Superconductivity and cobalt oxidation state in metastable Na{sub x}CoO{sub 2-{delta}}{center_dot}yH{sub 2}O (x{approx_equal}1/3; y{approx_equal}4x)
Superconductivity and cobalt oxidation state in metastable Na{sub x}CoO{sub 2-{delta}}{center_dot}yH{sub 2}O (x{approx_equal}1/3; y{approx_equal}4x)
复制标题
亚稳态 Na{sub x}CoO{sub 2-{delta}}{center_dot}yH{sub 2}O (x{approx_equal}1/3; y{approx_equal}4x) 中的超导性和钴氧化态
DOI：
10.1103/physrevb.72.134515
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发表时间：
2005
影响因子：
3.7
通讯作者：
S. Short
中科院分区：
文献类型：
--
作者：
P. W. Barnes;M. Avdeev;J. Jorgensen;D. Hinks;H. Claus;S. Short
本文报道了已知超导体Na{sub x}CoO{sub 2}{center_dot}yH{sub 2}O （x{approx_equal}1/3;y{approx_equal}4x）亚稳态形式的合成和超导性质。由于合成方法独特，我们获得了这种亚稳态钴酸盐超导体。在Na{sup +}脱插反应中，我们使用溴水溶液代替了传统的溴-乙腈混合物。使用这种方法，我们将样品氧化到钴酸钠变得不稳定的程度，如果不加以控制，就会导致其他产物的形成。该化合物具有与报道的超导体相同的结构，但它表现出超导转变温度（T{sub c}）作为时间函数的系统变化。在合成后，这种化合物不是超导体，即使它含有适量的Na{sup +}和H{sub 2}O。样品在{约}90 h后呈现超导状态，T{sub c}值较低，在约260 h后T{sub c}持续增加，达到最大值（4.5 K），随后T{sub c}急剧下降，在约100 h后变为非超导状态。相应的随时间变化的中子粉末衍射数据表明，亚稳态钴酸盐表现出的超导性变化与CoO{sub 2}层中氧空位的缓慢形成相对应。实际上，这些缺陷的形成不断降低钴的氧化态，导致样品在其超导生命周期中进化。因此，圆顶状超导相图被映射为使用单个样品的钴氧化态的函数。基于钴的形式氧化态，这个圆顶的宽度非常窄——大约0.1价单位宽。有趣的是，Na{下标x}CoO{下标2}{center_dot}yH{下标2}O中T{下标c}的最大值出现在钴氧化态接近+3.5时。因此，我们推测最大T{sub c}出现在与钴平均氧化态+3.5相关的电荷有序绝缘态附近。«少
We report the synthesis and superconducting properties of a metastable form of the known superconductor Na{sub x}CoO{sub 2}{center_dot}yH{sub 2}O (x{approx_equal}1/3;y{approx_equal}4x). We obtained this metastable cobaltate superconductor due to the unique way it was synthesized. Instead of using the conventional bromine-acetonitrile mixture for the Na{sup +}-deintercalation reaction, we use an aqueous bromine solution. Using this method, we oxidize the sample to a point that the sodium cobaltate becomes unstable, leading to formation of other products if not controlled. This compound has the same structure as the reported superconductor, yet it exhibits a systematic variation of the superconducting transition temperature (T{sub c}) as a function of time. Immediately after synthesis, this compound is not a superconductor, even though it contains appropriate amounts of Na{sup +} and H{sub 2}O. The samples become superconducting with low T{sub c} values after {approx}90 h. T{sub c} continually increases until it reaches a maximum value (4.5 K) after about 260 h. Then T{sub c} drops drastically, becoming nonsuperconducting approximately 100 h later. Corresponding time-dependent neutron powder diffraction data shows that the changes in superconductivity exhibited by the metastable cobaltate correspond to slow formation of oxygen vacancies in the CoO{sub 2} layers. In effect, the formation of thesemore » defects continually reduces the cobalt oxidation state causing the sample to evolve through its superconducting life cycle. Thus, the dome-shaped superconducting phase diagram is mapped as a function of cobalt oxidation state using a single sample. The width of this dome based on the formal oxidation state of cobalt is very narrow--approximately 0.1 valence units wide. Interestingly, the maximum T{sub c} in Na{sub x}CoO{sub 2}{center_dot}yH{sub 2}O occurs when the cobalt oxidation state is near +3.5. Thus, we speculate that the maximum T{sub c} occurs near the charge ordered insulating state that correlates with the average cobalt oxidation state of +3.5.« less