76
5 Combinatorial Studies of the Spinel and Rocksalt Regions . . .
for samples heated in oxygen, and in both of these phase diagrams, this boundary
matched up very well with the phases determined by visually examining the XRD
scans, as well as joining nicely with the boundary determined in the region to the
right of LiNi 0.5 Mn 1.5 O 4 . In air, insufficient data were collected to use the lever rule
in this region and so the lower spinel boundary in Fig. 5.3 is left incomplete.
5.6 Conclusions Regarding Spinel and Rocksalt
Li–Mn–Ni Oxides
The entire spinel and rocksalt solid-solution regions of the Li–Mn–Ni oxide pseudoternary system were determined at 800
◦ C when regular cooled in air and when
quenched or regular cooled in oxygen. All samples discussed here either contained
one or two phases; and no evidence for nonequilibrium behaviour was seen. Twophase fits in the coexistence regions were used to determine lattice parameters to
allow the drawing of tie-lines. The lever rule was used to determine the boundaries
and showed excellent agreement with visually identified phases. Milligram-scale
combinatorial samples can therefore be used to obtain a high degree of precision in
the mapping of pseudo-ternary phase diagrams.
The spinel samples near LiNi 0.5 Mn 1.5 O 4 quenched in oxygen showed oxygen
vacancies. It is likely that samples quenched in air with a lower oxygen partial
pressure also have vacancies. Since oxygen vacancy formation occurs simultaneously
as an oxygen loss resulting in an upwards motion of the lower spinel boundary, it is
important to recognize that both these effects are present in samples synthesized in
air and are difficult to differentiate. However, it has not yet been determined whether
or not bulk samples also show these deficiencies; it is possible that oxygen loss is
significantly more prominent in the combinatorial samples due to the large surface
area to volume ratios.
Comparing Figs. 5.2 and 5.3 shows the motion of single-phase boundaries with
synthesis conditions. Generally, the single-phase regions are much larger than previously known and increase in size with temperature (i.e., quenched) and they are also
larger when prepared in air. The ordered rocksalt boundary is higher in air than it is in
oxygen. In air, the lower oxygen partial pressure favors structures with lower metal
oxidation numbers (i.e., lower oxygen content) such that the boundary of the rocksalt and layered regions is expected to move upward, particularly when quenched.
The resulting lattice parameter contour plots show that the spinel samples made in
oxygen have oxygen site vacancies which are relieved during regular cooling.
The structure of the cubic rocksalt materials is more complex than previously suspected. The refinement of the XRD patterns showed ordering of lithium, manganese,
and metal site vacancies on the cubic lattice, and the extent of this ordering increased
during regular cooling.
5 Combinatorial Studies of the Spinel and Rocksalt Regions . . .
for samples heated in oxygen, and in both of these phase diagrams, this boundary
matched up very well with the phases determined by visually examining the XRD
scans, as well as joining nicely with the boundary determined in the region to the
right of LiNi 0.5 Mn 1.5 O 4 . In air, insufficient data were collected to use the lever rule
in this region and so the lower spinel boundary in Fig. 5.3 is left incomplete.
5.6 Conclusions Regarding Spinel and Rocksalt
Li–Mn–Ni Oxides
The entire spinel and rocksalt solid-solution regions of the Li–Mn–Ni oxide pseudoternary system were determined at 800
◦ C when regular cooled in air and when
quenched or regular cooled in oxygen. All samples discussed here either contained
one or two phases; and no evidence for nonequilibrium behaviour was seen. Twophase fits in the coexistence regions were used to determine lattice parameters to
allow the drawing of tie-lines. The lever rule was used to determine the boundaries
and showed excellent agreement with visually identified phases. Milligram-scale
combinatorial samples can therefore be used to obtain a high degree of precision in
the mapping of pseudo-ternary phase diagrams.
The spinel samples near LiNi 0.5 Mn 1.5 O 4 quenched in oxygen showed oxygen
vacancies. It is likely that samples quenched in air with a lower oxygen partial
pressure also have vacancies. Since oxygen vacancy formation occurs simultaneously
as an oxygen loss resulting in an upwards motion of the lower spinel boundary, it is
important to recognize that both these effects are present in samples synthesized in
air and are difficult to differentiate. However, it has not yet been determined whether
or not bulk samples also show these deficiencies; it is possible that oxygen loss is
significantly more prominent in the combinatorial samples due to the large surface
area to volume ratios.
Comparing Figs. 5.2 and 5.3 shows the motion of single-phase boundaries with
synthesis conditions. Generally, the single-phase regions are much larger than previously known and increase in size with temperature (i.e., quenched) and they are also
larger when prepared in air. The ordered rocksalt boundary is higher in air than it is in
oxygen. In air, the lower oxygen partial pressure favors structures with lower metal
oxidation numbers (i.e., lower oxygen content) such that the boundary of the rocksalt and layered regions is expected to move upward, particularly when quenched.
The resulting lattice parameter contour plots show that the spinel samples made in
oxygen have oxygen site vacancies which are relieved during regular cooling.
The structure of the cubic rocksalt materials is more complex than previously suspected. The refinement of the XRD patterns showed ordering of lithium, manganese,
and metal site vacancies on the cubic lattice, and the extent of this ordering increased
during regular cooling.