Structure and dynamics of superconducting Na x CoO 2 hydrate and its unhydrated analog
Structure and dynamics of superconducting Na x CoO 2 hydrate and its unhydrated analog
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DOI：
10.1103/physrevb.68.214516
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发表时间：
2003-07
影响因子：
3.7
通讯作者：
J. W. Lynn;Q. Huang;Craig M. Brown;Craig M. Brown;V. Miller;M. Foo;R. Schaak;Camille Y. Jones;Elizabeth A. Mackey;R. J. Cava
中科院分区：
文献类型：
--
作者：
J. W. Lynn;Q. Huang;Craig M. Brown;Craig M. Brown;V. Miller;M. Foo;R. Schaak;Camille Y. Jones;Elizabeth A. Mackey;R. J. Cava
利用中子散射研究了超导材料${\ mathm {Na}}_{0.3}{\ mathm {CoO}}_{2}\ensuremath{\cdot}1.4(\ mathm {H}/\ mathm {D}{)}}_{2}\ mathm {O}、${\ mathm {Na}}_{0.3}{\ mathm {CoO}}_{2}$的晶体结构和晶格动力学。${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_{2}$的结构由${\mathrm{CoO}}_{2}$和Na的交替层组成，与Na浓度较高时的结构相同。对于超导体，水在Na和${\ maththrm {CoO}}_{2}之间形成两个额外的层，使${P6}_{3}/mmc$空间群的$c$轴晶格参数从11.16 \AA{}增加到19.5 \AA{}。研究发现，钠离子的结构与母体化合物不同，而水的结构则与冰的结构相似。这两种地点都只有部分被占用。另一方面，这些结构中的${\ maththrm {CoO}}_{2}$层是坚固的，我们发现${\ maththrm {CoO}}_{2}$层厚度与超导转变温度${之间存在很强的负相关关系（T}_{C}$随厚度的减小而增加）。${\ mathm {Na}}_{0.3}{\ mathm {CoO}}_{2}$的声子密度表现出明显的声光带，其高能截止值为$\ensuremath{\sim}100\ mathm {meV}。超导体的晶格动力学散射以氢模式为主，其振动和弯曲模式与冰非常相似，支持水在超导体中插入并形成冰状层的结构模型。
Neutron scattering has been used to investigate the crystal structure and lattice dynamics of superconducting ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_{2}\ensuremath{\cdot}1.4(\mathrm{H}/\mathrm{D}{)}_{2}\mathrm{O},$ and the ``parent'' ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_{2}$ material. The structure of ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_{2}$ consists of alternate layers of ${\mathrm{CoO}}_{2}$ and Na and is the same as the structure at higher Na concentrations. For the superconductor, the water forms two additional layers between the Na and ${\mathrm{CoO}}_{2},$ increasing the $c$-axis lattice parameter of the hexagonal ${P6}_{3}/mmc$ space group from 11.16 \AA{} to 19.5 \AA{}. The Na ions are found to occupy a different configuration from the parent compound, while the water forms a structure that replicates the structure of ice. Both types of sites are only partially occupied. The ${\mathrm{CoO}}_{2}$ layer in these structures is robust, on the other hand, and we find a strong inverse correlation between the ${\mathrm{CoO}}_{2}$ layer thickness and the superconducting transition temperature ${(T}_{C}$ increases with decreasing thickness). The phonon density of states for ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_{2}$ exhibits distinct acoustic and optic bands, with a high-energy cutoff of $\ensuremath{\sim}100\mathrm{meV}.$ The lattice dynamical scattering for the superconductor is dominated by the hydrogen modes, with librational and bending modes that are quite similar to ice, supporting the structural model that the water intercalates and forms ice-like layers in the superconductor.