Enhanced acceptor concentration, proton conductivity, and hydration in multicomponent rare‐earth ortho‐niobates

Abstract

The structural properties of (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)1‐xCaxNbO4‐δ (x = 0 ‐ 0.05) series have been studied by X‐ray powder diffraction at room temperature. The thermal properties were investigated using thermogravimetry, from which the concentration of proton defects was determined. Additionally, dilatometry studies were carried out, from which the thermal expansion coefficients and the phase transition temperature between low‐ and high‐temperature polymorphs were determined. In order to determine the electrical properties, the electrochemical impedance spectroscopy measurements under atmospheres of dry synthetic air and saturated with water vapor and heavy water at 300 ‐ 800°C temperature range were performed. All the studied materials were single‐phase compositions and were stable in monoclinic I2/c crystal structure at room temperature. The water uptake studies revealed a small mass change under the switch from dry to humidified synthetic air atmospheres, suggesting the formation of proton defects. The phase transition from monoclinic to tetragonal crystal structure in the (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)1‐xCaxNbO4‐δ (x = 0 ‐ 0.05) series occurs above 650°C. A small amount of calcium dopant increases the total conductivity in all tested atmospheres by two orders of magnitude, for example, from 0.34 μScm−1 in La0.2Nd0.2Sm0.2Gd0.2Eu0.2NbO4 to 63.7 μScm−1 in (La0.2Nd0.2Sm0.2Gd0.2Eu0.2)0.95Ca0.05NbO4‐δ at 700°C in humid synthetic air atmosphere.This article is protected by copyright. All rights reserved

Structural phase transitions in multicomponent La0.2Nd0.2Sm0.2Gd0.2RE50.2NbO4 (RE5 = Ho, Y, Tb, Eu, Pr) oxides

“…To maintain the repeatability of the synthesis process and thus the possibility of comparing their properties, the materials were prepared in the same way as described in our previous work. 20 That is, the materials were synthesized using a three-step solid-state synthesis method, starting from the preparation of a stoichiometric mixture of reagents, which was ground in an agate mortar with the addition of isopropyl alcohol. Then, the powders were heated at a temperature of 750 • C (with a heating and cooling rate of 5 • C min −1 ) for 24 h. In the next step, the powders were ground again, sieved through a 100 µm sieve, uniaxially compressed under a pressure of 250 MPa and sintered at a temperature of 1400 • C (at a heating and cooling rate of 2 • C min −1 ) for 12 h. In the final stage, which was similar to the second stage, the prepared pellets were sintered at a temperature of 1500 • C (with a heating and cooling rate of 2 • C min −1 ) for 12 h to obtain dense samples.…”

“…23 Structural data presented in our previous work were used for refinement. 20 A separate CIF file was prepared for each compound using the VESTA software, 24 and then uploaded to the GSAS-II program. For the analysis in which only one of the polymorphic varieties of the compound occurred, appropriate assumptions were used in the program, such as the assumption that five different elements occupy the same position in the crystal lattice (constrains, make atoms equivalent).…”

“…Initially, this concerned metal alloys, but since 2015, when Rost et al 15 synthesized the first high-entropy oxide material (HEO), and scientists began developing new HEOs from other groups, such as ABO 3 [16][17][18] and ABO 4 . 19,20 By mixing different elements of different properties, one can obtain a new material with unknown properties, thanks to which we can, for example, improve its three key properties at the same time. So far, only one work has been reported on structural phase transitions in multicomponent rare-earth ortho-niobates.…”