Single-crystal growth and investigation of $\mathrm{N}{\mathrm{a}}_x\mathrm{C}\mathrm{o}{\mathrm{O}}_2$ and $\mathrm{N}{\mathrm{a}}_x\mathrm{Co}{\mathrm{O}}_2·y{\mathrm{H}}_2\mathrm{O}$

A systematic study of ${\mathrm{Na}}_x{\mathrm{CoO}}_2(x=0.50–0.90)$ and ${\mathrm{Na}}_x{\mathrm{CoO}}_2·y\mathrm{H}{\left(\mathrm{D}\right)}_2\mathrm{O}$ ($x=0.26–0.42$, $y=1.3$) has been performed to determine phase stability and the effect of hydration on the structural and superconducting properties of this system. We show that a careful control of the Na deintercalation process and hydration dynamics is possible in single crystals of this system. Furthermore, we give experimental evidence that the dependence of the superconducting transition temperature on Na content is much weaker than reported earlier. Implications of this effect for the understanding of the superconducting phase diagram are discussed.

Figure 1

DTA-TG analysis by melting single crystalline ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$ heated at $7.5°\mathrm{C}∕\min$ up to $1200–1250°\mathrm{C}$ in flowing oxygen. $T_1=1035°\mathrm{C}$, $T_2=1092°\mathrm{C}$. (a) The melting behavior of the compound, and (b) temperature dependence of the weight loss.

Figure 2

A typical ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$ single-crystal ingot obtained by the optical floating zone technique.

Figure 3

Two halves of an as-grown crystal ingot with the 001 surface. (a) The as-cleaved half ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$ showing the ${\mathrm{CoO}}_2$ inclusions gathering at the growth boundary and (b) the other half ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_2·y{\mathrm{H}}_2\mathrm{O}$ showing the removal of the ${\mathrm{CoO}}_2$ inclusions after deintercalation followed by hydration. (c) The enlarged ${\mathrm{CoO}}_2$ inclusions.

Figure 4

Schematic drawing of the layered structures Ref. 17 for (a) nonhydrated ${\mathrm{Na}}_{0.61}{\mathrm{CoO}}_2$ and (b) fully hydrated ${\mathrm{Na}}_{0.33}{\mathrm{CoO}}_{2}1.3{\mathrm{H}}_2\mathrm{O}$.

Figure 5

Three typical morphologies showing the layered structure of ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ single crystal. (a) After cutting an as-grown ${\mathrm{Na}}_{0.7}{\mathrm{CoO}}_2$ single crystal, (b) the cracked layers perpendicular to the c axis of the ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_2$ crystals after deintercalation, and (c) the “booklet”-like structure of the ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_{2}1.3{\mathrm{H}}_2\mathrm{O}$ after being hydrated by ${\mathrm{H}}_2\mathrm{O}$.

Figure 6

Single crystal x-ray diffraction patterns for ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_2·y{\mathrm{H}}_2\mathrm{O}$ measured by immersing the sample in deionized water. The initial crystal contained the phases of $y=0.6$ and 0.

Figure 7

Thermogravimetric analysis of an overhydrated ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_{2}1.8{\mathrm{H}}_2\mathrm{O}$ single crystal heated at $0.3\mathrm{K}∕\min$ in flowing oxygen. Inset: time dependence of the weight loss for the same specimen at room temperature in flowing oxygen. Initial mass of the sample was $112.965\mathrm{mg}$. The loss was approximately $19\mathrm{mg}$ after heating and rehydrating.

Figure 8

The 00l reflections showing the mixture of cells and the lattice constants for the various hydrates of ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_2·y{\mathrm{H}}_2\mathrm{O}$ under different ambient conditions. (a) From humid to dry air for 2 days, (b) from humid to dry air for 5 days, and (c) from dry to humid air for 5 days. Symbol $\mathbf{A}$ represents the phase of $y=0$ for $a=2.82\mathrm{\AA }$ and $c=11.195\mathrm{\AA }$, $\mathbf{B}$: $y=1.3$ for $a=2.82\mathrm{\AA }$ and $c=19.710\mathrm{\AA }$, $\mathbf{C}$: $y=0.3$ for $a=2.82\mathrm{\AA }$ and $c=12.478\mathrm{\AA }$, and $\mathbf{D}$: $y=0.6$ for $a=2.82\mathrm{\AA }$ and $c=13.815\mathrm{\AA }$.

Figure 9

Time dependence of the out-of-plane resistivity ${\rho }_{\mathrm{c}}$ of a ${\mathrm{Na}}_{0.3}{\mathrm{CoO}}_2·y{\mathrm{H}}_2\mathrm{O}$ single crystal during a hydration process. The number given as footnote at the plateaus is hydration time in hours.

Figure 10

Zero field Cooling (ZFC) magnetic characterization of the ${\mathrm{Na}}_x{\mathrm{CoO}}_{2}1.3{\mathrm{H}}_2\mathrm{O}$ single crystals showing the onset $T_c\left(x\right)$. Inset: optimum $T_c\approx 4.9\mathrm{K}$ with $x=0.42$ as ZFC and FC measurement.

Figure 11

Superconducting phase diagram as a function of $\mathrm{Na}$ content. Open circles correspond to the onset temperature, while closed square correspond to a linear low-temperatures extrapolation. The dashed line is a guide to the eye. The dashed bar marks the metal-insulator transition observed in nonhydrated ${\mathrm{Na}}_{0.5}{\mathrm{CoO}}_2$.