Crystallographic and magnetic structure of SrCoO2.5 brownmillerite:: Neutron study coupled with band-structure calculations
Crystallographic and magnetic structure of SrCoO2.5 brownmillerite:: Neutron study coupled with band-structure calculations
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DOI：
10.1103/physrevb.78.054404
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发表时间：
2008-08-01
影响因子：
3.7
通讯作者：
Rivas, J.
中科院分区：
文献类型：
--
作者：
Munoz, A.;de la Calle, C.;Rivas, J.
通过在10到623 K的温度范围内（跨越这种反铁磁氧化物的奈尔温度\(T_N = 537\ K\)）进行中子粉末衍射（NPD）测量，对具有钙钛矿型结构的\(SrCoO_{2.5}\)的晶体结构和磁结构进行了研究。该研究还辅以差示扫描量热仪（DSC）、直流磁化率和磁化强度测量。尽管从NPD数据对晶体结构进行精修在正交的\(Pnma\)和\(Ima2\)空间群中是可能的，但基于第一性原理的能带结构计算使我们能够明确地选择\(Ima2\)空间群作为\(SrCoO_{2.5}\)钙钛矿的基态。在\(Ima2\)空间群中，\(SrCoO_{2.5}\)的晶体结构被描述为沿\(a\)轴交替的共角\(Co1O_6\)八面体层和共顶点\(Co2O_4\)四面体层，形成沿\([0\ 0\ 1]\)方向延伸的链。在\(T_N = 537\ K\)以下，磁结构为\(G\)型，磁矩沿\(c\)轴方向。这种磁排列从\(T_N\)到10 K是稳定的。在\(T = 10\ K\)时，\(Co1\)和\(Co2\)原子的磁矩值分别为\(3.12(13)\ \mu_B\)和\(2.88(14)\ \mu_B\)，与\(Co^{2 + }L\)态相符，其中\(L\)代表配体空穴。在200 K以下，磁化率曲线显示零场冷却和场冷却曲线出现分歧，呈现出宽的极大值，这被解释为是由于反铁磁基体中存在铁磁团簇。这些不均匀性是合成过程所固有的，是通过从高温淬火\(SrCoO_{3 - x}\)成分的微晶样品产生的，在高温下通过衍射方法已确定存在立方的铁磁钙钛矿。
A study of the crystallographic and magnetic structure of SrCoO(2.5) with a brownmillerite-type structure has been carried out from neutron powder-diffraction (NPD) measurements at temperatures ranging from 10 to 623 K, across the Neel temperature (T(N)=537 K) of this anti ferromagnetic oxide. The study has been complemented with differential scanning calorimeter (DSC), dc susceptibility and magnetization measurements. Although the refinement of the crystal structure from NPD data is possible in the orthorhombic Pnma and Ima2 space groups, the support of ab-initio band-structure calculations has allowed us to select, without ambiguity, the Ima2 space group as the ground state for SrCoO(2.5) brownmillerite. In Ima2 the crystallographic structure of SrCoO(2.5) is described as layers of corner-sharing Co1O(6) octahedra alternating along the a axis with layers of vertex-sharing Co2O(4) tetrahedra, conforming chains running along the [0 0 1] direction. The magnetic structure below T(N)=537 K is G-type with the magnetic moments directed along the c axis. This magnetic arrangement is stable from T(N) down to 10 K. At T=10 K, the magnetic moment values for Co1 and Co2 atoms are 3.12(13) mu B and 2.88(14) mu B, respectively, compatible with a Co(2+)L state, where L stands for a ligand hole. The magnetic susceptibility curves show, below 200 K, a divergence of zero-field cooling and field cooling curves, displaying broad maxima which are interpreted as due to the presence of ferromagnefic clusters embedded into an anti ferromagnetic matrix. These inhomogeneities are inherent to the synthesis process, by quenching microcrystalline samples of SrCoO(3-x) composition from high temperature, where cubic, ferromagnetic perovskites have been identified by diffraction methods.