Synthesis of New Layered Oxides A_0.67[Ni_0.33Ti_0.67]O₂ (A=Li, Cu and Ag) by Topotactic Cation-exchange from Na_0.67[Ni_0.33Ti_0.67]O₂

Na_0.67[Ni_0.33Ti_0.67]O₂의 이온교환반응에의한 새로운 층상산화물 A_0.67[Ni_0.33Ti_0.67]O₂ (A=Li, Cu and Ag)의 합성

EXPERIMENTAL SECTION

Na0.67[Ni0.33Ti0.67]O2 was first prepared by solid state reaction by heating the mixture of stoichiometric amounts of starting materials, Na2CO3, NiO, TiO2 at 1000℃ for 12 h under Ar-stream7. Samples of A=Li, Cu and Ag were prepared by cationexchange reactions using Na0.67[Ni0.33Ti0.67]O2 as a precursor and molten salt AX following the equation6,

Each mixture of molten salt and Na-precursor (3 g; 30 mmol) was put into silica tube, sealed under vacuum, and subsequently heated for 10 days at 420℃ for Li- and Cu-substitution, or at 250℃ for Ag-substitution. As for Cu-derivative, an additional composition of Cu0.75[Ni0.365Ti0.625]O2 was prepared to compare the structures. In case of Cu, an excess CuCl of 5 mol% (2.07g; 21mmol) was used due to the difficulty to remove the remaining CuCl after reaction. When A=Li or Ag, about 50 mol% of excessive nitrates (30-40 mmol) were used to facilitate the reaction. KNO3 was also added for Ag0.67[Ni0.33Ti0.67]O2 to keep the oxidative ambient and thus prevented the reduction of Ag+ into Ag0. Products were recovered by washing out the remaining nitrates or chlorides with ethanol and dried under vacuum. The remaining trace of CuCl after reaction was removed with 0.5M aqueous ammonia solution. For Li0.67[Ni0.33Ti0.67]O2, a solution reaction was also performed. Na0.67[Ni0.33Ti0.67]O2 was introduced into the LiNO3-saturated n-hexanol and subsequently refluxed for 10h at 160℃ with vigorous stirring, resulting in low-temperature phase of Li0.67[Ni0.33Ti0.67]O2.

Particle morphology of samples was monitored by FE SEM using JSM6700 FE-SEM II, JEOL. Identification of crystalline phases and determination of lattice parameters were carried out by X-ray powder diffraction (XRD) analysis using a Siemens D5005 diffractometer equipped with curved graphite monochromator with CuKa radiation (λ=0.1506 nm). For electrical conductivity measurements, samples were prepared as pellets (d=7 mm, t=2 mm) by pressing at 20 MPa. Blocking Auelectrodes were deposited by DC-sputtering. The AC-conductivity was measured using HP 4192A LF impedance analyzer at the range of 5 Hz-13 MHz under Ar atmosphere.

 

RESULTS AND DISCUSSION

FE SEM photographs of A0.67[Ni0.33Ti0.67]O2 (A=Na, Li, and Ag) are presented in Fig. 1. They show topotactic features of the cation-exchange reactions. Na0.67[Ni0.33Ti0.67]O2 obtained from direct thermal reaction exhibited disk-shaped large particles of 20-30 μm, reflecting the hexagonal crystallographic feature. After the cation-exchange reactions, the product particles exhibited nano-scale platelet texture and this indicates the reactions were topotactic.

Fig. 1FE-SEM micrographs of layered oxides Na0.67[Ni0.33Ti0.67]O2 (a), Li 0.67[Ni0.33Ti0.67]O2 (b) and Ag0.67[Ni0.33Ti0.67]O2 (c), (d).

Fig. 2XRD patterns of Na0.67[Ni0.33Ti0.67]O2 (a) and Li0.67-[Ni0.33Ti0.67]O2 obtained from the molten-salt method at 420 (b), LT-Li0.67[Ni0.33Ti0.67]O2 from the reflux in n-hexanol solution (c) and after annealing at 160 ℃.

In Fig. 2, XRD patterns of Li0.67[Ni0.33Ti0.67]O2 are represented. While the precursor Na0.67[Ni0.33Ti0.67]O2 exhibited a primitive hexagonal lattice (type P2)7, Li0.67[Ni0.33Ti0.67]O2 obtained at 420 showed a structural conversion into type O3 with the periodicity of 3-layer unit along axis c and Li+ ions at octahedral sites. When the cation-exchange was done in n-hexanol solution at 160, slightly different XRD pattern of Li0.67[Ni0.33Ti0.67]O2 was obtained as shown in Fig. 2(c). It exhibited two supplementary peaks which could be indexed as (101) and (107) on the basis of structural type O6, though the broad peak pattern prevented accurate structural analysis. Since any atomic rearrangement involving the bond rupture of M-O cannot be expected at the temperature as low as 160 ℃6,8, the structural conversion in this condition should occur only by the cooperative movements of MO2 sheets as illustrated in Fig. 3. The low-temperture O6-phase of Li0.67[Ni0.33Ti0.67]O2 was a meta-stable phase, for it converted into type O3 after annealing for 12h at 450, where some transient bond rupture of M-O seems possible and thus gives rise to the conversion into type O3.

Fig. 3Schematic diagram of structural evolution from type P2 to O3. At low temperature, the structure changes by the cooperative movement of MO2 sheets (a), but when temperature is elevated, some bond rupture may be involved in the structural conversion (b).

Fig. 4 shows the XRD patterns of Cu+- and Ag+- phases. Cu0.75[Ni0.375Ti0.625]O2 was found to have a well-crystallized 3R-delafossite structure. Since its precursor Na0.75[Ni0.375Ti0.625]O2 has the structural type O37, the structural conversion from O3 to 3R can be interpreted by a simple topotatic process accompanied with cooperative movement of MO2 sheets to furnish linear site for Cu+ ion6. For x=0.67, the XRD pattern could be indexed with more complicate 6H-delafossite structure9 with small amount of decomposed impurity NiTiO310. The appearance of 6H-delafossite Cu0.67[Ni0.33Ti0.67]O2 can be rationalized by the elevated reaction temperature where the atomic rearrangement is possible, though the reason why the product did not crystallize in 2Hdelafossite which is closely related with P2 is not yet understood. Anyway, the fact that both compositions exhibit delafossite structure signifies that the covalency in Cu-O is sufficiently strong for maintaining the linear symmetry O-Cu-O.

Fig. 4XRD patterns of Cu0.75[Ni0.375Ti0.625]O2 (a) and Cu0.67-[Ni0.33Ti0.67]O2 (b) obtained at 420, and Ag0.67[Ni0.33Ti0.67]O2 at 250 ℃. Peaks labeled with asterisk(*) correspond to NiTiO310, due to the partial decomposition.

On the other hand, the XRD pattern of Ag0.67-[Ni0.33Ti0.67]O2 could be indexed in six-layered lattice but not interpreted by any delafossite structure. Systematic absence condition -h+k+l 3n was obeyed in this composition, indicating a rhombohedral symmetry. Since the structural conversion of Ag0.67-[Ni0.33Ti0.67]O2 could be carried out only by cooperative movement of MO2 sheets due to low reaction temperature, its rhombohedral symmetry suggests that some of Ag+ ions should be placed between the close-packed oxygen layers. It means that Ag-O bond should largely lose covalent character, and becomes no more sufficient to keep the linear symmetry of O-Ag-O bond. In such circumstances, the balance of covalency in competitive bond A-O-(Ni2+0.33Ti4+0.67) should be sensitively shifted to O-(Ni2+0.33Ti4+0.67) from A-O for A=Ag, whereas Cu-O bond is considered still highly covalent enough to hold the linear coordination.

In Table 1, the ac-conductivity values at 25 ℃ and 150 ℃ are listed for A0.67[Ni0.33Ti0.67]O2 (A=Na, Li, Ag, and Cu). Na0.67[Ni0.33Ti0.67]O2 exhibited a good ionic conduction as reported earlier11,12. For Li0.67[Ni0.33Ti0.67]O2, the ionic conductivity was nearly zero, indicating that the reduced unit-cell volume and thus increased covalency of Li-O as well as the trigonal diffusion window of LiO6 octahedron should establish an energy barrier too high for Li+ ions to cross over. In case of Cu0.67[Ni0.33Ti0.67]O2, Though the inter-sheet space for Cu+ ions is largely increased, the ionic conduction of Cu+ ion is greatly restricted by the strong covalency of the linear bond O-Cu-O. It is worthy noting that no delafossite oxide with linear bond O-A-O has been reported to exhibit significant ionic conduction. In this sense, the enhanced ionic conductivity found in Ag0.67[Ni0.33Ti0.67]O2 strongly supports the presence of Ag+ ions within the rock-salt layer, AgxO2, where Ag-O is highly ionic. Such Ag+ ions are therefore expected to diffuse within the rock-salt layer, leading to the enhanced ionic conduction. A systematic study on Agx(Ni,Ti)O2 focused on the correlation of crystallographic features and the conduction mechanism is now under progress in our lab.

Table 1AC-conductivity of some layered oxides A0.67Ni0.33Ti0.67O2 (A=Na, Li, Cu, Ag)