Structural transition in ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ with $x$ near 0.75 due to Na rearrangement

We report neutron powder diffraction measurements on a series of ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ samples with $x$ near 0.75 which were prepared under different synthesis conditions. Two different crystal structures for the samples are observed at room temperature. The two structures belong to the space group $P{6}_3∕mmc$ and differ primarily by a shift of a large fraction of the Na ions from a high-symmetry position to a lower-symmetry position. Close inspection of the refinement parameters indicates that the presence of either structure depends sensitively on the Na content $x$, with $x\simeq 0.75$ as the critical concentration separating the two phases. By raising the temperature to around $T\simeq 323\mathrm{K}$, the high-symmetry structure can be converted to the lower-symmetry structure. The transition is reversible, but there is significant hysteresis. We discuss the effects of this structural transition on the bulk magnetic and transport properties.

Figure 1

(Color online) Portions of the neutron powder diffraction pattern of ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ samples annealed under different conditions. The graphs from top to bottom correspond to the following samples: crushed crystal CC measured at room temperature (RT), powder P-900 quenched in air from $900°\mathrm{C}$ measured at RT, powder P-900 measured at $350\mathrm{K}$, and powder P-750 quenched in air from $750°\mathrm{C}$ measured at RT. The curves are the calculated intensity with the structural models H2 and H1, corresponding to the crystal structures shown on Fig. 2.

Figure 2

(Color online) Models for the crystal structure used in the refinements for ${\mathrm{Na}}_x\mathrm{Co}{\mathrm{O}}_2$ ($x$ near 0.75), space group $P{6}_3∕mmc$. The Na(2) atoms are (a) at the $6h(2x,x,1∕4)$ site for structure H1 and (b) at the $2c(2∕3,1∕3,1∕4)$ site for structure H2. The numbers shown in the lower figure of the Na planes are the refined occupancy of each Na site for the P-900 sample.

Figure 3

(Color online) Top panel: scans through the the (109) peak of sample P-900 at various temperatures. Bottom panel: temperature dependence of the intensity of the (109)-H1 peak. These data demonstrate that at high temperatures (above $340\mathrm{K}$), model H1 is the stable structure.

Figure 4

(Color online) Resistivity versus temperature on a single crystal prepared under the same conditions as the crushed crystal sample CC. The magnetic field is parallel to the $c$ axis, and the current is parallel to the $ab$ plane.

Figure 5

Magnetic susceptibility versus temperature on the same sample as in Fig. 4. A magnetic field of $5\mathrm{T}$ was applied approximately parallel to the $ab$ plane.

Figure 6

(Color online) Coordination of Na(1) and Na(2) in models H1 and H2. The Na(1) ions are located at the $2b$ site, center of $\mathrm{Na}{\mathrm{O}}_6$ triangular polyhedron, and are coordinated to six oxygen atoms which are associated with two nearest $\mathrm{Co}{\mathrm{O}}_6$ octahedra (directly above and below). (a) The Na(2) ions randomly occupy the $6h$ site, shifting away from the $2c$ site—i.e., the center of the triangular polyhedron. (b) The Na(2) ions are located at the $2c$ site, the center of the triangular mesh, and are coordinated to six oxygen atoms which are associated with to six nearest $\mathrm{Co}{\mathrm{O}}_6$ octahedra (above and below $\mathrm{Co}{\mathrm{O}}_2$ blocks).

Figure 7

(Color online) Lattice parameters at room temperature as a function of Na content $x$. The change in the lattice constants as $x$ increases above $x\simeq 0.75$ corresponds to the change from the H1 structure to the H2 structure.