Interplay of valence states and magnetic interactions in the perovskite system LaNi1–x Rh x O3

Highlights: • A phase transformation from R-3c to Pnma correlate with a MIT for 0.10x• Oxidation state Ni^II/Rh^IV dominates samples with x>~0.4 in LaNi_1-xRh_xO₃. • Long range structural and magnetic ordering is absent for all samples. • Rh-rich (x>0.60) samples show weak ferromagnetism, correlating with mixed valence. • Conduction mechanisms strongly depend on Rh-content (x) and temperature. We report on the structural, magnetic and electronic properties of Rh substituted LaNiO₃, LaNi_1–xRh_xO₃ (0≤x≤1). Increased Rh contents are associated with increased structural distortion, and we observe a phase transformation and an immiscibility gap between 0.15≤x≤0.25, separating the R-3c and Pnma perovskite structure regimes. Structural evaluation by synchrotron X-ray diffraction, X-ray absorption spectroscopy and DC magnetic measurements suggest variations in oxidation state for the B-site cations, with Ni(II)/Rh(IV), in addition to Rh(III), dominating for high Rh-contents. Although ordering of Ni(II)/Rh(IV) is possible, no B-site ordering between Ni and Rh is observed for any composition. A metal-to-insulator transition occurs upon Rh-substitution, correlating with the structural transition. DC- and AC magnetic measurements reveal that several of the compositions are magnetically frustrated with both antiferro- and ferromagnetic behaviour at low temperatures. The antiferromagnetic interactions are dominating, while the ferromagnetic interactions increase with increasing Rh-content, to a maximum at x=0.80 and x=0.90 in LaNi_1–xRh_xO₃. The mixed valence states are assumed to contribute with a ferromagnetic super-exchange interaction between Ni(II) and Rh(IV).