Oxygen Ion and Proton Transport in Alkali-Earth Doped Layered Perovskites Based on BaLa₂In₂O₇

Abstract

Inorganic materials with layered perovskite structures have a wide range of physical and chemical properties. Layered perovskites based on BaLa_nIn_nO_3n+1 (n = 1, 2) were recently investigated as protonic conductors. This work focused on the oxygen ion and proton transport (ionic conductivity and mobility) in alkali-earth (Sr²⁺, Ba²⁺)-doped layered perovskites based on BaLa₂In₂O₇. It is shown that in the dry air conditions, the nature of conductivity is mixed oxygen–hole, despite the dopant nature. Doping leads to the increase in the conductivity values by up to ~1.5 orders of magnitude. The most proton-conductive BaLa_1.7Ba_0.3In₂O_6.85 and BaLa_1.7Sr_0.15In₂O_6.925 samples are characterized by the conductivity values 1.2·10⁻⁴ S/cm and 0.7·10⁻⁴ S/cm at 500 °C under wet air, respectively. The layered perovskites with Ruddlesden-Popper structure, containing two layers of perovskite blocks, are the prospective proton-conducting materials and further material science searches among this class of materials is relevant.

1. Introduction

The layered perovskite family includes such classes of perovskite-related structures as Ruddlesden-Popper (RP), Dion–Jacobson (DJ), and Aurivillius structures (Figure 1). Compounds with a Ruddlesden-Popper structure have the general formula A_n+1B_nO_3n+1, in which the rock salt layers AO alternate with perovskite blocks (ABO₃)_n. This type of structure was described for the first time for the compositions Sr₂TiO₄ [1] and Sr₂Ti₂O₇ [2] by S.N. Ruddlesden and P. Popper in 1957–1958. Aurivillius phases are characterized by the formula A_n−1Bi₂B_nO_3n+3, which describes alternation of perovskite layers A_n−1B_nO_3n+1 with layers of a fluorite structure, formed by bismuth and oxygen ions [Bi₂O₂]²⁺. This crystal structure was first described in 1949 by B. Aurivillius [3]. Dion–Jacobson phases can be described by the formula A’A_n−1B_nO_3n+1. The structure of these compounds includes perovskite blocks A_n−1B_nO_3n+1 separated by layers, in which only metal cations A’ are presented. The mixed niobates of alkali and alkaline earth metals with such type of structure were synthesized for the first time by M. Dion [4] and A.J. Jacobson [5,6] in the first half of the 1980s.

The materials with layered perovskite-related structures have many various applications due to their different physical–chemical properties. They are known as photocatalysts [7,8,9,10,11,12,13], and include materials for solar hydrogen production [14,15,16,17], ferroelectrics [18,19,20,21,22], and phosphors [22,23,24,25,26,27,28]. Some of RP and DJ materials are capable of water molecule intercalation into the interlayer space. Ion exchange leads to the formation of protonated forms of compositions, for example H_xLn_1−xTiO₄∙nH₂O [29,30,31,32] or H_1−xLa_xCa_2−xNb₃O₁₀ [33,34]. Recently, the possibility of dissociative incorporation of water molecules into crystal lattice was proven for the RP compositions based on AA′BO₄ (A = Ba, Sr, A′ = La, Nd, B = In, Sc) and AA′₂B₂O₇ (A = Ba, A′ = La, Nd, B = In). The materials based on the monolayer perovskites BaNdInO₄ [35,36,37,38,39,40], BaLaScO₄ [41], SrLaInO₄ [42,43,44,45,46], and BaLaInO₄ [47,48,49,50,51,52] were investigated as protonic conductors in the temperature range of 300–500 °C. It is shown that doping of the cationic sublattices is a successful way to improve the oxygen ion and proton transport [53]. Two-layer perovskites BaLa₂In₂O₇ [54] and BaNd₂In₂O₇ [55] are also investigated as proton conductors. The acceptor doping of the lanthanum sublattice of BaLa₂In₂O₇ allows for an increase in the ionic conductivity of the matrix composition [56]. In this work, the effect of acceptor dopant (Sr²⁺, Ba²⁺) concentration on the ionic transport (O²⁻, H⁺) of BaLa_2−xM_xIn₂O_7−0.5x solid solutions, including the effect on the oxygen and proton mobility, was found.

2. Results and Discussion

2.1. XRD and TG Investigations

The X-ray diffraction (XRD) analysis was applied to establish the homogeneity ranges of the solid solutions BaLa_2−xM_xIn₂O_7−0.5x (M = Sr, Ba). The single-phase compositions were obtained in the ranges 0 ≤ x ≤ 0.20 and 0 ≤ x ≤ 0.30 for the Sr- and Ba-doped solid solutions, respectively (tetragonal symmetry, P4₂/mnm space group). The introduction of ions with bigger ionic radius (${\mathrm{r}}_{{\mathrm{La}}^{3+}}$ = 1.216 Å, ${\mathrm{r}}_{{\mathrm{Sr}}^{2+}}$ = 1.31 Å, ${\mathrm{r}}_{{\mathrm{Ba}}^{2+}}$ = 1.47 Å [57]) leads to the increase in the lattice parameters and unit cell volumes of doped compositions (

Table 1. Lattice parameters, unit cell volume, and water uptake for BaLa_2−xSr_xIn₂O_7−0.5x.

Sample	a, b (Å)	c (Å)	Vcell (Å3)	Water Uptake (mol)
0	5.914(9)	20.846(5)	729.3365	0.17
0.05	5.915(2)	20.869(0)	730.1977	0.15
0.10	5.916(3)	20.870(4)	730.5183	0.18
0.15	5.916(4)	20.871(3)	730.5745	0.18
0.20	5.917(2)	20.872(1)	730.8001	0.19

and

Table 2. Lattice parameters, unit cell volume, and water uptake for BaLa_2−xBa_xIn₂O_7−0.5x.

Sample	a, b (Å)	c (Å)	Vcell (Å3)	Water Uptake (mol)
0	5.914(9)	20.846(5)	729.3365	0.17
0.05	5.915(1)	20.859(0)	729.8232	0.16
0.10	5.916(3)	20.870(4)	730.5183	0.17
0.15	5.920(4)	20.899(3)	732.5442	0.19
0.20	5.927(5)	20.940(1)	735.7357	0.20
0.25	5.941(5)	20.949(1)	739.5330	0.21
0.30	5.956(6)	20.954(9)	743.5025	0.22

). Figure 2a represents the example of a full-profile analysis for the BaLa_1.8Ba_0.2In₂O_6.9 composition.

The possibility for water uptake was investigated by the thermogravimetry (TG) method. The amounts of water uptake for each composition are presented in the Table 1 and Table 2. As can be seen, water uptake for all undoped and doped compositions is close, and it is in the range 0.15–0.22 mol water per formula unit. It should be added that these values are comparable with water uptake for acceptor-doped classic perovskites. The TG curve coupled with MS water signal for the hydrated composition BaLa_1.8Ba_0.2In₂O_6.9∙0.2H₂O are presented in Figure 3.

The dissociative incorporation of water for the classic perovskites ABO_3−δ is due to the interaction of oxygen vacancies ${\mathrm{V}}_{\mathrm{o}}^{••}$ (appearing by the acceptor doping) or ${\mathrm{V}}_{\mathrm{o}}^{\times }$ (own structural defects) with water molecules:

$${\mathrm{V}}_{\mathrm{o}}^{••}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_{\mathrm{o}}^{\times }\iff 2{\left(\mathrm{OH}\right)}_{\mathrm{o}}^{•},$$

(1)

$${\mathrm{V}}_{\mathrm{o}}^{\times }+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{O}}_{\mathrm{o}}^{\times }\iff 2{\left(\mathrm{OH}\right)}_{\mathrm{o}}^{•}+{\mathrm{O}}_{\mathrm{i}}^{″},$$

(2)

where ${\mathrm{V}}_{\mathrm{o}}^{••}$ is the oxygen vacancy, ${\mathrm{O}}_{\mathrm{o}}^{\times }$ is the oxygen atom in the regular position,${\left(\mathrm{OH}\right)}_{\mathrm{o}}^{•}$ is the hydroxyl group in the oxygen sublattice, and ${\mathrm{O}}_{\mathrm{i}}^{″}$ is the oxygen atom in the interstitial position.

Accordingly, the amount of water uptake must increase with increase in the oxygen vacancy concentration in the crystal structure of a doped complex oxide:

$$2\mathrm{MO}\stackrel{{\mathrm{La}}_2{\mathrm{O}}_3}{\to }2{\mathrm{M}}_{\mathrm{La}}^{\prime }+2{\mathrm{O}}_{\mathrm{o}}^{\times }+{\mathrm{V}}_{\mathrm{o}}^{••},$$

(3)

where ${\mathrm{M}}_{\mathrm{La}}^{\prime }$:Sr or Ba atoms in La sites; ${\mathrm{V}}_{\mathrm{o}}^{••}$: oxygen vacancy; and ${\mathrm{O}}_{\mathrm{o}}^{\times }$: oxygen atom in a regular position. However, the water uptake for the monolayer perovskites based on BaLaInO₄ is much bigger than oxygen vacancy concentration, and it depends on the space size between perovskite blocks in the structure [53]. In this case, the dissociative intercalation of water can be described as the incorporation of hydroxyl groups into space between perovskite blocks:

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_{\mathrm{o}}^{\mathrm{\times }}\iff {(\mathrm{OH})}_{\mathrm{o}}^{•}+{(\mathrm{OH})}_{\mathrm{i}}^{\prime },$$

(4)

where ${(\mathrm{OH})}_{\mathrm{o}}^{•}$ is the hydroxyl group in the regular oxygen position and ${(\mathrm{OH})}_{\mathrm{i}}^{\prime }$ is the hydroxyl group located in the rock salt block. It is also shown that the water uptake for the two-layer BaLa₂In₂O₇ composition is lower compared to the monolayer BaLaInO₄ composition (0.17 vs. 0.62 mol per formula unit, respectively) [55]. In other words, monolayer and two-layer barium–lanthanum indates have different hydration relationships. The obtained results concerning water uptake for BaLa_1−xM_xIn₂O_7−x (M = Sr, Ba) also prove this. Obviously, the presence in the structure of two octahedral layers of perovskite blocks means less structural flexibility of the crystal lattice during hydration, and this factor is more significant than the increase in the lattice parameter and oxygen vacancies concentration during doping. In contrast to the previously described hydrated solid solutions based on BaLaInO₄∙nH₂O, the investigated hydrated solid solutions based on BaLa_2−xM_xIn₂O_7−0.5x∙nH₂O do not change the symmetry; these solid solutions are described by the same P4₂/mnm space group as anhydrous BaLa_2−xM_xIn₂O_7−0.5x (M = Sr, Ba) phases also (Figure 2b).

2.2. Electrical Conductivity Investigations

The general view of the temperature dependencies of conductivities is the same for the all investigated compositions of BaLa_2−xM_xIn₂O_7−0.5x (M = Sr, Ba). As an example, the temperature dependencies of conductivity obtained under different oxygen and water partial pressure are presented in Figure 4 for the composition BaLa_1.75Ba_0.25In₂O_6.875.

As can be seen, the values obtained under low pO₂ (~10⁻⁵ atm, Ar) and low pH₂O (3.5·10⁻⁵ atm, dry atmosphere) conditions are lower than under dry air (pO₂ = 0.21 atm, pH₂O = 3.5·10⁻⁵ atm) conditions. The difference is about 0.5 order of magnitude over the whole investigated temperature range. The effect of high water partial pressure (pH₂O = 2·10⁻² atm, wet atmosphere) on the conductivity values is more significant at the temperatures below ~500 °C. In this temperature region (300–500 °C), conductivity values under wet conditions are higher that under dry conditions by about 0.4 order of magnitude under air and 0.8 order under Ar. In addition, the conductivity values under wet air and wet Ar are very close at T < 500 °C. Figure 5 represents the concentration dependencies of conductivities for both solid solutions BaLa_2−xM_xIn₂O_7−0.5x (M = Sr, Ba) at 500 °C under dry air. As can be seen, the maximum for the concentration dependencies of conductivities is observed for both solid solutions. The difference in the conductivity values under different oxygen partial pressure (air and Ar) indicates the mixed ionic–electronic nature of electrical conductivity (Figure 4). Consequently, the detailed analysis of the dependence of conductivity on oxygen partial pressure is necessary.

The σ−pO₂ dependencies obtained under dry and wet conditions for the doped compositions are presented in Figure 6. All dependencies for Sr and Ba compositions have the same regularities, which are discussed below. It should be noted that the shape of σ−pO₂ dependencies for undoped and doped compositions are also similar [54].

Under dry air and oxidizing conditions (pO₂ > 10⁻⁴ atm), the nature of conductivity is mixed ionic–hole, and the process of the appearance of holes can be written by the following equation:

$${\mathrm{V}}_{\mathrm{o}}^{••}+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{O}}_2\iff {\mathrm{O}}_{\mathrm{o}}^{\times }+2{\mathrm{h}}^{•},$$

(5)

where ${\mathrm{V}}_{\mathrm{o}}^{••}$ is the oxygen vacancy, ${\mathrm{O}}_{\mathrm{o}}^{\times }$ is the oxygen atom in the regular position, and ${\mathrm{h}}^{•}$ is the hole. The area of predominant oxygen ionic conductivity is observed at pO_2, lower than 10⁻⁴ atm. The conductivity values obtained under the Ar atmosphere are localized in the electrolytic region (black open symbols in Figure 6), which allows us to consider the values obtained in dry argon as oxygen ion conductivity values.

The concentration dependencies of oxygen ion conductivity obtained at different temperatures are presented in Figure 7. As can be seen, these dependencies exhibit a similar shape to the dependencies of total (mixed ionic–electronic) conductivities (Figure 5). The maxima on the curves are recorded for the Sr-doped composition at x = 0.1 and for Ba-doped compositions at x = 0.15–0.25. In the other words, doping by both Sr²⁺ and Ba²⁺ ions leads to an increase in the oxygen ionic conductivity values. The possible reasons are the increase in the oxygen vacancies concentration and the expansion of space for ionic transport (increase in the lattice parameters and unit cell volumes) during doping. At the same time, the conductivity decreases in the area of high dopant concentrations. The most possible cause may be an association of point defects and the formation of the clusters:

$${\mathrm{M}}_{\mathrm{La}}^{\prime }+{\mathrm{V}}_{\mathrm{o}}^{••}\to {\left({\mathrm{M}}_{\mathrm{La}}^{\prime }·{\mathrm{V}}_{\mathrm{o}}^{••}\right)}^{•}\mathrm{or}$$

(6)

$${\mathrm{M}}_{\mathrm{La}}^{\prime }+{\left({\mathrm{M}}_{\mathrm{La}}^{\prime }·{\mathrm{V}}_{\mathrm{o}}^{••}\right)}^{•}\to {\left(2{\mathrm{M}}_{\mathrm{La}}^{\prime }·{\mathrm{V}}_{\mathrm{o}}^{••}\right)}^{\times },$$

(7)

The observed decrease in the oxygen vacancy mobility at the high dopant concentrations (Figure 8) proves this assumption. It should be noted that the similar effect of decreasing the oxygen ion conductivity and mobility is observed for the Ba-doped solid solution based on monolayer BaLaInO₄ composition [47].

The interaction of water molecules with the crystal lattice of layered perovskites based on BaLa₂In₂O₇ can be expressed by Equation (4) or by the following reaction:

$${\mathrm{BaLa}}_2{\mathrm{In}}_2{\mathrm{O}}_7+\frac{\mathrm{x}}{2}{\mathrm{H}}_2\mathrm{O}\iff {\mathrm{BaLa}}_2{\mathrm{In}}_2{\mathrm{O}}_{7+\frac{\mathrm{x}}{2}}{\left(\mathrm{OH}\right)}_{\mathrm{x}}^{}.$$

(8)

Consequently, the hydrated forms of complex oxides can be written, for example, as BaLa₂In₂O_6.83(OH)_0.34 (undoped composition). The effect of the water partial pressure on conductivity is more pronounced in the electrolytic region (Figure 6). The increase in the proton concentration during decreasing temperature leads to the decrease in the hole concentration and, consequently, in the hole conductivity:

$${\mathrm{h}}_{}^{•}+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_{\mathrm{o}}^{\times }\iff \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.{\mathrm{O}}_2+{\left(\mathrm{OH}\right)}_{\mathrm{o}}^{•}.$$

(9)

The predominance of protonic transport is achieved below ~450 °C. The good comparability between conductivity values obtained under wet Ar (black open signs in Figure 6) and values obtained from σ–pO₂ dependencies at pO₂ = 10⁻⁵ atm should be noted.

Proton conductivity values were calculated as the difference between the conductivity values in wet and dry Ar, and its concentration dependencies for the solid solutions BaLa_2−xM_xIn₂O_7−0.5x are presented in Figure 9. As can be seen, the proton conductivity increases with increasing dopant concentration. However, the most significant increase is observed in the area of “small” dopant concentrations (x ≤ 0.10 for Sr-doped and x ≤ 0.15 s for Ba-doped solid solutions). The further increase in the dopant concentration leads to the small increase in conductivity (Ba-doped solid solution) or results in no effect (Sr-doped solid solution).

The concentration dependencies of proton mobilities at 300 °C are presented in Figure 10. As can be seen, the increase in the proton mobility is observed in the area of “small” dopant concentration, and it can be associated with the increase in the oxygen ion mobility in this area (Figure 8). The further increase in the dopant concentration leads to the decrease in the proton concentration (Sr-doped solid solution) or to an insufficient increase (Ba-doped solid solution). Obviously, the cluster formation occurs in the wet atmosphere also:

$${\mathrm{M}}_{\mathrm{La}}^{\prime }+{(\mathrm{OH})}_{\mathrm{o}}^{•}\to {({\mathrm{M}}_{\mathrm{La}}^{\prime }\cdot {(\mathrm{OH})}_{\mathrm{o}}^{•})}^{\times },$$

(10)

and this process is more significant for the compositions with smaller dopant size ${\mathrm{M}}_{\mathrm{La}}^{\prime }$ (that is, for the Sr-doped compositions). It should be noted that the same effect of decreasing the proton mobility under “high” dopant concentrations is observed for the Ba-doped solid solution based on the monolayer BaLaInO₄ composition [47].

Thus, the acceptor doping of two-layer perovskite BaLa₂In₂O₇ is a successful way for improving proton conductivity and obtaining novel high-conductive electrolytic materials. The doping by the Sr²⁺- and Ba²⁺- ions on the La³⁺ sublattice leads to the increase in the ion conductivity by up to ~1.5 orders of magnitude. The most proton-conductive compositions BaLa_1.7Ba_0.3In₂O_6.85 and BaLa_1.7Sr_0.15In₂O_6.925 have close conductivity values of 1.2 × 10⁻⁴ S/cm and 0.7 × 10⁻⁴ S/cm at 500 °C under wet air, respectively.

3. Materials and Methods

Two-layer perovskites BaLa_2−xM_xIn₂O_7−0.5x (M = Sr, Ba) were prepared by the solid-state method. The carbonates BaCO₃ and SrCO_3, and oxides In₂O₃ and In₂O₃ (99.99% purity, REACHIM, Moscow, Russia) were initially dried, then weighed (analytical balance (Sartorius AG, Göttingen, Germany)) and mixed in stoichiometric quantities. The agate mortar was used for milling the powders. The calcination was performed at 800, 900, 1000, 1100, 1200, and 1300 °C. The time of each temperature treatment was 24 h.

The X-ray analysis was performed using a Bruker Advance D8 Cu K_α diffractometer (Bruker, Billerica, MA, USA) with step of 0.01°, scanning rate of 0.5°/min.

The thermogravimetry (TG) and mass spectrometry (MS) analysis were performed using STA 409 PC Netzsch Analyser connected with QMS 403 C Aëolos mass spectrometer (Netzsch, Selb, Germany). The heating of initially hydrated samples was conducted in the temperature range of 40–1100 °C, with the rate of 10 °C/min under a flow of dry Ar.

The measurements of electrical conductivity were performed by impedance spectroscopy method (impedance spectrometer Z-1000P, (Electrochemical Instruments (Elins), Chernogolovka, Russia). The investigations were conducted from 1000 to 200 °C with 1°/min cooling rate under dry air or dry Ar conditions. The dry gas (air or Ar) was produced by circulating the gas through P₂O₅ (pH₂O = 3.5·10⁻⁵ atm). The wet gas (air or Ar) was obtained by bubbling the gas at room temperature first through distilled water, and then through saturated solution of KBr (pH₂O = 2·10⁻² atm). The humidity of the gas was controlled by a Honeywell HIH-3610 H₂O sensor (Honeywell, Freeport, TX, USA). The dependencies of conductivities vs. partial oxygen pressures pO₂ were obtained by using the electrochemical method for producing different pO₂ with oxygen pump from Y-stabilized ZrO₂ ceramic. The values of the resistance were recorded after 3–5 h of equilibrium.

4. Conclusions

It this paper, the members of the layered perovskites family with a two-layer Ruddlesden-Popper structure were obtained, and theirs transport properties were investigated. It is shown that doping of the lanthanum sublattice of BaLa₂In₂O₇ by the ions Sr²⁺ and Ba²⁺ with bigger ionic radius leads to the increase in lattice parameters and unit cell volumes of doped compositions. The possibility for dissociative incorporation of water molecules into crystal lattice is proven. The water uptake is in the range of 0.15–0.22 mol water per formula unit of complex oxide for all undoped and doped compositions. In the dry air conditions, the nature of conductivity is mixed oxygen–hole, regardless of the dopant nature (Sr²⁺, Ba²⁺). Doping leads to the increase in the conductivity values by up to ~1.5 orders of magnitude. The most proton-conductive BaLa_1.7Ba_0.3In₂O_6.85 and BaLa_1.7Sr_0.15In₂O_6.925 samples are characterized by the conductivity values of 1.2·10⁻⁴ S/cm and 0.7·10⁻⁴ S/cm at 500 °C under wet air, respectively. The acceptor-doped layered perovskites based on BaLa₂In₂O₇ are prospective proton-conducting materials, and further material science searches among this class of materials are relevant.