Search

We have studied the structure and proton conductivity of rhombohedral La6−xMoO12−δ (x = 0.5, 0.6, 0.7, 1) lanthanum molybdates prepared via mechanical activation of lanthanum and molybdenum oxides, followed by thermal annealing at 1650 °C. The La6−xMoO12−δ (x = 0.5, 0.6) materials were phase-pure and had a complex rhombohedral structure (R1). An increase in the molybdenum concentration leads to a decrease in the degree of rhombohedral distortion and proton conductivity in the La6−xMoO12−δ (x = 0.5, 0.6, 0.7, 1) series. The proton conductivity at the optimal composition La6−xMoO12−δ (x = 0.5) is ~4.0 × 10−5 S/cm at 500 °C in wet air. A comparative analysis shows that, in the Ln6−xMoO12−δ (Ln = La, Nd, Sm, Gd, Dy, Ho, Er, Tm, Yb; x = 0–1) series, proton conductivity decreases with the Ln ionic radii decreasing regardless of the structural type. Because of this, the high proton conductivity is demostrated by the stable La6−xMoO12−δ (x = 0.5, 0.6) materials, with an inherently deficient oxygen sublattice, which crystallize in a large-volume, complex rhombohedral cell (R1).

The synthesis of the Ln 6 MoO 12 (Ln = Sm, Ho - Yb) (3:1) rare-earth molybdates from binary oxides at room temperature has been studied by XRD and electron spin resonance spectroscopy (ESR). The mechanical activation of 3Ln 2 O 3 + MoO 3 (Ln = Sm, Ho, Yb) mixtures containing unmilled or premilled MoO 3 initiates the formation of Ln 6 MoO 12 (Ln = Sm, Ho - Yb) at room temperature, which is accompanied by a reduction in the ESR signal from Mo 5+ paramagnetic ions located on the surface of the activated MoO 3 and, hence, by a reduction in the amount of MoO 3 in the mixture, due to reaction with Ln 2 O 3 (Ln = Sm, Ho, Yb)·The major phase resulting from the mechanochemical synthesis is a cubic phase with the bixbyite structure ( Ia¯3, no. 206) for Ln 6−x MoO 12−δ (Ln = Dy –Yb; x = 0, 0.5) and the Sm 2 O 3 type structure ( C 2/m, no. 12) for Ln 6 MoO 12 (Ln = Gd, Sm). The high-temperature synthesis of Ln 6−x MoO 12−δ (Ln = Ho – Yb; x = 0, 0.5) and Ho 10 Mo 2 O 21 from precursors prepared at room temperature has been studied at 1200 °C and heat treatment times of 4, 40, 80, and 160 h. The bixbyite phase disappears because it is metastable and is formed due to kinetic factors in the stability field of lower symmetry phases: tetragonal (T) and rhombohedral ( R 3 ¯ ). We have located the stability range of the rhombohedral ( R 3 ¯ ) phase as a function of Ln below 1200 °C using different heating time. We have found conditions for the synthesis of the tetragonal phase T, which exists only in the case of the intermediate lanthanides. The tetragonal phase in phase-pure form has been synthesized for the first time via prolonged (160 h) heat treatment of Ho 10 Mo 2 O 21 at 1200 °C. The electrical conductivity of the samples with different % of tetragonal phase has been measured by impedance spectroscopy in dry and wet air. The Arrhenius plot of conductivity for pure tetragonal phase has the form of straight line over the entire temperature range studied, 440–900 °C (Е а = 1.37 eV, σ 600°C = 1 × 10 −5 S cm −1 ) in dry and wet air. The absence of electrode dispersions at low frequencies and conductivity growth in a wet air makes us assume the predominantly electronic conductivity type of the Ho 10 Mo 2 O 21 tetragonal phase in the temperature range 440–900 °C. In multiphase samples surface electronic conductivity is observed up to 620–650 °С in wet air, which is associated with the presence of defects at grain boundaries of crystallographically related phases (bixbyite, rhombohedral and tetragonal phases).

The tolerance factor is a good criterion to understand the structural transitions in Ln2-xCaxScMO7-δ (Ln = La, Sm, Ho, Yb; M = Nb, Ta; x = 0, 0.05, 0.1). Decreasing the Ln ionic radius in Ln2ScNb(Ta)O7 leads to a morphotropic transition from a pyrochlore to a fluorite-like structure. Ca2+-doping leads to a pyrochlore-to-fluorite transition in Ln2-xCaxScMO7-δ (Ln = La, Sm) and a fluorite-to-pyrochlore transition in Ho2-xCaxScNbO7-δ. Proton contribution to the total conductivity was observed for Ln2-xCaxScNb(Ta)O7-δ (Ln = La, Sm; x = 0, 0.05, 0.1) 3+/5+ pyrochlores and the maximum proton contribution was shown by Sm1.9Ca0.1ScMO6.95 (M = Nb, Ta), which are located at the boundary between pyrochlores and fluorites (comparative study of electrical conduction and oxygen diffusion). Proton conduction of Sm1.9Ca0.1ScNbO6.95 and Sm1.9Ca0.1ScTaO6.95 pyrochlores persists up to 800 and 850 °C, respectively. The conductivity of fluorite-like Ho2-xCaxScNbO7-δ (x = 0, 0.05) and Yb2ScNbO7 is dominated by the oxygen ion transport, in accordance with their energy activation values 1.09-1.19 eV. The dielectric permittivity and TG studies were used for the investigation of oxygen vacancy dynamics and water incorporation into the Ln2-xCaxScNb(Ta)O7-δ (Ln = La, Sm, Ho, Yb; x = 0, 0.05, 0.1) lattice. It is shown that oxygen vacancy-related dielectric relaxation in the range of 550-650 °C (ambient air), typical of pyrochlores and fluorites with pure oxygen ion conductivity, decreases and disappears for proton-conducting oxides.

Samarium molybdates Sm 6-x MoO 12-δ (x = 0, 0.5) and samarium tungstate Sm 6 WO 12 – potential mixed electron-proton conductors have been studied by X-ray diffraction, Raman spectroscopy, SEM and impedance spectroscopy (in ambient air and in dry and wet air). Solid solutions differing in structure have been obtained in the Sm 2 O 3 -MoO 3 system at 1600 °C. The samarium molybdate Sm 6 MoO 12 has the fluorite structure ( Fm¯3m ). The less samarium rich solid solution Sm 5.5 MoO 11.25 crystallizes in a rhombohedral ( R 3 ¯ ) structure. The morphotropic transformation is due to the change in the chemical composition of the solid solution with decreasing Sm 3 + concentration. The total conductivity of the cubic fluorite phase Sm 6 MoO 12 at 750 °C in air (1.48 × 10 − 3 S/cm, E a = 1.22 eV) is an order of magnitude higher than that of rhombohedral Sm 5.5 MoO 11.25 (2.34 × 10 − 4 S/cm, E a = 1.11 eV). At low temperatures (T 6 MoO 12 and Sm 5.5 MoO 11.25 in air deviates from linearity, suggesting that there is a proton contribution to its conductivity at these temperatures, like in the case of the Sm 5.4 Zr 0.6 MoO 12.3 zirconium-doped molybdate. Below ~ 500 °C, Sm 6 MoO 12 fluorite and fluorite-like Sm 6 WO 12 have identical Arrhenius plots of conductivity in ambient air. The region of dominant proton conductivity is wider for Sm 6 WO 12 than Sm 6 MoO 12 , reaching temperatures as high as 750 °С for the former. The absolute values of total conductivity obtained for samarium tungstate and molybdate at 400 °С in wet air are virtually identical and close to 3 × 10 − 6 S/cm, which suggests the conductivity of both compounds is dominated by protons at low temperatures and the proton transport numbers are similar.

The formation of Ln6−xMoO12−δ (Ln = La, Gd, Dy, Ho, Er, Tm, Yb, Lu; x = 0, 0.5) rare-earth molybdates from mechanically activated oxide mixtures has been studied in the range 900–1600 °C. The morphotropy and polymorphism (thermodynamic phase and kinetic (growth-related) transitions) of the Ln6−xMoO12−δ (Ln = La, Gd–Lu; x = 0, 0.5) molybdates have been analyzed in detail. As a result we have observed two new types of oxygen ion- and proton-conducting materials with bixbyite (Ia, no. 206) and rhombohedral (R, no. 148) structures in the family of Ln6−xMoO12−δ (Ln = La, Gd–Lu; x = 0, 0.5) molybdates. The heavy rare-earth molybdates Ln6−xMoO12−δ (Ln = Er, Tm, Yb; x = 0, 0.5) have been shown for the first time to undergo an order–disorder (rhombohedral–bixbyite) phase transition at 1500–1600 °C, and we have obtained compounds and solid solutions with the bixbyite structure (Ia). The stability range of the rhombohedral phase (R) increases with decreasing Ln ionic radius across the Ln6−xMoO12−δ (Ln = Er, Tm, Yb, Lu) series. We have detected a proton contribution to the conductivity of the rhombohedral La5.5MoO11.25 (2 × 10−4 S cm−1 at 600 °C in wet air) and high-temperature polymorph Yb6MoO12−δ bixbyite structure, (Ia) below 600 °C. At these temperatures, rhombohedral (R) Yb6MoO12 seems to be an oxygen ion conductor (Ea = 0.53–0.58 eV). The total conductivity of rhombohedral (R) Yb6MoO12 exceeds that of bixbyite Yb6MoO12−δ by more than one order of magnitude and is 3 × 10−5 S cm−1 at 500 °C. According to their high-temperature (T > 600 °C) activation energies, the lanthanum and ytterbium molybdates studied here are mixed electron–ion conductors.

Sm2&minus, xCaxZr2O7&minus, x/2 (x = 0, 0.05, 0.1) and Gd2&minus, x/2 (x = 0.05, 0.1) mixed oxides in a pyrochlore&ndash, fluorite morphotropic phase region were prepared via the mechanical activation of oxide mixtures, followed by annealing at 1600 &deg, C. The structure of the solid solutions was studied by X-ray diffraction and refined by the Rietveld method, water content was determined by thermogravimetry (TG), their bulk and grain-boundary conductivity was determined by impedance spectroscopy in dry and wet air (100&ndash, 900 &deg, C), and their total conductivity was measured as a function of oxygen partial pressure in the temperature range: 700&ndash, 950 &deg, C. The Sm2&minus, x/2 (x = 0.05, 0.1) pyrochlore solid solutions, lying near the morphotropic phase boundary, have proton conductivity contribution both in the grain bulk and on grain boundaries below 600 &deg, C, and pure oxygen&ndash, ion conductivity above 700 &deg, C. The 500 &deg, C proton conductivity contribution of Sm2&minus, x/2 (x = 0.05, 0.1) is ~ 1 &times, 10&minus, 4 S/cm. The fluorite-like Gd2&minus, x/2 (x = 0.1) solid solution has oxygen-ion bulk conductivity in entire temperature range studied, whereas proton transport contributes to its grain-boundary conductivity below 700 &deg, C. As a result, of the morphotropic phase transition from pyrochlore Sm2&minus, x/2 (x = 0.05, 0.1) to fluorite-like Gd2&minus, x/2 (x = 0.05, 0.1), the bulk proton conductivity disappears and oxygen-ion conductivity decreases. The loss of bulk proton conductivity of Gd2&minus, x/2 (x = 0.05, 0.1) can be associated with the fluorite structure formation. It is important to note that the degree of Ca substitution in such solid solutions (Ln2&minus, xCax)Zr2O7&minus, &delta, (Ln = Sm, Gd) is low, x &lt, 0.1. In both series, grain-boundary conductivity usually exceeds bulk conductivity. The high grain-boundary proton conductivity of Ln2&minus, x/2 (Ln = Sm, Gd, x = 0.1) is attributable to the formation of an intergranular CaZrO3-based cubic perovskite phase doped with Sm or Gd in Zr sublattice.

We have compared (Ln 2- x Zr x )Zr 2 O 7 + x/2 (Ln = Nd, Sm) pyrochlorelike solid solutions with interstitial oxide ion conduction and Ln2(Zr 2- xLnx)O 7- δ (Ln = Nd, Sm) pyrochlorelike solid solutions with vacancymediated oxide ion conduction in the symmetric systems Nd2O3-ZrO2 (NdZrO) and Sm2O3- ZrO2 (SmZrO). We have studied their structure, microstructure, and transport properties and determined the excess oxygen content of the (Sm 2- xZrx)Zr2O7 + x/2 (x = 0.2) material using thermal analysis and mass spec� trometry in a reducing atmosphere (H 2 /Ar-He). The Ln 2 ± x Zr 2 ± x O 7 ± x/2 (Ln = Nd, Sm) solid solutions have almost identical maximum oxygen vacancy and interstitial conductivities: (3-4) × 10 -3 S/cm at 750°C. The lower oxygen vacancy conductivity of the Ln2(Zr 2- xLnx)O 7- δ (Ln = Nd, Sm; 0 < x ≤ 0.3) solid solutions is due to the sharp decrease in it as a result of defect association processes, whereas the interstitial oxide ion con� ductivity of the (Ln 2- xZrx)Zr2O 7+ x/2 (Ln = Nd, Sm; 0.2 ≤ x < 0.48) pyrochlorelike solid solutions is essen� tially constant in a broad range of Ln 2 O 3 concentrations.

We have studied the structure and transport properties of ten (Nd 2 − x Zr x )Zr 2 O 7+ x /2 ( x = 0–1.27) solid solutions, which lie in the ZrO 2 –Nd 2 Zr 2 O 7 isomorphous miscibility range. Major attention has been focused on the pyrochlore-like (Nd 2 − x Zr x )Zr 2 O 7+ x /2 solid solutions with x = 0–0.78, which are thought to be potential interstitial oxide ion conductors. The X-ray and neutron diffraction results demonstrate that the (Nd 2 − x Zr x )Zr 2 O 7+ x /2 ( x = 0–1.27) solid solutions undergo an order–disorder (pyrochlore–defect fluorite) structural phase transition. The (Nd 2 − x Zr x )Zr 2 O 7+ x /2 ( x = 0.2–0.78) have the bulk conductivity, ∼(1.2–4) × 10 –3 S/cm at 750 °C, which is two orders of magnitude higher than that of the ordered pyrochlore Nd 2 Zr 2 O 7 . An attempt has been made to determine the interstitial oxygen content of (Nd 2 − x Zr x )Zr 2 O 7+ x /2 ( x = 0.2; 0.67) in a reducing atmosphere using thermogravimetry and mass spectrometry. It has been shown that no reduction occurs in the NdZrO system, where neodymium has only one oxidation state, 3+.

We have studied the new compounds with fluorite-like (Ho 2 RNbO 7 (R = Lu, Sc)) and pyrochlore-like (Sm 2 ScTaO 7 ) structure as potential oxide ion conductors. The phase formation process (from 1200 to 1600 °C) and physical properties (electrical, thermo mechanical, and magnetic) for these compounds were investigated. Among the niobate materials the highest bulk conductivity is offered by the fluorite-like Ho 2 ScNbO 7 synthesized at 1600 °C: 3.8 × 10 −5 S/cm at 750 °C, whereas in Sm system the highest bulk conductivity, 7.3 × 10 −6 S/cm at 750 °C, is offered by the pyrochlore Sm 2 ScTaO 7 synthesized at 1400 °C. In Sm 2 ScTaO 7 pyrochlore we have observed the first-order phase transformation at ∼650–700 °C is related to rearrangement process in the oxygen sublattice of the pyrochlore structure containing B-site cations in different valence state and actually is absent in the defect fluorites. The two holmium niobates show Curie–Weiss paramagnetic behavior, with the prevalence of antiferromagnetic coupling. The magnetic susceptibility of Sm 2 ScTaO 7 is a weak function of temperature, corresponding to Van Vleck paramagnetism.

High-density (Yb0.9Ca0.1)2Ti2O6.9, (Yb0.8Ca0.1Tb0.1)2Ti2O7 − δ, and (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ solid solutions have been prepared through co-precipitation followed by firing for 4 h at 1500 and 1550 °C, and their crystal structure (XRD), microstructure (SEM), and oxygen ion conductivity (impedance spectroscopy) have been studied in relation to the firing temperature and precipitant used. As in the case of (Yb0.9Ca0.1)2Ti2O6.9 and (Yb0.8Ca0.1Tb0.1)2Ti2O7 − δ, the optimal synthesis temperature for (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ is 1500 °C. The bulk oxygen ion conductivity of the pyrochlore-like solid solutions (Yb0.9Ca0.1)2Ti2O6.9 is a stronger function of synthesis temperature than that of the (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ and (Yb0.8Ca0.1Tb0.1)2Ti2O7 − δ solid solutions with more complex A sublattice. The rise of the synthesis temperature from 1500 to 1550 °C has detrimental effect on the grain boundary conductivity of the (Yb0.9Ca0.1)2Ti2O6.9 and (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ ceramics. That effect is connected with a considerable grain-boundary segregation of a calcium-containing phase in the (Yb0.9Ca0.1)2Ti2O6.9 and (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ. The bulk and grain boundary conductivity of (Dy0.8Ca0.1Tb0.1)2Ti2O7 − δ are independent of the precursor synthesis conditions (homogeneous and non-homogeneous co-precipitation).

We have studied terbium substitution for ytterbium in (Yb0.9 − xTbxCa0.1)2Ti2O7 − δ (x = 0.1, 0.2, 0.3, 0.4) pyrochlore solid solutions synthesized through coprecipitation followed by firing at 1550°C. The results indicate that only a small amount of terbium (less than 10%) can be incorporated into the pyrochlore structure of (Yb0.9Ca0.1)2Ti2O6.9 because of the large difference in ionic radius between the terbium and ytterbium cations: Δr = r(TbCN 83+) − r(YbCN 83+) = 0.055 A. The oxygen ion conductivity of the (Yb0.9 − xTbxCa0.1)2Ti2O7 − δ solid solutions has been determined by impedance spectroscopy in air in the temperature range 300 to 900°C. At high temperatures (t > 640°C), their bulk conductivity was essentially independent of the Yb/Tb ratio. The observed decrease in density and microstructural changes were insignificant. At relatively low temperatures (t < 640°C), the bulk conductivity decreased slightly, and the decrease depended little on terbium concentration.

Low-temperature ordering transitions in polycrystalline high temperature conductors (Dy 1 − x Ca x ) 2 Ti 2 O 7 − δ ( x = 0, 0.1) prepared using co-precipitation, mechanical activation and solid-state reactions at 1400 or 1600 °C have been studied by impedance spectroscopy at low frequencies and thermal mechanical analysis (TMA). The dielectric permittivity and loss tangent of the ceramics obtained have been measured as a function of temperature at low frequencies (0.5−500 Hz). The results provide evidence for the relaxation of point defects, most likely oxygen vacancies, at 500−600 °C and an antiferroelectric low-temperature phase transition of the second order, associated with re-arrangement process in the oxygen sublattice of pyrochlore structure. The temperature of the antiferroelectric transition is 700 to 800 °C, depending on the synthesis procedure and ceramic composition. Calcium doping of Dy 2 Ti 2 O 7 leads to the formation of additional oxygen vacancies and, in the case of the samples prepared via co-precipitation, increases the peaks in permittivity due to the relaxation process and ordering transition by three or six times, respectively.

We have studied the δ-phase to defect fluorite F * (order–disorder) transition in the R 4 M 3 O 12 (R = Sc, Tm, Lu; M = Zr, Hf) compounds. The temperature of the δ– F * phase transition in Tm 4 Zr 3 O 12 is ∼1600 °C. The rate of this transition in R 4 Zr 3 O 12 (R = Sc, Tm, Lu) decreases markedly with decreasing difference in ionic radius between the R 3+ and Zr 4+ , leading to stabilization of the δ-phases R 4 Zr 3 O 12 with R = Sc and Lu at high temperatures (∼1600 °C). During slow cooling (5 °C/h), the high-temperature defect fluorites F *-R 2 Hf 2 O 7 (R = Tm, Lu) decompose reversibly to form the δ-phases R 4 Hf 3 O 12 . Some of the materials studied exhibit microdomains formation effects, typical of the fluorite-related oxide compounds in the R 2 O 3 –MO 2 (M = Ti, Zr, Hf) systems of the heavy rare earths. The high-temperature defect fluorites F *-R 4 M 3 O 12 (R = Tm, Lu; M = Zr, Hf) as a rule contain antiphase microdomains of δ-R 4 Zr 3 O 12 . After slow cooling (5 °C/h), such microdomains are large enough for the δ-phase to be detected by X-ray diffraction. The conductivity data for R 4 M 3 O 12 (R = Sc, Tm, Lu; M = Zr, Hf) and Ln 2 Hf 2 O 7 (Ln = Dy, Lu) prepared by different procedures show that the rhombohedral phases δ-R 4 M 3 O 12 (R = Sc, Tm, Lu; M = Zr, Hf) are poorer conductors than the defect fluorites, with 740 °C conductivity from 10 −6 to 10 −5 S/cm. The conductivity drops with decreasing rare-earth ionic radius and, judging from the E a values obtained (1.04–1.37 eV), is dominated by oxygen ion transport. The highest conductivity, ∼6 × 10 −4 S/cm at 740 °C, is offered by the rapidly cooled F *-Dy 2 Hf 2 O 7 . In the fluorite homologous series, oxygen ion conductivity decreases in the order defect pyrochlore > defect fluorite > δ-phase.

New LANTIOX high-temperature conductors with the pyrochlore structure, (Ln1� xAx)2Ti2O7� d (Ln = Dy, Ho, Yb; A = Ca, Mg, Zn; x = 0, 0.01, 0.02, 0.04, 0.07, 0.1), have been prepared at 1400-1600 8C using mechanical activation, co-precipitation and solid-state reactions. Acceptor doping in the lanthanide sublattice of Ln2Ti2O7 (Ln = Dy, Ho, Yb) with Ca 2+ ,M g 2+ and Zn 2+ increases the conductivity of the titanates except in the (Ho1� xCax)2Ti2O7� d system, where the conductivity decreases slightly at low doping levels, x = 0.01-0.02. The highest conductivity in the (Ln1� xAx)2Ti2O7� d (Ln = Dy, Ho, Yb; A = Ca, Mg, Zn) systems is offered by the (Ln0.9A0.1)2Ti2O7� d and attains maximum value for (Yb0.9Ca0.1)2Ti2O6.9 and (Yb0.9Mg0.1)2Ti2O6.9 solid solutions:� 2 � 10 � 2 and 9 � 10 � 3 Sc m � 1 at 750 8C, respectively. Ca and Mg are best dopants for Ln2Ti2O7 (Ln = Dy, Ho, Yb) pyrochlores. Using impedance spectroscopy data, we have determined the activation energies for bulk and grain-boundary conduction in most of the (Ln1� xAx)2Ti2O7� d (Ln = Dy, Ho; A = Ca, Mg, Zn) materials. The values obtained, 0.7-1.05 and 1-1.4 eV, respectively, are typical of oxygen ion conductors. We have also evaluated defect formation energies in the systems studied.

We report the first synthesis of pyrochlore-like Ln 2 TiO 5 with Ln = Er–Lu via cooling at a rate as slow as 5 °C/h in the range 1600–820 °C. The materials thus prepared differ in structure from the oxygen-ion-conducting Ho 2 TiO 5 pyrochlore: the X-ray diffraction patterns of the pyrochlore-like Ln 2 TiO 5 (Ln = Er–Lu) show weak 511 and 531 pyrochlore superstructure peaks and an unidentified peak at 2 θ = 19.6° (Cu Kα) (5% intensity). The Ln 2 TiO 5 (Ln = Er, Tm) pyrochlores prepared via very slow cooling followed by quenching in liquid nitrogen from 840 °C contain, in addition to a pyrochlore-like phase, a small amount (up to 5%) of the low-conductivity high-temperature hexagonal phase β-Ln 2 TiO 5 . The pyrochlore Ho 2 TiO 5 contains microdomains of the hexagonal phase β-Ho 2 TiO 5 , which does not affect its high oxygen ion conductivity. The conductivity of the pyrochlore-like Ln 2 TiO 5 with Ln = Er and Yb is substantially lower than that of the fluorite-like Ln 2 TiO 5 (Ln = Er, Yb) and oxygen-ion-conducting pyrochlore Ho 2 TiO 5 obtained via furnace cooling (130 °C/h), which can be understood in terms of oxygen ordering in these compounds and the presence of the low-conductivity high-temperature phase β-Ln 2 TiO 5 in the pyrochlore-like Ln 2 TiO 5 (Ln = Er, Yb) at measurement temperatures (∼500–840 °C).

Ho2Ti2O7 and LnYTi2O7 (Ln = Dy, Ho) pyrochlores have been synthesized using hydroxide coprecipitation, mechanical activation, and firing at 1600°C. The bulk and grain-boundary components of their conductivity have been determined for the first time by impedance spectroscopy. The 740°C bulk conductivity of Ho2Ti2O7 is 4 × 10−4 S/cm, and that of HoYTi2O7 is 1 × 10−3 S/cm, with activation energies Ea = 1.01 and 1.17 eV, respectively, suggesting that these materials are new oxygen-ion conductors. The bulk conductivity of DyYTi2O7 (3 × 10−4 S/cm at 740°C, Ea = 1.09 eV) is almost one order of magnitude lower than that of HoYTi2O7 (1 × 10−3 S/cm at 740°C, Ea = 1.17 eV).

The Er2+xTi2−xO7−δ (x = 0.096; 35.5 mol% Er2O3) solid solution and the stoichiometric pyrochlore-structured compound Er2Ti2O7 (x = 0; 33.3 mol% Er2O3) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650–1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800–1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10−3 S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10−10 Pa, the Er2+xTi2−xO7−δ (x = 0, 0.096) samples are purely ionic conductors.

We have studied the structure, microstructure, and electrochemical properties in air of (Pr-2 - Zr-x(x))Zr2O7 + (x/2) (X = 0.15, 0.32, 0.78), Pr2Zr2O7, and Pr-2(Zr-2 - Pr-x(x))O-7 - (x/2) (x = 0.1, 0.4, 1) materials. The solid solutions were prepared through coprecipitation followed by heat treatment of the precursors at 1550 degrees C for 4 h. According to XRD data, the extent of the pyrochlore-like Pr-2 xZr(2) th xO(7) th (x/2) solid solutions at 1550 degrees C is -6 mol.%, which is considerably smaller than that in the NdZrO and SmZrO systems at this temperature. Among the pyrochlores, the highest bulk conductivity was offered by the (Pr2-xZrx)Zr2O7 + x/2 (X = 0.15): 7.15 x 10-3 S/cm at 800 C (Ea= 0.66 eV).The pyrochlore-like Pr-2(Zr-2,Pr-x)O-7 x/2 (X = 0.1) had lower conductivity (3.97 x 10-3S/cm at 800 C). The highest bulk conductivity among the materials studied was found in the Pr(2)03-rich fluorite-like Pr-2(Zr-2 - Pr-x(x))O-7 - (x/2) with x = 1: similar to 0.217 S/cm at 800 degrees C (E-a = 0.0.31 eV). The temperature-dependent conductivity of the Pr2O3-rich fluorite-like solid solutions Pr-2(Zr-2 - Pr-x(x))O-7 (x/2) with x = 0.4 and 1 had a break at 560 C, suggesting a change in the mechanism of ion transport at this temperature. 2014 Elsevier B.V. All rights reserved.

Data are presented on the evolution of the pyrochlore structure in the Ln2+xHf2−xO7−δ (Ln = Sm, Eu; x = 0.096) solid solutions and Ln2Hf2O7 (Ln = Gd, Tb) compounds prepared from mechanically activated oxide mixtures. Sm2.096Hf1.904O6.952 is shown to undergo pyrochlore-disordered pyrochlore-pyrochlore (P-P1-P) phase transformations in the temperature range 1200–1670°C. The former transformation leads to a rise in 840°C conductivity from 10−4 to 3 × 10−3 S/cm in the samples synthesized at 1600°C, and the latter leads to a drop in 840°C conductivity to 6 × 10−4 S/cm in the samples synthesized at 1670°C. The reduction in the conductivity of Sm2.096Hf1.904O6.952 is accompanied by the disappearance of the assumed superstructure. In the range 1300–1670°C, Eu2+xHf2−xO7−δ (x = 0.096) and Ln2Hf2O7 (Ln = Gd, Tb) have a disordered pyrochlore structure. The highest 840°C conductivity is offered by Eu2.096Hf1.904O6.952, Gd2Hf2O7, and Tb2Hf2O7 synthesized at 1670°C: 7.5 × 10−3, 5 × 10−3, and 2.5 × 10−2 S/cm, respectively.

(Yb1−xCax)2Ti2O7 and (Yb1−xBax)2Ti2O7 (x = 0, 0.05, 0.1) have been synthesized using hydroxide coprecipitation and mechanical activation of oxide mixtures, and their electrical conductivity has been measured from 350 to 1000°C. The pyrochlore titanate (Yb0.9Ca0.1)2Ti2O7 synthesized at 1400°C from a mechanically activated oxide mixture has the highest conductivity, ∼0.1 S/cm at 1000°C, among the oxygen-ion-conducting pyrochlores studied so far. The (Yb0.95Ca0.05)2Ti2O7 and (Yb0.9Ca0.1)2Ti2O7 samples prepared by reacting coprecipitated powder mixtures at 1400°C have a lower conductivity, as do the (Yb1−xBax)2Ti2O7 (x=0.05, 0.1) samples prepared using mechanical activation.

The Lu 2+ x Ti 2− x O 7− x /2 ( x = 0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu 2 O 3 ) nanoceramics with partly disordered pyrochlore-type structure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes have oxide-ion conductivity in the interval of 1.0 × 10 − 3 – 2.5 × 10 − 3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO 2 /Y 2 O 3 ceramics. The conductivity of Lu 2+ x Ti 2− x O 7− x /2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearly stoichiometric Lu 2+ x Ti 2− x O 7− x /2 ( x = 0.096; 35.5 mol% Lu 2 O 3 ) material containing ∼ 4.8 at.% Lu Ti anti-site defects.

Nanocrystalline Lu2O3-TiO2 (33.3–44 mol % Lu2O3) materials with a partially disordered pyrochlore structure, prepared via heat treatment in the range 1400–1750°C, are found to possess high oxygen ionic conductivity. Their 740°C conductivity is 10-3 to 10-2 S/cm, depending on the heat-treatment temperature and composition, which is comparable to that of the well-known fluorite solid electrolyte ZrO2-9 mol % Y2O3.

The ordering processes in the rare-earth titanates Ln2Ti2O7 with Ln = Lu, Yb, Tm, and Gd are studied by x-ray diffraction, thermal analysis, IR spectroscopy, electron microscopy, and electrical conductivity measurements. The compounds are prepared via hydroxide coprecipitation, followed by freeze-drying and heat treatment in the temperature range 350–1700°C. The compounds Ln2Ti2O7 with Ln = Lu, Yb, and Tm are found to have the fluorite structure between 600 and 800°C. Above 800°C, they undergo a fluorite-to-pyrochlore transformation, with cation disordering and the formation of LnTi + TiLn antistructure pairs. Gd2Ti2O7 has the pyrochlore structure over the entire temperature range studied and contains no antistructure defects. In contrast to Gd2Ti2O7 , the compounds Ln2Ti2O7 with Ln = Lu, Yb, and Tm undergo a high-temperature pyrochlore-to-fluorite phase transition around 1700°C. The 750°C conductivity of Ln2Ti2O7 (Ln = Lu, Yb, Tm) samples sintered at 1700°C is 5 × 10–3 to 10–2 S/cm, which is two orders of magnitude higher than that of ceramics of the same composition prepared at lower temperatures (950 or 1400°C). The conductivity of the Gd2Ti2O7 ceramic prepared at 1500°C is two orders of magnitude lower than that of Ln2Ti2O7 with Ln = Lu, Yb, and Tm.

We have studied the effect of niobium doping on the electrical conductivity of Yb 2 Ti 2 O 7 -based oxygen ion conductors. Yb 2 [Ti 1 − x Nb x ] 2 O 7 (x = 0.01, 0.04, 0.1) and (Yb 0.8 Tb 0.1 Ca 0.1 ) 2 [Ti 1 − x Nb x ] 2 O 6.9 (x = 0; 0.05; 0.1) pyrochlore solid solutions were synthesized through coprecipitation followed by firing at 1550 °C for 4 h. The materials were examined by XPS, XRD, scanning electron microscopy and impedance spectroscopy. Yb 2 (Ti 0.99 Nb 0.01 ) 2 O 7 was shown to have the highest oxygen ion conductivity in air (2.3 × 10 − 3 S/cm at 750 °C), which is however markedly lower than that of undoped Yb 2 Ti 2 O 7 . In the (Yb 0.8 Tb 0.1 Ca 0.1 ) 2 [Ti 1 − x Nb x ] 2 O 6.9 (x = 0; 0.05; 0.1) system, the highest conductivity is offered by (Yb 0.8 Tb 0.1 Ca 0.1 ) 2 [Ti 0.95 Nb 0.05 ] 2 O 6.9 (σ = 4.44 × 10 − 3 S/cm at 650 °C). Additional oxygen vacancies created by Ca doping in pyrochlore structure reduce the detrimental effect of Nb 4 + doping on the oxide ion transport up to 5% Nb. The conductivity of the Yb 2 (Ti 0.99 Nb 0.01 ) 2 O 7 and (Yb 0.8 Tb 0.1 Ca 0.1 ) 2 [Ti 0.95 Nb 0.05 ] 2 O 6.9 solid solutions was measured both in air and under reducing conditions (5% H 2 in N 2 and CO 2 atmospheres). A comparative study of both these compositions under 5% H 2 in N 2 atmosphere showed that the transport mechanism was not affected by complex doping of the lanthanide and titanium sublattices in the Yb 2 Ti 2 O 7 -based materials and was related to oxygen vacancies. Conductivity measurements in CO 2 were done to ensure correct evaluation of the ionic conductivity of (Yb 0.8 Tb 0.1 Ca 0.1 ) 2 [Ti 0.95 Nb 0.05 ] 2 O 6.9 , because in air it seems to be a mixed p -type and ionic conductor.