In situ neutron diffraction study of the high-temperature redox chemistry of Ln3−xSr1+xCrNiO8−δ (Ln = La, Nd) under hydrogen

Abstract

International audienceThe chemical reduction of the K2NiF4-type oxides, Ln2Sr2CrNiO8−δ (Ln = La, Nd) and Nd2.25Sr1.75CrNiO8−δ, has been investigated in situ under a dynamic hydrogen atmosphere at high temperature using neutron powder diffraction. The high count-rate and high resolution of the D20 diffractometer at ILL, Grenoble allowed real-time data collection and structure refinement by full-pattern Rietveld analysis with a temperature resolution of 1 °C. Excellent agreement was obtained with the results of thermogravimetric analysis of these materials, which are potential fuel-cell electrodes. The neutron study revealed that oxygen is lost only from the equatorial anion site; the reduction of La2Sr2CrNiO8−δ yields a pure Ni(II) phase, La2Sr2CrNiO7.5en route to a mixed Ni(II,I) oxide, La2Sr2CrNiO7.40, whereas hydrogen reduction of Nd2Sr2CrNiO8−δ and Nd2.25Sr1.75CrNiO8−δ proceeds continuously from Ni(III) to an average oxidation state of 1.80 for the nickel ion. The data collected throughout a subsequent heating/cooling cycle in air indicated that the reduced phases intercalate oxygen reversibly into the equatorial vacancies of the K2NiF4-type structure. The retention of I4/mmm symmetry, along with the absence of the formation of any impurities throughout the heating/cooling cycles under reducing and oxidizing atmospheres, demonstrates both the reversibility and the strongly topotactic character of the oxygen deintercalation/intercalation chemical redox process and establishes the excellent structural stability of these layered mixed-metal oxides over a wide range of oxygen partial pressures

Formation and Decomposition of Iron Nitrides Observed by in situ Powder Neutron Diffraction and Thermal Analysis

“…As sample container a bottom-closed tube of silica glass (12 mm outer diameter) was used for in situ neutron diffraction. [15,37,53,54] Additionally, the HTF provides the possibility of using predissociated ammonia through the modified gas supply. This is also advantageous for reactions of metals and ammonia, because the reactivity of the ammonia is increased, which is easily realized by preheating the ammonia before it comes into contact with the sample.…”

Formation and Decomposition of Metastable α′′‐Fe16N2 from in situ Powder Neutron Diffraction and Thermal Analysis

“…[26] In this in situ neutron diffraction experiment we used a high temperature resistive furnace (HTF) due to its expected larger temperature homogeneity over the sample. A configuration similar to those previously described was chosen, [88][89][90][91] but the gas supply was modified to requirements of using preheated gas. The measurement cell consists of a silica glass tube (480 mm length, 12 mm outer diameter, 1 mm wall thickness) closed at the bottom and connected to the gas supply head part of D20.…”

“…Further description can be found in references. [25,[88][89][90][91] Temperature control is realized with a thermocouple type K [92] inserted into the reaction tube and positioned off-beam close above the sample. The reactions were performed in a well defined gas flow of argon controlled with a mass flow controller (MF1, MKS Instruments Germany).…”

Enhancing Electrochemical Water‐Splitting Kinetics by Polarization‐Driven Formation of Near‐Surface Iron(0): An In Situ XPS Study on Perovskite‐Type Electrodes

“…[24] High chemical stability of such phases in hydrogen atmosphere was reported for Cr/Ni containing Ruddlesden–Popper oxides. [27]…”