New mixed-valent, Ni1+/Ni2+, metastable nickelate, La3Ni2O6, was synthesized by low-temperature reduction of La3Ni2O7 with CaH2. The crystal structure of La3Ni2O6 (space group: I4/mmm, a = 3.9686(1) A and c = 19.3154(6) A) was determined from powder neutron diffraction data by Rietveld analysis. The structure can be described as an intergrowth of LaO2 fluorite and double infinite layer (LaNiO2)2 blocks and represents the n = 2 homologue of the T'-type series Lan+1NinO2n+2. Such double T'-type structural arrangement has never been observed before. The 3d9/3d8 electronic configuration of Ni1+/Ni2+ and the presence of NiO2 infinite layers resemble electronic and structural features of the superconducting cuprates. X-ray absorption spectroscopy supports the 1+/2+ oxidation state and planar coordination of Ni in agreement with the structure determination.

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