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

The n = 1 Ruddlesden-Popper perovskite LaSrCoO3.5-δHx 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 LnSrCoO3.5-δHx 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─AxAlGa, 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 LnSrCoO3.5-δ, and at T = 350 °C, hydrogen-free reduction continues to yield LnSrCoO3.5-δ, whereas hydride reduction results in mixed-phase LnSrCoO3.5-δ and LnSrCoO3.5-δHx. 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 LnSrCoO3.5-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 LnSrCoO3.5-δ and LnSrCoO3.5-δHx 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.