Reduction of Sr2MnO4 Investigated by High Temperature in Situ Neutron Powder Diffraction under Hydrogen Flow

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• 作者：Thibault Broux ; Mona Bahout ; Olivier Hernandez ; Florent Tonus ; Serge Paofai ; Thomas Hansen ; Colin Greaves
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：January 18, 2013
• 年：2013
• 卷：52
• 期：2
• 页码：1009-1017
• 全文大小：537K
• 年卷期：v.52,no.2(January 18, 2013)
• ISSN：1520-510X

文摘

This experiment emphasizes the first example of two-phase sequential Rietveld refinements throughout a solid/gas chemical reaction monitored by Neutron Powder Diffraction (NPD) at high temperature. The reduction of the n = 1 Ruddlesden鈥揚opper (RP) oxide Sr₂MnO₄ heated under a flow of 5% H₂鈥揌e has been investigated throughout two heating/cooling cycles involving isothermal heating at 500 and 550 掳C. Oxygen loss proceeds above T 470 掳C and increases with temperature and time. When the oxygen deintercalated from the 鈥淢nO₂鈥?equatorial layers of the structure results in the Sr₂MnO_3.69(2) composition, the RP phase undergoes a first order I4/mmm 鈫?P2₁/c, tetragonal to monoclinic phase transition as observed from time-resolved in situ NPD. The phase transition proceeds at 500 掳C but is incomplete; the weight ratio of the P2₁/c phase reaches 41% after 130 min of isothermal heating. The fraction of the monoclinic phase increases with increasing temperature and the phase transition is complete after 80 min of isothermal heating at 550 掳C. The composition of the reduced material refined to Sr₂MnO_3.55(1) and does not vary on extended heating at 550 掳C and subsequent cooling to room temperature (RT). The symmetry of Sr₂MnO_3.55(1) is monoclinic at 550 掳C and therefore consistent with the RT structure determined previously for the Sr₂MnO_3.64 composition obtained from ex situ reduction. Consequently, the stresses due to phase changes on heating/cooling in reducing atmosphere may be minimized. The rate constants for the reduction of Sr₂MnO_4.00 determined from the evolution of weight ratio of the tetragonal and monoclinic phase in the time-resolved isothermal NPD data collected on the isotherms at 500 and 550 掳C are k₅₀₀ = 0.110 脳 10^鈥? and k₅₅₀ = 0.516 脳 10^鈥? min^鈥? giving an activation energy of 163 kJ mol^鈥? for the oxygen deintercalation reaction.