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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.

The earlier obtained data on the transport properties of cesium-conducting solid electrolytes based on cesium orthophosphate in the Cs3–2xMx II PO4 (MII = Mg, Ca, Sr, Ba), Cs3–3x PO4 (Mx III = Sc, Y, La, Sm, Nd) and Cs3–xP1–xZx VI O4 (Z = S, Cr, Mo, W) systems are analyzed. It is shown that, in addition to the conventional jump mechanism, the “paddle wheel” mechanism can play an important role in the ionic transport. This mechanism is associated with the orientation disordering of [PO4] tetrahedrons at the elevated temperatures, which leads to their rotation promoting “pushing” cesium ions into the accessible neighboring positions.

New solid electrolytes with a high rubidium-cation conductivity, which are based on Rb3PO4 with partial substitution of three-charged yttrium, lanthanum, and neodymium cations for Rb+ cations, are synthesized and studied. Introduction of M3+ cations leads to an abrupt increase in the conductivity of rubidium orthophosphate due to the formation of cationic vacancies and stabilization of high-temperature cubic β-modification of Rb3PO4. In all systems, the highest conductivity is observed in the single-phase region of β-phase existence: it is ~10–1 S cm–1 at 700°C and above 10–2 S cm–1 at 400°C. The factors, which have an effect on the transport properties of studied electrolytes, are considered.

Solid electrolytes with rubidium-cation conductivity in the Rb1−x Al1 − x E x O2 (E = Si, Ti, Ge) systems are synthesized. Their phase composition, temperature and concentration dependences of conductivity are studied. A high rubidium-cation conductivity (3 × 10−3 S cm−1 at 300°C, 1.4 × 10−2 S cm−1 at 700°C) is caused by the formation of rubidium vacancies as a result of Rb+ + Al3+ → E4+ + VRb substitution and stabilization of disordered structure of γ-KAlO2 type.

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.

Solid lithium electrolytes in the Li4-3xFexGeO4 system were synthesized. Their phase composition, thermal behavior, and electrical conductivity were studied in the temperature interval 300–750°C. Introduction of Fe3+ ions into lithium orthogermanate leads to the formation of a γ-Li3PO4-type structure and to a sharp increase in the conductivity, with a maximum reached at x = 0.075–0.15: about 10−1 S cm−1 at 300°C and more than 1 S cm−1 at 700°C. The main current carriers are interstitial Li+ cations weakly bound with the rigid framework. Owing to high conductivity, the electrolytes studied are of interest for use in high-temperature electrochemical devices.

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.

New potassium-conducting solid electrolytes in the mixed ferrite-aluminate K1.80(Fe1 − x Al x )1.90V0.10O4 and K1.85(Fe1 − x Al x )1.925P0.075O4 are synthesized and studied. In the first system, the conductivity only slightly depends on the composition; therefore, the composition of solid electrolyte can be optimized with respect to the conductivity-stability characteristics. In the second system, extremes and considerable negative deviations from the additivity are observed in the conductivity isotherms. Possible reasons for this phenomenon are discussed.

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.

New potassium-conducting solid electrolytes in the mixed ferrite-aluminate system K0.85Pb0.075(Fe1 − xAlx)O2 are synthesized and studied. The electrolytes exhibit high ionic conductivity in the studied temperature range of 350 to 750°C (approximately 10−2 S/cm at 400°C and approximately 10−1 S/cm at 700°C). An increase in the conductivity with increasing concentration of iron in the specimens is a general tendency. However, in a wide range of compositions (from x = 0.2 to x = 0.9), the conductivity only slightly depends on x. Possible reasons for the effect of Fe/Al ratio in the structure of solid electrolytes on their transport properties are discussed.

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.

Data on transport properties of potassium conducting solid electrolytes based on K 3 PO 4 in the systems K 3 − 2x M x PO 4 (M = Mg, Ca, Sr, Ba, Zn, Cd, Pb) and K 3 − x P 1 − x E x O 4 (E = S, Cr, Mo, W) have been analyzed. The results indicate that, in addition to the usual hopping mechanism, a substantial significance for ion transport in similar phases belongs to the “paddle wheel” mechanism. This mechanism is due to orientation disorder of the PO 4 tetrahedra at elevated temperatures, which promotes moving K + ions to the neighboring vacant positions.

New solid rubidium-conducting electrolytes based on rubidium monoferrite in the system of Rb2 − 2xFe2−xVxO4 are synthesized and studied. It is found that introduction of V5+ ions causes a drastic decrease in the electronic conductivity component prevalent in pure RbFeO2 with a simultaneous increase in the ionic conductivity. The latter becomes predominant at an increase in the concentration of vanadium. The optimum compositions of the studied electrolytes feature a very high cationic rubidium conductivity (∼1.8 × 10−2 S cm−1 at 200°C, more than 10−1 S cm−1 at 700°C). The results are compared with the data obtained earlier for similar systems based on RbGaO2 and RbAlO2.

New potassium-conducting solid electrolytes in the mixed gallate-ferrite systems (1 − x)Ga2O3 · xFe2O3 · 0.25TiO2 · K2O and 1.5[(1 − x)Ga2O3 · xFe2O3] · TiO2 · 2K2O are synthesized and studied. The electrolytes exhibit high ionic conductivity in the test temperature range of 300 to 750°C (above 10−2 S/cm at 300°C and above 10−1 S/cm at 700°C). An increase in the conductivity with increasing concentration of iron in the specimens is a general tendency. Possible reasons for the effect of Ga/Fe ratio in the structure of solid electrolytes on their transport properties are discussed.

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 highly conducting rubidium cationic solid electrolytes were synthesized on the basis of rubidium aluminates by partial substitution of Rb+ cations with Pb2+, Cd2+, and Ca2+ divalent ions and studied. All these three additives essentially increase electroconductivity of RbAlO2 especially in the low-temperature range. The values of electroconductivity and its activation energy are close for the optimum compositions in three studied systems. The conductivity of rubidium monoaluminate is increased by the studied additives due to formation of additional rubidium vacancies by substitution 2 Rb+ → Me2+ + V′Rb.

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.

The effect of partial substitution of Zr4+ ions for Ge4+ ions in highly conducting lithium-cationic solid electrolyte Li3.75Ge0.75P0.25O4 is studied. It is found that the introduction of zirconium ions considerably raises the conductivity of basic electrolyte in the high-temperature range. For the optimal composition, the conductivity is 2.82 × 10−1 S cm−1 at 400°C and 1.55 S cm−1 at 700°C. Possible reasons for the effects are discussed.

New cesium-conducting solid electrolytes based on cesium monoferrite in the Fe2O3-TiO2-Cs2O system are synthesized and studied. It is found that the introduction of titanium dioxide significantly reduces the electronic component of conductivity, which prevails in pure CsFeO2, and raises the ionic conductivity. The latter becomes predominant with increasing concentration of TiO2. The effect of dimensional factor on the characteristics of electrolyte is shown. The optimal compositions studied have very high cesium-cationic conductivity: it is above 10−2 S cm −1 at 300°C.

New potassium-monoaluminate-based solid electrolytes with high potassium-cationic conductance are synthesized, by partial substitution of penta-charged cations Nb5+ and Ta5+ for Al3+ cations, and studied. Both additives increased drastically the KAlO2 conductivity over the entire studied temperature range. The reason for the increase in the potassium monoaluminate conductivity, on the introducing of the studied additives, is the formation of potassium vacancies as a result of the substitution reaction Al3+ → Nb5+(Ta5+) + 2V’c, as well as broadening of the region of existence of the KAlO2 high-temperature γ-modification whose transport properties exceed those of the low-temperature β-form.

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.

We have synthesized and characterized new cesium-phosphate-based solid electrolytes with the general formula Cs3−4xMxIVPO4 (M = Zr, Hf, Ce) and high cesium ion conductivity. The increase in the conductivity of Cs3PO4 upon the introduction of the dopants studied is primarily due to the formation of cesium vacancies and the broadening of the stability range of the high-temperature phase of cesium phosphate, which has good transport properties.

New solid electrolytes with high potassium cationic conductivity based on potassium monoaluminate were synthesized and studied in the K1−2xBaxAlO2 and K1−2xPbxAlO2 systems. A partial substitution of potassium cations with double charged positive cations results in a considerable increase in the conductivity of KAlO2 in the whole studied range of temperatures. The electric characteristics are somewhat higher in the bariumcontaining system, which is in all probability connected with the effect of the size factor: the size of the Ba2+ ion is higher than that of Pb2+, which must correlate with the increase of the effective dimensions of the migration channels in the electrolyte structure. The main cause of the conductivity increase at the introduction of the studied additives is the formation of potassium vacancies at the substitution of 2K+ → Ba2+(Pb2+) + V′K.

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.

Potassium ion conducting solid electrolytes based on potassium monoferrite have been prepared by substituting pentavalent phosphorus cations for Fe3+. The highest conductivity of K2 − 2xFe2 − xPxO4 is achieved in the range x = 0.05−0.10: 7.1 × 10−3 S/cm at 300°C and 1.6 × 10−1 S/cm at 700°C. The rise in the conductivity of KFeO2 upon phosphorus doping is due to the formation of potassium vacancies.

The crystal structure of the low-temperature forms of the Cs3PO4 and Rb3PO4 orthophosphates has been determined for the first time by neutron diffraction using the Rietveld method. Cs3PO4 and Rb3PO4 are shown to be isostructural with K3PO4: orthorhombic cell (sp. gr. Pnma, Z = 4); lattice parameters a = 1.23177(6) nm, b = 0.88948(4) nm, c = 0.64197(3) nm for Cs3PO4; a = 1.17362(2) nm, b = 0.81046(1) nm, c = 0.615167(9) nm for Rb3PO4.

Highly conductive potassium-cation solid electrolytes based on potassium monoferrite with cations Nb5+ and Ta5+ partially replacing cations Fe3+ are synthesized and studied. Both additives cause a sharp increase in the conductivity of KFeO2 at temperature below ∼700°C. The two solid electrolytes have similar electric characteristics, which is apparently associated with the close sizes of the doping cations. The increase in the conductivity of potassium monoferrite with the introduction of dopants studied is associated with the formation of potassium vacancies at the substitution Fe3+ → Nb5+ (Ta5+) + 2 V′c and also with the elimination of a phase transition typical of pure KFeO2.

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.

Novel highly conductive potassium-cation solid electrolytes based on potassium monoferrite are synthesized via partial substitution of Ba2+ and Pb2+ for K+ and studied. Both additives lead to an abrupt increase in the conductivity of KFeO2. In the barium-containing system, the conductivity is slightly higher and the activation energy is somewhat lower. This can be explained by the effect of dimensional factor, because the Ba2+ cation is larger than the Pb2+ cation. The reasons for the conductivity increase in potassium monoferrite upon introducing the additives are the formation of potassium vacancies in the course of substitution K+ → M2+ + V’K and the extension of temperature range of existence of the high-temperature KFeO2 modification.

The crystal structure of the low-temperature form of K3PO4 has been determined for the first time using neutron diffraction (Rietveld method) and Raman spectroscopy: orthorhombic cell (sp. gr. Pnma, Z = 4), lattice parameters a = 1.12377(2) nm, b = 0.81046(1) nm, c = 0.59227(1) nm. The structure is made up of isolated [PO4] tetrahedra, with the potassium ions in between.

New high-conductance potassium-cation solid electrolytes based on potassium aluminate are synthesized by means of partial substitution of five-charged phosphorus cations for three-charged aluminum cations and investigated. The maximum conductivity in the system K2 − 2xAl2 − xPxO4 is found to equal 5 × 10−3 S cm−1 at 200°C and ∼1 S cm−1 at 700°C, which is one of the best values for potassium solid electrolytes. The principal factors responsible for the high conductance are the stabilization of a high-temperature form of potassium aluminate and the formation of additional vacancies in the potassium sublattice, which occurs during the substitution process Al3+ → P5+ + 2VK′.

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.

New potassium-conducting solid electrolytes based on potassium monogallate in the K2−2xGa2−xVxO4 system are synthesized and studied. It is found that an introduction of V5+ ions leads to a considerable increase in the KGaO2 conductivity due to the formation of vacancies in the potassium sublattice. The conductivity for optimal compositions is approximately 10−3 S cm−1 at 400°C and above 10−2 S cm−1 at 700°C. The results are compared with early obtained data for potassium monogallate dopped with four-charged cations.

Rubidium monoaluminate RbAlO2 has been studied by powder neutron diffraction and differential scanning calorimetry. A structural phase transition has been found at 1050°C. It is shown that the low-temperature modification RbAlO2 has the orthorhombic structure (Pnma, a = 0.5570(2) nm, b = 1.1189(4) nm, c = 1.5818(6) nm) close to the crystal structures of low-temperature modifications RbGaO2 and RbFeO2, not a face-centered cubic structure, as assumed previously.

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.

The crystalline structure of low- and high-temperature modifications of KAlO2 is studied by a neutron diffraction method combined with a full-profile Rietveld analysis. At low temperatures KAlO2 has an orthorhombic structure (spatial group Pbca), which turns tetragonal (spatial group P41212) at 540°C. During a phase transition along the c axis there open new conduction channels with a large cross-section of voids, which is a probable reason for the conduction jump. The presumed anisotropy of conduction in the low-temperature modification disappears during the phase transition.

Solid electrolytes K3 – 2x M x PO4 (M = Ca, Sr, Ba) are synthesized and the temperature and concentration dependences of their electroconductivity are studied. Adding calcium and strontium stabilizes the high-temperature β-form of K3PO4 at room temperature, while barium-containing solid electrolytes undergo an eutectoid decomposition below 430°C. Maximum electroconductivity is exhibited by K3 – 2x Sr x PO4 (7.1 × 10–3 and 1.25 × 10–1 S cm–1 at 300 and 700°C).

The crystal structure and vibrational dynamics of lithium atoms in Li3.75Ge0.75V0.25O4 and Li3.70Ge0.85W0.15O4 solid electrolytes in the superionic state are investigated using neutron diffraction and nuclear magnetic resonance (NMR) spectroscopy. It is found that, in the crystal lattice, lithium ions occupy four nonequivalent positions in the tetrahedral and octahedral oxygen ion environment with vacancies in the octahedra. These findings are in good agreement with the NMR data on the dynamic inhomogeneity of lithium cations in the lattice. It is shown that the origin of the superionic state in the studied compounds is associated primarily with the geometric factor, i.e., with an increase in the size of cavities in the oxygen polyhedra surrounding lithium cations.

Crystalline structure of low- and high-temperature modifications of solid lithium-conducting electrolytes Li3.75Ge0.75V0.25O4 and Li3.70Ge0.85W0.15O4 are studied by powder neutron diffraction analysis with use made of Rietveld full-profile analysis. The structure of either composition is derivative from γ-Li3PO4. The high conductivity of the electrolytes is connected with the presence of interstitial lithium cations, whose positions are occupied but partially. Phase transitions in the electrolytes consist of the leveling of the occupation degrees of these positions and make practically no impact on the rigid cage of the lattice.

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.

The electrical conductivity of silicon nitride and its solid solutions with calcium nitride and aluminum nitride was measured in the ranges 400–900 and 1000–1300°C. The conduction mechanisms were found to be substantially different in these temperature ranges. The Si3N4–Ca3N2solid solutions exhibited high ionic conductivity between 400 and 900°C. The densest and most oxidation-resistant materials were obtained in the Si3N4–AlN–Ca3N2system (Al introduced as fine powder and then nitrided).

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.