Redox chemistry and magnetism of LaSrM0.5Ru0.5O4±δ (M = Co, Ni and Zn) Ruddlesden–Popper phases

Abstract

The n = 1 Ruddlesden–Popper (RP) phases LaSrM_0.5Ru_0.5O_4±δ (M = Co, Ni and Zn) have been prepared by solid state reactions and structurally characterized by powder X-ray and electron diffraction. All the samples adopt the tetragonal I4/mmm space group with random M and Ru cation occupation on the B-sites. The potential causes of no cation ordering are discussed. A combined analysis of the tolerance factors, the distortion of the octahedral coordination of M and Ru cations and the magnetic interactions between M and Ru cations provide a better understanding for forming a phase with 3D cation ordering on the B-sites in the n = 1 RP phases. The investigation of XPS spectra suggests that the transition element species exist as mixed ion pairs, Ru^(4−δ)+–Ru⁴⁺ ↔ Co²⁺–Co³⁺ in LaSrCo_0.5Ru_0.5O₄, and Ru⁴⁺–Ru^(4+δ)+ ↔ Ni⁺–Ni²⁺ in LaSrNi_0.5Ru_0.5O₄, which is consistent with cation disorder over the B sites. LaSrCo_0.5Ru_0.5O₄ shows a weakly ferromagnetic behaviour below 50 K; LaSrNi_0.5Ru_0.5O₄ is evidenced by the presence of long-range magnetic ordering at a Néel temperature of 125 K, and LaSrZn_0.5Ru_0.5O₄ exhibits a paramagnetic behaviour down to 5 K. Due to atomic disorder, Ru4d, O2p covalent coupling is weakened, strengthening the intraatomic spin–spin coupling among the π* electrons. Charge transfer between Ru and Co or Ru and Ni, as well as the increasing overlap of both nearest-neighbour and next-nearest-neighbour Ru 4d electrons due to atomic disorder, favour the formation of ferromagnetic interactions. Although antiferromagnetism is dominant, particularly in LaSrNi_0.5Ru_0.5O₄, ferromagnetic interactions are stronger in the title compounds than in the related La₂MRuO₆ (M = Co, Ni) double perovskites where the B-site cations are ordered.