Coupled Electronic and Magnetic Phase Transition in the Infinite-Layer Phase LaSrNiRuO4

Topochemical reduction of the ordered double perovskite LaSrNiRuO6 with CaH2 yields LaSrNiRuO4, an extended oxide phase containing infinite sheets of apex-linked, square-planar Ni1+O4 and Ru2+O4 units ordered in a checkerboard arrangement. At room temperature the localized Ni1+ (d9, S = 1/2) and Ru2+ (d6, S = 1) centers behave paramagnetically. However, on cooling below 250 K the system undergoes a cooperative phase transition in which the nickel spins align ferromagnetically, while the ruthenium cations appear to undergo a change in spin configuration to a diamagnetic spin state. Features of the low-temperature crystal structure suggest a symmetry lowering Jahn–Teller distortion could be responsible for the observed diamagnetism of the ruthenium centers.