Perovskites with partially filled e_g orbitals at the octahedrally coordinated atom respond to this impending violation of orbital degeneracy in a variety of ways. In SrFeO₃, the e_g electrons are delocalized and metallic behavior ensues. In LaMnO₃, the e_g electrons are localized and the octahedra distort through cooperative Jahn–Teller distortions. In CaFeO₃, the orbital degeneracy is removed through charge disproportionation into an ordered pattern of Fe³⁺ and Fe⁵⁺. The RNiO₃ (R = trivalent rareearth ion) perovskites are another system with partial filling of eg orbitals. The phase diagram between critical temperatures for insulator-to-metal (TIM) and antiferromagnetic-to-paramagnetic (TN) transitions in these nickelates is shown in the figure below (SmNd approximates Pm).

(a) What is the oxidation state, d-electron count and occupation of the t_2g and e_g orbitals for the low-spin nickel in these compounds?

(b) Would you expect the nickel-centered octahedra in LaNiO₃ to be symmetric or to be distorted as a result of a cooperative Jahn–Teller distortion?

(c) Whereas LaNiO₃ is a paramagnet and a metallic conductor at all temperatures, the lanthanoid RNiO₃ phases undergo a metal–insulator transition upon cooling. What stabilizes metallic LaNiO₃ even at low temperatures and why?

(d) Why does the insulator-to-metal transition temperature T_IM increase along the rare-earth series?

(e) Which changes in the crystal structure of YNiO₃ would you expect upon cooling through T_IM? How would the Ni–O distances allow you to see whether the low-temperature paramagnetic insulating phase is stabilized by a cooperative Jahn–Teller distortion or by a charge disproportionation?

Transcribed Image Text:

T. (K) 400 200 0 Lu paramagnetic insulator Tm T TN Gd Eu Sm Sm Nd antiferromagnetic insulator RNIO paramagnetic metal Nd Pr R+ radius La