The Ni1+/Ni2+ states of nickelates have the identical (3d(9)/3d(8)) electronic configuration as Cu2+/Cu3+ in the high temperature superconducting cuprates, and are expected to show interesting properties. An intriguing question is whether mimicking the electronic and structural features of cuprates would also result in superconductivity in nickelates. Here we report experimental evidence for a bulklike magnetic transition in La4Ni3O8 at 105 K. Density functional theory calculations relate the transition to a spin density wave nesting instability of the Fermi surface.
Analysis of the structural parameters of phases that adopt brownmillerite-type structures suggests the distribution of the different complex ordering schemes adopted within this structure type can be rationalized by considering both the size of the separation between the tetrahedral layers and the tetrahedral chain distortion angle. A systematic study using structural data obtained from La1−x
A
x
MnO2,5 (A = Ba, Sr, Ca,) phases, prepared by the topotactic reduction of the analogous La1−x
A
x
MnO3 perovskite phases, was performed to investigate this relationship. By manipulating the A-cation composition, both the tetrahedral layer separation and tetrahedral chain distortion angle in the La1−x
A
x
MnO2,5 phases were controlled and from the data obtained a “structure map” of the different brownmillerite variants was plotted as a function of these structural parameters. This map has been extended to include a wide range of reported brownmillerite phases showing the structural ideas presented are widely applicable. The complete structural characterization of La1−x
A
x
MnO2,5 0.1 ≤ x ≤ 0.33, A = Ba; 0.15 ≤ x ≤ 0.5 A = Sr, and 0.22 ≤ x ≤ 0.5 A = Ca is described and includes compositions which exhibit complex intralayer ordered structures and Mn2+/Mn3+ charge ordering.
Sodium ion batteries (NIBs) are one of the versatile technologies for low cost rechargeable batteries. O3-type layered sodium transition metal oxides (NaMO2, M = transition metal ions) are one of the most promising positive electrode materials, capacity-wise. However, the use of O3 phases is limited due to their low redox voltage and associated multiple phase transitions which are detrimental for long cycling. Herein, we proposed a simple strategy to successfully combat these issues. It consists in the introduction of a larger, non-transition metal ion Sn 4+ in NaMO2 to prepare a series of NaNi0.5Mn0.5-ySnyO2 (y=0-0.5) compositions with attractive electrochemical performances, namely for y=0.5, which shows a single phase transition from O3 P3 at the very end of the oxidation process. Na-ion NaNi0.5Sn0.5O2/C coin cells are shown to deliver an average cell voltage of 3.1 V with an excellent capacity retention as compared to an average step-wise voltage of ~2.8 V and limited capacity retention for the pure NaNi0.5Mn0.5O2 phase. This study potentially shows the way to manipulate the O3 NaMO2 for facilitating their practical use in NIBs.
The crystal structure of the Sr 2 Fe 2 O 5 brownmillerite has been investigated using electron diffraction and high resolution electron microscopy. The Sr 2 Fe 2 O 5 structure demonstrates two-dimensional order: the tetrahedral chains with two mirror-related configurations (L and R) are arranged within the tetrahedral layers according to the -L-R-L-Rsequence, and the layers themselves are displaced with respect to each other over 1/2[111] or 1/2[111 j ] vectors of the brownmillerite unit cell, resulting in different ordered stacking variants. A unified superspace model is constructed for ordered stacking sequences in brownmillerites based on the average brownmillerite structure with a ) 5.5298(4)Å, b ) 15.5875(12)Å, c ) 5.6687(4)Å, and (3 + 1)-dimensional superspace group I2/m(0βγ)0s, q ) βb* + γc*, 0 e β e 1/2, 0 e γ e 1.
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