Proton Conductivity of La₂(Hf_2−xLa_x)O_7−x/2 “Stuffed” Pyrochlores

Featured Application

The proposed materials can be applied in solid oxide fuel cells (SOFCs).

Abstract

The design of new oxygen- and proton-conducting materials is of paramount importance for their possible utilization in solid oxide fuel cells. In the present work, La₂(Hf_2–xLa_x)O_7–x/2 (x = 0, 0.1) ceramics were prepared using ball milling of oxide mixtures (La₂O₃ and HfO₂) followed by high-temperature annealing at 1600 °C for 10 h in air. La₂Hf₂O₇ ceramics exhibit an ordered pyrochlore-type structure, whereas La₂(Hf_1.9La_0.1)O_6.95 has a defect pyrochlore structure type with oxygen vacancies at the 48f positions. The oxygen ion and proton conductivity of La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) and four-probe DC measurements in a dry and a wet atmosphere (air and nitrogen). The use of two distinct conductivity measurement techniques ensured, for the first time, the collection of reliable data on the proton conductivity of the La₂(Hf_1.9La_0.1)O_6.95 “stuffed” hafnate pyrochlore. La₂Hf₂O₇ was found to be a dielectric in the range 400–900 °C, whereas the La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore had both oxygen ion and proton conductivities in this temperature range. The proton conductivity level was found to be equal to ~8 × 10⁻⁵ S/cm at 700 °C. Clearly, the proton conductivity of the La₂(Hf_1.9La_0.1)O_6.95 “stuffed” hafnate pyrochlore is mainly due to the hydration of oxygen vacancies at 48f positions.

1. Introduction

The (Ln = La-Lu; M = Ti, Zr, Hf) systems with a pyrochlore structure can be used as potential oxygen ion-conducting electrolytes for solid oxide fuel cell (SOFC) applications [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. The most promising electrolytes among pyrochlores are rare-element (RE)-based zirconates: the Li-doped gadolinium zirconate Gd_1.7Li_0.3Zr₂O_6.7, with a conductivity of 1 × 10^–3 S/cm at 450 °C (~8 × 10^–3 S/cm at 700 °C) [1], and (Nd_2−xZr_x)Zr₂O_7+x/2 (x = 0.4) (1.78 × 10⁻² S/cm at 700 °C) [5]. It can be seen from previous results [4,5] that the solid solutions in broad isomorphism ranges with a variable Ln/M ratio (Ln = La-Lu; M = Ti, Zr, Hf) in the titanate, zirconate, and hafnate systems can be used to prepare materials with high oxygen ion conductivity. The undoubted advantage of some RE zirconate solid solutions with the pyrochlore structure is pure oxygen ion conductivity persisting in a wide temperature range (600–900 °C) and a wide range of oxygen partial pressures, and absence of p-type conductivity at high oxygen partial pressure [4]. Undesirable electronic transport of materials used in PCFCs and PCECs leads to a decrease in their effectiveness [6,7].

The Ln₂Hf₂O₇ (Ln = Sm, Eu, Gd) compounds have the pyrochlore structure, and the highest oxygen ion conductivity is reached for Gd₂Hf₂O₇: ~1.3 × 10⁻³ S/cm at 750 °C [8,12]. The Ln₂Hf₂O₇ (Ln = Dy, Y, Ho, Yb) heavy RE hafnates have a fluorite structure, and the Dy- and Ho-based compounds have the highest conductivity among them [8]. These results were essentially confirmed by Sardar et al. [9], who were able to separate the total conductivity onto the bulk (1.0 × 10⁻⁴ S/cm at 700 °C) and grain boundary (4.66 × 10⁻⁵ S/cm at 700 °C) components for the Ho₂Hf₂O₇ fluorite prepared by the conventional solid-state reaction technique. Recently, Shlyakhtina et al. [16] synthesized Eu₂(Hf_2−xEu_x)O_7−x/2 (x = 0.1) stuffed pyrochlores, which showed the highest oxygen ion conductivity among the hafnate family: ~7.5 × 10⁻⁴ S/cm at 700 °C. This is, however, lower than the conductivity of the RE zirconates and titanates [16,17,18,19,20]. The authors of [21] investigated the electronic structure of La₂Hf₂O₇ based on first-principle calculations with a particular focus on the anion Frenkel (anti-Frenkel) defect pair (${\mathrm{V}}_{\mathrm{O}}^{••}+{{\mathrm{O}}^{″}}_{\mathrm{i}}$) in terms of defect structures with correlated formation energies. Such anion Frenkel pair (${\mathrm{V}}_{\mathrm{O}}^{••}+{{\mathrm{O}}^{″}}_{\mathrm{i}}$) defects are the most stable ones in pure La₂Hf₂O₇. It is worth noting that, whereas the oxygen ion conductivity of the RE hafnates and related solid solutions has been the subject of intense research [8,9,10,11,12,13,14,15,16], there are few data on their proton conductivity, except for the Nd₂(Hf_2−xNd_x)O_7−x/2 (x = 0; 0.1) system [22].

The proton conductivity of the Ln₂Hf₂O₇ hafnates and related solid solutions is essentially unexplored. Proton conduction was first found comparatively recently in Nd₂(Hf_2-xNd_x)O_7−x/2 (x = 0; 0.1) neodymium hafnates [22]. Proton conductivity was shown both in neodimium hafnate (1.25 × 10⁻⁶ S/cm at 700 °C) and Nd₂(Hf_1.9Nd_0.1)O_6.95 “stuffed” pyrochlore solid solution (~1 × 10⁻⁴ S/cm at 700 °C) [22]. Producing oxygen vacancies via acceptor substitutions is a well-known approach for raising the conductivity of various oxygen ion conductors [23,24,25,26]. The formation of vacancies in La₂(Hf_2−xLa_x)O_7−x/2 (x = 0.1) as a result of substitution of the trivalent lanthanide on the tetravalent metal site can be represented as follows:

$${\mathrm{La}}_2{\mathrm{O}}_3\stackrel{\hspace{1em}{\mathrm{HfO}}_2\hspace{1em}}{\to }2\mathrm{L}{{\mathrm{a}}^{\prime }}_{\mathrm{Hf}}+{\mathrm{V}}_{\mathrm{O}\left(48\mathrm{f}\right)}^{••}+3{\mathrm{O}}_{\mathrm{O}}^{\mathrm{x}}$$

(1)

The Ln₂O₃-MO₂ (M = Ti, Zr, Hf) systems have large isomorphism regions around the Ln₂Ti₂O₇ (Ln = Sm-Lu), Ln₂Zr₂O₇ (Ln = La-Eu), and Ln₂Hf₂O₇ (Ln = La-Tb) pyrochlores. In a certain range of Ln₂O₃ concentrations, the pyrochlore solid solutions in these systems can transform into fluorite solid solutions (order–disorder transition) [27,28,29,30,31,32]. In the zirconate systems, these order–disorder ranges are symmetric regarding Ln₂Zr₂O₇, whereas in the titanate systems they are rather broad and are located mainly at high degrees of Ln substitution for Ti at synthesis temperatures below 1700 °C. The investigation of the broad isomorphism ranges in the hafnate systems is complicated by the high synthesis temperatures of the solid solutions (T_syn. ≥ 1700 °C) [22,32].

As shown in a precision XANES spectroscopy study [4], the broad isomorphism range in the Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0–0.67) “stuffed” titanate system includes three different regions: (1) at x = 0–0.1, where only a P1 pyrochlore exists; (2) x~0.13–0.56, where P1 and P2 pyrochlore phases coexist, (P1—thulium-deficient; P2—thulium-enriched), and (3) x~0.56–0.67, where a defect fluorite F prevails. According to XRD data, in the Yb₂(Ti_2−xYb_x)O_7−x/2 system, two pyrochlore phases coexist in a range of x = 0.16–0.37 [31].

In this work, ball milling was used to activate a starting oxide mixture for high-temperature synthesis of La₂Hf₂O₇ and La₂(Hf_1.9La_0.1)O_6.95 solid solution with the aim of ascertaining whether they have proton conductivity. La₂(Hf_1.9La_0.1)O_6.95 falls in the La₂Hf₂O₇–La₂O₃ isomorphous miscibility range, but a low degree of substitution (x = 0.1) simplifies the situation because in most of the Ln₂M₂O₇–Ln₂O₃ systems, only one solid solution exists at such degrees of substitution; in turn, this significantly facilitates interpretation of structural data and allows one to use diffraction techniques, without resorting to spectroscopy (Raman scattering or XANES) [2,3,4].

2. Materials and Methods

La₂Hf₂O₇ and La₂(Hf_1.9La_0.1)O_6.95 were synthesized by reacting appropriate oxide mixtures (La₂O₃ + HfO₂) after mechanical activation in an Aronov ball mill [33,34]. The parameters of the Aronov ball mill were as follows: an oscillation amplitude of 0.5 cm, a frequency of 50 Hz, a vial volume of 120 cm³, and a ball/powder weight ratio of 15. The La₂O₃ powder resource was annealed at 1000 °C for 2 h to remove absorber gases and then placed in a desiccator after cooling to 850 °C. The calcined mixtures were milled and pressed at 650 MPa and then sintered at 1600 °C for 10 h in air. The relative densities of the obtained ceramic samples were determined by measuring their weight and dimensions; they were found to be 92% above the theoretical levels for both La₂Hf₂O₇ and La₂(Hf_1.9La_0.1)O_6.95 compounds. In the case of La₂(Hf_2–xLa_x)O_7–x/2 (x = 0.1) “stuffed” hafnate pyrochlore, the ceramic’s relative density was determined by hydrostatic weighing in toluene (94.8%). The phase composition features and structural properties of the powder samples were analyzed by the X-ray diffraction (XRD) technique using a DRON-3M diffractometer (Bragg-reflection geometry, Cu K_α radiation) in the range 2θ = 10°−75° at ambient condition.

Scanning electron microscopy (SEM—using a JEOL JSM-6390LA device) was employed to analyze the morphology of the ceramic samples as well as to estimate their chemical compositions. Energy dispersive X-ray microanalysis (EDX) was used to obtain X-ray concentration maps of elements.

The La₂(Hf_2–xLa_x)O_7–x/2 (x = 0, 0.1) ceramics were subject to thermogravimetric analysis in air using a NETZSCH STA 449C system (50–1000 °C, heating rate of 10 K/min, Al₂O₃ plate). To verify whether the La₂(Hf_1.9La_0.1)O_6.95 ceramics contained structurally bound water, which might be responsible for proton conduction, we performed TG scans from 50 to 1000 °C in an oxygen atmosphere at a heating rate of 10 °C/min. To assess the reproducibility of the observed effects, the sample was reheated after cooling in the oxygen atmosphere.

The disk-shaped ceramic samples (diameter ~9 mm and thickness ~2 mm) were fabricated for electrical measurements. Electrode contacts to the sample were made by firing ChemPur C3605 paste with colloidal platinum at 950 °C for 0.5 h. The conductivity of La₂Hf₂O₇ and La₂(Hf_1.9La_0.1)O_6.95 was studied by impedance spectroscopy analysis (EIS) via a two-probe AC method in both dry and wet air atmospheres. A combination of P-5X potentiostat/galvanostat and a frequency response analyzer module (Electrochemical Instruments Ltd., Chernogolovka, Russia) was purposefully used to provide electrical conductivity measurements. The impedance spectra between 100 and 900 °C were recorded over a frequency range of 10⁻¹–5 × 10⁶ Hz at a signal amplitude of 150 mV. The dry atmosphere was created by passing air through a KOH, while the wet atmosphere was regulated by passing air through a water saturator held at 20 °C, which ensured a constant humidity of ~0.023 atm (2.3% H₂O). Air flow rate was 130 mL/min. To obtain stable water vapor pressure before measurement, the sample was kept at each temperature for 40 min. The impedance data fitting was performed using Zview software (Scribner Associates, Southern Pines, NC, USA).

In addition, the total conductivity of La₂(Hf_1.9La_0.1)O_6.95 the was evaluated by the four-probe DC method. These measurements were performed in a temperature range of 500–900 °C in dry and in wet oxidizing and reducing atmospheres (air and N₂). Wet atmospheres (P_H2O = 0.02 atm) were generated by bubbling gases through water thermostatted at a temperature of 18 °C.

3. Results and Discussion

3.1. Study of the La₂Hf₂O₇ and La₂(Hf_1.9La_0.1)O_6.95 Structure by the X-ray Diffraction Method

The pyrochlore structure of La₂Hf₂O₇ has two distinct cation sites—those of La, in the center of a distorted LaO₈ cube (scalenohedron), and Hf, in the center of a distorted HfO₆ octahedron (trigonal antiprism)—and three distinct anion sites: 48f (O1), 8a (O2), and 8b (O3). Each O1 oxygen (48f) is surrounded by two La³⁺ and two Hf⁴⁺ cations, each O2 oxygen (8a) is coordinated by four La³⁺ cations, and each O3 oxygen, which is always empty in ordered stoichiometric pyrochlores, is coordinated by four Hf⁴⁺ cations. La₂Hf₂O₇ is the most ordered pyrochlore phase in the Ln₂Hf₂O₇ (Ln = La-Tb) series, without cation disordering. In the case of La₂(Hf_1.9La_0.1)O_6.95, the Hf-to-La substitution leads to a decrease in the intensity of the 111, 311, 331, and 531 pyrochlore superstructure reflections compared with La₂Hf₂O₇ (Figure 1a, scans 1 and 2, respectively). In addition, the cubic pyrochlore cell parameter increases from 10.7731(2) Å for nominally stoichiometric La₂Hf₂O₇ to 10.7833 (1) Å for La₂(Hf_1.9La_0.1)O_6.95 because the ionic radius of lanthanum in the center position of the distorted octahedra exceeds that of hafnium (R La³⁺_CN6= 1.032Å; R Hf⁴⁺_CN6 = 0.71Å).

A detailed analysis of the phase composition and crystal structure of the obtained materials was carried out by refining diffraction patterns by the Rietveld method (by means of the TOPAS software). The results of such a refinement are summarized in

Table 1. Rietveld refinement of positions for La₂(Hf_2−xLa_x)O_7−x/2 (x = 0, 0.1) (sp.gr. Fd-3m) at T = 300 K.

c	Site	x	y	z	Occ.	Rexp, % Rwp, % Rp, % GOF	Unit Cell Parameter a, Å	Crystal Density, g/cm3
La2Hf2O7	LaLa(16d)	0.5000	0.5000	0.5000	1	9.61
	HfHf(16c)	0.0000	0.0000	0.0000	1	12.42
	LaHf(16c)	0.0000	0.0000	0.0000	0	9.93	10.7731(2)	7.93
	O(1)(48f)	0.3586	0.1250	0.1250	1	1.26
	O(2)(8a)	0.1250	0.1250	0.1250	1
La2(Hf1.9La0.1)O6.95	LaLa(16d)	0.5000	0.5000	0.5000	1	9.45
	HfHf(16c)	0.0000	0.0000	0.0000	0.95	15.08
	LaHf(16c)	0.0000	0.0000	0.0000	0.05	11.88	10.7833(1)	7.86
	O(1)(48f)	0.3770	0.1250	0.1250	0.992	1.6
	O(2)(8a)	0.1250	0.1250	0.1250	1

and in Figure 1b,c.

According to the XRD data, the La₂(Hf_1.9La_0.1)O_6.95 sample is a single-phase pyrochlore with no detectable secondary phase. This result agrees well with Durand’s data [35], according to which the homogeneity region of this solid solution extends to x = 0.16 at a temperature of 1600 °C.

Similar results were previously obtained for Nd₂(Hf_2−xNd_x)O_7−x/2 (x = 0, 0.1) synthesized at 1600 °C [22]. The incorporation of 5% La into the Hf sublattice leads to the formation of oxygen vacancies and a defect pyrochlore structure according to the following scheme:

$$\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3+\mathrm{H}{\mathrm{f}}_{\mathrm{H}\mathrm{f}}^{\mathrm{x}}+1/2{\mathrm{O}}_{\mathrm{O}}^{\mathrm{x}}\to \mathrm{L}{{\mathrm{a}}^{\prime }}_{\mathrm{H}\mathrm{f}}+1/2{\mathrm{V}}_{\mathrm{O}}^{••}+1/2\mathrm{L}{\mathrm{a}}_2\mathrm{H}{\mathrm{f}}_2{\mathrm{O}}_7$$

(2)

According to Mullens et al. [2], only one pyrochlore phase exists in the Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0–0.67) system at a low substitution degree, x = 0–0.1, unlike at higher degrees of substitution (x > 0.1) [2]. As shown for A₂(B_2−xA_x)O_7−x/2 “stuffed” pyrochlores [2] using the Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0–0.67) system as an example, at low degrees of Ti-for-Tm substitution (at x = 0–0.1), oxygen vacancies are formed predominantly in position 48f, opposed to the change in the vacancies formation in two positions (48f and 8b) in Y₂(Zr_yTi_1−y)₂O₇ [36] and Y₂Ti_2−xHf_xO₇ [37] as a result of isovalent substitutions in the tetravalent metal sublattice. Note that in the case of the formation of anti-site pairs in nominally stoichiometric pyrochlores at high temperatures (e.g., in the oxygen- and proton-conducting neodymium hafnate Nd₂Hf₂O₇ synthesized at 1700 °C), oxygen vacancies are formed in position 48f [22].

It can be assumed that there a similar situation obtains for the Ln₂(Hf_2−xLn_x)O_7−x/2 (Ln = La, Nd; x = 0.1) hafnate systems and the Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0–0.1) titanate system. La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore was shown to contain oxygen vacancies only in position 48f (Table 1).

3.2. Microstructure of La₂Hf₂O₇ and La₂(Hf_1.9La_0.1)O_6.95

Table 2. Average La/Hf ratio on the outer and fracture surfaces of ceramics: SEM/EDX ten point analysis of La₂(Hf_2−xLa_x)O_7−x/2 (x = 0, 0.1).

Composition	Analysing Area	Nominal Stoichiometry La/Hf	La/Hf Mean	Standard Error of the Mean
La2(Hf1.9La0.1)O6.95	Outer surface	1.11	1.34	0.04
	Fracture surface		1.24	0.05
La2Hf2O7	Outer surface	1.00	1.57	0.13
	Fracture surface		1.05	0.04

presents the La/Hf ratio on the outer and fracture surfaces of La₂Hf₂O₇ and La₂(Hf_1.9La_0.1)O_6.95. The La/Hf ratio on the fracture surfaces is similar to the intended one, whereas the outer surface is found to be enriched with lanthanum. Note that the nominally stoichiometric La₂Hf₂O₇ ceramic is visually inhomogeneous and two-colored; its microstructure is presented in Figure 2a,b. Its surface is white, which correlates with the La₂O₃ excess detected by EDS/EDX (energy dispersive X-ray spectroscopy X-ray analysis), and there is a brown layer in its interior. The micrograph of a fracture surface of the ceramic in Figure 2a demonstrates its inhomogeneity. Nevertheless, the La₂Hf₂O₇ ceramic is dense: its relative density is 7.4 g/cm³, i.e., 93.3% of its theoretical density. The La₂(Hf_1.9La_0.1)O_6.95 ceramic is light brown in colour and more homogeneous than the La₂Hf₂O₇ ceramic prepared under the same conditions. The La/Hf ratio is near the expected one (Table 2). The microstructure of the La₂(Hf_1.9La_0.1)O_6.95 ceramic is shown in Figure 2c,d. Note the La excess is observed on the outer surface of the La₂Hf₂O₇ compared to La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore (Table 2). The density of the La₂(Hf_1.9La_0.1)O_6.95 ceramic is 7.27 g/cm³, which is 92.5% of its theoretical density, whereas its density determined by hydrostatic weighing is higher: 94.8%.

The average La/Hf ratio value on the fracture surface exceeds the nominal stoichiometry within the standard error of the mean (SEM), but it is close to it and significantly less than this ratio on the outer surface (Table 2). In addition, visual information about the inhomogeneity of the color of the samples cannot serve as a basis for considering the samples as polyphasic; however, based on the EDX analysis data, we can conclude that there is a gradient in the La/Hf ratio. This inhomogeneity is apparently associated with significant inertia of the system, as a result of which the diffusion of atoms proceeds slowly.

3.3. Oxygen Ion and Proton Conductivity of La₂(Hf_2−xLa_x)O_7−x/2 (x = 0; 0.1) in Dry and Wet Air

Electrochemical impedance spectroscopy data for the La₂Hf₂O₇ ceramic demonstrate that its impedance response corresponds to dielectric behaviour. No significant conductivity was detected in the range 300–900 °C with our measurement equipment. Therefore, La₂Hf₂O₇ is a dielectric, unlike Nd₂Hf₂O₇, studied previously [22], whose conductivity in wet air is ~1.25 × 10⁻⁶ S/cm at 700 °C, approaching the proton conductivity of La₂Zr₂O₇ [38]. The negligible conductivity of La₂Hf₂O₇ is attributable to the high covalence of the La–Hf bonds and the perfect structure of this pyrochlore phase. Figure 3 shows Arrhenius plots for a number of Ln₂(Hf_1.9Ln_0.1)O_6.95 ((1) Ln = La; (2) Nd [22]; (3) Sm [16]; (4) Eu) [16] “stuffed” pyrochlores in dry and wet air. La₂(Hf_1.9La_0.1)O_6.95 has considerable proton conductivity in the range 400–750 °C. The impedance spectra of La₂(Hf_1.9La_0.1)O_6.95 at 530 and 700 °C in dry and wet air are presented in Figure S1. Apparent activation energy (E_a) of total conductivity temperature dependences of Ln₂(Hf_1.9Ln_0.1)O_6.95 (Ln = La, Nd, Sm, Eu) “stuffed” hafnate pyrochlores in dry and wet air are presented in

Table 3. Apparent activation energy (E_a) of total conductivity temperature dependences of Ln₂(Hf_1.9Ln_0.1)O_6.95 (Ln = La, Nd, Sm, Eu) stuffed hafnate pyrochlores in dry and wet air.

Composition Ln2(Hf1.9Ln0.1)O6.95	Atmosphere	Temperature Range, °C	Ea (±0.01), eV	References
La	Dry air	300–600	0.77	This work
		600–900	1.04
	Wet air	300–600	0.66
		600–900	0.82
Nd	Dry air	550–900	1.25	[22]
	Wet air	550–900	1.16
Sm	Dry air	400–900	0.74	[16]
	Wet air	400–900	0.74
Eu	Dry air	400–900	1.14	[16]
	Wet air	400–900	1.14

. The conductivity of La₂(Hf_1.9La_0.1)O_6.95 in wet air is ~8 × 10⁻⁵ S/cm at 700 °C (Figure 3), which differs very little from the conductivity of Nd₂(Hf_1.9Nd_0.1)O_6.95 under the same conditions [22]. The conductivity of Ln₂(Hf_1.9Ln_0.1)O_6.95 (Ln = Sm, Eu) is not higher in wet air [16], which indicates that Sm- and Eu-based “stuffed” hafnate pyrochlores are purely oxygen ion conductors (Figure 3). The Ln₂(Hf_1.9Ln_0.1)O_6.95 (Ln = La, Nd) “stuffed” pyrochlores, in which the major defects are oxygen vacancies in position 48f, have proton conductivity due to interaction of the oxygen vacancies in position 48f with water according to the Equation (3):

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{V}}_{\mathrm{O}}^{••}+{\mathrm{O}}_{\mathrm{O}}^{\mathrm{x}}\to 2\mathrm{O}{\mathrm{H}}_{\mathrm{O}}^{•}$$

(3)

It is obvious (Table 3) that below 800 °C the activation energies for conduction in the Ln₂(Hf_1.9Ln_0.1)O_6.95 (Ln = La, Nd) “stuffed” hafnate pyrochlores in wet air are lower than those in dry air. This is usual proton conductor behaviour. Ln₂(Hf_1.9Ln_0.1)O_6.95 (Ln = Sm, Eu) “stuffed” pyrochlores have the same activation energy (Table 3) for conductivity, which is independent of humidity.

3.4. DSC and TG Characterization of La₂(Hf_1.9La_0.1)O_6.95 “Stuffed” Pyrochlore

No significant proton conductivity for the Ln₂(Hf_1.9Ln_0.1)O_6.95 (Ln = Sm, Eu) materials is attributed to the low basicity of Sm₂O₃ and Eu₂O₃ and the low hydration degree of oxygen vacancies in position 48f in their pyrochlore solid solutions compared to La- and Nd-counterparts [39]. The La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore powder was heated twice up to 1000 °C. Figure 4a shows DSC and TG curves of this sample during the first (curves 1 and 1a) and second (curves 2 and 2a) heating cycles. The total weight loss between 50 and 1000 °C is 0.41% in the first and 0.32% in the second heating cycles. The TG curve obtained during the first heating has two steps. A sharpest weight loss is observed between 50 and 650 °C, with ΔM₁ = 0.33%. According to Colomban [40], this temperature range corresponds to the removal of surface water and hydroxyl ions. Above 650 °C, water was removed rather slowly, up to 1000 °C. This weight loss is related to the structurally bounded water and interstitial protons [40]. The weight loss was about ΔM₁ = 0.08% at 650–1000 °C. After cooling in the same oxygen atmosphere, the sample was reheated, and the weight loss between 50 and 650 °C was 0.27%, i.e., smaller than in the first heating cycle. The slope of the second weight loss step, between 650 and 1000 °C, also decreased, and the weight loss was ΔM₁ = 0.05%, i.e., also smaller than in the first heating cycle. Thus, the second heating showed that, during cooling in an oxygen atmosphere, the material regained most of the physically and structurally bound water. The present results demonstrate that La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore powder rehydrates rather easily during cooling and the water is then present in the form of not only physically bound water but structurally bound water and interstitial protons.

Figure 4b displays TG curves of the La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore powder (curve 1) and pure La₂Hf₂O₇ powder (curve 2). It can easily be seen that La₂Hf₂O₇ loses surface water and hydroxyl ions up to 650 °C and above this temperature there is no weight loss, while La₂(Hf_1.9La_0.1)O_6.95 loses structurally bound water and protons in the 650–1000 °C temperature interval (ΔM₁ = 0.08%). In this work, the sharpest weight loss (50–650 °C) for La₂Hf₂O₇ (Figure 4b, curve 2) is associated with the release of H₂O and CO₂. This can be partially assigned to the decomposition of La-containing carbonates and hydroxycarbonates formed due to CO₂ trapping from air during cooling and located at grain boundaries or in pores. The exoeffect on the DSC curve of La₂(Hf_1.9La_0.1)O_6.95 near 800 °C (Figure 4a, curve 1) can be caused by the structure relaxation after the removal of bulk CO₃²⁻ groups. It should be noted that after the second heating this effect disappeared (Figure 4a, curve 2).

3.5. Four-Probe Proton Conductivity Measurements for the La₂(Hf_1.9La_0.1)O_6.95 Stuffed Pyrochlore in Dry and Wet Nitrogen and Air

The four-probe conductivity measurement results for La₂(Hf_1.9La_0.1)O_6.95 are presented in Figure 5. In the measurements, we used a bar-shaped sample cut from the center of the ceramic pellet. It can be seen that the conductivity in both wet air and wet N₂ exceeds that in the dry atmospheres. The conductivity of La₂(Hf_1.9La_0.1)O_6.95 in wet air is ~8 × 10⁻⁵ S/cm at 700 °C, in agreement with the two-probe measurement results (Figure 3). The total conductivity in wet nitrogen exceeds that in dry nitrogen, indicating that the La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore has proton conductivity. Clearly, the oxygen vacancies in position 48f of the La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore interact with water. A crucial role is played by the ability of the oxygen vacancies in position 48f, i.e., in the center of the Ln³⁺₂Hf⁴⁺₂ (Ln = La, Nd) tetrahedra in the pyrochlore structure, to be hydrated, which is due to the basicity of lanthanum and neodymium compounds. The lower basicity of the Ln₂(Hf_1.9Ln_0.1)O_7−x/2 (Ln = Sm, Eu) “stuffed” pyrochlores makes them purely oxygen ion conductors [16].

La₂(Hf_1.9La_0.1)O_6.95 “stuffed” hafnate pyrochlores demonstrated proton contribution ~1× 10⁻⁵ S/cm at 600 °C, whereas the conductivity of Ca-doped zirconate pyrochlores is higher by ~1–1.5 orders of magnitude. For example, the conductivity of (La_2−xCa_x)Zr₂O_7−δ (x = 0.05) in wet air is 7 × 10⁻⁴ S/cm at 600 °C while that of (Nd_2-xCa_x)Zr₂O_7-δ (x = 0.05) is 2.5 × 10⁻⁴ S/cm at 600 °C) and that of (Sm_2−xCa_x)Zr₂O_7−δ (x = 0.05) is 7.5 × 10^–4 S/cm at 600 °C [41,42,43,44,45]. The comparison of the proton conductivity of the Ca-doped zirconates and “stuffed” hafnate pyrochlores indicates that the higher basicity of cations around an oxygen vacancy in position 48f as a result of Ca-substitution, a more basic cation, for the lanthanide increases the hydration at this position and, as a result, proton conductivity rises by almost two orders of magnitude [41,42,43,44,45,46,47,48]. Another factor contributing to the decrease in the conductivity of the “stuffed” hafnate pyrochlores is the presence of more covalent and strong Hf-O bonds in hafnates relative to zirconates [32].

4. Conclusions

La₂Hf₂O₇ and a La₂(Hf_1.9La_0.1)O_6.95 solid solution were prepared using ball milling of oxide mixtures followed by high-temperature annealing at 1600 °C for 10 h in air. The prepared La₂Hf₂O₇ ceramic is nonuniform in composition, with an excess of lanthanum oxide on its surface. The La₂Hf₂O₇ ceramic is a dielectric, whereas the La₂(Hf_1.9La_0.1)O_6.95 ceramic is more homogeneous and has a defect pyrochlore structure type with oxygen vacancies at the 48f positions of the pyrochlore structure. La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore exhibits both oxygen ion and proton transference. The use of impedance spectroscopy in different atmospheres (dry and wet air and nitrogen) with two-probe AC and four-probe DC measurements provided reliable data on the proton conductivity of the La₂(Hf_1.9La_0.1)O_6.95 “stuffed” hafnate pyrochlore. The La₂(Hf_1.9La_0.1)O_6.95 “stuffed” pyrochlore has been shown to have both oxygen-ion and proton conductivity in the range 600–900 °C, with a proton conductivity of ~8 × 10⁻⁵ S/cm at 700 °C. The conductivity in the wet N₂ atmosphere was higher than in the dry N₂ atmosphere in the temperature range of 600–900 °C, which indicates hydration of oxygen vacancies at 48f positions. In the air atmosphere, the difference was smaller, since some of the oxygen vacancies serve to move oxygen ions.

Clearly, the excess conductivity in a wet atmosphere relative to a dry one for the La₂(Hf_1.9La_0.1)O_6.95 “stuffed” hafnate pyrochlore is due solely to proton conductivity, without any contribution from surface conduction.