Effect of high oxygen deficiency in nano-confined bismuth sesquioxide

Bismuth sesquioxide in its cubic form, i.e. δ-Bi2O3, is the fastest oxygen ionic conductor known which has important applications in energy technologies. However, the material is unstable as it undergoes high-density polymorphic transitions and degradation. In this work, we show that δ-Bi2O3 can be stabilized both at high and low temperatures (T < 775 °C) under low oxygen partial pressure (pO2 < 10−5 atm), where the material is nanostructured in multilayered thin film coherent heterostructures with yttrium stabilized zirconia. Density Functional Theory calculation confirms such a form of metastability, also showing that high oxygen defect concentration favors the cubic phase. Moreover, high oxygen deficiency in the nanoionics leads to an unexpected ‘two-regime’ conductivity with high values (σ > 1 S cm−1 at 600 °C) at high pO2 and lower ionic conductivity (σ ∼ 0.1 S cm−1 at 600 °C) at low pO2. Ionic conductivity at low pO2 occurs with high activation energy (Ea > 1.5 eV), suggesting a drastic decrease in mobility for high concentration of defects.


Introduction
Defective metal oxides (MeOx) that can transport oxygen at high and intermediate temperatures are the corecomponents of key energy technologies such as fuel cells, electrolysers and sensors [1,2]. Besides such wellestablished applications, oxygen defective metal oxides are spreading in new fields, as in MEMS, ICT and Biomedical technologies [3][4][5]. Such an attainment relies on the high tunability of the defective chemistry of MeOx that can bring to the material wide range of properties either as the result of combination of functions or even by the arising of new properties that do not belong to constitutive components [6]. Among the different strategies of tuning, 'nanostructuring' is probably the most promising one. This is the case of nanometric ionic conductive materials (nanoionics) [7] for miniaturized energy devices [8], novel memory device concepts [9], ionotronics [10] and minutuirized actutators [11]. These are made by reducing the shape to the ionic component to a nanoscale and introducing interfaces between functional components [6].
For energy technologies, the search for high oxygen ionic conductivity has been the focus of intense investigation for the last decades, as it has been seen as resolute for reducing the operative temperature of solid oxide fuel cells and other energy devices [12]. Among the fast oxygen ionic conductors, bismuth sesquioxide in its cubic form, i.e. δ-Bi 2 O 3 , is the fastest one [13]. However, the material is also chemically unstable with highdensity polymorphism: up to five phases can be found only in the Bi 2 O 3 composition at high oxygen partial pressure (pO 2 ), e.g. in air and pure oxygen. Under low pO 2 , the oxide gets reduced to the highly volatile metallic form [13]. On the other side, polymorphism can be retained by doping, i.e. cubic phase stabilization [14], degradation under low pO 2 is difficult to prevent and it limits performances and applications [15].
In our previous work, we have shown that nanostructuring of bismuth oxide (both doped and undoped) in multilayered heterostructures with other stable materials can lead to cubic stabilization at low pO 2 and unexpected electrical properties [13,16]. Nanostructuring can stabilize the bismuth oxide component beyond its thermodynamical stability, e.g. in highly reducing conditions (H 2 ) and at high temperatures [13]. We also found that the pure bismuth sesquioxide, with no doping, does not undergo to polymorphism in air at different temperatures and we observed that such a form of metastability depends on interface and lattice strain at the heterostructures [16]. Particularly, in YSZ/δ-Bi 2 O 3 heterostructures with different numbers of interface and thicknesses, the YSZ component constrains the bismuth oxide in an overall large lattice that is able to host the oxygen defects in a larger extend than expected for the pure material [16]. Despite such results, the configuration of the defects in the heterostructure as well as their influence on the electrical properties at low pO 2 remains unexplored.
In the present paper, we analyse the thermochemical stability and electrical properties of YSZ/δ-Bi 2 O 3 heterostructures with thin layers of ca. 1, 2, 4 nm, giving a new insight on the structural factors that can influence ionic conductivity at the multilayer interface. We also characterize the crystal structure of the multilayered material by using x-ray diffraction and electron microscopy. To clarify some theoretical aspects of the complex defective chemistry occurring at the nanoionic, we escort the experimental results with some ab-initio calculations, significantly to define the effect of high defect concentration on the metastable cubic bismuth sesquioxide under low pO 2 and high temperatures.

Thin films deposition
The heterostructures were deposited by Pulsed Laser Deposition (PLD) technique onto NdGaO 3 single crystal (100) oriented. The alternative layers (YSZ/δ-Bi 2 O 3 ) in the heterostructures were made by using a multi-target carousel in the vacuum chamber with the base pressure of 10 −7 mbar. The ablation on the targets was made with A KrF excimer laser (Coherent Lambda Physik GmbH) with a wavelength of 248 nm and a pulse duration of 25 ns. The laser energy was kept at 2 J cm −2 at a repletion rate of 2 Hz. The substrate temperature during the deposition was at 600°C with oxygen partial pressure about 10 −2 mbar.

Thin films characterization
Structural analysis was carried out by x-ray Diffraction (XRD). θ-2θ scans and rocking curve analysis were performed using the Rigaku Smartlab diffractometer with x-ray beam monochromatized with a Cu (λ=1.5418 Å) rotating anode.
Electrochemical impedance spectroscopy (EIS, Solartron 1260) measurements were performed in 400°C-750°C temperature range in air, O 2 and H 2 -N 2 atmosphere with fixed ratio of 3 vol% hydrogen. Other mixtures were made by mixing N 2 and H 2 -N 2 mixtures. For the electrochemical characterization, the frequency varied from 50 Hz to 1 MHz with 0.2 V alternating voltage signal.
Microstructure and thickness of the samples were measured by scanning electron microscopy (Zeiss SUPRA FEGSEM). Preparation of lamellas for scanning transmission electron microscopy (STEM) analysis was carried out by FIB (FEI Helios 600 Dual beam FIB/SEM) and STEM images were acquired at 300 kV by using a probe corrected TEM (FEI Analytical Titan), equipped with an energy dispersive x-ray (EDX) detector for chemical analysis.

Atomic-scale calculations
Atomic-scale calculations have been proved very successful in elucidating material properties [17]. Here, we perform Density Functional Theory (DFT) calculations to explain the role of vacancies with respect to stability of polymorphic phases of Bi 2 O 3 and diffusion barriers. All calculations have been performed using the GPAW code [18,19] and the Atomistic Simulation Environment (ASE) [20]. All structure have been fully optimized until the forces where under 0.01 eV Å −1 , using PBEsol [21] as exchange-correlation functional, with a k-point density of at last 3 kpoints/Å. The Climbing Image Nudged Elastic Bands (CI-NEB) method [22,23] has been used to estimate the diffusion barriers for various concentrations and orderings of vacancies.

Results and discussion
Coherency and reduced thickness of the epitaxial fluorite structures in the multilayer are necessary conditions to obtain the chemical stabilization of highly defective metal oxides [24][25][26]. For bismuth sesquioxide, such conditions are achieved by depositing nanometric multilayer on single crystal substrates that present compatible lattice parameter with materials in the multilayer. This is the case for NGO (100) perovskite orthorhombic structure which is only 3.8% larger than YSZ (111), thus ensuring a coherent growth of the bottom layer in the epitaxial film [16]. For the cubic δ-Bi 2 O 3 over the cubic YSZ fluorite layers, the theoretical lattice mismatch at room temperature is relatively low (5.14 Å for YSZ versus 5.66 Å for δ-Bi 2 O 3 ), resulting in a coherent heterostructure with a slightly compression of the δ-Bi 2 O 3 layers [16]. In NGO (substrate)-YSZ-Bi 2 O 3 sequence, the deposited multilayer structure results highly coherent and, depending on the number and thickness of the layers, an ordered and epitaxial heterostructure is arranged along the (111) orientation. Figure 1 shows a schematic representation of the multilayer architecture and three examples of XRD results for NGO(100)/[YSZ/δ-Bi 2 O 3 /YSZ] N=20 heterostructures, at a constant number of interfaces N=20, with total thickness of 20, 40 and 80 nm. The diffraction patterns show the (111) reflection at 2θ between 29 and 30°with a typical sub-patterning around the main peaks for all the samples. Subpattern oscillations are a typical feature of the heterostructures multilayers [25]. Figure 1 also shows a slightly relaxation of the layers at from the thinner to thicker samples, with a nearly linear shift of the peaks to higher angles (see dashed line figure 1). Since the lattice parameter mismatch between the components is ca. 10%, i.e. of δ-Bi 2 O 3 is 5.66 Å (lattice bulk) the lattice parameter of YSZ is 5.14 Å (lattice bulk), thin samples tend to be more strained with the increasing the number of the interfaces, especially in thin film samples (e.g. in the 18 nm sample in figure 1). Conversely, when layers became thicker (for example in the 40 to 80 nm samples, figure 1), the heterostructure tends to adapt to the lattice parameter of YSZ. These two effects also indicate that that the δ-Bi 2 O 3 component is prone to strain more than YSZ in the multilayered arrangement [26]. The XRD patterns clearly also indicate a high coherency of the nanoscaled materials, confirming previous results on pure Bi 2 O 3 and Er-stabilized bismuth sesquioxide [13,16,27].
For the electrical properties, impedance spectroscopy characterization is carried out using in-plane electrodes configuration, especially to evaluate the effect of the layers´thickness and interfaces on the total conductivity [13]. Figure 2 show the Arrhenius and the Brower-like (conductivity as function of pO 2 ) plots of the total conductivity for the 40 and 80 nm thick samples in air and in low pO 2 conditions, i.e. in N 2 and in N 2 -H 2 mixtures. Measurements are a collection of long (2-5 h) isothermal measurements. Due to the high impedance, thin samples below 20 nm could not be measured consistently and are not included. Figure 2 illustrates a collection of results on the conductivity as function of the temperature and pO 2 . Data are compared with electrical conductivity of YSZ and Bi 2 O 3 bulk materials and to similar N=20 heterostructure reported in a previous publication [16]. In air, i.e. at pO 2 =0.21 atm, we register a significant enhancement of the conductivity with respect of both the constitutive materials (YSZ and δ-Bi 2 O 3 ) and the 40 nm sample [16]. Results also show very large conductivity at low temperatures, i.e. 10 −1 S cm −1 at 400°C. Such high values of conductivity are often measured in bismuth oxides based compounds, although they often appear rather unconsistent in the literature [27][28][29]. Interestingly, no change in conductivity is observed at T=700°C, as expected by the α− to δ−phase transition of Bi 2 O 3 (dashed line figure 2 top). However, a change in activation energy at ca. 600°C is seen in the heterostructure, which, in the case of the 80 nm sample, changes from 0.35 eV at high temperatures to 0.69 eV at low temperatures. Changes in activation energy are generally observed in stabilized Bi 2 O 3 by dopants [14] and not in YSZ. However, activation energies in the 80 nm heterostructure results lower than previously measured in bulk, thin film and in the 40 nm heterostructure (between 0.8-1.5 eV [16]). About the nature of the conductivity, both Bi 2 O 3 and YSZ in air, are expected to be pure ionic conductors. However, due to the presence of several interfaces (N=20) and the reduced size of the layers (ca. 2 nm each in the 40 nm sample and 4 nm for the 80 nm sample), formation of electronic defects and space charge regions is also possible [6]. Polarized species can especially gather at the YSZ/Bi 2 O 3 interfaces and lead to lateral conductivity in the thin samples. However, this is not observed in the 40 nm sample, where conductivity is lower and the activation energies higher, in the range of 0.77 to 1.4 eV [16]. Electronic defects in ionic compounds as ZrO 2 and Bi 2 O 3 , such as small polarons, are unlikely formed under high pO 2 , as they are induced either by a partial reduction of cations in the materials or by formation of other interstitial species in the lattice [6]. In both cases, the concentration of the charge carriers is strictly depended on the oxygen partial pressure at the environment.
To check the reactivity of the heterostructure toward the pO 2 we show in figure 2 the Arrhenius plot carried out in N 2 and H 2 -N 2 3 vol.%. For both the 80 nm and the 40 nm samples, the electrical properties are comparable in terms of conductivity and activation energies (only 80 nm sample shown in figure 2 top). The conductivity decays especially at low temperatures, as result of a large increased in the activation energy, i.e. from 0.7 eV in air to ca. 1.5 eV in both N 2 and H 2 -N 2 . For the 40 nm sample in air and in N 2 -H 2 resulted less affected by the change in pO 2 than the 80 nm samples. Such a result is surprising as YSZ is expected to introduce n-type polarons at high temperature and very low pO 2 only, with an increase of total conductivity as addition of n-type conductivity to the pure ionic conductivity [30]. Electrical properties of Bi 2 O 3 under low pO 2 are unknown, as the material generally degrades below 10 −4 atm [31]. The Arrhenius plot in figure 2 shows that the ionic transport is nearly pO 2 independent. To clarify such effects in the condition mechanisms, several cycles were carried out both changing the temperature and tuning the pO 2 concentration in the environment. Particularly, figure 2-bottom show the Brower-like plots for the 40 and 80 nm samples N=20, in Air, O 2 , N 2 and several N 2 -H 2 gas mixtures. The conductivity behaviour of the sample appears rather uncommon for pure ionic conductors such as YSZ and Bi 2 O 3 . A rapid change of conductivity is registered for all the sample from high pO 2 to low pO 2 at any temperatures at around 10 −3 atm, depending on the temperatures. The change is particularly fast in the 40 nm sample, where several cycles (3) were carried out by increasing and decreasing the pO 2 . During the cycling the samples showed reversibility of the changes. Moreover, the conductivity appeared rather constant at low pO 2 , suggesting the presence of pure ionic conductivity under reducing conditions. Such a kind of electrical properties are not seen in YSZ and they can be attributed either to the Bi 2 O 3 component or to the YSZ/Bi 2 O 3 interfaces.
Further structural characterization was carried out by XRD by varying the pO 2 and using STEM-HAADF to check the structural integrity of the sample after the electrochemical characterization (figure 3). Both XRD peak in situ characterization (figure 3 up) of the 40 nm sample indicated that the heterostructure preserves the crystallinity with a slight change in the lattice size. Similar outcomes have been also reported in our previous work on pure and stabilized Bi 2 O 3 in YSZ and CGO heterostructures [13]. The atomic arrangement of the We perform DFT calculations, to better explain, at the atomistic level, the role of vacancies and their concentration in the stability of Bi 2 O 3 and ionic conductivity. Bi 2 O 3 shows a variety of polymorphs, from the most stable α-phase (monoclinic), to the βand δ-phases (tetragonal and cubic, which have an energy of around 20 and 70 meV above the one of the α-phase [32]). As the energy difference between these polymorphic structures is rather low, a change in the external conditions during the synthesis, such as pO 2 , strain or nanoconfinement, could be enough to favour one higher energetic polymorph with respect to the ground state one. Figure 4, shows the energy difference between the δand α-phases as a function of the concentration of the oxygen vacancies. Although at a low concentration of vacancies, the α-phase is more favourable, increasing the concentration of vacancies, the metastable δ-phase becomes the thermodynamically most stable phase. Because of the nanoconfinement between the YSZ and Bi 2 O 3 layers, the Bi 2 O 3 phase stays trapped in its δ-phase also after the synthesis, at pO 2 which should, instead, favour the transition towards the α-phase.
To explain the change in the ionic conductivity as a function of pO 2 , we calculate the diffusion barriers of oxygen vacancies in Bi 2 O 3 . To reduce the interaction between the neighboring vacancies, we now consider a supercell of δ-Bi 2 O 3 built by repeating the unit cell of Bi 2 O 3 2×2×2 times. It has been previously shown that the (110) vacancy alignment is the most favorable in δ-Bi 2 O 3 [33]. We have calculated the oxygen vacancy diffusion barriers in different vacancies configurations for one and two vacancies, as shown in figure 5. While the diffusion barrier for one vacancy is of the order of 0.9 eV, the average value for diffusion barrier for two vacancies, which are close to each other is around 1.5 eV. Because of the preferred distribution, the energy barrier for diffusion increases together with the vacancy concentration. A possible explanation of the difference in the activation energy at high and low pO 2 can be related with the diffusion barriers in these two cases: the one vacancy configuration can well approximate the high pO 2 limit, and the two vacancies structure well correlates with the activation energy in the low pO 2 limit. This (bulk) model, however, does not consider the role of the interfaces, which can also have an effect in modifying the activation barriers.

Conclusions
Cubic sesquioxide is stabilized in YSZ heterostructures as thin film multilayers for a total thickness below 100 nm. The materials show remarkably high electric conductivity and low activation energy values in air that are superior to the material's components. At low pO 2 below 10 −5 atm, the heterostructure has rather stable conductivity, although lower than in air. Such properties are not expected by mixing two of properties and they likely rise by nanostructuring Bi 2 O 3 and exposing it to low pO 2 conditions. A possible explanation for the different activation energies at high and low pO2 has been provided using DFT results and it is connected with the energy barriers for vacancy diffusion being higher for high concentration of vacancies compared to a low vacancy concentration configuration.