Crystal Structures of Ln4Ni3O8 (Ln = La, Nd) Triple Layer T'-type Nickelates

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• 作者：Viktor V. Poltavets ; Konstantin A. Lokshin ; Mark Croft ; Tapas K. Mandal ; Takeshi Egami ; Martha Greenblatt
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：December 10, 2007
• 年：2007
• 卷：46
• 期：25
• 页码：10887 - 10891
• 全文大小：177K
• 年卷期：v.46,no.25(December 10, 2007)
• ISSN：1520-510X

文摘

Single-phase Ln₄Ni₃O₈ (Ln = La, Nd) nickelates were synthesized and their crystal structures were determined byRietveld refinement of powder neutron diffraction data. The crystal structures of these mixed-valent Ni¹⁺/Ni²⁺ phasesbelong to the T'-type and are built by intergrowth of LnO₂ fluorite layers with triple NiO₂ infinite-layer structuralblocks. The major driving force of transformation of the LnO rock-salt block of the parent Ln₄Ni₃O_10- Ruddlesden-Popper phases to the fluorite arrangement in the reduced Ln₄Ni₃O₈ phases is attributed to internal structural stress.This transformation allows longer Ni-O bonds in Ln₄Ni₃O₈ without overstretching of the Ln-O bonds, especially inthe equatorial plane. The observed displacement of Ni atoms from the outer NiO₂ planes toward the Ni atom of thecentral NiO₂ plane in Ln₄Ni₃O₈ is ascribed to large electrostatic repulsion from the fluorite part of the structure.X-ray absorption spectra near the K-edge of Ni suggest that the charge density on the nickel ion is similar for allmembers of the T'-type Ln_n+1Ni_nO_2n+2 homologous series, which correlates with nearly constant Ni-O bond lengthsobserved in all the reduced nickelates. This suggests that the formal changes in the valence state of Ni affect thecovalency of the Ni-O bond.