Structural peculiarities of La2Ge1-xCrxMgO6-delta (0<0 <= 5): a superior oxide-ion electrolyte for low-temperature solid-oxide fuel cells

The double perovskite La2MgGeO6 has been modified by substitution of Ge by Cr to introduce oxygen vacancies. A specimen of composition La2Ge0.55Cr0.45MgO6 has been studied by neutron diffraction, in the 300-873 K temperature range. The perovskite structure can be defined in the rhombohedral R3 space group. At 295 K, the unit-cell parameters are a = 5.5115 (2), c = 13.3485 (7) angstrom and V = 351.16 (3) angstrom(3). This double perovskite exhibits two distinct crystallographic sites for Mg and (Cr,Ge), statistically distributed at the octahedral sites. It presents a conspicuous deficiency at O1 sites, accounting for the excellent ionic conduction properties. The Bond-Valence Energy Landscape (BVEL) map at 873 K shows that oxygen atoms present a higher mobility around the (Ge/Cr)O-6 octahedra than the MgO6 ones; therefore, the "bottleneck" points for oxygen mobility are placed between the (Ge/Cr)O-6 octahedra around the La3+ and Mg2+ cations. The dark-red samples prepared in air show evidence of oxidation of some Cr3+ to Cr4+ to give a polaronic component to a conductivity of 10(-2) Scm(-1) at 300 degrees C for La2Ge0.5Cr0.5MgO6-delta; Arrhenius plots of conductivity obtained on cooling from 900 degrees C to 25 degrees C in air give an activation energy of similar to 0.25 eV. The fuel-cell performance at 600 degrees C gave a power density of 606 mW cm(-2), a threefold increase over the output compared with LSGM electrolyte. (c) 2023 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).