Structural disorder and antiferromagnetism in LaNi1–x Pt x O3

Highlights: • B-site ordering is absent for x≤0.05 (R-3c), but present for x≥0.20 (P2₁/n). • Non-equimolar compositions (0.20≤x≤0.40) form Pt-rich and Pt-depleted domains. • The system is best described by the formula ${\text{La}}_2{\text{Ni}}_y^{\text{II}}{\text{Ni}}_{2-2y}^{\text{III}}{\text{Pt}}_y^{\text{IV}}{\text{O}}_6$, 0≤y≤1. • La₂NiPtO₆ is antiferromagnetic with T_N $\approx$ 40K. • For 0.20≤x≤0.40, AFM ordering is "diluted" by the Pt-depleted domains. We report on the B-site substitution of Pt for Ni in the system LaNi_1–xPt_xO₃. The system can only be synthesized between x≤0.50, with LaNiO₃ (x=0.00) and the stoichiometric double perovskite La₂NiPtO₆ (x=0.50) as the end members. Higher Pt-contents (x>0.50) are unachievable due to the preference of Pt to being in oxidation state+IV in octahedral coordination. Upon introducing Pt into LaNiO₃, a phase transformation from rhombohedral (R-3c) to monoclinic (P2₁/n) symmetry is observed for 0.075≤x≤0.125, where all monoclinic samples are B-site ordered, and Pt show a strong preference for the Pt-site (2c-site) for all compositions (x≥0.20). Powder X-ray diffraction analysis reveal disorder of the Pt-distribution in several of the non-equimolar samples (0.20≤x≤0.40), which point toward cluster formation with domains of high and low Pt-content within each sample. La₂NiPtO₆ further show an antiferromagnetic transition at ∼ 40K. A similar transition is observed for all monoclinic samples (x≥0.20), however, the transition is less pronounced for lower x. This is explained in terms of the structural disorder, i.e. by the coexistence of antiferromagnetic domains with long range order and paramagnetic domains dominated by short range antiferromagnetic interactions.