Phase Selectivity in LnSrCoO4 (Ln = La–Gd) Using Hydrogen-Free and Hydride Reduction Methods

Abstract

The n = 1 Ruddlesden–Popper perovskite LaSrCoO_3.5-δH_x has been explored as an oxygen ion conductor and catalyst due to its ability to contain both oxygen vacancies and anionic hydrogen. Here, we compare the phase selectivity and formation of LnSrCoO_3.5-δH_x from LnSrCoO₄ (Ln = La, Pr, Nd, Sm, Eu, and Gd) using two distinct topochemical methods, CaH₂ hydride reduction and the recently reported amalgam-based hydrogen-free reduction─A_xAlGa, with A = Li, Na, and K─with a focus on the A = Na case in particular. These methods are coined as amalgam-based for their inspiration and similarities to sodium–mercury amalgams. At T = 300 °C, both techniques yield LnSrCoO_3.5-δ, and at T = 350 °C, hydrogen-free reduction continues to yield LnSrCoO_3.5-δ, whereas hydride reduction results in mixed-phase LnSrCoO_3.5-δ and LnSrCoO_3.5-δH_x. Temperature-dependent magnetization measurements show that the hydrogen-free reductions have less parasitic ferromagnetic contributions than those prepared by the CaH₂ method. Powder X-ray diffraction demonstrates that the amalgam-based method produces pure phase LnSrCoO_3.5-δ over a greater than 50 °C temperature range, whereas the CaH₂ technique results in mixed phase products within that range. In addition to extending the range of known LnSrCoO_3.5-δ and LnSrCoO_3.5-δH_x materials to the smaller rare-earth ions Ln = Sm, Eu, and Gd, our results demonstrate the solid-state chemical control that is possible with emerging topotactic methods.