smallest activation energies (32–35 kJ mol–1) take place in the systems with Zr and Hf.

Solid solutions of the structure γ-K4P2O7, which have the composition (K1 – xRb x )3.8M0.1P2O7(M = Ca, Sr, Cd, Ba) and (K1 – xRb x )4 – 2yCa y P2O7(y= 0.025–0.20), are synthesized. The solutions' electroconductivity is studied in the temperature range 400–600°C. The concentration dependences of the conductivity and the activation energy for conduction have extremums at x≈ 0.6, which points to the polyalkaline effect. The effect increases with decreasing radius of cation M2+and with increasing concentration of calcium cations, which is explained by an increased energy nonequivalence of positions in the cation sublattice accessible to alkali ions.

Transport numbers for potassium and rubidium cations in solid electrolytes (K1 – xRb x )3.8M0.1P2O7(M = Ca, Cd, Ba) are measured, and the co-cation nature of the electrolytes" conduction is confirmed. The concentration dependences of transport numbers are used to draw a conclusion about the ratio between mobilities of potassium and rubidium cations. The presence of an extremum in the concentration dependence of the activation energy for the rubidium-cation constituent of conduction is confirmed by the NMR method. The obtained results are explained by assuming that the polyalkaline effect is largely due to an ordering of mobile alkali cations.

Solid solutions of cubic structure based on K3PO4are synthesized in the systems K3 – xP1 – xE x O4(E = S, Cr, Mo, W). Approximate boundaries of single-phase regions and the temperature and concentration dependences of electroconductivity are studied. The transport numbers, measured by a modified Tubandt method, confirm a potassium cationic character of conduction. Electric parameters of the electrolytes are compared with those of sodium and rubidium conducting solid solutions in similar systems. Factors that affect transport properties of synthesized phases are discussed.

Samples of systems (Na 1-x K x )3.8M0.1P2O7 (M = Ca, Sr, Cd; 0 ≤x ≤ 1) are synthesized. It is established that solid solutions with the structure of γ-K4P2O7 form in these systems. At room temperature, the solutions exist atx ≥ 0.7. In the calcium-containing system, they form a continuous series at temperatures exceeding ∼500°C. The co-cation character of conduction of the solutions is confirmed by measured transport numbers. The concentration dependences of the electroconductivity and its activation energy point to a polyalkali effect, which decreases with an increase in the M2+ ion radius. Partial conductivities of sodium and potassium cations in the (Na 1-x K x )3.8Ca0.1P2O7 system are calculated and their dependences on the balance between alkali cations in the solid solution are discussed

Solid solutions based on rubidium pyrophosphate are synthesized in the Rb4 -2xMxP2O7 systems (M = Ca, Sr, Ba, Pb). The temperature and concentration dependences of their rubidium-cation conductivity are investigated. The X-ray data for the low and high-temperature forms of Rb4P2O7 and also for solid solutions based on it are reported. The effect exerted by modifying cation M2+ on electrical properties of synthesized solid solutions is considered

Regions of the formation of tetragonal solid solutions with the γ-LiAlO2 structure and the co-cation conduction are studied in the systems 0.9Na1-xLixAlO2-0.1EO2 (E = Ti, Ge, Si) and 0.8Na1-xLixAlO2-0.2TiO2. The single-phase regions of solid electrolytes narrows down with decreasing size of cations of a rigid lattice. No polyalkaline effect exists in the above systems. In the single-phase region of solid solutions, variations in the conductivity is caused largely by variations in the partial conductivity of the more mobile sodium cation.

Solid solutions Cs3 – 2xMxPO4 (M = Ba, Sr, Ca, Mg) are synthesized and their thermal behavior and electroconductivity are examined. Adding elements of Subgroup IIA of the periodic table into cesium orthophosphate shifts the phase transition, which occurs in pure Cs3PO4 at 450–620°С, towards lower temperatures and raises the cesium cation conductivity at low temperatures. The electroconductivity of a high-temperature modification of Cs3PO4 is weakly dependent on the presence and concentration of such additives, which points to structural disordering of the cesium sublattice.

Sm1-xCaxCr1-yCuyO3 perovskite solid solutions were prepared, and their properties were studied. The solid solutions, unreactive with a ZrO2-based solid electrolyte, are candidate electrode and interconnection materials for high-temperature fuel cells