Rapid synthesis of dielectric tantalum-based oxynitrides

Journal Pre-proof Rapid synthesis of dielectric tantalum-based oxynitrides

Duan Li, Liang Zeng, Bin Li, Xuejin Yang, Qiuping Yu, Zhongshuai Wu PII:

S0264-1275(19)30854-8

DOI:

https://doi.org/10.1016/j.matdes.2019.108416

Reference:

JMADE 108416

To appear in:

Materials & Design

Received date:

15 August 2019

Revised date:

7 December 2019

Accepted date:

9 December 2019

Please cite this article as: D. Li, L. Zeng, B. Li, et al., Rapid synthesis of dielectric tantalum-based oxynitrides, Materials & Design(2019), https://doi.org/10.1016/ j.matdes.2019.108416

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© 2019 Published by Elsevier.

Journal Pre-proof

Rapid Synthesis of Dielectric Tantalum-based Oxynitrides Duan Li1,*, Liang Zeng1, Bin Li2,*, Xuejin Yang1, Qiuping Yu1, Zhongshuai Wu1 1

Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha 410073, P R China

2

School of Materials, Sun Yat-sen University, Guangzhou 510275, P R China

Bin Li

[email protected]

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[email protected]

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Duan Li

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*Co-corresponding author.

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Abstract:

Perovskite-type oxynitride with a general formula of ATa(O,N)3 (A=Sr, Ba) is a class of promising dielectric

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material due to their very high permittivity. Conventional synthesis routes for these materials always require

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multiple processing steps, long durations and elevated temperatures. In this study, thermodynamic calculations were employed to predict the feasible synthesis reactions, possible impurities and reasonable processing parameters. Then, ceramic powders of SrTaO2N and BaTaO2N were fabricated through direct calcination of SrCO3/BaCO3 and Ta2O5 with urea as nitrogen source by using a modified pressureless spark plasma sintering set-up. High-purity oxynitrides can be obtained within 10 min. Thermal stability, corrosion resistance and dielectric property were evaluated. The results showed that SrTaO2N was thermally stable up to 475°C in air while that temperature for BaTaO2N was 605°C. The oxynitrides possessed a good resistance to hot water and strong acid/alkali. BaTaO2N had a very high room-temperature relative permittivity up to 9550 with a dielectric loss down to 0.001 at 100 Hz, while the values for SrTaO2N were 3141 and 0.017 respectively. The temperature dependence of permittivity for BaTaO2N was weak at -10~200°C. The efficient synthesis method enabled the fast 1

Journal Pre-proof preparation of the tantalum-based oxynitride materials for energy storage applications. Key words: Perovskite; Oxynitride; Urea; Spark Plasma Sintering; Permittivity

1. Introduction

The fast development of energy storage industry has shown a high demand on ceramic capacitors and memory

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devices with good performances [1-3]. Perovskite oxides such as BaTiO3, Pb(Zr,Ti)O3 (PZT) and

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Pb(Mg1/3Nb2/3)O3 (PMN) are currently most widely used dielectric ceramics due to their large permittivity (>1000)

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[3-6]. However, BaTiO3 is weak in bearing the corrosion of strong acid and its dielectric property is closely

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associated with temperature [4]. As for PZT and PMN, the toxicity of lead and high vapor pressure bring about environmental pollution and human health problems [2,5]. On this account, to develop high-quality dielectric

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materials with a good chemical stability and environmental harmony is necessary.

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During the past decades, perovskite-type oxynitrides with the general formula of AB(O,N)3 (A=rare earth,

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alkaline earth or alkali metal; B=transition metal) are becoming the focus of attentions owing to their particular multifunctionality [7-10]. They commonly adopt a crystal structure similar to perovskite oxides ABO3 in which oxygen atoms are partially replaced by nitrogen. The introduction of nitrogen varies the band gap and structure of their parent oxides and thus offers novel optical [11,12], dielectric [1,13], ferroelectric [14,15], magnetic [16,17] and photocatalytic properties [18,19]. Noticeably, tantalum-based oxynitrides ATa(O,N)3 (A=Ca, Sr, Ba) have exhibited remarkable dielectric performance [1,2,5]. For instance, SrTaO2N and BaTaO2N possessed large permittivity values of ~3000 and ~5000 respectively [1]. It is pointed out that the Ta(O,N)6 octahedra tilting induced by the cis arrangement of nitrogen atoms leads to the generation of local dipoles and further gives rise to their superior dielectric behaviour [20-22]. In addition, SrTaO2N and BaTaO2N ceramics are non-toxic and thermally stable in air (up to 600ºC), water and concentrated acids [1,2]. 2

Journal Pre-proof Synthesis of high-quality SrTaO2N and BaTaO2N powders is a key step for further making desired ceramic bulks and devices. The most popular fabrication route for tantalum-based oxynitrides is ammonolysis of their corresponding binary metal oxides with ammonia gas. Prior to it, the oxide precursors can be obtained by either the traditional solid state reaction (SSR) or soft chemistry methods (SCMs) [23-25]. To achieve sufficient nitridation, a duration of up to several tens of hours is often required [2,22,25,26]. Chen et al. [22] reported synthesis of Sr2Ta2O7 by calcining SrCO3 and Ta2O5 at 1300ºC for several times with intermittent grinding, each

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time for ~10 hours’ period. The following ammonolysis was carried out at 1000ºC for 90~120 h in every 10 h to

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finally obtain the SrTaO2N oxynitride. Balaz et al. [26] also prepared BaTaO2N in the similar way through

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heating BaCO3-Ta2O5 mixture in ammonia at 950°C for 24 h. Zhang et al. [2] adopted a citrate route to fabricate

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SrTaO2N during which SrCO3-TaCl5-citric acid solution was heated and stirred at 150ºC to form the viscous gel.

1000ºC consumed 12 h.

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Subsequent firing of the gel at 350ºC resulted in the formation of Sr2Ta2O7 and the following nitridation process at

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In order to avoid long processing time and excess reaction cycles, some direct methods for synthesising

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oxynitrides were recently revealed. Chen et al. [27] synthesized SrTaO2N based on the direct reactions between SrCO3 and TaN at the temperatures above 1000°C. The obtained oxynitride was slightly off-stoichiometric with lower nitrogen content. Clarke et al. [28] utilized a high-temperature synthesis route where SrTaO2N powder was prepared via the direct reaction between TaON and SrO (1500ºC, 3 h, N2 atmosphere). The obtained product was phase-pure with the presence of anionic deficiencies rather than the stoichiometric composition. Sun et al. [25,29] manufactured single-phase SrTaO2N/BaTaO2N through multi-step intermediate reactions starting from Ta3N5 and SrCO3/BaCO3 mixture. Unfortunately, these above-mentioned methods still require fabrication of Ta3N5 or TaON from the beginning. In comparison, it is noteworthy that urea can be used as nitriding agent instead of ammonia gas (so called “urea route”). Gomathi et al. [30] heated the mixture of BaCO3/SrCO3, Ta2O5 and urea to obtain crystalline SrTaO2N and BaTaO2N in conventional furnace (950ºC, 3 h, N2 atmosphere). As urea usually 3

Journal Pre-proof decomposes at a relatively low temperature which is far below the required nitridation temperature, it is still challenging to precisely control the reaction progress and the purity of the oxynitride products. Spark plasma sintering (SPS), similar to hot pressing, is a more advanced technique for fast densification of materials. Both Joule heating effect and field effect give rise to efficient heat transfer and homogeneous temperature distribution [7,31,32]. Modifications of regular SPS apparatus into a pressureless sintering mode can realize rapid consolidation of bulk materials with complex geometry or highly-porous structure. It has been

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successfully applied in densifying ceramics such as α-sialon [33], zirconia [34], hydroxyapatite [35], SiC-C

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composites [36], porous silicon nitride ceramics [37], and gradient silicon carbide foams [38]. Similarly, synthesis

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of ceramic powders using pressureless SPS machine may also exhibit advantages including shorter processing

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duration and lower cost. However, there is few research related to this topic so far.

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In the present work, we designed a novel synthesis strategy, i.e. urea route combined with pressureless SPS for fast fabricating perovskite-type tantalum-based oxynitrides. Ceramic powders of SrTaO2N and BaTaO2N with

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nanosized grains were successfully synthesized within 10 min, which consumed much shorter time than any other

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reported method. With optimization, the oxynitride phase purity was as good as coventionally obtained products. Finally, the thermal stability, corrosion resistance and dielectric property of the oxynitride powders were carefully examined. To the best of our knowledge, this is the first report on the minute-scale synthesis of perovskite-type oxynitrides with resulting high purity and excellent performance.

2. Material and Methods

2.1 Synthesis process SrTaO2N and BaTaO2N oxynitrides were synthesized by urea route and pressureless SPS. Typically, high-purity powders of SrCO3/BaCO3 (99.99%, Shanghai ST-NANO Co., Ltd.) and Ta2O5 (99.99%, Shanghai ST-NANO Co., Ltd.) with excess urea (99.999%, Shanghai Aladdin Bio-Chem Technology Co., Ltd.) in a molar ratio of 4

Journal Pre-proof SrCO3/BaCO3:Ta2O5:urea=2:1:x were mixed in anhydrous ethanol, followed by ball milling for 4h. Then the mixture was dried in an oven overnight and well ground in an agate mortar. Calcination of the mixture was carried out in a modified SPS set-up (SPS-3.20 mkII, Sumitomo Coal Mining, Tokyo, Japan) filled with nitrogen gas. A standard synthesis cycle was designed as follows: roughly 10 g of powder mixture was loaded into an alumina crucible which was placed inside a covered cylindrical graphite crucible with an inner diameter of 40 mm and an outer diameter of 50 mm (see the schematic diagram in Figure S1). The pressureless condition was built to ensure

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fast heating the crucible as well as the powders without applying mechanical pressure. The temperature was

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automatically raised to 600°C over a period of 3 min, and from there onwards it was monitored and regulated by

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an optical pyrometer focused centrally on the outer surface of the graphite crucible. Various urea contents,

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calcination temperatures, heating rates and dwelling times were tested under 0.7 atm nitrogen atmosphere to

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obtain oxynitride products with a better purity (see Table S2 & Figure S2). The products prepared under optimised condition were named as STON-2 and BTON-2 (see Table 1). To discover the advantages of pressureless SPS -1

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set-up, the oxynitrides were also fabricated using a conventional furnace with a heating rate of 10°C·min and

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the samples were labeled as STON-1 and BTON-1. After firing, all the products were naturally cooled down to room temperature and collected for analysis.

2.2 Characterization

Thermodynamics of the possible reactions were calculated by a commercial software (HSC Chemistry 6.0, Outokumpu Research Oy., Pori, Finland).

/

values of the novel compounds (SrTaO2N, BaTaO2N,

Sr2Ta2O7, Ba5Ta4O15, Ta3N5 and TaON) were estimated to be -1119.1/115.1, -1193.6/162.2, -3360.2/182.7, -3321.6/1498.8, -1261.5/184.8 and -464.0/155.0 (Units: kJ·mol-1/J·mol-1·K-1), respectively. Phase composition was determined by a powder X-ray diffractometer (XRD, D8 Advance, Bruker/Axs Corp., Germany), using CuKα radiation over a 2θ range of 10~80°. For Rietveld refinement, a fine scan mode was used from 10° to 100° with a 5

Journal Pre-proof step size of 0.02°and a counting time of 2.0 s per step. The phase percentage of the products was calculated by using TOPAS 4.2 software package. A combustion analyzer (LECO TC500, LECO Corp., USA) was employed to check the O/N/C content. UV-Visible diffuse reflectance spectra of the samples was collected by the UV-2600 (Shimadzu Corporation, Japan) spectrometer operating in the range of 200~800 nm. Morphology of the oxynitride samples was investigated by a scanning electron microscope (SEM, Tescan MIRA3, Tescan Ltd., Czech Republic) and a transmission electron microscope (TEM, Technai G2 F20 S-TWIN 200kV, FEI Corp., USA) equipped with

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an energy-dispersive spectroscopy (EDS) detector. EDS mapping of Sr/Ba/Ta/O/N elements were done in

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HAADF-STEM mode and contents of the Ba/Sr/Ta elements were quantitatively evaluated.

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The thermal stability of the synthesized oxynitrides was examined by thermal gravimetric and differential

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scanning calorimetry device (TG-DSC, DSC204F1, Netzsch, Germany) under flowing air, with a heating rate of

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10°C·min-1 from room temperature up to 1300°C. The chemical stability of the oxynitrides were assessed by studying the phase composition and microstructure changes with immersing the powders in water (80°C), H2SO4

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(pH=1) and NaOH (pH=12) solution for 12 h, respectively. The dielectric behavior in terms of relative

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permittivity and dielectric loss were measured using a dielectric spectrum measurement system (Partulab VDMS-2000) with a WK6500 impedance analyzer over the frequency range from 102 to 106 Hz at a temperature range of -10~200°C. To form the electrode contacts, the oxynitride powders were well packed, uniaxially pressed into pellets, heated in nitrogen at 700°C for 2h and covered by Ag-pasted electrodes on both sides.

3. Results and discussion

3.1 Thermodynamic analysis Thermodynamic calculations were conducted to predict feasibility of the urea route as well as the side reactions and possible by-products. Prior to calculation, all the assumed reactions involved are listed as below: 2SrCO3(s)+Ta2O5(s)+CO(NH2)2(s)→2SrTaO2N(s)+3CO2(g)+2H2O(l) 6

(1)

Journal Pre-proof 2BaCO3(s)+Ta2O5(s)+CO(NH2)2(s)→2BaTaO2N(s)+3CO2(g)+2H2O(l)

(2)

3CO(NH2)2(s)→4NH3(g)+3CO(g)+N2(g)

(3)

2SrCO3(s)+Ta2O5(s)→Sr2Ta2O7(s)+2CO2(g)

(4)

5BaCO3(s)+2Ta2O5(s)→Ba5Ta4O15(s)+5CO2(g)

(5)

Sr2Ta2O7(s)+2NH3(g)→2SrTaO2N(s)+3H2O(g)

(6)

Ba5Ta4O15(s)+4NH3(g)→4BaTaO2N(s)+6H2O(g)+BaO(s)

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SrCO3(s)→SrO(s)+CO2(g)

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BaCO3(s)→BaO(s)+CO2(g)

(8) (9) (10)

Ta3N5(s)+3CO2(g)→3TaON(s)+3CO(g)+N2(g)

(11)

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3Ta2O5(s)+10NH3(g)→2Ta3N5(s)+15H2O(g)

(7)

(12)

TaON(s)+BaO(s)→BaTaO2N(s)

(13)

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TaON(s)+SrO(s)→SrTaO2N(s)

(14)

Ta2O5(s)+9CO(g)→2TaC(s)+7CO2(g)

(15)

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2TaON(s)+3CO2(g)→Ta2O5(s)+3CO(g)+N2(g)

Ta2O5(s)+3CO(g)→2TaO(g)+3CO2(g)

(16)

3Ta2O5(s)+10NH3(g)→6TaN(s)+15H2O(g)+2N2(g)

(17)

Among all, reactions 1 & 2 are the overall reactions for producing SrTaO2N and BaTaO2N, respectively [30]. Reactions 3~9 describe their corresponding stepwise occurrence according to Ref. [18,24,30,39], while the side reactions can probably take place according to reactions 10~17 [25,27,29]. It should be mentioned that the decomposition of urea is quite complex as discussed in Ref. [40] and thus we simplify the final reaction as reaction 3 to indicate the partial loss of nitrogen source at <700°C. When it comes to the overall reactions 1 & 2, it is noticeable that the by-products are ideally only CO2 and H2O which can be easily removed. Previous investigations [22,23,41] have revealed a two-step formation of the oxynitrides: the carbonates reacted with Ta2O5 7

Journal Pre-proof to form the binary oxides (reactions 4 & 5), followed by ammonolysis with NH3 (reactions 6 & 7) which is produced from the decomposition of urea (reaction 3). Simultaneously, decomposition of the carbonates into their corresponding monoxides (reactions 8 & 9) together with the interactions between Ta2O5 and gas mixture (CO2, NH3 & CO) may give rise to the solid-state by-products BaO, SrO, Ta3N5, TaON, TaC and TaN (reactions 10~17) . For verification, the HSC Chemistry software was employed. All the thermodynamic values of standard enthalpy of formation (

, kJ·mol-1) and standard molar entropy (

, J·mol-1·K-1) were taken from the

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database of the software. The Gibbs free energy (ΔG) values as a function of temperature were given

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automatically. Unfortunately, due to a lack of data regarding some novel compounds (SrTaO2N, BaTaO2N,

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Sr2Ta2O7, Ba5Ta4O15, Ta3N5 and TaON), reactions involving these substances were analyzed manually by using the /

values, where Tmin is the minimal

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thermodynamic equation Tmin=  r H m /  r Sm with the estimated

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temperature required for activating the reaction. The corresponding results were then shown in Figure 1 and Table S1. Some implications can be obtained: firstly, it shows that reactions (1) & (2) can occur at above 1204°C and

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781°C, respectively. The feasibility of the urea route is thus indicated from thermodynamic point of view.

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Secondly, in reactions 4 & 6, formation of Sr2Ta2O7 happens at 1013°C and the further ammonolysis requests a temperature of >1139°C. Thirdly, urea tends to encounter dissociation into NH3 at >200°C (reaction 3). As this value is far away from the onset temperatures for reactions 1, 2 & 6, kinetics should be considered for the sake of prohibiting or delaying the decomposition of urea and release of NH3. Finally, reactions 8~12 and 17 have reasonable onset temperature values (1200°C, 1500°C, 203°C, 1424°C, 388°C and 1000°C) while it is quite difficult to stimulate reactions 13~16 at below 2000°C. On this account, impurity phases of Sr2Ta2O7, Ba5Ta4O15, SrO, Ta3N5 and TaN are most likely to be generated at below 1300°C.

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Figure 1. ΔG-T curves of thermodynamic calculations for the possible reactions. These reactions involved some , kJ·mol-1) and standard molar entropy (

, J·mol-1·K-1)

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substances that have their enthalpy of formation (

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values appeared in the database of HSC Chemistry software. All the results were directly given by the software

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value was less than zero.

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over the temperature range of 0~2000°C. The reaction can be judged to take place if the Gibbs free energy (ΔG)

Based on the above analysis, the overall synthesis conditions for SrTaO2N and BaTaO2N oxynitrides can be predicted as below: i) excessive urea is helpful for providing abundant ammonia and increasing the yield; ii) it is necessary to apply high heating rates for dynamically delaying the earlier decomposition of urea and the formation of other by-products from urea; and iii) a synthesis temperature range of 800~1200°C is suitable, which will not only facilitate the occurrence of the overall reactions but also benefit the creation of less impurities.

3.2 Rapid synthesis of tantalum-based oxynitrides The processing parameters such as urea content, heating rate, calcination temperature and dwelling time played an important role in affecting the purity of the products during the pressuless calcination process. The optimisation 9

Journal Pre-proof was conducted based on abundant trials (see Table S2 & Figure S2). A finally decent combination of these parameters was x=8 (urea content), 1000°C (calcination temperature), 300°C·min-1 (heating rate) and 1min (dwell time), which was displayed in Table 1 as samples STON-2 & BTON-2. Meanwhile, STON-1 & BTON-1 made by conventional pressureless furnace with a much lower heating rate (10°C·min-1) and longer dwell time (60 min) were also compared.

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Table 1 Synthesis parameters, phase composition and chemical composition of the oxynitride products. Synthesis parameters Dwell time

(°C·min-1)

(min)

10

60

Phase composition

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STON-1

Heating rate

-

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Products

Chemical composition

a

Sr(Ba)/Tab

N/Oc

C(wt%)c

-

-

-

0.93

0.38

0.105

-

-

-

0.86

0.33

0.093

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SrTaO2N: 96.9wt% Sr2Ta2O7: 3.1wt%

STON-2

300

1

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(Rp=6.62%, Rwp=8.84%,

BTON-2

a

300

60

BaTaO2N: 97.8wt% Ba5Ta4O15: 2.2wt%

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(Rp=7.22%, Rwp=9.47%, Rewp=4.08%, GOF=2.32)

Calculated based on the Rietveld refinement of the XRD results.

b c

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BTON-1

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Rewp=3.44%, GOF=2.57)

Based on the EDS results (all in molar ratio).

Based on the element analysis results (all in molar ratio).

Figure 2 listed the rough scan of XRD patterns for all the synthesized oxynitides. Remarkably, samples STON-2 and BTON-2 have shown standard diffraction peaks of the oxynitride phases. The existence of their corresponding oxides (Sr2Ta2O7 and Ba5Ta4O15) was also detected, and the low peak intensity revealed a minor 10

Journal Pre-proof concentration. Further Rietveld refinement results (see Figure 3) confirmed the co-existence of 96.9wt% SrTaO2N (space group=I4/mcm, ICSD #411137) and 3.1wt% Sr2Ta2O7 (space group=Cmcm, ICSD_#601) for STON-2 as —

well as 97.8wt% BaTaO2N (space group=Pm3m, ICSD_#62619) and 2.2wt% Ba5Ta4O15 (space group=P4/mbm, ICSD_#16028) for BTON-2. Phase composition of the conventionally heated samples STON-1 & BTON-1 was complex (see Figure 2). The characteristic peaks were identified as Sr2Ta2O7, Ba5Ta4O15 and Ta3N5 with almost no oxynitride phases. This difference can be also proved by their SEM images displayed in Figure 4. The

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morphology of STON-1 powder product was anomalous while STON-2 was more regular with appearing

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uniformly spherical or polyhedral shape. This phenomenon is more obvious for crystals of BTON-2 which seemed

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like cubes while the BTON-1 displayed irregular morphology. The high homogeneity, purity and crystallinity of

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STON-2 & BTON-2 samples were indicated. TEM analysis results are more convincing, see Figure 5. From the

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low magnification images (Figure 5a & 5c), the crystals with an estimated size range of 50~150 nm were visible, implying the size of the synthesized oxynitride powders was in nano-scale. The high-resolution micrographs

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shown in Figure 5b & 5d illustrated distinct lattice fringes and diffraction spots (see the inset FFT images).

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Indexing of the fringes revealed the (020) & (112) planes for STON-2 and (101) & (110) planes for BTON-2. To more accurately evaluate the element content of the products, EDS and combustion method were applied for samples STON-2 & BTON-2 and the data was shown in Table 1. The molar ratio of Sr/Ta=0.93 or Ba/Ta=0.86 was close to stoichiometry. In terms of N/O ratio, values of 0.38 and 0.33 were obtained for STON-2 and BTON-2 respectively. They were still less than the theoretical one (0.5). Also, carbon content was only about 0.1wt% for both samples, which indicated a very limited carbon contamination. Considering the phase composition analysed in Figure 2, these results are reasonable due to the fact that impurities of Ba5Ta4O15 and Sr2Ta2O7 possessed much lower N/O ratio than the targeted oxynitrides. In addition, partial nitrogen loss may also contribute to the N/O composition deviation as reported that nitrogen in oxynitrides was easily lost in the heat treatment at above 1000°C under Ar or N2 atmosphere [25, 42, 43]. 11

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Figure 2. XRD patterns of the synthesized oxynitides. Samples STON-1 and BTON-1 with a slow heating rate

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resulted in mass impurity phases including Sr2Ta2O7, Ba5Ta4O15 and Ta3N5. STON-2 and BTON-2 obtained by optimisation displayed distinct diffraction peaks of SrTaO2N and BaTaO2N phases. The presence of minor oxide

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impurities for STON-2 and BTON-2 was also found.

Figure 3. Rietveld refinement patterns for XRD patterns of (a) STON-2 and (b) BTON-2. The blue vertical lines

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Journal Pre-proof denote the standard Bragg positions of SrTaO2N (space group=I4/mcm, ICSD #411137) and Sr2Ta2O7 (space —

group=Cmcm, ICSD_#601) in Fig. 3a, and BaTaO2N (space group=Pm3m, ICSD_#62619) and Ba5Ta4O15 (space group=P4/mbm, ICSD_#16028) in Fig. 3b. The inserted crystal structures represent tetragonal SrTaO2N and cubic

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BaTaO2N respectively.

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Figure 4. SEM images of the synthesized oxynitides (a) STON-1, (b) STON-2, (c) BTON-1 and (d) BTON-2.

shapes.

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The morphology of STON-1 & BTON-1 was anomalous while STON-2 & BTON-2 exhibited more regular

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Journal Pre-proof Figure 5. TEM images of the synthesized oxynitides (a)(b) STON-2 and (c)(d) BTON-2. The evident lattice fringes were indexed to (020) & (112) planes for STON-2 and (101) & (110) planes for BTON-2. A good crystallinity with a crystal size of 50~150 nm was also discovered.

The above-mentioned results have suggested the tantalum-based oxynitrides with a high purity and nano-scale crystal size can be achieved by pressureless SPS set-up. The entire processing time needed (exclusive of cooling)

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was only 6 min, which is much shorter than any other calcination method reported so far. The conventional

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pressureless furnace with a much slower heating rate, however, would result in a poor quality of the products. As

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discussed in thermodynamic analysis, urea suffered decomposition into ammonia when temperature went up to

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more than 200°C and caused a big loss of nitrogen source [30]. Kinetically increasing the heating rate could

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“jump” this stage and retain nitrogen. Thus, the advantages of the synthesis approach used in this study are: i) a high heating rate is available to keep nitrogen source; and ii) a much shorter dwell time is available to minimize

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the decomposition of the oxynitrides as well as to suppress the crystal size.

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3.3 Thermal stability, water/acid/alkali resistance and dielectric property The performances of the oxynitrides, including thermal stability, water/acid/alkali resistance and dielectric property, were investigated based on the synthesized oxynitrides STON-2 and BTON-2. Thermal stability was evaluated by calorimetry from room temperature to 1300°C in air. Their corresponding TG and DSC curves were plotted in Figure 6. In terms of STON-2 (Figure 6a), three stages of weight and energy change can be distinguished. During the first stage (<475°C), a slight weight fluctuation was visible mainly due to the evaporation of physically and chemically bonded moisture. Then at 475~705°C there was a significant mass gain of 3.2wt%. The concurrent heat release process was revealed by the DSC curve. This can be explained by the total oxidation (replacing nitrogen atoms by oxygen) of SrTaO2N to its corresponding oxide form (Sr2Ta2O7, or 14

Journal Pre-proof SrTaO3.5) according to the fact that the theoretical conversion would bring about ~3.17% weight increase, which is in accordance with the measured value. The final stage (> 705°C) witnessed a gradual mass loss of the oxide as an exothermic process which took place in two steps, i.e., at 705~1029°C a gentle decline and at >1029°C a relative drastic decrease. It can be ascribed to the dissociation of the oxide at elevated temperatures and the temperature controlled the kinetics. With regard to BTON-2 (Figure 6b), the TG/DSC curves are similar to those of STON-2 and their thermal behaviors are alike. The sample enjoyed an evident weight increase since 605°C, kept relatively

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steady at 726~948°C and encountered a sudden drop afterwards. The weight loss of ~0.5% at around 400ºC can be

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attributable to the pyrolysis of some organic intermediate by-products according to Ref. [40]. It is noteworthy that

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the starting temperature for oxidation of BaTaO2N was much higher (605°C). Hence, the investigation suggests

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that these oxynitrides are stable in air up to 475°C for SrTaO2N and 605°C for BaTaO2N.

Figure 6. TG-DSC curves of the synthesized oxynitides (a) STON-2 and (b) BTON-2. Generally three stages for weight changes can be found: obvious growth, slight decline and evident drop. Each downward DSC peak indicated an exothermic process. 15

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Corrosion resistance ability test was performed by immersing and stirring the oxynitride powders in hot water (80ºC), acid (pH=1) and alkali (pH=10) for 12 h, respectively. The optical observation revealed no distinct color change of the solutions or immersed samples. XRD and UV-Vis diffuse reflectance tests were conducted to study the composition/structure change and the results were displayed in Figure 7. Surprisingly, the samples kept almost the same shape and position of the characteristic XRD peaks for original STON-2 and BTON-2 powders without

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the appearance of any other noticeable substances. Also, the UV-Vis diffuse reflectance spectra indicated almost

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the same reflection curves for all the samples individually. With measurement, the weight change of the oxynitride

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powders before and after corrosion test was negligible. Further check by optical and SEM images also implied no

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observable differences from the as synthesized powders (see Fig. 8). It should be noticed that the colors of the

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powders were different from those of standard SrTaO2N and BaTaO2N. The reasons can be attributed to the non-stoichiometric O/N ratio especially a lower N content [25, 29] as well as different crystal size and aggregation

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profile of the oxynitrides. Hence, a good resistance ability of the tantalum-based oxynitrides with regard to hot

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water, strong acid and strong alkali can be then deduced. The reason is that the introduction of nitrogen into the crystal structure made the oxynitrides inherit performances both from perovskite oxides and nitrides. The perovskite oxides are normally resistant to water and strong alkali while the nitrides are stable in strong acid. In addition, formation of an oxide-rich surface around the oxynitride crystallite also contributed to their corrosion stability [1].

16

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Figure 7. (a) XRD patterns and (b) UV-Vis diffuse reflectance spectra of the synthesized oxynitrides before and

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after corrosion tests in water/acid/alkali. No noticeable phase composition change was observed compared with

individually.

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Fig.2. The UV-Vis diffuse reflectance spectra also indicated almost the same reflection curves for all the samples

17

Journal Pre-proof Figure 8. Optical and SEM images of the synthesized oxynitrides (a) STON-2 and (b) BTON-2 after corrosion tests in water/acid/alkali. The tested samples were in powder form. No noticeable appearance/microstructure variations were observed compared with Fig.4.

Dielectric property of the synthesized oxynitrides was evaluated in terms of relative permittivity (εr) and dielectric loss (tanδ). Figure 9a illustrated the dielectric curves of STON-2 & BTON-2 measured at 25°C over the

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frequency of 100Hz~1MHz. It can be seen that the relative permittivity decreased with increasing frequency while

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the trend for dielectric loss was opposite. For STON-2, the permittivity was over 1000 at < 600 Hz. The maximal

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permittivity was 3141 at 100 Hz with a loss of 0.017, while at 1 MHz the corresponding values were 51.9 and

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1.16 respectively. A much higher permittivity and lower dielectric loss was observed for BTON-2, with εr being up

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to 9550 and tanδ down to 0.001 at 100 Hz. Its permittivity kept above 1000 in the entire measured frequency range and the minimal value was about 1070 at 1 MHz. The influence of the minor oxide impurities on the

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dielectric property could be very limited according to the logarithmic model [44, 45]. Figure 9b displayed the

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relative permittivity of sample BTON-2 as a function of temperature at selected frequencies. Clearly, all the εr values encountered approximately linear increase with elevated temperature while the maximum can reach up to 12269 at 200°C/100Hz. Nevertheless, a moderate permittivity dependence on temperature was suggested owing to the slight growth of εr.

In comparison, Kim et al. [5] prepared SrTaO2N and BaTaO2N oxynitrides by ammonolysis of their corresponding binary metal oxides, and the resulted room-temperature dielectric permittivity values were ~2900 and ~4900 respectively. Sun et al. [25] directly synthesized of SrTaO2N powders from SrCO3 and Ta3N5. With pressureless sintering, the oxynitride bulks possessed a room-temperature dielectric constant of 30~300 (RD=56.5~82.8%) at 100 Hz. Hosono et al. [41] also adopted ammonolysis method to fabricate SrTaO2N and the measured εr value was ~120 at 100 Hz (room temperature, RD=71.1%). It can be found that the dielectric values 18

Journal Pre-proof of STON-2 and BTON-2 in this work was much higher than those of conventionally manufactured oxynitrides and even comparable with the typical BaTiO3 dielectric or PbTiO3-type ferroelectrics at their Curie temperatures

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[46-48].

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Figure 9. Dielectric property of the synthesized oxynitrides in powder compact form: (a) relative permittivity and

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dielectric loss of STON-2 and BTON-2 over 100Hz to 1MHz at 25°C; (b) relative permittivity of BTON-2 over the temperature range of -10~200°C at 100Hz, 1kHz, 10kHz, 100kHz and 1MHz.

Previous studies have revealed the very large relative permittivity values of perovskite-type tantalum oxynitrides were closely associated with O/N anion ordering and displacements of Ta5+ accompanied by TaO4N2 octahedral tilting [1,2]. The ABO2N-type oxynitride compounds have exhibited two possible anion ordering configurations in a BO4N2 octahedron: cis-type (the two N ions occupy adjacent sites) and trans-type (the two N ions occupy opposite sites) [14,49]. Both of the cis- and trans- configurations were predicted to form polar octahedral units [1]. SrTaO2N with a tetragonal structure has been confirmed to display cis-type from the energetically stable point of view [14,50]. Introduction of nitrogen in the octahedral may induce displacements of 19

Journal Pre-proof Ta5+ ions and structural arrangements that possessed a local dipole [1], which would give rise to high dielectric permittivity under an applied electric field. The case in barium tantalum oxynitride is more complicated. Although BaTaO2N owns a cubic crystal structure, the local symmetry in the crystal could be lower as suggested by first-principles calculations [51,52]. Hence, different Ta-O and Ta-N bond lengths were still formed, supported by the argument that the cubic space group represents only the statistical average of a random distribution of anions [52]. Meanwhile, we believe that the phase purity and distribution of oxynitrides is also important for their high

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permittivity values. Figure 10 showed the EDS mapping of Sr/Ba/Ta/O/N elements obtained in HAADF-STEM

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mode. It can be clearly seen that these elements were uniformly distributed in the crystals, indicating a good

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homogeneity from the microscopic perspective. Furthermore, considering the low packing density (<50% TD) of

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the specimens during dielectric measurements, there should be substantial contribution of grain boundary [1,2].

Figure 10. STEM-EDS elemental maps of the synthesized oxynitrides (a) STON-2 and (b) BTON-2. The size of the studied crystals is less than 500 nm. It can be apparently seen that the Sr/Ba/Ta/O/N elements were homogeneously distributed.

On the other hand, dielectric loss for both compounds was large at higher frequency range. It was found that for BTON-2 the dielectric loss was less than 0.01 at 25°C below 1 kHz, but surprisingly, at 1 MHz the tanδ values of both compounds were in the order of 0.7~1.2. To explain it, electric conductivity of the specimens was measured 20

Journal Pre-proof and the average value was 1.076×10-5 S·cm-1 (25°C). Thus, the high dielectric loss at higher frequency range can be generally attributed to electrical conduction within the crystals [1,5,46] and anion deficiencies due to nitrogen loss [2,46]. The carbon contamination of the samples can contribute to the electrical conduction (see Table 1) but the influence is limited due to a very low content (<1wt%). Besides, the low packing density of the specimens undoubtedly provided a significant external contribution to the dielectric loss [1]. It is expected that further

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consolidation of the oxynitrides to denser bulks would significantly improve their dielectric performance.

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4. Conclusions

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In summary, a smart synthesis approach that combined urea route with pressureless SPS was designed for rapid

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fabrication of SrTaO2N and BaTaO2N ceramic powders. Thermodynamic analysis predicted that excessive urea was helpful for providing abundant ammonia and increasing the yield. To prevent urea from earlier decomposition,

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it is necessary to dynamically apply high heating rates. SrTaO2N and BaTaO2N powders were synthesized within

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10 min. With optimization of the processing parameters, the purity of the oxynitride phase can achieve up to

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96.9% for SrTaO2N and 97.8% for BaTaO2N. They both possessed a crystal size of 50~150 nm. The synthesized SrTaO2N can withstand a temperature of less than 475°C in air while BaTaO2N was more stable at below 605°C. Meanwhile, the oxynitrides suffered no optical or structural change with immersing in hot water/strong acid/strong alkali. At 25°C/100Hz, SrTaO2N possessed a relative permittivity of 3141 with a dielectric loss of 0.017, while BaTaO2N had a much higher permittivity (9550) and a lower loss (0.001). A small permittivity dependence on temperature for BaTaO2N was found. The O/N ordering in cis configuration together with the good oxynitrides phase purity and distribution contributed to their high permittivity. The large dielectric loss values at higher frequency range can be attributed to the presence of anion deficiencies and low packing density of the oxynitride bulks. The excellent overall performances of the fabricated oxynitrides together with the efficient synthesis method make it promising for the large-scale production and practical application in energy storage 21

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Acknowledgements

We gratefully acknowledged the financial support from the National Natural Science Foundation of China (Grant No.51702361).

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Data availability statement

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The data supporting the findings of this study will be made available on request.

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[1]

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Figure Captions Figure 1. ΔG-T curves of thermodynamic calculations for the possible reactions. These reactions involved some substances that have their enthalpy of formation ( entropy (

, kJ·mol-1) and standard molar

, J·mol-1·K-1) values appeared in the database of HSC Chemistry software. All the results

were directly given by the software over the temperature range of 0~2000°C. The reaction can be

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judged to take place if the Gibbs free energy (ΔG) value was less than zero.

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Figure 2. XRD patterns of the synthesized oxynitides. Samples STON-1 and BTON-1 with a slow

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heating rate resulted in mass impurity phases including Sr2Ta2O7, Ba5Ta4O15 and Ta3N5. STON-2 and BTON-2 obtained by optimisation displayed distinct diffraction peaks of SrTaO2N and

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BaTaO2N phases. The presence of minor oxide impurities for STON-2 and BTON-2 was also

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found.

Figure 3. Rietveld refinement patterns for XRD patterns of (a) STON-2 and (b) BTON-2. The blue

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vertical lines denote the standard Bragg positions of SrTaO2N (space group=I4/mcm, ICSD #411137) and Sr2Ta2O7 (space group=Cmcm, ICSD_#601) in Fig. 3a, and BaTaO2N (space —

group=Pm3m, ICSD_#62619) and Ba5Ta4O15 (space group=P4/mbm, ICSD_#16028) in Fig. 3b. The inserted crystal structures represent tetragonal SrTaO2N and cubic BaTaO2N respectively.

Figure 4. SEM images of the synthesized oxynitides (a) STON-1, (b) STON-2, (c) BTON-1 and (d) BTON-2. The morphology of STON-1 & BTON-1 was anomalous while STON-2 & BTON-2 exhibited more regular shapes.

Figure 5. TEM images of the synthesized oxynitides (a)(b) STON-2 and (c)(d) BTON-2. The 27

Journal Pre-proof evident lattice fringes were indexed to (020) & (112) planes for STON-2 and (101) & (110) planes for BTON-2. A good crystallinity with a crystal size of 50~150 nm was also discovered.

Figure 6. TG-DSC curves of the synthesized oxynitides (a) STON-2 and (b) BTON-2. Generally three stages for weight changes can be found: obvious growth, slight decline and evident drop. Each

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downward DSC peak indicated an exothermic process.

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Figure 7. (a) XRD patterns and (b) UV-Vis diffuse reflectance spectra of the synthesized oxynitrides before and after corrosion tests in water/acid/alkali. No noticeable phase composition

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change was observed compared with Fig.2. The UV-Vis diffuse reflectance spectra also indicated

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almost the same reflection curves for all the samples individually.

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Figure 8. Optical and SEM images of the synthesized oxynitrides (a) STON-2 and (b) BTON-2 after corrosion tests in water/acid/alkali. The tested samples were in powder form. No noticeable

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appearance/microstructure variations were observed compared with Fig.4.

Figure 9. Dielectric property of the synthesized oxynitrides in powder compact form: (a) relative permittivity and dielectric loss of STON-2 and BTON-2 over 100Hz to 1MHz at 25°C; (b) relative permittivity of BTON-2 over the temperature range of -10~200°C at 100Hz, 1kHz, 10kHz, 100kHz and 1MHz.

Figure 10. STEM-EDS elemental maps of the synthesized oxynitrides (a) STON-2 and (b) BTON-2. The size of the studied crystals is less than 500 nm. It can be apparently seen that the Sr/Ba/Ta/O/N elements were homogeneously distributed. 28

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Tables

Table 1 Synthesis parameters, phase composition and chemical composition of the oxynitride products. Synthesis parameters Products

Heating rate -1

STON-1

Chemical composition Phase compositiona

Dwell time

(°C·min )

(min)

10

60

-

Sr(Ba)/Tab

N/Oc

C(wt%)c

-

-

-

0.93

0.38

0.105

-

-

-

0.86

0.33

0.093

SrTaO2N: 96.9wt% Sr2Ta2O7: 3.1wt% 300

1

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STON-2

(Rp=6.62%, Rwp=8.84%,

10

60

-

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BTON-1

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Rewp=3.44%, GOF=2.57)

BaTaO2N: 97.8wt% 300

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Ba5Ta4O15: 2.2wt%

BTON-2

1

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(Rp=7.22%, Rwp=9.47%,

Rewp=4.08%, GOF=2.32)

b c

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Calculated based on the Rietveld refinement of the XRD results. Based on the EDS results (all in molar ratio).

Based on the element analysis results (all in molar ratio).

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a

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Graphical abstract

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Highlights

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 Urea route combined with pressureless SPS approach was adopted for fast

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synthesizing ATaO2N (A=Sr, Ba) within 10 min.

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 The obtained SrTaO2N and BaTaO2N powders had a high oxynitride phase content of 96.9% and 97.8% respectively.

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 SrTaO2N and BaTaO2N are chemically stable in water/acid/alkali with suffering no optical or structural change.  The permittivity of BaTaO2N was up to 9550 with a dielectric loss of 0.001 at 100 Hz/25°C.

30