Synthesis and Characterization of Pure Infinite Layer Ni⁺ Nickelates: LnNiO₂ (Ln = La, Nd, Pr) and La₃Ni₂O₆

A synthesis method of pure low valence nickelates using a custom built H2 circulation apparatus is described. Pure infinite layer LnNiO2 (Ln = La, Nd, Pr) and La3Ni2O6 nickelates have been successfully prepared using this method and characterized by x-ray diffraction. Resistivity of La3Ni2O6 was measured as a function of temperature and pressure up to ∼2 GPa and revealed significant pressure-induced changes in both magnitude and pressure dependence of resistivity. The existence of a hidden insulator-metal transition in La3Ni2O6 is proposed at pressures above 100 GPa.