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The capability of sodium hydride as a reducing agent in oxide deintercalation reactions is explored. The Ni(III) perovskite LaNiO3 can be reduced topotactically to LaNiO2, isostructural with the “infinite layer” cuprates, using solid sodium hydride in a sealed evacuated tube at 190 ≤ T/°C ≤ 210, and a similar infinite-layer phase is prepared by reduction of NdNiO3. Structural characterization indicates the coexistence of incompletely reduced regions, with five-coordinate Ni centers due to the introduction of oxide anions between the NiO2 3- sheets, giving samples with a refined stoichiometry of LaNiO2.025(3). Neutron powder diffraction and magnetization measurements indicate that the lamellar Ni(I) phase does not show the long-range antiferromagnetic ordering characteristic of isoelectronic Cu(II) oxides. This may be due either to the influence of the interlamellar oxide defect regions or to the reduced covalent mixing of Ni 3d and O 2p levels.

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The preparation and crystal structures of the n = 2 Ruddlesden−Popper phases Sr2 - x Ln1+ x Mn2O7 (0 ≤ x ≤ 0.5, Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, and Er) are described. The crystal chemistry and stability of this structure is governed by the size of the lanthanide cation. Partial ordering of the Sr2+ and Ln3+ cations occurs between the two available A cation (A = Ln3+, Sr2+) sites, with the smaller lanthanides preferring the site in the rock-salt layer over that in the perovskite block. This ordering is almost complete for the small lanthanides (Tb−Er), and these ordered compounds can be prepared as single phases. Cation disorder in compounds of the larger lanthanides is accompanied by a subtle separation into two n = 2 Ruddlesden−Popper phases, which is apparent only upon detailed inspection of Rietveld refinements of the X-ray profiles. In these cases, the two-phase model is found to be superior to a single phase model with strain broadening included. For a particular lanthanide, both the ease of synthesis of single phases and the extent of cation ordering depend on the manganese oxidation state.

Ordering of the tetrahedral site vacancies in two crystals of refined compositions K 0.93(1) Fe 1.52(1) Se 2 and K 0.862(3) Fe 1.563(4) Se 2 produces a fivefold expansion of the parent ThCr 2 Si 2 unit cell in the ab plane which can accommodate 20% vacancies on a single site within the square FeSe layer. The iron charge state is maintained close to +2 by coupling of the level of alkali metal and iron vacancies, producing a potential doping mechanism which can operate at both average and local structure levels. 65 K 0.93(1) Fe 1.52(1) Se 2 (crystal 1) and K 0.862(3) Fe 1.563(4) Se 2 (crystal 2) corresponding to formal iron charge states of 2.02(2) and 2.008(7), respectively (Table 1). The different K and Fe vacancy levels in the two crystals thus give rise to the same iron charge