6.5 The R, N, S, and M phases
83
6.5 The R, N, S, and M phases
Figure 6.5 shows two three-phase regions side-by-side between the spinel and layered
single-phase regions when samples were heated in oxygen and quenched. The two
regions were the NSM and NSR triangles. R was an ordered rocksalt structure, S was
a spinel structure, and N and M were both layered structures (labels were chosen to
represent nickel-rich or manganese-rich layered structures to distinguish them). The
large array of samples shown in Fig. 5.1b made it possible to identify samples near
each corner by matching lattice parameters to those obtained with the three-phase
fits. Figures 6.8 and 6.9 (left) show XRD scans of quenched samples near the R,
S, N, and M corners. Fitting these patterns allowed the determination of the lattice
parameters included in Table 6.1 and their coordinates in the Gibbs triangle are listed
in Table 6.2. In the scattering angle range 43–46
◦ , each of the four single-phase
samples have a single peak without K α 1,2 peak splitting. As such, this region is ideal
to illustrate the phases present in the three-phase regions.
Bulk quenched samples synthesized at each of the R, N, and M corners using
the one-pot synthesis (Sect. 2.1.2) were scanned with the JD-2000 diffractometer.
Table 6.3 shows the results of Rietveld refinement. The R phase only has 10 % vacancies such that the average Mn oxidation state is 3.6+, assuming that nickel is in the
2+ state. The density of this R phase sample was found to be 5.4447 ± 0.0090 g/mL
using a helium pycnometer and this value corresponds to 8.8 ± 2.0 % vacancies on
the 4b sites, in good agreement with the XRD results. In the previous chapter, an
ordered rocksalt structure synthesized in oxygen and cooled at the regular rate was
found to have approximately 30 % vacancies on the 4b sites and Mn in the 4+ state.
These results suggest that the structure of this phase changed during slow cooling
and further study is required to understand this fully.
The N-layered phase had a hexagonal structure with significant disorder in both the
lithium and transition metal layers such that there was a 30 % nickel occupation on the
lithium layers. By contrast, the M-layered phase was far more ordered with very little
nickel on the lithium layers. XRD scans of this structure also show superlattice peaks
in the range 20–35
◦ consistent with ordering on
√
3 ×
√
3 lattices in the transition
metal (TM) layers. Although the compositions of the TM layers were not in the 1:2
ratio needed for this ordering, it was quite close. As such, if one
√
3 ×
√
3 lattice
was randomly occupied with 0.708Li and 0.292Ni while the other two contained
1.866Mn and 0.134Ni, the resulting scattering contrast would be sufficient to give
rise to the weak ordering peaks seen here, consistent with the reasoning of Lu et
al. [45]. The ordering peaks should, therefore, not necessarily be interpreted as the
presence of Li 2 MnO 3 and the sample may yet be single-phase as suggested here.
Although the ordering seen in the M sample is expected to be coupled with monoclinic distortions in the hexagonal lattice, the fact that a high-quality fit was obtained
for a hexagonal structure shows that the extent of monoclinic distortion was small
and it was completely masked by the peak broadening due to the diffractometer.
The fact that the oxygen occupancy converges to a value above 100 % implies that
there are metal site vacancies in the M-layered structure. Both of these issues will
be resolved in Chap. 8.
83
6.5 The R, N, S, and M phases
Figure 6.5 shows two three-phase regions side-by-side between the spinel and layered
single-phase regions when samples were heated in oxygen and quenched. The two
regions were the NSM and NSR triangles. R was an ordered rocksalt structure, S was
a spinel structure, and N and M were both layered structures (labels were chosen to
represent nickel-rich or manganese-rich layered structures to distinguish them). The
large array of samples shown in Fig. 5.1b made it possible to identify samples near
each corner by matching lattice parameters to those obtained with the three-phase
fits. Figures 6.8 and 6.9 (left) show XRD scans of quenched samples near the R,
S, N, and M corners. Fitting these patterns allowed the determination of the lattice
parameters included in Table 6.1 and their coordinates in the Gibbs triangle are listed
in Table 6.2. In the scattering angle range 43–46
◦ , each of the four single-phase
samples have a single peak without K α 1,2 peak splitting. As such, this region is ideal
to illustrate the phases present in the three-phase regions.
Bulk quenched samples synthesized at each of the R, N, and M corners using
the one-pot synthesis (Sect. 2.1.2) were scanned with the JD-2000 diffractometer.
Table 6.3 shows the results of Rietveld refinement. The R phase only has 10 % vacancies such that the average Mn oxidation state is 3.6+, assuming that nickel is in the
2+ state. The density of this R phase sample was found to be 5.4447 ± 0.0090 g/mL
using a helium pycnometer and this value corresponds to 8.8 ± 2.0 % vacancies on
the 4b sites, in good agreement with the XRD results. In the previous chapter, an
ordered rocksalt structure synthesized in oxygen and cooled at the regular rate was
found to have approximately 30 % vacancies on the 4b sites and Mn in the 4+ state.
These results suggest that the structure of this phase changed during slow cooling
and further study is required to understand this fully.
The N-layered phase had a hexagonal structure with significant disorder in both the
lithium and transition metal layers such that there was a 30 % nickel occupation on the
lithium layers. By contrast, the M-layered phase was far more ordered with very little
nickel on the lithium layers. XRD scans of this structure also show superlattice peaks
in the range 20–35
◦ consistent with ordering on
√
3 ×
√
3 lattices in the transition
metal (TM) layers. Although the compositions of the TM layers were not in the 1:2
ratio needed for this ordering, it was quite close. As such, if one
√
3 ×
√
3 lattice
was randomly occupied with 0.708Li and 0.292Ni while the other two contained
1.866Mn and 0.134Ni, the resulting scattering contrast would be sufficient to give
rise to the weak ordering peaks seen here, consistent with the reasoning of Lu et
al. [45]. The ordering peaks should, therefore, not necessarily be interpreted as the
presence of Li 2 MnO 3 and the sample may yet be single-phase as suggested here.
Although the ordering seen in the M sample is expected to be coupled with monoclinic distortions in the hexagonal lattice, the fact that a high-quality fit was obtained
for a hexagonal structure shows that the extent of monoclinic distortion was small
and it was completely masked by the peak broadening due to the diffractometer.
The fact that the oxygen occupancy converges to a value above 100 % implies that
there are metal site vacancies in the M-layered structure. Both of these issues will
be resolved in Chap. 8.