Local structure and ionic transport in acceptor-doped layered perovskite BaLa 2 In 2 O 7

Materials with perovskite or perovskite-related structure have many applications because of theirs different physical and chemical properties. These applications are extremely diverse and cover different fields including hydrogen energy. Layered perovskites with Ruddles-den-Popper structure constitute a novel class of ionic conductors. In this paper, the effect of acceptor doping on the local structure and its relationship with transport properties were shown for layered perovskites based on BaLa 2 In 2 O 7 for the first time. The geometric factor (the increase in the unit cell volume due to the increase in the ionic radii of cations) plays major role in the area of small dopant concentration ( x < 0.15). The concentration factor (the increase in the oxygen vacancy concentration) is more significant in the area of big dopant concentration ( x > 0.15). The acceptor doping is a promising way of improving the oxygen-ionic


Introduction
Materials with perovskite or perovskite-related structure have many applications because of theirs different physical and chemical properties. These applications are extremely diverse and cover fields from biomedical applications, including bone tissue engineering [1−5], to energetics, including hydrogen energy [6−10]. The perovskite structure has a high tolerance to various kinds of substitutions. However, a significant change in the sizes and charges of ions leads to a change in the structure to the perovskite-related structure. Layered perovskites represent one of the large classes of perovskite-related materials and include such structural classes as Ruddlesden-Popper [11,12], Dion-Jacobson [13], and Aurivillius [14−16] structures. They can exhibit photocatalytic [17−19], ferroelectric [20−22], and luminescent [23−25] properties.

Experimental
The solid solutions BaLa2-xSrxIn2O7-0.5x and BaLa2-xBaxIn2O7-0.5x were obtained by a solid state method. The carbonates BaСО3, SrСО3 and the oxide In2О3 were initially dried and then weighed and mixed in stoichiometric quantities. The chemical reactions can be written as: The reagents were milled in an agate mortar and then calcined at 800−1300 °С with the steps of 100 °С and the duration of calcination 24 h. The intermediate regrindings followed every heating step.
The Bruker Advance D8 diffractometer with Cu Kα radiation was used for the monitoring of the phase purity of samples. The samples were prepared for XRD by heat treated at 1100 °C for 4 h and then cooled in dry Ar (pH2O = 3.5·10 −5 atm). Ar atmosphere was used to avoid any carbonization of the samples.
Raman spectra were collected on the modular confocal Raman microscopy system Alpha 300 AR (WiTec, Germany). The 10x objective lens (numerical aperture 0.2) were used to focus the blue laser (l = 488 nm, 1800 g/mm exposure time 5 seconds, averaging three spectra) to a spot size around 3 μm. The spectral resolution was 1.22 cm −1 . The RayShield coupler with a specialized narrow band filter was used to cut off the Rayleigh scattering.

Results and Discussion
The homogeneity ranges of the solid solutions BaLa2-xSrxIn2O7-0.5x and BaLa2-xBaxIn2O7-0.5x were established using X-ray analysis. The compositions with x ≤ 0.2 mol for BaLa2-xSrxIn2O7-0.5x and x ≤ 0.3 mol for BaLa2-xBaxIn2O7-0.5x were single-phase. All samples belong to the P42/mnm space group (tetragonal symmetry). The Raman spectra of all investigated compositions can be divided in two general regions. The first region includes the bending and stretching vibrations of polyhedra containing cations with bigger ionic radii (barium, strontium, lanthanum). This is a region of low, 120−200 cm − [49,50,52,53]. The second region includes the tilting/bending and stretching vibrations of In-contained polyhedra and locates higher than 200 cm −1 wavenumbers. The tilting/bending vibrations of polyhedra [InO6] are described by the ν5, ν6, ν7, ν8, and ν9 bands. The stretching vibrations of In-contained polyhedra should be located in the higher wavenumbers. The stretching vibrations of In−O bonds appear around 400 cm −1 for the monolayer perovskite BaLaInO4 [49,50]. The spectra of two-layer perovskites Srn+1TinO3n+1 [54] and Srn+1RunO3n+1 [55] contain two signals corresponding to the M−O stretching vibrations with lower and higher wavenumbers than the wavenumbers for their monolayer analogs. Based on this, ν10, ν11 and ν12 bands can be assigned to In−O stretching vibrations.
Comparable analysis of the Raman spectra of solid solutions BaLa2-xBaxIn2O7-0.5x and BaLa2-xSrxIn2O7-0.5x show that they are all similar to each other. On the one hand, doping leads to the increase in the oxygen vacancy concentration in the crystal lattice:  Consequently, the bond length M−O should decrease in the vacancy-containing polyhedra, and the blue shift in the Raman spectra can be expected. On the other hand, doping by the ions with bigger ionic radii leads to the increase in the unit cell volume (Figure 1c) which could be due to increase of the bond length M−O. Based on this, the red shift in the Raman spectra can be expected. Obviously, the absence of significant shifts in the spectra of doped compositions is the resulting effect of the overlay of these processes ( Figure 2). Meanwhile, the acceptor doping of layered perovskite BaLa2In2O7 is accompanied by the changes in the ionic radii of cations (geometric factor) and the concentration of oxygen vacancies (concentration factor). Both of these factors should affect the ionic conductivity. It should be noted that the effect of acceptor doping on the local structure of monolayer perovskite BaLaInO4 compared with that of the two-layer BaLa2In2O7 perovskite was more pronounced [49,50]. The monolayer perovskite structure contains the octahedra layers bonded only by axial oxygens and non-bonded by apical oxygens, in contrast with two-layer structure where perovskite blocks contain the octahedrons connected by all six vertices. Obviously, the crystal lattice of a monolayer perovskite is more flexible and the changes in the local structure are more evident. Figure 3 represents the dependences of oxygen-ionic conductivity and mobility for the solid solutions BaLa2-xBaxIn2O7-0.5x and BaLa2-xSrxIn2O7-0.5x obtained in the previous work [46]. As can be seen, the maximum in the conductivity and mobility curves is observed at a relatively small (0.1−0.15) dopant concentration. The most probable reason of oxygen mobility increasing in the area of small dopant concentration is the increase in the unit cell volume, i.e. in the space for ionic transfer in the crystal lattice. The presence of significant dopant concentration can lead to the formation of defect associates because of the interaction between defects with opposite charges: Consequently, the oxygen mobility should decrease. As we can see (Figure 3), the decrease in the oxygen mobility determines the decrease in the oxygen conductivity despite the increase in the oxygen vacancy concentration and the increase in the unit cell volume.
Therefore, we can conclude, that ionic conduction in the acceptor-doped layered perovskite BaLa2In2O7 is determined by several factors, including geometric (dopant radius) and concentration (dopant concentration) factors. The geometric factor (the increase in the unit cell volume due to the increase in the ionic radii of cations) plays major role in the area of small dopant concentration (x < 0.15). The concentration factor (the increase in the oxygen vacancy concentration) is more significant in the area of big dopant concentration (x > 0.15), where formation of defect associates is more probable.

Conclusions
In this paper, the local structure of solid solutions BaLa2-xBaxIn2O7-0.5x and BaLa2-xSrxIn2O7-0.5x was investigated. It was shown that several factors, including dopant radius and dopant concentration affect the changes in the oxygen ionic conductivity. The increase in the unit cell volume due to the increase in the ionic radii of cations (geometric factor) plays major role in the area of small dopant concentration (x < 0.15). The increase in the oxygen va-cancy concentration (concentration factor) is more significant in the area of big dopant concentration (x > 0.15), where formation of defect associates is more probable. The acceptor doping is a promising way of improving the oxygen-ionic conductivity of layered perovskite BaLa2In2O7.

Supplementary materials
Supplementary materials are available.