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Electrical conductivity of polycrystalline Ln 2 (M 2 – x Ln x )O 7 – δ (Ln = Sm–Gd; M = Zr, Hf; 0 ≤ x ≤ 0.286) materials
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The class of oxygen-ion-conducting rare-earth pyrochlores has been considerably extended. New solid electrolytes, Ln2Ti2O7 (Ln = Dy-Lu) and Ln2Hf2O7 (Ln = Eu, Gd) pyrochlores, are intrinsic ionic conductors at elevated temperatures, as are the well known Ln2Zr2O7 (Ln = Sm-Gd) zirconates, which suggests that oxygen ion conduction in the rare-earth p...
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... mol % Ln 2 O 3 ) pyroo chloreelike solid solutions was studied by Shlyakhtina et al. [29,39]. The conductivity data obtained at 1 Hz, 10 Hz, 100 Hz, …, and 1 MHz for the compounds and solid solutions in these systems (Ln = Sm-Gd, x = 0- 0.286) are presented in Table 2 With increasing deviation from stoichiometry, х, in the Ln 2 (Zr 2 -x Ln x )O 7 -δ (Ln = Sm-Gd) series, the conductivity of all the solid solutions studied to date tends to drop (Fig. 8) [2,29,39]. The highest ionic conductivity offered by nominally stoichiometric Ln 2 Zr 2 O 7 (Ln : Zr = 1 : 1, Ln = Sm-Gd) is due to the intrinsic cation and anion disorder of the pyrochlore structure [schemes (1)-(3)] because of the small size mismatch between Ln 3+ = Sm 3+ -Gd 3+ and Zr 4+ and the pronounced Zr-O bond ionicity [48,49]. ...
Context 2
... Ln 2 ± x Zr 2 ± x O 7 ± δ (Ln = Sm-Gd) solid solutions with high values of x (x = 0.161-0.286), which lie just at the boundary of the wide isomorphism region at high Ln 2 O 3 concentrations (Ln = Sm-Gd) and are similar in structure to fluorite (Fm3m), possess lower conductivity (Table 2). High ionic conductivity was found in pyrochloreelike intermediate lanthanide zirconates, Ln 2 ± x Zr 2 ± x O 7 ± x/2 with Ln = Eu, Gd, which typically undergo kinetic (growthhrelated) trann Ln Zr ' ...
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Citations
... Pyrochlore-type oxides are increasingly recognized for their potential in oxygen-ion conduction, particularly within the temperature range of 400-550 o C, making them promising candidates for various applications such as energy conversion and storage devices, oxygen separation membranes, electrolyte for IT-SOFCs, and oxygen sensing systems. [13][14][15][16][17][18][19][20][21] The pyrochlore structure, characterized by A 2 B 2 O 7 with space group Fd−3m consists of two types of polyhedra, AO 8 and BO 6 with two unoccupied O-sites (8(b) site) within the BO 6 polyhedra. This arrangement facilitates the transport of oxygen ions through a hopping mechanism via the vacancy sites, notably the 8(b) sites. ...
The pyrochlore-based system is gaining significant attention as a solid electrolyte in electrochemical energy devices, particularly solid oxide fuel cells (SOFC) due to its high oxygen-ion conductivity at the Intermediate temperature range (400−650 o C). In this study, we investigate the Gd 2 − x Sr x Ti 2 O 7 , pyrochlore system doped with strontium (Sr), where, x = 0, 0.02 and 0.04, 0.06, 0.08 and 0.1 to develop an oxygen-ion conductor as an electrolyte for intermediate temperature SOFCs (IT-SOFCs). Structural information is collected using the X-ray diffraction technique and confirms the cubic pyrochlore phase with Fd−3m symmetry accompanied by superstructure peaks (111) (311) (511) and (111) planes across all compositions. The structural data are simulated using Rietveld Refinement. Microstructural features of as-calcined and sintered samples studied by Scanning Electron Microscopy; confirm non-spherical grains with high non-uniformity in particle size distribution. of as-calcined samples and highly dense sintered samples. Elemental composition is confirmed by EDAS. Raman spectroscopy reveals detailed insights into the dopant-induced local restructuring in the Gadolinium Titanate lattice. Few intense Raman modes related to E g +F 2g and A lg involve the modulation of crystal structure through the vibration of oxygen along < 100 > cubic axis. Ionic conductivity and activation energy data are extracted through AC impedance measurements. The electric modulus study reveals the ionic relaxation and ion hopping dynamics and their effect on ionic conductivity. M'' relaxation peak and its distribution in relaxation time are analyzed using the Kohlrausch-Williams-Watts (KWW) fit. The presence of dopants induced structural deformations and oxygen vacancies in the GTO host lattice. This led to the disordering of vacancies and modifications in the stretching exponent 'β' and activation energy. Cooperative hopping dynamics through ion-vacancy interactions are found to be a notable influence on ionic conductivity. The optimized dopant composition of GSTO−4 exhibits the highest conductivity peak (σ = 4.3 x 10 − 3 S/cm@650 o C). This suggests that apart from vacancy concentration and energy barriers for single-ion hopping, the cooperative dynamics of oxygen ions play a crucial role in determining the ionic conductivity values. Consequently, the GSTO−4 system demonstrates the potential for application as an electrolyte in intermediate temperature-SOFCs.
... Cubic pyrochlore complex oxides A 2 B 2 O 7 have also attracted much attention due to their ability to exhibit various types of geometrically frustrated magnetism [10,11]. Furthermore, complex oxide compounds and solid solutions from this family are prospective thermal barrier coatings [12][13][14], ion conductors [15,16], matrices for the immobilization of nuclear wastes [17,18], neutron-absorbing materials [19,20], and catalysts [21,22]. ...
The influence of Yb3+ cations substitution for Pr3+ on the structure and catalytic activity of (Pr1-xYbx)2Zr2O7 powders synthesized via coprecipitation followed by calcination is studied using a combination of long- (s-XRD), medium- (Raman, FT-IR, and SEM-EDS) and short-range (XAFS) sensitive methods, as well as adsorption and catalytic techniques. It is established that chemical composition and calcination temperature are the two major factors that govern the phase composition, crystallographic, and local-structure parameters of these polycrystalline materials. The crystallographic and local-structure parameters of (Pr1-xYbx)2Zr2O7 samples prepared at 1400oC/3 h demonstrate a tight correlation with their catalytic activity towards propane cracking. The progressive replacement of Pr3+ with Yb3+ cations gives rise to an increase in the catalytic activity. A mechanism of the catalytic cracking of propane is proposed, which considers the geometrical match between the metal–oxygen (Pr–O, Yb–O, and Zr–O) bond lengths within the active sites and the size of adsorbed propane molecule to be the decisive factor governing the reaction route.
... Similar to the oxygen separation membranes, a high hydrogen mobility and surface reactivity as well as a high electronic conductivity are required for hydrogen separation membrane materials. This allows it to reach high hydrogen permeation fluxes for obtaining pure hydrogen including its production in catalytic membrane reactors for fuel transformation reactions [42,55,58,[82][83][84][85][86][87][88][89]. There are advantages in using triple (H + /O 2− /e − )-conducting materials for hydrogen separation membranes, since the presence of the oxide-ionic component of the conductivity can enable the following features: ...
... Similar to the oxygen separation membranes, a high hydrogen mobility and surface reactivity as well as a high electronic conductivity are required for hydrogen separation membrane materials. This allows it to reach high hydrogen permeation fluxes for obtaining pure hydrogen including its production in catalytic membrane reactors for fuel transformation reactions [42,55,58,[82][83][84][85][86][87][88][89]. There are advantages in using triple (H + /O 2− /e − )conducting materials for hydrogen separation membranes, since the presence of the oxide-ionic component of the conductivity can enable the following features: ...
... The pyrochlore structure A2B2O7 is a derivative of the fluorite structure in which half of the cubes are replaced by octahedra (more precisely, it consists of the alternating AO8 polyhedra and BO6 trigonal antiprisms). Pyrochlores possessing a highly mixed ionic-electronic conductivity such as doped Pr2Zr2O7, Gd2Ti2O7, Er2RuMnO7, etc., are used in SOFC cathodes [164,225,226], oxygen [83,227,228] and hydrogen separation membrane [229,230]. They contain high amounts of oxygen vacancies providing fine oxygen transport characteristics. ...
Oxygen and hydrogen mobility are among the important characteristics for the operation of solid oxide fuel cells, permselective membranes and many other electrochemical devices. This, along with other characteristics, enables a high-power density in solid oxide fuel cells due to reducing the electrolyte resistance and enabling the electrode processes to not be limited by the electrode-electrolyte-gas phase triple-phase boundary, as well as providing high oxygen or hydrogen permeation fluxes for membranes due to a high ambipolar conductivity. This work focuses on the oxygen and hydrogen diffusion of mixed ionic (oxide ionic or/and protonic)-electronic conducting materials for these devices, and its role in their performance. The main laws of bulk diffusion and surface exchange are highlighted. Isotope exchange techniques allow us to study these processes in detail. Ionic transport properties of conventional and state-of-the-art materials including perovskites, Ruddlesden-Popper phases, fluorites, pyrochlores, composites, etc., are reviewed.
... Pyrochlore structure A2B2O7 is a derivative of the fluorite structure in which a half of cubes are replaced by octahedra (more precisely, it consists of the alternating AO8 polyhedra and BO6 trigonal antiprisms). Pyrochlores possessing a high mixed ionic-electronic conductivity such as doped Pr2Zr2O7, Gd2Ti2O7, Er2RuMnO7, etc. are used in SOFC cathodes [128,234,235], oxygen [236][237][238] and hydrogen separation membranes [239,240]. They contain high amounts of oxygen vacancies providing fine oxygen transport characteristics. ...
... Similar to the oxygen separation membranes, a high hydrogen mobility and surface reactivity as well as a high electronic conductivity are required for hydrogen separation membrane materials. This allows to reach high hydrogen permeation fluxes for obtaining pure hydrogen including its production in catalytic membrane reactors for fuel transformation reactions [41,54,62,170,236,304,305,[321][322][323][324]. There are advantages in using triple (H + /O 2-/e -) conducting materials for hydrogen separation membranes since the presence of oxide-ionic component of the conductivity can enable the following features: ...
Oxygen and hydrogen mobility are among the important characteristics for operation of solid oxide fuel cells, permselective membranes and many other electrochemical devices. This, along with other characteristics, enables reaching a high power density of solid oxide fuel cells and a high oxygen or hydrogen permeation fluxes for membranes. This work focuses on oxygen and hydrogen diffusion of mixed ionic (oxide ionic or/and protonic) – electronic conducting materials for these devices and its role in the performance. Ionic transport properties of conventional and state-of-the-art materials are reviewed.
... Materials with pyrochlore structure Ln 2 M 2 O 7 (Ln 2 O 3 + 2MO 2 ) (Ln = La -Lu; M = Ti, Zr, Hf) are attractive for development of solid-oxide fuel cells and thermal-barrier coatings because of their stability in redox-atmosphere and high-temperature stability [1][2][3][4][5]. The Ln 2 M 2 O 7 -based solid solutions have wide homogeneity areas Ln 2 ± x M 2 ± x O 7 ± δ , whose width is determined by the synthesis temperature and the nature of the oxide-dopant (Ln 2 O 3 or MO 2 ). ...
... Upon the introducing of cations other than those of the pyrochlore lattice (lanthanide or metal), singlephase materials are also formed, however, the substitution limit in each particular case is determined by the ionic radius ratio, the synthesis method, and the thermal treatment temperature; generally, it is lower than in the case of doping with the lattice inherent cations [5]. ...
... At higher temperatures, it is the oxygen-ionic character that dominated in conductivity. Importantly, practically all REE titanates are distinguished in the absence of hole conductance at higher oxygen partial pressures [5]. ...
... Oxides and solid solutions with the pyrochlore structure A 2 B 2 O 7 (or A 2 B 2 O 6 O', where A and B are rare earth or transition elements) attracted a lot of attention as materials for many applications such as oxygen [1][2][3][4] and hydrogen [5,6] separation membranes, solid oxide fuel cell/electrolyzer electrolytes [1,[7][8][9] and electrodes [8,10,11], catalysts for various transformation processes [12,13], pigments [14,15], etc. [16][17][18]. The prospects of using pyrochlores in various electrochemical devices are provided by their high ionic or mixed ionic-electronic conductivity, depending on their composition and synthesis conditions [1,[8][9][10][11]19,20]. ...
... Oxides and solid solutions with the pyrochlore structure A 2 B 2 O 7 (or A 2 B 2 O 6 O', where A and B are rare earth or transition elements) attracted a lot of attention as materials for many applications such as oxygen [1][2][3][4] and hydrogen [5,6] separation membranes, solid oxide fuel cell/electrolyzer electrolytes [1,[7][8][9] and electrodes [8,10,11], catalysts for various transformation processes [12,13], pigments [14,15], etc. [16][17][18]. The prospects of using pyrochlores in various electrochemical devices are provided by their high ionic or mixed ionic-electronic conductivity, depending on their composition and synthesis conditions [1,[8][9][10][11]19,20]. ...
... Oxides and solid solutions with the pyrochlore structure A 2 B 2 O 7 (or A 2 B 2 O 6 O', where A and B are rare earth or transition elements) attracted a lot of attention as materials for many applications such as oxygen [1][2][3][4] and hydrogen [5,6] separation membranes, solid oxide fuel cell/electrolyzer electrolytes [1,[7][8][9] and electrodes [8,10,11], catalysts for various transformation processes [12,13], pigments [14,15], etc. [16][17][18]. The prospects of using pyrochlores in various electrochemical devices are provided by their high ionic or mixed ionic-electronic conductivity, depending on their composition and synthesis conditions [1,[8][9][10][11]19,20]. ...
Synthesis and study of materials based on bismuth cerates and titanates were carried out. Complex oxides Bi1.6Y0.4Ti2O7 were synthesized by the citrate route; Bi2Ce2O7 and Bi1.6Y0.4Ce2O7—by the Pechini method. The structural characteristics of materials after conventional sintering at 500–1300 °C were studied. It is demonstrated that the formation of a pure pyrochlore phase, Bi1.6Y0.4Ti2O7, occurs after high-temperature calcination. Complex oxides Bi2Ce2O7 and Bi1.6Y0.4Ce2O7 have a pyrochlore structure formed at low temperatures. Yttrium doping of bismuth cerate lowers the formation temperature of the pyrochlore phase. As a result of calcination at high temperatures, the pyrochlore phase transforms into the CeO2-like fluorite phase enriched by bismuth oxide. The influence of radiation-thermal sintering (RTS) conditions using e-beams was studied as well. In this case, dense ceramics are formed even at sufficiently low temperatures and short processing times. The transport characteristics of the obtained materials were studied. It has been shown that bismuth cerates have high oxygen conductivity. Conclusions are drawn about the oxygen diffusion mechanism for these systems. The materials studied are promising for use as oxygen-conducting layers in composite membranes.
... Under the action of radiation, the structure can undergo a structural phase transition from pyrochlore to a structure of defective fluorite, without being subjected to amorphization. In this regard, Gd 2 Zr 2 O 7 is considered as a matrix for radioactive waste disposal [18][19][20][21][22]. Gadolinium zirconate is also used as a material for thermal barrier coatings [20][21][22], catalysts [23,24], solid electrolytes for SOFCs [25,26], and sensors [27]. ...
Composites of (1-x)Gd2Zr2O7·xMgO were prepared by mixing gadolinium zirconate with freshly precipitated Mg(OH)2 followed by heat treatment at 1500 °C. Small concentrations of magnesium oxide dissolved in the complex oxide matrix of Gd2Zr2O7. This led to decrease in the lattice parameters of the matrix phase and a complex redistribution of Gd and Zr over the A and B sublattices. According to the impedance spectroscopy results of the studied samples, for (1-x)Gd2Zr2O7·xMgO (x = 0.05, 0.07, 0.10), the ionic conductivity was slightly higher than that for the undoped Gd2Zr2O7. The share of dominant ion transport did not change upon doping with magnesium oxide. The composites showed chemical resistance in a lithium halide (LiCl) melt and interacted with LiCl-xLi2O (x = 2 wt.%, 4 wt.%) melts at 650 °C with the formation of a Gd2O3 phase or a mixture of phases (Gd2O3, Li2ZrO3, ZrO2, LiGdO2, or LiGdCl2) on the ceramic surface, respectively.
... Recently, pyrochlore structured oxides with rare-earth ions at one cationic site have been explored to draw interesting magnetic properties driven by geometrical frustration of spin-spin interactions. These materials have also been used as electrolytes [7], nuclear reactor materials [8][9][10], and solid catalysts [11]. In these constituents, the nature of spin fluctuations is linked by different spin configurations. ...
Ho2Ti2O7 is recognized for its striking spin-ice behaviour as well as for prominent quantum nature. In present report, structural driven magnetic properties of the spin frustrated magnet Ho2(Ti1-xGex)2O7 (x = 0, 0.2, and 0.4) pyrochlore were examined by partially substitution of Ti by Ge. Single phasic pyrochlore superstructure was noted by X-ray diffraction and Raman spectroscopicinvestigations. A monotonically decreasing cell parameter along with microstrainconfirms the perfectly stabilization of Ge-ion in cationic lattice. Furthermore, Raman spectroscopy also exhibits a blue shift and hardening of phonon mode due to anharmonic interaction. The electron microscopic images displayed homogenously distributed densely packed grains with decreasing grains size from ∼6.2 μm to ∼3.8 μm as on increasing x value.The weakening of ferromagnetic interaction due to enhancement of chemical pressure in Ho2Ti2O7 has been revealed by magnetic studies. The observed effect occurs due to variation in exchange and dipolar interactions among the Ho spins. No spin glass-like transition occurs even on cooling sample down to 2 K.
... The pyrochlore oxides with the general formula of A 2 3+ B 2 4+ O 7 , where A 3+ is a rareearth ion and B 4+ is usually a transition metal, have attracted great attention as materials for nuclear waste disposal [1][2][3], thermal barrier coating materials [4][5][6], catalysts [7,8], and for other important technological applications such as solid electrolytes [9,10], electrodes [11], and magnets [12][13][14]. ...
Materials with pyrochlore structure A2B2O7 have attracted considerable attention owing to their various applications as catalysts, sensors, electrolytes, electrodes, and magnets due to the unique crystal structure and thermal stability. At the same time, the possibility of using such materials for electrochemical applications in salt melts has not been studied. This paper presents the new results of obtaining high-density Mg2+-doped ceramics based on Gd2Zr2O7 with pyrochlore structure and comprehensive investigation of the electrical properties and chemical stability in a lithium chloride melt with additives of various concentrations of lithium oxide, performed for the first time. The solid solution of Gd2−xMgxZr2O7−x/2 (0 ≤ x ≤ 0.10) with the pyrochlore structure was obtained by mechanically milling stoichiometric mixtures of the corresponding oxides, followed by annealing at 1500 °C. The lattice parameter changed non-linearly as a result of different mechanisms of Mg2+ incorporation into the Gd2Zr2O7 structure. At low dopant concentrations (x ≤ 0.03) some interstitial positions can be substituted by Mg2+, with further increasing Mg2+-content, the decrease in the lattice parameter occurred due to the substitution of host-ion sites with smaller dopant-ion. High-density ceramics 99% was prepared at T = 1500 °C. According to the results of the measurements of electrical conductivity as a function of oxygen partial pressure, all investigated samples were characterized by the dominant ionic type of conductivity over a wide range of pO2 (1 × 10–18 ≤ pO2 ≤ 0.21 atm) and T < 800 °C. The sample with the composition of x = 0.03 had the highest oxygen-ion conductivity (10−3 S·cm−1 at 600 °C). The investigation of chemical stability of ceramics in the melt of LiCl with 2.5 mas.% Li2O showed that the sample did not react with the melt during the exposed time of one week at the temperature of 650 °C. This result makes it possible to use these materials as oxygen activity sensors in halide melts.
... Therefore, the 48f oxygen x parameter can be used to measure the structural order/disorder degree. In this study, an increase in x indicated order/disorder transition, which could be attributed to the sublattice transition of cation, because the cation sublattice is disordered earlier than the anion sublattice, i.e., the lattice of titanate ceramics tends to be short-range disordered and long-range ordered after ageing [25][26][27]. In such cases, an increase in disorder would disrupt the ordered arrangement and concentration of 48f-sites oxygen vacancies in the ceramics, preventing oxygen ions from hopping along the 48f sites and increasing resistivity during ageing [27,28]. ...
... In this study, an increase in x indicated order/disorder transition, which could be attributed to the sublattice transition of cation, because the cation sublattice is disordered earlier than the anion sublattice, i.e., the lattice of titanate ceramics tends to be short-range disordered and long-range ordered after ageing [25][26][27]. In such cases, an increase in disorder would disrupt the ordered arrangement and concentration of 48f-sites oxygen vacancies in the ceramics, preventing oxygen ions from hopping along the 48f sites and increasing resistivity during ageing [27,28]. Nevertheless, the resistance drift occurs largely in the preliminary stage, followed by a stabilization of the resistance value, as reflected by the resistance-temperature plots obtained for A 2 Ti 2 O 7 (A = Sm, Eu, Y) ceramics after 75 h and 400 h of ageing ( Fig. 5b-d). ...
In this study, a series of novel negative temperature coefficient thermistor materials based on A2Ti2O7 (A = Sm, Eu, Y) titanate ceramics have been developed for high-temperature applications. The A2Ti2O7 (A = Sm, Eu, Y) ceramics showed cubic pyrochlore structures with dense morphology in X-ray diffraction and scanning electron microscopic analyses. These ceramics showed typical NTC characteristics in a wide temperature range of 673–1473 K, accompanied by resistance drift rates of 2.71%, 1.63%, and 4.95% for 400 h of ageing at 1173 K. The proposed ceramics have high resistivity and thermal constants B that ensure their better sensitivity at high temperatures and also show unique oxygen insensitivity. These properties outperform those of traditional high-temperature NTC materials composed of perovskite-type or scheelite-type ceramics, making A2Ti2O7 (A = Sm, Eu, Y) ceramics promising candidates for fabricating high-temperature NTC thermistors.
