Rapid synthesis,structure,and magnetic properties of the non-stoichiometric oxynitride EuTaO 1.75 N 1.25

Introduction

Perovskite oxynitrides ABO_3−xN _x (A = rare earth, alkaline earth or alkali metal; B = transitional metal; 0 < x < 3) have shown novel electric properties [1], high permittivity [2,3], magnetic properties [4], and photocatalytic activity [5]. They commonly crystallize in cubic structure with distortions. The substitution of oxygen ions by nitrogen ions strongly alters the properties of perovskites [6]. Many perovskite oxynitrides can be fabricated by calcinating the mixtures of regents or oxide precursors in ammonia gas flow at high temperature [7]. Novel approaches for low-cost, low-temperature, and/or time-saving preparation of the perovskite oxynitrides were reported [8].

Europium tantalum oxynitride compounds have been particularly investigated. Pastrana et al. [9] reported magnetic properties of the perovskite EuWO_1.58N_1.42 phase. Yang et al. [10] found the excellent flexibility of the electron and giant magnetoresistance effects in EuWO_1+xN_2−x, while changing the ratio of anions can adjust Eu and W oxidation states and hole/electron doping.

In this study, we focused on the non-stoichiometric oxynitride EuTaO_1.75N_1.25. Band structural of EuTaO_1.75N_1.25 were calculated by first-principles calculations. A urea route together with pressureless spark plasma sintering equipment was adopted for rapid synthesis. The non-stoichiometric oxynitride with a formula of EuTaO_1.75N_1.25 was obtained. The magnetic properties of EuTaO_1.75N_1.25 were investigated.

Experimental

The density functional theory (DFT) calculations were performed using the Vienna ab-initio simulation package (VASP). The projector-augmented wave (PAW) method was used to represent the core-valence interaction. The plane wave energy cut-off was set to 500 eV. The generalised gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional were used in our calculations. The first Brillion zone k-point sampling utilises the Monkhorst–Pack scheme with an automated mesh determined by 20 times of the reciprocal lattice vectors. The energy and force criterion for convergence of the electron density are set at 10–5 eV and 0.02 eV Å⁻¹, respectively.

Powders of Eu₂O₃ and Ta₂O₅ with urea were mixed with anhydrous ethanol. Then the mixture was dried in an oven and well ground. The dry regent was calcined in a nitrogen atmosphere using a modified pressureless SPS equipment (SPS-3.20 mk II, Sumitomo Coal Mining, Tokyo, Japan). The temperature was raised to 600°C over 3 min, then to 1100°C at 300°C min⁻¹ and kept for 1 min. The products were collected after the furnace was cooled to room temperature. First-principles calculations were performed by using the Vienna Ab initio Simulation Package (VASP) to investigate the electronic properties of EuTaO_1.75N_1.25.

The O/N content was determined by a combustion analyser (LECO TC500, LECO Corp., USA). The phase composition was determined by X-ray diffraction (XRD, D8 Advance, Brucker/Axs Corp., Germany), collecting data at 0.02° steps over the 2θ range of 5-110° with a counting time of 1.0 s per step. Rietveld refinement was done with the Full-prof program package. Morphology of the product was observed with scanning electron microscope (SEM, Tescan MIRA3, Tescan Ltd., Czech Republic) and transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN 200 kV, FEI Corp., USA). Thermal stability was tested on a thermogravimetric and differential scanning calorimetry (TG-DSC, DSC204F1, Netzsch, Germany) in an oxygen flow. Magnetic susceptibility was measured by a Quantum Design XL-SQUID magnetometer over 2–300 K at 0.1 T. Magnetisation at different temperatures were measured in the magnetic field range of −6 to 6 T.

Results and discussion

The calculated DOS (Density of States) and band structure of EuTaO_1.75N_1.25 was shown in Figure 1. The results revealed a split in spin polarisation of EuTaO_1.75N_1.25, which was caused by electronic of Eu−4f. The Energy level of spin-lower electronic was obviously higher than that of spin-upper electronic, which indicated that spin-lower state may be unstable. Also, the number of electronic calculated in two cases were 22 (spin upper) and 16 (spin lower), respectively. Therefore, the result of spin-upper state was more convincible. In spin-upper state (Figure 1(c)), the calculated band gap of EuTaO_1.75N_1.25 was 1.21 eV, which was significantly lower than that of EuTaO₄ (3.66 eV) [11]. The bottom of the conduction band (CBM) at G point and the top of the valence band (CVM) at G point indicated a direct bandgap for EuTaO_1.75N_1.25 oxynitride. OPEN IN VIEWER

Figure 2 showed the Rietveld refinement results of the XRD pattern with randomly distributed O and N. More details were listed in Table 1. The oxynitride crystallized in a cubic structure with a space group . Compared with considering Eu³⁺ (1.06 Å, CN:8), the experimental bond distance (2.848 Å) for Eu−(O|N) is closer to the sum of the ionic radii of Eu²⁺ (1.25 Å, CN:8) and O²⁻ (1.38 Å, CN:4)/N³⁻ (1.46 Å, CN:4). Octahedral tilting was identified using the method by Barnes et al. [12]. The ionic radii were obtained according to Shannon Previtt [13] cationic radii in oxides. Based on the space groups originated from cis- and trans- anion ordering and octahedral tilting in the mixed anion perovskites proposed by Porter et al. [14], it was deduced that there was no anion ordering. The anion composition of the synthesised oxynitride was determined by hot gas extraction. The results showed that the anion content of nitrogen and oxygen was 4.41 and 7.01 wt.% respectively, which indicated a chemical formula of EuTaO_1.75N_1.25. The non-stoichiometric O/N ratio can be induced by the presence of oxygen vacancies. OPEN IN VIEWER

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Table 1

Rietveld refinement results including unit-cell parameters, atomic coordinates, and main interatomic distances for EuTaO_1.75N_1.25.

Formula			EuTaO1.75N1.25
Space group
Crystal system			Cubic
Cell parameter (Å)			4.0283(7)
Volume (Å3)			65.370(1)
Rietveld R-factors			R p		6.42%
			R wp		4.75%
			GOF		2.35
Atoms	Wyck.	Occ.	x/a	y/b	z/c
Eu	1a	1.00	0	0	0
Ta	1b	1.00	1/2	1/2	1/2
O/N	3c	1.00	1/2	1/2	0
d(Eu−O|N) (Å)		Polyhedral volume (Å3)	d(Ta−O|N)(Å)		Polyhedral volume (Å3)
2.848 × 12		10.8955	2.014 × 6		54.4773

Figure 3(a) showed the results of thermogravimetric study. The decomposition of the oxynitride can be divided into three steps. A slight weight fluctuation occurred during the first step (<345°C) was mainly due to the removal of the physical and chemical combination of moisture. At 345–515°C, the mass increased significantly by 6.4 wt-%. The DSC curve reflected the heat release process which can be attributed to the total oxidation of EuTaO_1.75N_1.25. The difference (1.51 wt-%) between the experimental weight gain and the theoretical value (4.89 wt-%) can be due to the oxygen vacancy, which was very close to the weight loss of oxygen (1.39 wt-%). In the last step (>515°C), the sample showed a gradual weight loss in a two-step exothermal process. One possible explanation is the dissociation and kinetics of the oxide at high temperatures were controlled by temperature. Another is the removal of N₂ suggested by Rachel et al. [15] and Gendre et al. [16] The results showed that EuTaO_1.75N_1.25 was stable in the air below 345°C. OPEN IN VIEWER

The composition of oxidised EuTaO_1.75N_1.25 was determined by X-ray diffraction as shown in Figure 3(b). Amorphous EuTaO₄ was obtained after EuTaO_1.75N_1.25 oxidised in air at 500°C for 2 h. However, when heated in air at 1000°C for 2 h, EuTaO_1.75N_1.25 transformed to europium oxide containing crystal EuTaO₄ and a handful of Eu₅Ta₄O₁₅. As a result, the decomposition of EuTaO_1.75N_1.25 produced intermediate products as amorphous EuTaO₄ and final products as crystal EuTaO₄ and a handful of Eu₅Ta₄O₁₅.

SEM and TEM images were illustrated in Figure 4. The SEM image displayed regular cubic grains with a size of 50–200 nm. The ED pattern was indexed as (100) plane. Considering that anion ordering would lower the symmetry of crystal structure, there is no evidence of ordering between the oxygen and nitrogen atoms. Figure 5 showed the EDS mapping of Eu/Ta/O/N elements obtained in HAADF-STEM mode. It can be seen that these elements were homogenously distributed in the crystals, which indicated a good homogeneity from the microscopic perspective. OPEN IN VIEWER

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Figure 5

STEM-EDS elemental maps for EuTaO_1.75N_1.25. The size of the studied crystal is <500 nm. It can be obviously seen that the Eu/Ta/O/N elements were homogeneously distributed.

The magnetic properties of EuTaO_1.75N_1.25 were investigated. Figure 6(a) showed the temperature dependence of the magnetic susceptibility. In the temperature range of 30–300 K, the magnetic susceptibility conformed to a Curie–Weiss law: χ = 7.75/T−3.32. The effective magnetic moment is 7.82 μ_B. The value of the positive Weiss constant was 3.32 K, and the deviation of magnetic susceptibility below 10 K can be ascribed to the beginning of ferromagnetic behaviour, which was confirmed by the irreversibility found in the zero-field cooling (ZFC) susceptibility measurements. The estimated Curie temperature (T_c) was 9.9 K. In most of the trivalent cations R³⁺ (R = lanthanide), the crystal field effects leading to deviations are banned with ⁸S_7/2 ground term. Therefore, the deviations from the Curie–Weiss behaviour observed at low temperatures were caused by the beginning of magnetic cooperative interactions [17]. Figure 6(b) showed the field dependence of the magnetisation at different temperatures. The oxynitride performed a typical ferromagnetic behaviour below 15 K and a paramagnetic behaviour over 30 K. The saturated moment was 6.82 μ_B at 6 T. The effective magnetic moment (7.82 μ_B) and saturated moment (6.82 μ_B) are both smaller than the theoretical value (7.94 μ_B and 7 μ_B) of the Eu²⁺ cation. This phenomenon can be attributed to the existence of Eu³⁺ confirmed by the non-stoichiometric O/N ratio. The ferromagnetic behaviour of the Eu²⁺ was revealed by the hysteresis loop at 2 K depicted in Figure 6(c). The Eu²⁺ cation behaved as a soft magnetic cation because of the isotropic character of its ⁸S_7/2 ground state without anisotropic effect. The remanence and the coercive field were too low to be detected due to isotropic character. OPEN IN VIEWER

Conclusions

EuTaO_1.75N_1.25 can be rapidly synthesised within 10 min.