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C: Energy Conversion and Storage; Energy and Charge Transport

Effect of Thermal Ageing on the Phase Stability of 1YbO- xScO-(99-x)ZrO (x = 7, 8 mol.%) 2

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Vandana Shukla, Kantesh Balani, Anandh Subramaniam, and Shobit Omar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05672 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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The Journal of Physical Chemistry

Effect of Thermal Ageing on the Phase Stability of 1Yb2O3- xSc2O3-(99-x)ZrO2 (x = 7, 8 mol.%) Vandana Shukla, Kantesh Balani, Anandh Subramaniam and Shobit Omar* Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, India

*

Corresponding Author. Tel: (+91) 51 26797427, Fax: (+91) 51 22597505

E-mail address: [email protected]

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Abstract The present work studies the effect of thermal ageing and 1 mol.% Yb2O3 co-doping on the metastable tetragonal phases present in 8-9 mol.% Sc2O3-ZrO2. Dense ceramic discs of xYb2O3–(y-x)Sc2O3–(100-x-y)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%) are prepared using conventional solid-state route. XRD, TEM and Raman spectroscopy analysis confirmed the metastable t′-phase in 8Sc2O3-92ZrO2 and t″-phase in 9Sc2O3-91ZrO2. Although t′-phase is retained on Yb2O3 co-doping in 8Sc2O3-92ZrO2, the tetragonality reduces. However, co-doping does not affect the t″-phase present in 9Sc2O3-91ZrO2. After ageing at 650ºC for 2000 h in air, a decrease in unit-cell parameters ratio and weak XRD peak-splitting are observed in 8Sc2O392ZrO2 and 1Yb2O3-7Sc2O3-92ZrO2 suggesting a reduction in unit-cell tetragonality, though t′-phase still exists in both the compositions. In compositions possessing t″-phase, a slight lattice contraction is detected on thermal ageing. The tetragonality induced by oxygen-ion shuffling inside the unit-cell increases after ageing in all the compositions. This shuffling is lower in the aged sample of 1Yb2O3-7Sc2O3-92ZrO2 compared to 8Sc2O3-92ZrO2. However, it remains similar in both 1Yb2O3-8Sc2O3-9ZrO2 and 9Sc2O3-91ZrO2, suggesting a negligible co-doping effect on t′′-phase. These results demonstrate the effectiveness of Yb2O3 co-doping in lowering the tetragonality of metastable t′-phase, which may positively affect the conductivity of ScSZ.


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1. Introduction Development of solid-oxide fuel cells (SOFCs) operating in the intermediate temperature (IT) range of 500-700°C is often limited by the lack of suitable electrolyte materials that exhibit not only high oxygen-ion conductivity but also good long-term phase stability in the working conditions.1-2 The 8 mol.% Y2O3-ZrO2 material has been conventionally used as an electrolyte in SOFCs for operation at high-temperatures (~1000°C). This material suffers from insufficient oxygen-ion conductivity at lower temperatures, which has motivated researchers to look for alternatives.3-4 Besides discovering novel oxygen-ion conducting materials, the research thrust in the last couple of decades has been directed to comprehend the underlying mechanisms/factors that govern the ionic conduction process in conventional materials such as CeO2, ZrO2, Bi2O3, etc.5-7 These investigations are expected to aid in the formulation of strategies for the enhancement of ionic conductivity in these materials.3-4, 8-9 Among the wellknown materials, Sc2O3 stabilized ZrO2 (ScSZ) materials have been extensively investigated because of their excellent oxygen-ion conductivity (>15 mS.cm-1 at 600°C) and high ionic transference number (close to unity).6, 10-12 During thermal treatment, this system undergoes several phase transformations with sluggish kinetics. This results in a complex assemblage of several stable and metastable phases with poorly defined phase boundaries.13-15 It is important to comprehend these structural transformations, as this aspect plays a key role in determining the ionic conductivity in the Sc2O3-ZrO2 system.16 Although the focus of numerous studies 11, 17-20

have been to examine the co-doping effect on the stable phases, the metastable tetragonal

phases present in ScSZ have hardly received any consideration. The present work examines the influence of Yb2O3 co-doping on the metastable phases of t′ and t′′ existing in ScSZ. Further, the stability of these phases present in co-doped samples on thermal ageing has been investigated.



In ScSZ system, achieving phase stability is challenging below 1200 °C. This is because of the slow rates of diffusional transformation and cation diffusionless transformation which leads to various stable/metastable phases such as monoclinic (m), tetragonal (t), cubic (c) and rhombohedral (β, γ, δ) phases.13, 15-16 Among them, cubic fluorite is the most desirable phase with respect to electrical conductivity. According to Badwal et al. 16, a stable cubic phase exists above 9 mol.% Sc2O3 in ScSZ system with the maximum conductivity is found to be in 9.3Sc2O3-90.7ZrO2. At 10-11 mol.% Sc2O3, a considerable amount of the β-phase (orderedrhombohedral phase) exists along with the cubic phase at room temperature. It has been shown in several studies that in this compositional range, the high-temperature cubic phase partially transforms into poor conducting β-phase on cooling below 600°C.6, 16, 21 In ScSZ, co-doping with different metal oxides such as Yb2O3, CeO2, Y2O3, Bi2O3, Nb2O5, etc. has been extensively studied to stabilize the cubic phase over the β-phase and to improve the ionic conductivity.11, 17-20 A slight amount of the second dopant is sufficient to suppress the longrange oxygen vacancy ordering resulting in a linear Arrhenius plot of conductivity versus temperature, without a step, and in a higher conductivity in IT range.12, 14-16, 18 However, the conductivity in ScSZ compositions with 10-11 mol.% Sc2O3 is expected to be severely affected by the defect interactions that lead to the formation of local defect complexes. These complexes trap the oxygen vacancies and effectively reduce the number of mobile oxygen-ions, thus resulting in a decrease of the ionic conductivity.22 As a result, the concentration regime of 810 mol.% Sc2O3 in ZrO2 appears to be more practically relevant where the conductivity of highest magnitude and temporal stability have been observed.11, 23-24 In ScSZ compositions with ≤ 9 mol.% Sc2O3, tetragonal phase exists which, according to Yashima et al.14, is of three types, i.e. t, t′ and t″. First one is the stable tetragonal (t) phase with the thermodynamic stabilization (i.e., change in Gibbs free energy, ΔGt−m = Gt − Gm < 0 at T ≤ To where To is a t↔m phase transformation temperature). The processes of thermal ageing or 4 ACS Paragon Plus Environment

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mechanical grinding results in the partial transformation of t-phase to the m-phase, which has a lower conductivity. The other two forms of tetragonal are metastable t′ and t″-phases with kinetic stabilization (i.e., the activation energy to form metastable phase, ΔG* > 0). The increase in energy barrier (ΔG*) leads to the rise in the non-transformable nature of these nonequilibrium phases. Thus, it takes longer time to transform into stable states. Badwal et al.10 and Araki et al.25 studied the effect of thermal ageing (at 1000°C for 1000 h) on the microstructure of 7 to 9 mol.% ScSZ compositions. Badwal et al.10 reported the presence of t′-phase in this doping region, whereas Araki et al.25 found t′-phase in 7Sc2O393ZrO2 (7ScSZ) and t″-phase in 9Sc2O3-91ZrO2 (9ScSZ). After the long-term ageing, 7ScSZ was reported to exhibit a significant decrease in the ionic conductivity, which is attributed to the decomposition of t′-phase into stable t-phase precipitates in a cubic matrix. On the other hand, thermal ageing was shown to convert t″-phase present in the 9ScSZ sample into t′-phase. Further, Yashima et al.26 reported that the reversible phase transformation occurs between the t"- and t'-phase at temperatures below 1200°C. The effect of high-temperature (>700ºC) ageing on the phase stability in various zirconia-based compositions has been well studied in the literature16, 23, 25, 27-28. However, the influence of thermal ageing at lower temperatures (500700 °C) on the ScSZ compositions containing the tetragonal phase, remains unexplored. As the compositions with 8-10 mol.% Sc2O3 are prone to contain the tetragonal phase, from the electrolyte perspective, it is essential to stabilize the cubic phase and improve the ionic conductivity. In the present work, 1 mol.% Yb2O3 has been substituted for Sc2O3 in 8Sc2O392ZrO2 (8ScSZ) and 9ScSZ to study its effect on the metastable tetragonal phases of t′- and t′′ present in both these compositions. Yb2O3 as a co-doping oxide has been selected as the ionic radius of Yb3+ ( rYb3,VIII = 0.985 Å) is close to that of Sc3+ ( rSc3,VIII = 0.87 Å) and Zr4+ ( rZr4 ,VIII = 0.84 Å), and it has also been successful in suppressing the β-phase formation in 12Sc2O35 ACS Paragon Plus Environment


88ZrO2.17, 29 The study includes the effect of long-term thermal ageing (650°C for 2000 h) on the stability of phases observed. The role of Yb2O3 co-doping is established by comparing the phase formation in co-doped compositions with that in 8 and 9 mol.% ScSZ samples. A detailed structural characterization using various techniques has been performed for identifying the phases present in the tested samples.

2. Experimental Polycrystalline bulk samples of xYb2O3–(y-x)Sc2O3–(100-x-y)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%) were synthesized via conventional solid-state reaction and pressureless sintering methods. The nomenclature used for the tested compositions is given in Table 1. The starting powders of Yb2O3 (99.9 % purity, Alfa Aesar, India), Sc2O3 (99.9 % purity, Alfa Aesar, India) and ZrO2 (99 % purity, Samics Research Materials Pvt. Ltd., India) were heated to 1000 °C for 1 h to compensate for the loss on ignition. The stoichiometric amounts of powders were weighed and ball-milled for 24 h in ethanol medium using 1-2 drops of ammonium polyacrylate as a dispersant. The ball-milled slurries were then dried overnight at 120°C. The obtained agglomerated powders were ground using agate mortar and pestle followed by passing the powder through a sieve with the aperture opening of 90 µm. The fine-sized powders were then calcined at 1300 °C for 10 h with the ramp rate of 2°C/min for both the heating and cooling cycles. The calcined powders were again ball milled for 24 h in ethanol followed by drying, grinding and sieving. For pressing disk-shaped pellets, approximately 2 wt.% polyvinyl alcohol (binder) was added to the powder. The calcined powders were then uniaxially pressed, followed by cold isostatic pressing at 300 MPa for 5 min. The obtained green pellets were finally sintered at 1550°C for 10 h with an intermediate binder burn-out step at 500°C for 2 h. The % relative theoretical density of all the pellets obtained through the Archimedes method was determined to be above 93% (reported in Table 1). The thermal ageing study was conducted by placing the 6 ACS Paragon Plus Environment

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sintered ceramic samples inside the furnace at 650°C for 2000 h in air. The thermocouple was kept in close vicinity to the samples to minimize the temperature measurement error. After 2000 h, the samples were slowly cooled to room temperature. A detailed structural examination was carried out on these samples using multiple characterization techniques. From now on, the term "ageing" has also been used to denote thermal ageing. Further, the samples subjected to thermal ageing are described as aged samples, while the ones without the thermal ageing are referred to as unaged samples.


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Table 1: Nomenclature and structural characteristics of the unaged and aged samples of xYb2O3–(y-x)Sc2O3–(100-x-y)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%) compositions.

Composition

% Relative Average grain Nomenclature Theoretical size (µm) Density

Lattice Parameter (Å) Unaged Sample

Lattice Parameter (Å) Aged Sample

Unit-cell Parameter Ratio (ct/(√2at)) Unaged

a

c

a

Sample

Aged Sample

c

8Sc2O3–92ZrO2

8ScSZ

94.3

4.4 ± 1.4

3.5975(1) 5.1123(2)

3.6015(1)

5.1042 (2)

1.005

1.002

1Yb2O3–7Sc2O3–92ZrO2

1Yb7ScSZ

95.9

6.8 ± 1.3

3.6021(1) 5.1141(1)

3.6077(1)

5.0981(3)

1.004

0.999

9Sc2O3–91ZrO2

9ScSZ

93.1

4.9 ± 1.4

5.0946(1)

5.0878(1)

1

1Yb2O3–8Sc2O3–91ZrO2

1Yb8ScSZ

93.6

6.8 ± 1.5

5.0978(1)

5.0869(1)

1


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X-ray diffraction (XRD) patterns were collected on the powders obtained by crushing the sintered pellets using a laboratory diffractometer (PANalytical, X’pert Powder XRD, Netherlands) having a PIXcel1D line detector and CuKα incident radiation. The scan time per step was fixed to 0.5 s with the step size of 0.026º in the 2θ range of 25º to 80º. The lattice parameters of the observed phases were estimated by the whole-pattern fitting method using MDI Jade software. The goodness of fit value (whole pattern fitting, Rwp) obtained after fitting was found to be less than 7. The phase formation was further investigated by Raman spectroscopy on sintered unaged and aged pellets in the wavenumber range from 100 to 1100 cm-1 using a Raman system (Princeton Instruments, Acton spectrapro® SP-2500, Japan). For acquiring the spectra, polarized He-Ne laser (λ= 633 nm) at 40 mW was used with the total accumulative time of 3 minutes. Raman lines corresponding to naphthalene crystal were used for the Raman shift calibration. The surface morphology of the as-sintered samples was studied using a scanning electron microscope (SEM, JEOL JSM 6400, Japan) at an accelerating voltage of 20 kV. The microstructural features present in the unaged and aged samples were further examined using transmission electron microscopy (TEM, FEI tecnai G2 20U twin transmission electron microscope, USA). Ultrasonic disk cutter was used to cut samples into circular disks of diameter 3 mm which were then polished up to a thickness of ~100 μm using SiC abrasive papers. The polished thin-disks were then dimpled to a thickness of ~25 μm in dimple grinder followed by ion-milling in precision ion polishing system (Gatan, Model 691).



3. Results and discussion 3.1 Surface Morphology Analysis via SEM The representative backscattered electron (BSE) micrograph of the as-sintered 1Yb7ScSZ sample is shown in Figure 1. The microstructure appears to be dense with only a few isolated small pores (of size ~50 nm) present primarily at the grain boundary triple junctions and along the grain boundaries. The density estimated through micrographs (~94%) correlates well with the % relative theoretical density obtained (> 93%) in these samples. The average grain size calculated using the mean linear intercept method30 is found to be in the range of 4.4 to 6.8 µm for all the compositions as given in Table 1. SEM micrographs of all the tested samples of xYb2O3–(y-x)Sc2O3–(100-x-y)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%) are similar and the average grain size lie within the error range. It implies that the co-doping of Yb2O3 in ScSZ system does not significantly affect the size of grains. Further, no evidence of any impurity (glassy phase) is found in the BSE micrograph of all the tested samples.

Figure 1: Representative backscattered electron micrograph of the as-sintered 1Yb7ScSZ sample. 10 ACS Paragon Plus Environment

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In order to systematically elicit the effect of Sc and Yb doping, first, the phase identification of unaged samples is presented in section 3.2, and consequently, the influence of ageing on phase evolution is presented in Section 3.3. 3.2 Phase identification in unaged bulk samples The presence of complicated phase chemistry in ScSZ does not allow to distinguish between various metastable and stable phases present in ScSZ compositions using only a single characterization technique. For example, conventional XRD (and most of the TEM analysis) does not allow to separate the c-phase from the metastable t″-phase for which the ratio of the unit-cell parameters (cc/ac) is also unity. Here the subscript c denotes the cubic unit cell. This oxygen-induced transition cannot be detected by XRD owing to the small scattering factor of the oxygen atom.31 Similarly, the difference between the metastable t′- and t″-phases cannot be recognized from the Raman spectra. Therefore, in the present work, a complementary structural analysis through XRD, TEM and Raman Spectroscopy has been performed for the phase identification in the tested compositions. 3.2.1 XRD Analysis The XRD patterns collected on the powder obtained by crushing sintered dense samples of xYb2O3–(y-x)Sc2O3–(100-x-y)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%) at room temperature is shown in Figure 2. All the diffraction peaks detected in XRD profiles for all the compositions were indexed using the standard powder diffraction cards (Joint Committee on Powder Diffraction Standards) with the reference code of 01-01-089-5481 (for t-phase) and 01-0895485 (for c-phase).32



Figure 2: XRD profiles collected on the powders obtained by crushing sintered pellets of the unaged and aged samples of (a) 8ScSZ (b) 1Yb7ScSZ (c) 9ScSZ, and (d) 1Yb8ScSZ. The XRD profile of 8ScSZ reveals distinct splitting of peaks at 2θ around 35° (002, 110), 50° (112, 200), 60° (103, 211) and 75° (004, 220) corresponding to various hkl reflections (shown in brackets) of tetragonal phase (with the space group of P42/nmc). The presence of other phases could not be detected from the XRD pattern. Similar XRD pattern is also found in 1Yb7ScSZ indicating the existence of the tetragonal phase. However, the absence of peak splitting in the XRD pattern of 9ScSZ and 1Yb8ScSZ (shown in Figure 2(c, d)) suggest that at 9 mol.% total doping concentration, the formation of the high-symmetry cubic fluorite structure (space group Fm3m ) is favoured over the lower-symmetry t-phase which is observed in 8ScSZ


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and 1Yb7ScSZ. Further, no additional peaks associated with any impurities are detected in the XRD profile of all the tested compositions.

Figure 3: The high-angle portion (2θ≈60°) of the XRD profile collected on the sintered samples of (a) 8ScSZ (b) 1Yb7ScSZ (c) 9ScSZ, and (d) 1Yb8ScSZ. Here, open symbols are the data points, and the solid line represents the fit. The high-angle portion (2θ ≈ 60°) of the XRD profiles is shown in Figure 3. The peak splitting at around 60° is observed in 8ScSZ and 1Yb7ScSZ compositions, which correspond to (103) and (211) diffraction planes, respectively, of the tetragonal phase. However, this peak splitting is absent in 9ScSZ and 1Yb8ScSZ samples. Instead, a single sharp peak is noticed in 9ScSZ and 1Yb8ScSZ samples, which corresponds to (311) diffraction plane of the c-phase.



The lattice parameters estimated for all the compositions are given in Table 1. The lattice parameters found in our work for 8ScSZ are similar to that reported by Fujimori et al.32 In 1Yb7ScSZ, a slight increase is observed in both the lattice parameters of the tetragonal phase. Similarly, the lattice parameter of the cubic phase detected in 1Yb8ScSZ is also found to be higher than in 9ScSZ composition. These results imply a successful dissolution of larger-sized Yb3+ in ScSZ system. According to Viazzi et al.33, for the metastable t′-phase, the unit-cell parameters ratio (ct/(√2at)) tends to lie in the range of 1.00-1.01. Here the subscript t denotes the tetragonal unit cell. On the other hand, this value is greater than 1.01 for the stable tetragonal t-phase. For the metastable t″-phase, the ratio ct/(√2at) is unity, which is also the case for the cubic phase. Based on these values, it is evident from Table 1 that the 8ScSZ composition contains metastable t′– phase rather than the stable t–phase as the ratio is estimated to be 1.005. Despite the increase in cell volume, the ct/(√2at) value in 1Yb7ScSZ is found to be similar to that of 8ScSZ, i.e. 1.004 suggesting the existence of the metastable t′-phase in 1Yb7ScSZ. The ratio of the unit-cell parameters is unity for both the 9ScSZ and 1Yb8ScSZ, implying the presence of either cubic or metastable t"-phase in these compositions. It is worth mentioning here that XRD is insensitive to distinguish between these phases, which can be attributed to the small scattering factor of the oxygen compared to zirconium and other rare earth cations.31 From the XRD analysis, it can be inferred that t′-phase exists in both 8ScSZ and 1Yb7ScSZ compositions while 9ScSZ and 1Yb8ScSZ samples contain either c-phase or t"-phase. Further, these results indicate that a phase boundary exists in 8-9 mol.% doping region in ScSZ. 3.2.2 TEM Analysis Figure 4 shows the representative bright-field images (BFI) and corresponding selected area diffraction patterns (SADPs) of all the tested compositions. The selected region of the 14 ACS Paragon Plus Environment

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microstructure, where the SAD aperture lies, is shown by a dotted circle in Figure 4 (a-d). The whole grain could not be captured in any of the micrographs because of the large size of grains (≈ 4 to 6 µm). The micrographs obtained from various samples reveal that the grain boundaries are sharp, without the presence of impurities (glassy phase).

Figure 4: TEM micrograph and the corresponding SADP of unaged samples of (a) 8ScSZ, (b) 1Yb7ScSZ, (c) 9ScSZ, (d) 1Yb8ScSZ (Here, the SAD aperture is shown as a dotted circle). The BFI of 8ScSZ (shown in Figure 4(a)) consists of various bands having an average thickness of ~ 0.2 µm formed inside the grain. Besides, a twin-like structure, which is considered to be the characteristic feature of dopant-rich t′-phase, is also detected.16, 34 The SADP of 8ScSZ collected from the twin-like microstructural region was indexed to the tetragonal phase with the [101]t zone axis.32 Figure 4(b) shows the microstructure of 1Yb7ScSZ, which also contains the twin-like feature indicating the existence of t′-phase. However, the average thickness of the bands in 1Yb7ScSZ is comparatively less (~0.09 µm). 15 ACS Paragon Plus Environment


The SADP obtained from the grain of 1Yb7ScSZ sample was indexed to [210]t zone axis of the tetragonal phase. The diffraction pattern also reveals some weak secondary reflections along with the primary reflections having the same zone axis of [210]t. The reason for the appearance of weak secondary reflections in the 1Yb7ScSZ sample could be because of the two different variants of bands lying under the SAD aperture. The twin relationship of variants is absent in both the 8ScSZ and 1Yb7ScSZ samples as spot splitting cannot be observed in their respective diffraction pattern.35 Thus, the twin-like feature present in the grains reflects the presence of t'-phase in 8ScSZ and 1Yb7ScSZ. Figure 4(c) shows the BFI of the 9ScSZ sample in which a triple grain boundary junction can be seen. No twin-like structure is found indicating the absence of t′-phase. Instead, a mottled contrast is seen inside the grain. The obtained SAD pattern was indexed to the [100]c zone axis of c-phase reported by Fujimori et al32. It is to be noted that the c-phase cannot be differentiated from the metastable t″-phase (which also possesses axial c/a ratio of 1) through the BFI or SAD pattern.36 Like 9ScSZ sample, the twin-like feature is found absent in the microstructure of 1Yb8ScSZ sample. The SADP of 1Yb8ScSZ sample was indexed according to the zone axis [121]c of c-phase (as shown in Figure 4(d)). Thus, from the TEM analysis, the presence of metastable tetragonal t′-phase is evident in 8ScSZ and 1Yb7ScSZ. No twin-like structure and the obtained SADP confirm the presence of either cubic or metastable t"-phase in 9ScSZ and 1Yb8ScSZ samples. 3.2.3 Raman Spectroscopy Analysis Raman spectroscopy was performed on the sintered bulk samples to observe any local structural changes induced by the displacement of oxygen-ions inside the unit cell. Figure 5(ad) shows the Raman spectra collected on the sintered samples of xYb2O3–(y-x)Sc2O3–(100-xy)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%) at room temperature. Four distinct broad bands 16 ACS Paragon Plus Environment

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are observed in the Raman spectrum of all the compositions. These bands were deconvoluted into six peaks using the Lorentzian peak profile function. In all the compositions, these six peaks are detected around 155, 260, 370, 477, 595 and 631 cm-1 in the measured Raman shift range of 100-1100 cm-1. The Raman peaks are related to the six Raman active modes 15 (A1g+2B1g+3Eg) of tetragonal t-phase (Space group D4h ). Based on the description provided

by Bouvier et al.37, the Raman peaks located at 155, 260, and 477 cm-1 are assigned to E1g(3), E1g(2) and E1g(1) modes, respectively. The peaks centred at 631 and 370 cm-1 are ascribed to the B1g modes while the peak present at 595 cm-1 corresponds to A1g mode of t-phase. Further, it can be observed that the peaks located at 150 and 260 cm-1 are quite broad with relatively weak intensity than the peak present at 477 cm-1. This can be interpreted as the lower vibration motion of Zr-OI/II with E1g(1) and E1g(2) modes compared to E1g(3) mode revealing the presence of metastable tetragonal phase (t′ or t″) instead of stable t-phase in all the compositions. These two metastable tetragonal phases (t' and t") cannot be differentiated by the Raman spectra. It is important to note that metastable t″-phase is a distorted form of c-phase in which the cell parameters ratio ct/(√2at) equals 1, and the oxygen-ions are displaced alternatively along the z-axis making the symmetry tetragonal. On the other hand, in the t′phase, besides the presence of oxygen displacement, the ratio ct/(√2at) is slightly greater than unity.38



Figure 5: Raman spectra of the unaged and aged samples of (a) 8ScSZ, (b) 1Yb7ScSZ, (c) 9ScSZ, and (d) 1Yb8ScSZ collected at room temperature. As described in literature32, 39, the intensity height ratio of the Raman peaks located at 477 cm-1 and 631 cm-1 (and are labelled as I4 and I6 respectively) can provide the measure of oxygen-ion displacement along the z-axis inside the unit cell of zirconia-based solid-solutions. The Raman spectrum in the wave number range of 320-800 cm-1 is fitted using the Lorentzian peak profile function (shown in Figure 6) to evaluate the influence of ytterbia co-doping on the local structural changes in 8ScSZ and 9ScSZ compositions. The best fit to the Raman spectra of all the samples was achieved using four peaks with the R2-value of >0.98. The height of the profile fit was considered to be the height of the peak. The height was measured using the common baseline for the peaks at 477 and 631 cm-1.39 The numerical values of the Raman peak 18 ACS Paragon Plus Environment

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height ratio (I4/I6) for all the tested compositions are provided in Table 2. The reported ratio for each composition is the average of values estimated from at least 5 Raman spectra.

Figure 6: The de-convoluted Raman spectra (range 320-800 cm-1) of the unaged and aged samples of (a) 8ScSZ, (b) 1Yb7ScSZ, (c) 9ScSZ, and (d) 1Yb8ScSZ. Here, open symbols are the data points, while the solid line represents the profile fit. The substitution of 1 mol.% ytterbia in 8ScSZ reduces the intensity ratio from ~0.36 to 0.33, suggesting a slight decrease in the displacement of oxygen-ion along z-axis inside the unit-cell. However, not much change in the magnitude of this ratio has been observed on Yb 2O3 codoping in 9ScSZ composition. Although 8ScSZ and 9ScSZ possess different forms of 19 ACS Paragon Plus Environment


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metastable tetragonal phases, they show a similar intensity ratio of Raman peaks. It can be inferred from an analysis of the data that, Yb2O3 co-doping lowers the tetragonal-symmetry in 8ScSZ which possesses a metastable t'-phase. However, the oxygen sublattice in metastable t″-phase existing in 9ScSZ remains unaffected on substituting 1 mol.% Yb2O3. Table 2: The estimated value of intensity height ratio (I4/I6) of the Raman peaks in the spectrum collected over the unaged and aged samples of xYb2O3–(y-x)Sc2O3–(100-x-y)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%). The summary of phases identified from the detailed analysis of XRD, TEM and Raman spectroscopy is also given. Sample ID

Peak height ratio, (I4/I6)

Phase present after ageing

Unaged Samples

Aged Samples

8ScSZ

0.36±0.01

0.43±0.02

t′

1Yb7ScSZ

0.33±0.02

0.39±0.02

t′

9ScSZ

0.36±0.02

0.45±0.02

t″

1Yb8ScSZ

0.37±0.01

0.45±0.01

t″

3.2.4. Effect of Sc and Yb Doping on Unaged stabilized Zirconia Samples In light of evidence obtained through XRD, TEM and Raman spectroscopy analysis, it can be concluded that a metastable t′-phase (rather than the stable tetragonal t-phase) exist in both the 8ScSZ and 1Yb7ScSZ samples. Further, the extent of tetragonality in the t′-phase present in 8ScSZ is lowered on co-doping with 1 mol.% Yb2O3. Three observations can be made to support this inference – (1) the band thickness in the twin-like structure is lower in 1Yb7ScSZ compared to that in 8ScSZ, (2) slightly smaller unit-cell parameters ratio ct/(√2at) found in 1Yb7ScSZ, and (3) the displacement of oxygen-ions inside the unit cell in the direction of zaxis is lower in the co-doped composition. Although XRD profile and TEM micrograph with SAD pattern suggest a similar length of unit-cell parameters in 9ScSZ and 1Yb8ScSZ samples, the presence of characteristics Raman peaks of tetragonal phase confirmed the existence of t"20 ACS Paragon Plus Environment

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phase instead of c-phase in these compositions. The shuffling of oxygen-ions along the z-axis remain unaffected in t"-phase in 9ScSZ on co-doping with 1 mol.% Yb2O3. 3.3 Influence of long-term thermal ageing on phase formation All the sintered polycrystalline samples were kept inside the tube furnace and were subjected to 650°C for 2000 h. The influence of thermal ageing on the phase formation in the Yb2O3 codoped ScSZ was investigated by performing a detailed structural analysis via XRD, TEM and Raman spectroscopy. For comparison, similar studies were conducted on 8ScSZ and 9ScSZ samples. 3.3.1 XRD Analysis A comparison of XRD profiles collected on the powders of crushed unaged and aged samples of xYb2O3–(y-x)Sc2O3–(100-x-y)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%) at room temperature is provided in Figure 2. Like unaged samples, the XRD patterns of the aged 8ScSZ and 1Yb7ScSZ samples contain peak splitting at 2θ ~ 60° and 75° suggesting the presence of tetragonal phase. However, the peak splitting is not as prominent in aged samples when compared to unaged samples. A very weak hump of (103) reflection is observed on the lefthand side of (211) diffraction peak of t-phase in the aged samples of 8ScSZ and 1Yb7ScSZ at 2θ~60° (shown in Figure 7(a, b)). Also, peaks appear to broaden on thermal ageing in the aged samples of both 8ScSZ and 1Yb7ScSZ. The XRD pattern of aged specimens of 9ScSZ and 1Yb8ScSZ reveals no peak splitting indicating that both the compositions retained cubic symmetry after the 2000 h of ageing at 650°C. A single peak is detected at 2θ~60° in the aged samples of 9ScSZ and 1Yb8ScSZ (shown in Figure 7(c, d)) which corresponds to (311) reflection of the c-phase. Finally, the XRD patterns of all the aged samples do not contain any secondary peaks suggesting the absence of any impurity phase. 21 ACS Paragon Plus Environment


Figure 7: The high-angle portion (2θ≈60°) of the XRD profile collected on the aged samples of (a) 8ScSZ (b) 1Yb7ScSZ (c) 9ScSZ, and (d) 1Yb8ScSZ. Here, open symbols are the data points, while the solid line represents the profile fit. The estimated unit-cell parameters and their ratio ct/(√2at) of the aged samples are given in Table 1. As described earlier, the ratio ct/(√2at) is used to distinguish between the stable tphase and the metastable t′-phase present in the aged 8ScSZ and 1Yb7ScSZ samples. In 8ScSZ sample, the lattice parameter at increases while ct decreases after ageing such that the ratio ct/(√2at) declines from 1.005 to 1.002, indicating the reduction in the unit cell tetragonality. Nevertheless, the metastable t′-phase still exists even after the 2000 h of ageing as the obtained ratio lies in the range of 1-1.01. A similar trend is observed in 1Yb7ScSZ composition, where the unit-cell parameters ratio ct/(√2at) decreases significantly from 1.004 to 0.999. As the 22 ACS Paragon Plus Environment

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magnitude of the ratio is close to 1, it cannot be confirmed (from the value) whether t′- or t"phase exists in the aged 1Yb7ScSZ sample. However, the presence of metastable t′-phase is evident from the weak peak splitting observed in the XRD profile of the aged 1Yb7ScSZ sample, even though the unit cell tetragonality reduces after ageing. The lattice parameters estimated in the aged samples of 9ScSZ and 1Yb8ScSZ suggest a minor contraction of the unit cell on ageing. Similar lattice contraction on thermal ageing in air has also been reported in the co-doped 12ScSZ compositions containing high-symmetry cubic phase.12 3.3.2 TEM Analysis The bright-field images and selected area diffraction patterns of all the aged samples of xYb2O3–(y-x)Sc2O3–(100-x-y)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%) are shown in Figure 8. The SAD aperture is shown as the dotted circle in BFI while the superlattice reflections (if present) are encircled in SADPs. Like in unaged samples, no evidence of impurity segregation such as glassy phase near the grain boundary is found in all the aged samples. The twin-like feature (with the band thickness of ~0.2 µm) exists inside the grains of the aged 8ScSZ sample confirming the retention of metastable t′-phase. The obtained SADP is indexed corresponding to the [210]t zone-axis of tetragonal phase (shown in Figure 8(a)). The SADP of the aged sample also consists of superlattice reflections and diffuse intensity around each reflection. These features are absent in the SADP obtained from the unaged sample (shown in Figure 4(a)). Since only a single twin variant exists under the SAD aperture, the reason for the emergence of weak secondary reflections in [210] t zone axis on ageing may be related to the presence of periodicity in anisotropic lattice distortions. These strain fields appear owing to the difference in the size of dopant and host cations. Kondoh et al.40 observed the superlattice reflections in the SADP of 8YSZ before ageing, which are attributed to the periodic 23 ACS Paragon Plus Environment


and anisotropic nature of lattice distortions. However, the size mismatch is minor in the ScSZ system. Thus, in the present study, the absence of secondary reflections in unaged samples can be possibly explained by the existence of random weak distortions. These distortions become anisotropic and gain some periodicity on ageing to relax the structure which leads to superlattice reflections such as {001}, {122}, etc. in SADP. The weak diffuse scattering observed surrounding the principal spots in the aged samples indicates the initiation of the partial or short-range ordering of oxygen-ion vacancies to further relax the anisotropy of strain field.40-41

Figure 8: TEM micrograph and the corresponding SADP of the aged samples of (a) 8ScSZ, (b) 1Yb7ScSZ, (c) 9ScSZ, and (d) 1Yb8ScSZ. (Here, the SAD aperture is shown as a dotted circle). After 2000 h of ageing at 650°C, the twin-like bands are not found in 1Yb7ScSZ (shown in Figure 8(b)). These bands were distinctly observed in the unaged sample. The possible reason for their disappearance is the reduction in tetragonality on ageing as found from the value of 24 ACS Paragon Plus Environment

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ct/(√2at) ratio in the aged sample (~ 0.999) which is slightly lower than 1. The structural relaxation on ageing in 1Yb7ScSZ may lead to the disappearance of bands. The principal reflections observed in the SAD pattern are indexed according to the [102]t zone axis of the tetragonal phase. The presence of weak superlattice reflections can again be ascribed to the anisotropic periodic lattice distortion, while the occurrence of weak diffuse scattering on ageing corresponds to the partial or short-range ordering of oxygen vacancies. Figure 8(c) shows the BFI of a three-grain junction in the aged 9ScSZ sample. In the inset, a dark and bright mottled contrast is visible inside the grain in the aged 9ScSZ sample. The parallel straight lines observed in the micrograph are the artifacts created during the ion-milling step of sample preparation for the TEM analysis and are not the feature related to the aged 9ScSZ sample. This is evident in Figure 8(c) where these parallel lines continue to pass from one grain to another without interruption. The SADP of the aged 9ScSZ sample is indexed according to the zone axis [111]c of c-phase. A mottled appearance is also seen in the aged 1Yb8ScSZ sample (shown in Figure 8(d)). The principal reflections observed in SADP obtained from mottled zone are indexed according to the zone axis [111]c of c-phase. The SADP of the aged samples of both 9ScSZ and 1Yb8ScSZ also contain weak superlattice reflections such as {011} , {112} , {213} , {110} {211} , etc., which are forbidden in [111]c zone axis of cubic fluorite structure. The presence of superlattice reflections indicates the periodicity in anisotropic lattice distortion on ageing. Further, the diffuse scattering around the principal reflections, which is found to be lower in the aged 1Yb8ScSZ sample, arises from the partial or short-range ordering of oxygen-ion vacancies to further lower the anisotropy of the lattice distortion.36, 41



3.3.3 Raman Spectroscopy Analysis Like the Raman spectra of unaged samples, in the measured wavenumber range of 120-1100 cm-1, six peaks are identified centred around 155, 260, 370, 477, 595 and 631 cm-1 in all the aged samples (shown in Figure 5 (a-d)). The peaks located at 150, 260 and 477 cm-1 are assigned to Eg Raman active mode of the tetragonal phase. In all the aged samples, the intensity of the broad peak at 260 cm-1 appears to be lower than the peaks present at 477 and 628 cm-1 indicating the absence of stable t-phase and the presence of either the metastable tetragonal t′or t″-phase. As both XRD and TEM analysis showed the absence of t″-phase in the aged 8ScSZ and 1Yb7ScSZ samples, the existence of t′-phase can be confirmed in the 8 mol.% dopant containing compositions. Similarly, the cubic nature of the phase has been identified from the detailed XRD and TEM studies of the aged 9ScSZ and 1Yb8ScSZ samples suggesting the existence of t″-phase, which possesses the unit-cell parameters ratio of unity. From the Raman analysis, it can be inferred that the thermal annealing at 650°C increases the tetragonal symmetry induced by the displacement of oxygen atoms inside the unit-cell in both the metastable t′- and t″- phases. After ageing, the 1Yb7ScSZ composition exhibits a lower tetragonality compared to 8ScSZ. On the other hand, both 9ScSZ and 1Yb8ScSZ compositions show a similar amount of tetragonality after thermal ageing. 3.3.4. Effect of Thermal Ageing on the Phase Evolution of Sc and Yb Doped Zirconia Samples In summary, the present work discusses the effect of thermal ageing on the phase formation by describing two types of tetragonality existing in the metastable tetragonal phases. The first one is related to the distortion of the unit cell, which can be explained in terms of the ratio of unit-cell parameters. The second one is induced by the shuffling of oxygen-ion along the z-axis inside the unit cell. A comparison of various structural features observed in unaged and aged 26 ACS Paragon Plus Environment

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samples of Yb2O3 co-doped ScSZ is given in Table 3. Similar observations are also presented for ScSZ specimens. From the above analysis, it can be concluded that the 2000 h of thermal ageing at 650°C lowers the unit-cell tetragonality of the metastable t′-phase which is evident from the decrease in unit-cell parameters ratio, presence of weak-splitting of XRD peaks and the disappearance of twin-like structure in the TEM micrographs of 1Yb7ScSZ composition. The Yb2O3 co-doped composition appears to possess lower tetragonality in t′-phase compared to 8ScSZ after ageing, which highlights the benefit of using the co-doping approach in stabilizing the high-symmetry phase. However, the metastable t″-phase remains stable in both 9ScSZ and 1Yb8ScSZ with a slight lattice contraction (0.1%-0.2%) after ageing. The tetragonality induced by oxygen-ion displacement inside the unit-cell increases in all the tested compositions on thermal ageing. The oxygen-ion shuffling in the metastable t′-phase is higher in the 8ScSZ aged sample as compared to the Yb2O3 co-doped sample (aged). On the other hand, the tetragonality is nearly the same in both 9ScSZ and 1Yb8ScSZ compositions after the thermal ageing. These results demonstrate the effectiveness of Yb2O3 co-doping in reducing tetragonality in compositions containing the metastable t′-phase. However, not much benefit in terms of phase stabilization can be achieved in the metastable t″-phase.


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

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Table 3: Summary of the structural analysis performed on the unaged and aged samples xYb2O3–(y-x)Sc2O3–(100-x-y)ZrO2 (x = 0 and 1 mol.%; y = 8 and 9 mol.%) compositions using various characterization techniques. XRD

Raman spectroscopy

TEM

Sample Unaged

8ScSZ

1Yb7ScSZ

 XRD peak splitting.  Unit-cell parameters ratio ct/(√2at) = 1.005.  XRD peak splitting.  Larger unitcell volume than unaged 8ScSZ.  Unit-cell parameters ct/(√2at) = 1.004.  No XRD peak splitting.  ct/(√2at)= 1.

9ScSZ

Aged

Unaged

Aged

Unaged

 Minor XRD peak splitting.  Unit-cell parameters ratio ct/(√2at) = 1.002.  Lattice contraction on ageing.  Minor XRD peak splitting.  Unit-cell parameters ct/(√2at) = 0.999.  Lattice contraction on ageing.

 Six peaks are identified at ~ 153, 260, 369, 475, 596 and 632 cm-1.  Intensity height ratio (I4/I6) ~ 0.36.

 Six peaks are identified at ~ 153, 260, 370, 475, 595 and 632 cm-1.  Increase in the intensity height ratio (I4/I6) to 0.43 on ageing.

 Twin-like bands feature of thickness ~0.2 μm in BFI.  SADP indexed according to [101]t –zone axis.

 Retained twin-like bands of thickness ~0.2 μm after ageing.  SADP indexed according to [210]t – zone axis.  Presence of weak secondary reflections.  Weak diffuse scattering around principal reflections.

 Six peaks are identified ~152, 258, 368, 478, 595 and 631 cm-1.  Lower intensity height ratio (I4/I6~0.33) than 8ScSZ.

 Twin-like bands of slightly lower thickness ~0.09μm  SADP indexed according to [210]t –zone axis.

 Disappearance of twin-like bands  SADP indexed according to [102]t – zone axis  Presence of weak secondary reflections.  Weak diffuse scattering around principal reflections.

 No XRD peak splitting.  ct/(√2at) = 1.  Lattice contraction on ageing.

 Six peaks are identified at ~ 153, 259, 368, 478, 597 and 631 cm-1  Intensity height ratio (I4/I6) ~ 0.36.

 Six peaks are identified at ~ 152, 258, 368, 477, 595 and 631 cm-1  Increase in the intensity height ratio (I4/I6) to 0.39 on ageing.  After ageing, the I4/I6 is lower than that in 8ScSZ.  Six peaks are identified at ~ 153, 260, 369, 478, 597 and 631 cm-1.  Increase in the intensity height ratio (I4/I6) to 0.45 on ageing. 1.

 Mottled contrast in BFI.  SADP indexed according to [100]c–zone axis.

 Retained mottled appearance in BFI after ageing.  SADP indexed according to [111]c – zone axis.  Presence of weak secondary reflections.  Weak diffuse scattering around principal reflections.


Aged

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1Yb8ScSZ

 No XRD peak splitting.  Slightly larger unitcell volume. than 9ScSZ  ct/(√2at)= 1.

 No XRD peak splitting.  ct/(√2at)= 1.  Lattice contraction on ageing.

 Six peaks are identified at ~ 157, 261, 373, 475, 598 and 632 cm-1.  Similar intensity height ratio (I4/I6 ~ 0.36) as observed in 9ScSZ.

 Six peaks are identified at ~ 157, 261, 373, 475, 598 and 632 cm-1.  Increase in the intensity height ratio (I4/I6) to 0.452. on ageing.  After ageing, the increase in the ratio is same as in 9ScSZ.


 Mottled contrast in BFI.  SADP indexed according to [121]c –zone axis.

 Retained mottled appearance in BFI after ageing.  SADP indexed according to [111]c – zone axis.  Presence of weak secondary reflections.  Weak diffuse scattering around principal reflections.


4. Conclusions The effect of thermal ageing and the addition of 1 mol.% Yb2O3 were investigated on the metastable tetragonal t′- and t′′ phases present in bulk sintered samples of 8-9 mol.% ScSZ. A comprehensive structural analysis revealed the existence of t′- phase in 8ScSZ and t′′ phase in 9ScSZ composition. Although t′-phase is retained in 1Yb7ScSZ, the tetragonality of this phase is reduced compared to that in 8ScSZ. The reduction of tetragonality is evident from the smaller band thickness in the twin-like feature present in the microstructure, lower unit-cell parameters ratio ct/(√2at), and lesser displacement of oxygen-ions along the z-axis of unit-cell in the 1Yb7ScSZ composition. Likewise, the metastable t″-phase, which is present in 9ScSZ continues to exist in 1Yb8ScSZ sample. A slight lattice expansion with larger-sized Yb3+ substitution does not seem to affect the tetragonality induced by oxygen-ion shuffling in the metastable t″-phase. The thermal ageing performed at 650°C for 2000 h does not lead to any phase transition in any of the tested samples. However, the unit-cell parameters ratio ct/(√2at) in t′-phase is reduced in both the aged 8ScSZ and 1Yb7ScSZ samples. Further, a slight lattice contraction is observed in the t′′-phase of 9ScSZ and 1Yb8ScSZ after ageing. The SAD patterns obtained through TEM show the presence of weak superlattice reflections in all the aged samples, which is attributed to the periodicity of anisotropic distortions that arises because of the size mismatch between the host and dopant cations. The thermal ageing has shown to increase the extent of tetragonality induced by oxygen-ion displacement in all the compositions. This oxygen-ion shuffling is lower in the aged samples of 1Yb7ScSZ than 8ScSZ, which contain t′-phase. However, no appreciable difference in oxygen displacement is observed in t′′-phase of 1Yb8ScSZ when compared to that of 9ScSZ after ageing treatment. 30 ACS Paragon Plus Environment

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In summary, Yb2O3 co-doping reduces the tetragonality and improves the phase stability during ageing in t′-phase contained in 8ScSZ, while it does not have any significant effect on t′′-phase present in 9ScSZ.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgement Authors would like to acknowledge the financial aid from DST, MHRD, Govt. of India (Grant Number: EMR/2016/005438) for the support of this research.



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