High-Temperature Synthesis and Dielectric Properties of LaTaON₂

Abstract

The development of new synthetic methodologies of perovskite oxynitrides is challenging but necessary for the search of new compounds and the investigation of new properties. Here, we report a new method of preparation of the perovskite LaTaON 2 that has been investigated as a pigment and photocatalyst for water splitting. The synthesis proceeds through the solid-state reactions under N 2 at 1500 °C between La 2 O 3 , LaN, and Ta 3 N 5 or between LaN and TaON, which are completed after 3 h and lead to sintered, highly crystalline samples with particle sizes up to 1 μm. Nitrogen-deficient samples LaTaO 1+x N 2−x with x ≤ 0.35 are prepared by changing the N/O ratio in the mixture of reactants. Electron diffraction, synchrotron diffraction, and neutron diffraction studies on stoichiometric and nitrogen-deficient compounds indicate that they crystallize in the monoclinic space group I2/m with lattice parameters for LaTaON 2 of a = 5.71458( 7), b = 8.05987(10), c = 5.74772(6) Å, and β = 89.982(3)°. The three anion sites of the I2/m structure are partially occupied by oxygen and nitrogen, with a preference of nitride for two positions with occupancies of 77 and 88%. This anion distribution is different from that reported in previous studies of samples prepared by ammonolysis at lower temperature, suggesting that the synthesis conditions affect the anion order of this perovskite. Optical measurements indicate a band gap of about 1.9 eV, which is close to that observed in samples prepared by other methods. The determined dielectric permittivity for LaTaON 2 ε r ≈ 200, reported for the first time for a highly nitrided pseudocubic perovskite, is similar to that observed in perovskites with one nitrogen per formula.

High-Temperature Synthesis of Ferromagnetic Eu3Ta3(O,N)9 with a Triple Perovskite Structure

“…Considering the charge compensation, this stoichiometry is consistent with the presence of reduced Ta and Eu cations with the formal plausible composition Eu 2+ (Ta 0.37 4+ Ta 0.63 5+ )­O 2.37 N 0.63 . The existence of 100% of europium in the divalent state agrees with the observed effective magnetic moment of this compound (see below), whereas the +4 oxidation state of tantalum has been suggested in other perovskite oxynitrides coexisting with the more stable Ta 5+ cation. − The electron diffraction patterns of EuTaO 2.37 N 0.63 indicated a cubic perovskite cell of a ≃ 4.0 Å with the space group of aristotype Pm 3̅ m (Figure ). This result differs from our previously reported electron diffraction study of EuTaO 2 N prepared by ammonolysis, which showed additional reflections indicative of a tilted I2 / m superstructure with a , b = √2 a 0 and c = 2 a 0 . , The perovskite Eu 3 Ta 3 O 3.66 N 5.34 (phase II) was prepared with reaction 2 at the same temperature than EuTaO 2.37 N 0.63 , under N 2 with y = 1.8 (initial ratio N/O of 3.78).…”

“…The reaction used, the proportions of the reactants, the selected gas, and the maximum synthesis temperature determined the average nitrogen content of the sample per Eu or Ta mol, which was tuned from x = 0.63 to 1.78, and the phase composition. We have recently reported a similar synthetic approach for the preparation of LaTaON 2 and slightly nitrogen-deficient LaTaO 1.12 N 1.88 that we investigated for their dielectric properties . Both compounds were prepared either from LaN and TaON or from La 2 O 3 , LaN, and Ta 3 N 5 at 1500 °C.…”

“…We have recently reported a similar synthetic approach for the preparation of LaTaON 2 and slightly nitrogen-deficient LaTaO 1.12 N 1.88 that we investigated for their dielectric properties. 28 Both compounds were prepared either from LaN and TaON or from La 2 O 3 , LaN, and Ta 3 N 5 at 1500 °C. In the EuTaO 3−x N x samples, the syntheses performed at 1500 °C led to partial decomposition into TaN and Eu 3 TaO 6 phases; hence, a lower temperature of 1200 °C was selected.…”

Anionic and Magnetic Ordering in Rare Earth Tantalum Oxynitrides with an n = 1 Ruddlesden–Popper Structure

“…In all cases, these contents were below the nitrogen stoichiometry in the mixture of reactants (3, 3.25, 3.56, and 1.96 for La, Ce, Nd, and Eu samples, respectively), indicating the incorporation of extra oxygen in the samples during synthesis. Nitrogen loss at high temperatures has been observed in other tantalum perovskites such as SrTaO 2 N and LaTaON 2 and has been interpreted as a decomposition reaction releasing N 2 with partial reduction of Ta 5+ to Ta 4+ , analogous to the oxygen loss of transition metal oxides at high temperatures that produces reduced oxides together with O 2 . The corresponding cation ratios determined by EDX were La/Ta = 1.82(17), Ce/Ta = 1.81(10), Nd/Ta = 1.77(17), and Eu/Ta = 1.89(15), which agree with the nominal compositions within the experimental error.…”

“…EuTaO 2 N, EuNbO 2 N, and EuWO 1+ x N 2– x are ferromagnetic with T c values between 5 and 12 K because of Eu 2+ spin ordering. LaTiO 2 N and RHfO 2 N (R = La, Nd, Sm) compounds are visible light-active photocatalysts in water oxidation and reduction, whereas LaTaON 2 , in addition to a photocatalyst for water splitting, is a high-dielectric permittivity material …”