Evaluation of the effect of site substitution of Pr doping in the lithium garnet system Li₅La₃Nb₂O₁₂

Abstract

Li ion conducting garnets have been attracting considerable interest for use as the electrolyte in all solid-state batteries, due to their high ionic conductivity and wide electrochemical stability window. Consequently, there have been a number of doping studies aimed at optimising the conductivity, focusing on both doping in Li₇La₃Zr₂O₁₂ and Li₅La₃(Nb/Ta)₂O₁₂ systems. In this paper, we report a detailed study of Pr doping in Li₅La₃Nb₂O₁₂, and show that this is a rare example of an ambi-site dopant, being able to be doped onto either the La or Nb site. Interestingly the resultant Pr oxidation state is determined by the site substitution, with oxidation states of 3+ for the La site, and 4+ for the Nb site. While the conductivity is essentially unchanged for the La site substitution, Pr⁴⁺ substitution on the Nb site leads to a large increase in the conductivity associated with the increase in Li content (Li_5+xLa₃Nb_2−xPr_xO₁₂) up to 0.56 mS cm⁻¹ (at 50 °C) for x = 0.8. Overall, this work highlights the flexibility of these garnet materials to doping, and suggests that further consideration of site substitution be considered for other dopants.

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Introduction

Increased demand for portable electronic devices, electric vehicles and renewable energy has significantly expanded demand for efficient, safe, low cost and high-power energy storage. Lithium ion batteries (LIB) are principally used for these purposes, due to their high volumetric and gravimetric energy densities.¹ However, current LIBs have a variety of safety, thermal and efficiency issues, which are often attributed to the liquid organic electrolytes used in the cell. These liquid electrolytes also limit the cell potential by preventing the use of lithium metal anodes.² These disadvantages, and advantages, are covered in many review articles.^1,3–7 Therefore, the replacement of liquid electrolytes with a solid-state electrolyte (SSE) to give an all solid-state battery (ASSB) remains the key challenge to deliver this next leap in energy storage.⁸ Such batteries, when compared to traditional LIBs, offer the potential for increased safety, operate over a wider electrochemical window, have increased thermal stability and are ideally stable against Li metal, hence increasing energy density per cell.^8–12

As a result, there is considerable interest in the identification and optimisation of new solid state electrolyte materials. However, many of these electrolytes, such as LISICON, sulphides/glass and LiPON, are limited by either low ionic conductivity or a poor electrochemical stability window.^13–15 Of these reports, the lithium containing garnet type systems are one of the few which present with high lithium ion conductivity (reportedly >10⁻⁴ S cm^−1 16), a wide electrochemical stability window and are chemically stable against Li metal.^9,13,17–19 Hence these systems are attracting considerable interest for use in ASSBs.

However, there still remain challenges for these garnet systems, and so there continues to be interest in new doping studies in these materials. In particular, there is a need to further improve the ionic conductivity, as well as improve the interface between the electrolyte and electrodes.^9,10,13 Traditional garnet materials have the general formula A₃B₂X₃O₁₂,^13,20–22 where the A, B, C cations are located in 8, 6, 4 coordinate sites respectively. In the Li containing garnets, excess Li can be accommodated in interstitial sites, such that the general formula of these can be classed as A₃B₂Li_3+xO₁₂ (0 ≤ x ≤ 4), so that the Li content can be varied between 3 (as in Li₃Ln₃Te₂O₁₂), and 7 (as in Li₇La₃(Zr/Sn/Hf)₂O₁₂). Those systems with <7 lithium atoms crystallise in a cubic structure which has high Li-ion conductivity. In contrast, the stoichiometric Li₇La₃(Zr/Sn/Hf)₂O₁₂ systems crystallise in the less conductive tetragonal phase, although doping to reduce the Li content allows the formation of the cubic cell with enhanced conductivity.

For cubic garnets, such as Li₅La₃Nb₂O₁₂ (LLNO), ionic migration pathways have been attributed to two tetrahedral sites bridged by a single face sharing octahedron. As the lithium sites are not fully occupied in these systems, lithium partially occupies the interstitial octahedral sites 48 h and 96 g, in a disordered fashion. This gives a disordered Li sublattice and a migration pathway which leads to high ionic conductivity via the Li⁺ charge carrier, which is thought to follow the hopping mechanism of 24 d → 96 h → 48 g → 96 h → 24 d, as determined by high temperature neutron diffraction studies.²³

Of the wide variety of garnet systems, particular attention has been afforded to Li₇La₃Zr₂O₁₂ (LLZO). This stoichiometric system is tetragonal at room temperature, with comparatively low conductivity, and changes to a cubic cell at around 750° C; this temperature can be reduced to 350 °C by doping at the Zr site with Ce, leading to an increase in conductivity.²⁴ Stabilisation of the cubic cell at room temperature (c-LLZO) requires a reduction of the lithium content, often using Al or Ga substitution at the Li site.^16,25,26 Dopants at the Zr site (e.g. Nb, Ta^27,28) also can stabilise c-LLZO.

Although c-LLZO has been reported with conductivities >10⁻⁴ S cm⁻¹, these often require complex methodologies and/or equipment, such as field assisted sintering, which presents a barrier to commercial scalability.^16,29,30 Consequently, the majority of c-LLZO doping strategies lead to conductivities ∼10⁻⁴ S cm⁻¹, or lower, and are suboptimal compared with current LIB liquid electrolytes.^31–38 Therefore, it is of interest to investigate alternative doping strategies of other lithium garnet systems, such as Li₅La₃Nb₂O₁₂ (LLNO). This system was the first to show fast Li-ion conductivity, and outside of detailed examinations of the lithium migration pathways, has now been superseded by research on c-LLZO.^39,40 However, of the prior reports of doping into the LLNO system, many show room temperature conductivities of ∼10⁻⁴ S cm⁻¹ and are thus similar to c-LLZO.^13,41 Therefore, further doping studies on the LLNO system are warranted. Herein we report an examination of Pr doping in this system. Pr was chosen because of its ability to adopt different oxidation states (3+, 4+), and so potential, based on size considerations, to dope onto either the Nb or the La site according to the following formula Li_5+xLa₃Nb_2−xPr_xO₁₂ (Pr⁴⁺) and Li₅La_3−xPr_xNb₂O₁₂ (Pr³⁺). We confirm this ambi-site substitutional ability of Pr, and illustrate in particular that Pr doping on the Nb site leads to samples with high Li ion conductivity.

Methods

Synthesis

Li_5+xLa₃Nb_2−xPr_xO₁₂ garnets were prepared via the solid-state route from stochiometric quantities of Li₂CO₃, La₂O₃, Nb₂O₅ and Pr₆O₁₁ under air. A 40% mol excess of lithium was added to compensate for lithium loss during high temperature sintering. All powders were ball milled for 1 h (500 rpm) and heated to 950 °C (16 hours). Impure phases (x > 0.25) were subsequently ball milled with a 20% lithium mol excess and heated to 950 °C (12 hours). Samples where x ≥ 0.8 were synthesised in a dry room with a dewpoint between −45 °C to −64 °C (the elimination of humidity was found to be necessary to prepare good quality samples for these high Pr contents: it is known that moisture can be an issue in the synthesis of Li garnet systems).^42–44

Li₅La_3−xPr_xNb₂O₁₂ was also prepared via the solid-state route in either air or 5%H₂/N₂. Samples were heated to 950 °C for 16 hours, cooled to room temperature and reheated to 950 °C for 16 hours. After synthesis, all samples were stored in an argon glove box.

Characterisation

Samples were characterised by X-ray diffraction (XRD) using a Bruker D8 diffractometer with Cu Kα radiation. Cell parameters were determined from Rietveld refinement using GSAS II software.⁴⁵ In order to evaluate the Pr oxidation states X-ray absorption near edge spectroscopy (XANES) data were recorded at Diamond Light Source on beamline B18 and data interpreted via the Athena/Artemis software.⁴⁶ Scanning electron microscopy (SEM) was performed using a TM4000plus SEM, with elemental compositions evaluated by the corresponding AZtecOne EDX attachment. The Pr–LLNO garnet powders were placed on carbon tape, attached to the SEM stub and evaluated at 15 kV in back scattered electron mode.

Impedance spectroscopy

Pellets for conductivity measurements were prepared as follows: 10 mm diameter pellets were uniaxially hot pressed with an atlas heating platen to ca. 3 tonnes at 300 °C and, subsequently, heated to 1000–1050 °C for 12 hours under dry N₂ to densify the SSE membrane. During N₂ treatment sacrificial powders were used to protect pellets from Li loss and to prevent Al contamination from the Al₂O₃ crucible. Pellets were then polished, painted with gold electrodes and heated to 800 °C for 1 hour in air to cure the Au paste. The pellets were then quenched from 700 °C to limit any H⁺/Li⁺ on cooling, as has been shown to occur for garnet samples at lower temperatures in air.^26,47 A.C. impedance spectroscopy data were collected during heating over a temperature range of 50 °C to 250 °C (with 16 measurements taken), using a Hewlett Packard 4192A instrument. Data for the most conductive phases (∼10⁻⁴ S cm⁻¹) were also measured at room temperature. All measurements were conducted in air.

Results and discussion

Praseodymium doping on the Nb Site: Li_5+xLa₃Nb_2−xPr_xO₁₂

The powder XRD patterns for Li_5+xLa₃Nb_2−xPr_xO₁₂ (0 ≤ x ≥ 1) are shown in Fig. 1, illustrating pure garnet samples across the range. All patterns could be indexed based on a cubic garnet with space group Ia3̄d (no. 230).^22,39,40 Phases where x > 0.25 initially demonstrated Pr₂O₃ and La₂O₃ impurity phases which were overcome by addition of 20% excess lithium and reheating. Phases where x > 0.8 showed decreased conductivity (see later), hence the synthesis method was not undertaken past x = 1. The colour of the samples became increasingly yellow as a function of Pr content, which is attributed to the presence of Pr⁴⁺, which is commonly believed to be responsible for the colour in the yellow pigment Pr–ZrSiO₄ (although some debate persists behind the chemistry of Pr–ZrSiO₄).^48–50

Fig. 1 Powder XRD patterns for Li_5+xLa₃Nb_2−xPr_xO₁₂ (0 ≤ x ≥ 1) synthesised in air, showing the formation of single phase garnet samples across the range.

Rietveld refinements, based on the structural model from Cussen³⁹ showed a linear increase in lattice parameters with increasing Pr content (Fig. 2 and Table 1), in agreement with Vegard's law and so confirming the solid solution range. This increase is due to the larger ionic radius of Pr⁴⁺ compared to Nb⁵⁺.^39,41,51 The refined Pr occupancies on the Nb site were similar to the expected stoichiometric ratio across the series, however, did deviate slightly from the ideal ratio as the Pr content increased, see Table 1.

Fig. 2 Cell parameters vs. Pr content for Li_5+xLa₃Nb_2−xPr_xO₁₂.

Table 1 Pellet and conductivity data in addition to lattice parameters for Li_5+xLa₃Nb_2−xPr_xO₁₂ samples which were uniaxially hot pressed. The Arrhenius plot x = 1 was not linear, hence no activation energy available

Hot pressed samples	Pellet density (g cm^−1)	Refined density (g cm^−1)	Relative density (%)	Lattice parameters (Å)	Pr fractional site occupancies on the Nb site	Conductivity (50 °C) (S cm^−1)	Activation energy (eV)
LLNO reference^39	—	5.259	—	12.79432		∼ × 10^−6
LLNO	4.52	5.248	86	12.7880(3)		4.3 × 10^−6	0.32
x = 0.1	4.32	5.284	82	12.8066(3)	0.0649	5.6 × 10^−5	0.28
x = 0.25	4.50	5.300	85	12.8410(4)	0.1265	7.3 × 10^−5	0.32
x = 0.5	4.33	5.321	81	12.9048(3)	0.2568	1.6 × 10^−4	0.29
x = 0.75	4.72	5.361	88	12.9527(3)	0.3876	3.4 × 10^−4	0.37
x = 0.8	4.53	5.358	85	12.9741(5)	0.4179	5.6 × 10^−4	0.36
x = 1	3.50	5.366	65	13.0269(4)	0.5160	1.9 × 10^−4	—

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In order to try to confirm the Pr oxidation state, X-ray absorption near edge spectroscopy (XANES) data were collected for the Li_5+xLa₃Nb_2−xPr_xO₁₂ systems. However, the La L2 edge of La (∼5897 eV) interfered with the Pr L3 edge (∼5968 eV) to some degree. Nevertheless, when compared to the Pr³⁺ and Pr⁴⁺ reference (Pr₂O₃ and BaPrO₃ respectively), the peaks are qualitatively characteristic of Pr⁴⁺, see Fig. 3. These data show a peak at 5973 eV with a shoulder at 5980 eV, which closely matches the Pr⁴⁺ (BaPrO₃) reference (Fig. 3) and can be assigned to Pr⁴⁺. Further support for the assignment of Pr⁴⁺ in the Li_5.5La₃Nb_1.5Pr_0.5O₁₂ samples is provided by comparison with Pr_0.5Ce_0.5O₂, a Pr⁴⁺ reference used elsewhere,^52,53 which shows peaks at 5970 and 5980 eV.

Fig. 3 XANES spectrum for Li_5.5La₃Nb_1.5Pr_0.5O₁₂ compared to BaPrO₃ (Pr⁴⁺) reference.

Therefore, considering the XRD, XANES and refinement data, there is compelling evidence that Pr⁴⁺ has been doped into the LLNO structure at the Nb site (16a).

SEM and EDX

SEM and EDX on a singular grain of Li_5.5La₃Nb_1.5Pr_0.5O₁₂ demonstrated uniform Pr presence (in the expected stoichiometric ratio) when compared to La, Nb and O. However, for Pr (x) contents ≥0.5 Al contamination from the Al₂O₃ crucible became increasingly apparent²⁵ (Fig. 4).

Fig. 4 SEM image and EDX map of elemental distribution in Li_5.5La₃Nb_1.5Pr_0.5O₁₂. Scale bar at 10 μm in all images.

Praseodymium doping on the La site (Pr³⁺): Li₅La_3−xPr_xNb₂O₁₂

The successful synthesis of Li₅La_3−xPr_xNb₂O₁₂ garnets in air was limited to samples in the range 0 ≤ x ≤ 1. In contrast synthesis under 5%H₂/N₂ permitted a complete solid solution range for the replacement of La with Pr, 0 ≤ x ≤ 3. Both XRD patterns for air prepared samples (0 ≤ x ≤ 1) and 5%H₂ prepared samples (0 ≤ x ≤ 3) could be indexed on a cubic garnet cell with space group Ia3̄d (no. 230);^22,39,40 see Fig. 5 and 8 respectively. In some cases, particularly for the air-based synthesis, a small amount of Li₃NbO₄ impurity was observed marked in the respective figures (also see ESI†).

Fig. 5 The powder XRD patterns for Li₅La_3−xPr_xNb₂O₁₂ (0 ≤ x ≤ 1.5) synthesised in air. Phase pure samples are shown up to x = 1, but all show garnet type symmetry. Li₃NbO₄ impurity marked by arrow. See ESI† for expanded XRD pattern.

Li₅La_3−xPr_xNb₂O₁₂ samples prepared in air demonstrated an increasingly straw yellow/brown colour, which was distinctly different from the colour of the Li_5+xLa₃Nb_2−xPr_xO₁₂ samples discussed earlier. Cell parameters determined from Rietveld refinement indicated the expected decrease with increasing Pr content (due to the smaller size of Pr³⁺versus La³⁺) (Fig. 6) and were similar to the results for equivalent Li₅La_3−xPr_xNb₂O₁₂ phases synthesised under 5%H₂ (discussed below). The presence of the Li₃NbO₄ impurity in the air synthesised samples may suggest Nb site vacancies in the LLNO structure which have been filled by Pr⁴⁺, which could account for the yellow powder colour. It could also be the case the Pr⁴⁺ is located on the La site. Therefore, these data suggest that there may be some partitioning of the Pr over both sites however, more work is needed to confirm whether this is so and to what extent.

Fig. 6 Cell parameters vs. Pr content for air-based synthesis of Li₅La_3−xPr_xNb₂O₁₂.

In order to confirm the oxidation state of Pr in these samples, XANES data were collected on both x = 0.5 and 1, however, as outlined previously, in all cases the La L2 edge interfered somewhat with the Pr L3 edge. Nevertheless, the XANES spectra of the Li₅La_2.5Pr_0.5Nb₂O₁₂ phase, which was prepared in air, does demonstrate characteristics of the Pr⁴⁺ reference (which would be consistent with presence of Pr⁴⁺ on either the Nb or La site), such as the peak broadness and the shoulder noted at 5982 eV, however the main peak appears to be positioned in line with the Pr₂O₃ reference at 5969 eV, consistent with mainly La site substitution, see Fig. 7a. However, the XANES spectra of the higher Pr content phase, Li₅La₂Pr₁Nb₂O₁₂, possesses only an absorption peak which is consistent with the Pr³⁺ reference at 5970 eV, Fig. 7b. Therefore, these spectra are somewhat contradictory but indicate, potentially, the presence of Pr in different oxidation states for x = 0.5, but not when x = 1 where only Pr³⁺ is seen.

Fig. 7 (a) XANES spectra for Li₅La_2.5Pr_0.5Nb₂O₁₂ (air synthesised) compared to the Pr₂O₃ (Pr³⁺) reference. (b) XANES spectra for Li₅La₂Pr₁Nb₂O₁₂ (air synthesised) compared to the Pr₂O₃ (Pr³⁺) reference.

Fig. 8 The powder XRD patterns of Li₅La_3−xPr_xNb₂O₁₂ (0 ≤ x ≤ 3) prepared in a reducing atmosphere (5%H₂). Phase pure samples with garnet type symmetry were obtained for all. For the small Li₃NbO₃ impurity see ESI† for expanded XRD pattern.

The colours of the 5%H₂ synthesised garnets were increasingly vibrant green as the Pr content was increased, which indicates the presence of Pr³⁺. Cell parameters determined from Rietveld refinement show a linear decrease with increasing Pr content, consistent with the smaller ionic radius of Pr³⁺ compared to La³⁺, see Fig. 9. Despite the interference from the La L2 edge, the XANES data for these garnets are somewhat clearer than those derived from the Li_5+xLa₃Nb_2−xPr_xO₁₂ phases (Fig. 10a and b). The 5%H₂ prepared samples show a clear correlation with the Pr³⁺ reference (Pr₂O₃) in terms of peak intensity and characteristics, in both x = 0.5 and 1. Each phase has a strong absorption peak noted at 5970 eV, characteristic of Pr³⁺, which matches both the Pr₂O₃ reference and other Pr³⁺ references used elsewhere.⁵² When the Pr content is increased to x = 1, only increased absorbance occurs, as expected, but the peak characteristics remain unchanged. Hence, the characterisation data support the conclusion that only Pr³⁺ is present, and the different characteristics of the x = 0.5 phase prepared in air are unique to the synthesis method, as seen in the XANES data.

Fig. 9 Cell parameters vs. Pr content for 5%H₂ based synthesis of Li₅La_3−xPr_xNb₂O₁₂.

Fig. 10 (a) XANES spectra for Li₅La_2.5Pr_0.5Nb₂O₁₂ (5%H₂) compared to the Pr₂O₃ (Pr³⁺) reference. (b) XANES spectrum for Li₅La₂Pr₁Nb₂O₁₂ (5%H₂) compared to the Pr₂O₃ (Pr³⁺) reference.

The XANES and XRD data for the H₂ synthesised samples therefore support the conclusion that Pr³⁺ has successfully been doped into the LLNO structure at the La site (24c).

SEM and EDX

SEM/EDX data for Li₅La_3−xPr_xNb₂O₁₂ garnets, synthesised in air or H₂, showed uniform elemental distribution throughout the analysed grains, and have similar grain characteristics, see Fig. 11 and 12 respectively. The data for both air and H₂ synthesised samples demonstrated Pr and La present in the expected ratio. Hence EDX data supports the stoichiometric incorporation. However, the air synthesised samples did show some aluminium contamination, see Fig. 11, attributed to a reaction with the Al₂O₃ crucible.

Fig. 11 SEM images and EDX map of elemental distribution in Li₅La₂Pr₁Nb₂O₁₂ (air). The presence of Al in this air synthesised sample is attributed to partial substitution onto the Li site. Al incorporation is often apparent in higher lithium content systems, and so this may support the postulation that there is some Pr⁴⁺ partitioning onto the Nb⁵⁺ site in the air synthesised Li₅La_3−xPr_xNb₂O₁₂ samples. The corresponding H₂ synthesised sample, where only La site substitution is believed to occur, showed no Al contamination.

Fig. 12 SEM images and EDX map of elemental distribution in Li₅La_1.5Pr_1.5Nb₂O₁₂ (H₂). Since there is no increased Li content for Pr³⁺ doping on the La³⁺ site, therefore there is a decreased chance of Al exchange, hence no contamination from the alumina crucible is present.

Conductivity data for Nb site substitution: Li_5+xLa₃Nb_2−xPr_xO₁₂

Lithium ion conductivity was evaluated via impedance spectroscopy (a typical Nyquist impedance plot is shown in Fig. 13). A single non ideal semicircle was observed. A resistor, R1, in parallel with a constant phase element, CPE1, were used to fit the semicircle, and an inductor, L1, in series with R1/CPE1 was used to account for the inductance effect due to the limitations of the HP impedance spectroscopy equipment.

Fig. 13 Nyquist impedance plot for Li_5.75La₃Nb_1.25Pr_0.75O₁₂ at 30 °C which was fit the equivalent circuit in the top left.

A characteristic spike relative to the semi-circle is present in all plots. This is attributed to a sample – electrode double layer effect. This spike represents the Li-ion transfer resistance between the garnet electrolyte and the Au electrode and corresponds to the capacitive behaviour of the gold electrodes which block Li-ion diffusion. As the semi-circle and tail were obtained in high and low frequency regions respectively it can be considered that conduction is primarily ionic in nature.³¹

The ionic conductivity of the Li_5+xLa₃Nb_2−xPr_xO₁₂ samples increased in line with increased Pr and hence lithium content (Fig. 14). The highest ionic conductivity of 5.6 × 10⁻⁴ S cm⁻¹ at 50 °C was observed for x = 0.8, with a measured conductivity of 4.1 × 10⁻⁴ S cm⁻¹ at 21 °C. See Table 1 and Fig. 14. These conductivity values represent some of the highest ever reported for garnets with Li content <6 per formula unit.

Fig. 14 Arrhenius plots for Li_5+xLa₃Nb_2−xPr_xO₁₂ (x = 0.1–0.8) samples which were hot pressed prior to sintering under N₂.

Higher content Pr phases (x > 0.75) were, however, prone to degradation during the high temperature densification treatment under N₂, resulting in small Pr₂O₃, La₂O₃ and PrNb₅O₁₄ impurities. In particular the XRD patterns for the x = 1 sample after N₂ sintering showed a more severe phase degradation. This degradation most likely accounts for the low relative pellet density of this sample and the resultant lower conductivity associated with this phase, as well as the non-linear temperature dependence of the Arrhenius plot, see Table 1 and ESI.† Consequently, further studies of samples with higher Pr contents, x > 1, were not performed (see ESI† for comparative XRD patterns before and after sintering treatments).

The activation energies for all samples (Fig. 14) ranged between 0.24–0.38 eV, which are similar to reports for other garnets, see Table 1.^51,54

Conductivity data for La site substitution: Li₅La_3−xPr_xNb₂O₁₂

All samples where Pr was doped on the La site showed similar conductivities to LLNO consistent with no change in Li content for this series. A typical Nyquist plot is displayed in Fig. 15a and b for air-based and H₂ synthesis respectively. These plots were fitted to two R/CPE components, representative of overlapping bulk and grain boundary resistance.

Fig. 15 (a) Nyquist impedance plot for Li₅La_3−xPr_xNb₂O₁₂ at 130 °C for air-based synthesis. The fitted equivalent circuit is in the top left. R_b and R_gb relate to bulk and grain boundary resistivity respectively. (b) Nyquist impedance plot for Li₅La_3−xPr_xNb₂O₁₂ at 130 °C for H₂ synthesis. The fitted equivalent circuit is in the top left. R_b and R_gb relate to bulk and grain boundary resistivity respectively.

It was noted earlier that in the air synthesis samples, there may be some small level of Pr⁴⁺ substitution on the Nb site, which would be expected to give a small increase in Li content and hence conductivity. This was not observed, however these pellets proved difficult to densify, hence present with much lower relative densities than the corresponding undoped LLNO pellet. The influence of the poorer sintering may therefore mask any increase in conductivity from any Nb site substitution. Activation energies are similar to other reported garnets, see Table 2 and Fig. 16.^51,54

Fig. 16 Arrhenius plots for Li₅La_3−xPr_xNb₂O₁₂ samples in air (x = 0–1).

Table 2 Pellet and conductivity data in addition to lattice parameters for Li₅La_3−xPr_xNb₂O₁₂ samples synthesised in air

Sample	Pellet density (g cm^−1)	Refined density (g cm^−1)	Relative density (%)	Lattice parameters (Å)	Conductivity at 50 °C (S cm^−1)	Activation energy 50–210 °C (eV)
LLNO	4.52	5.248	86	12.7880(3)	4.3 × 10^−6	0.32
x = 0.5	3.08	5.289	58	12.7701(3)	9.3 × 10^−6	0.25
x = 1	3.21	5.317	60	12.7481(3)	6.9 × 10^−6	0.28

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The H₂ synthesised samples showed similar conductivities for all samples, with a small peak in the conductivity of the series (1.7 × 10⁻⁵ S cm⁻¹ (50 °C)) for x = 0.5, see Fig. 17 for Arrhenius plots. Activation energies fall within the 0.3–0.4 eV range, hence are similar to reports for other garnets, see Table 3.^51,54

Fig. 17 Arrhenius plots for Li₅La_3−xPr_xNb₂O₁₂ samples under H₂ (x = 0–3). The deviation from linearity at lower temperature for the x = 0.5 sample is attributed to partial water incorporation and subsequent loss on heating.

Table 3 Pellet and conductivity data in addition to lattice parameters for Li₅La_3−xPr_xNb₂O₁₂ samples synthesised under H₂

Sample	Pellet density (g cm^−1)	Refined density (g cm^−1)	Relative density (%)	Lattice parameters (Å)	Conductivity at 50 °C (S cm^−1)	Activation energy 50–210 °C (eV)
LLNO reference^39		5.259	—	12.79432	∼ × 10^−6
LLNO synthesized	4.52	5.248	86	12.7880(3)	4.3 × 10^−6	0.32
x = 0.5	3.87	5.286	73	12.7727(3)	1.7 × 10^−5	0.31
x = 1	3.40	5.316	64	12.7490(2)	9.2 × 10^−6	0.38
x = 1.5	3.47	5.346	65	12.7250(5)	6.0 × 10^−6	0.35
x = 2	3.91	5.373	73	12.7032(3)	1.1 × 10^−5	0.40
x = 2.5	3.72	5.407	69	12.6769(3)	8.1 × 10^−6	0.38
x = 3	4.01	5.435	74	12.6548(5)	4.9 × 10^−6	0.32

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Samples with Pr doping at the La site showed far lower densities than the samples with Pr doping on the Nb site (see Tables 1–3). Hence Pr doping on the Nb site appears to also enhance the sintering compared to Pr doping on the La site, which may be related to the higher lithium content in these Li_5+xLa₃Nb_2−xPr_xO₁₂ systems.

Conclusions

In summary, we have shown for the first time a dopant (Pr) that can be substituted onto both the La and Nb site in Li₅La₃Nb₂O₁₂. Furthermore, the oxidation state of the Pr is dictated by the site substitution; +3 on the La site, and +4 on the Nb site. Due to the resultant increase in Li content for Nb site substitution, a significant increase in conductivity is observed. These Li_5+xLa₃Nb_2−xPr_xO₁₂ samples show some of the highest conductivities for garnets with Li contents less than 6, with values up to 0.56 mS cm⁻¹ (at 50 °C) for x = 0.8. A final point of note, Pr doped phases at the Nb site (x ≤ 0.25) were a vibrant yellow, with a vibrant green colour obtained from H₂ based synthesis of Li_5+xLa₃Nb_2−xPr_xO₁₂ garnets when x ≥ 2.5. Hence the optical properties of these garnets may merit further investigation, especially with respect to potential interest as pigments (see ESI†), however the effect of moisture on the Pr oxidation states, and therefore the sample colour, requires further investigation. It may also be of interest to study the optical properties of these Pr garnets under U.V. light.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We would like to thank the University of Birmingham for the studentship funding of Mark Stockham and the EPSRC for funding the GENESIS project (under EP/R024006/1). We thank the Diamond Light Source for the award of beam time as part of the Energy Materials Block Allocation Group SP14239. We would also like to thank Prof. Emma Kendrick for the continued use of the dry room facilities. Raw experimental data can be found at: https://doi.org/10.25500/edata.bham.00000490.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0dt01497d

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