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ISUPOV et al.
a Bruker Avance 400 spectrometer using magic angle the lithium cations (Fig. 3). In particular, the NMR
spinning at 10 kHz. Samples of the mechanically actiꢀ spectrum of the sample prepared at 550 , that is,
vated mixture were heated in air at a rate of 10 /min under the conditions corresponding to the formation
°С
°С
to a preset temperature in an SNOL furnace, then of an Xꢀray amorphous phase, shows a rather strong
withdrawn, and stored in hermetically sealed ampules shoulder, indicating the presence of a band at a chemꢀ
until measurements. Peak positions were determined ical shift of –0.3 to –0.4 ppm, due to lithium in tetraꢀ
relative to the NMR signals of aqueous aluminum hedral oxygen coordination [12].
nitrate and lithium chloride.
Raising the heatꢀtreatment temperature to 600–
650
25 , 32.5
reflections with those from the best known crystalline
forms of lithium aluminate ( and ) suggests the
following: First of all, they differ dramatically in posiꢀ
tion from those of the lowꢀtemperature phase
°
С
gives rise to three broad XRD lines, at
2
θ
= 20 –
°
°
°
–35 , and 35 –37 . Comparison of these
°
°
°
RESULTS AND DISCUSSION
As shown previously [9], the activation of a mixture
of aluminum hydroxide and lithium carbonate in an
AGOꢀ2 planetary mill leads not only to size reduction
but also to aluminum hydroxide amorphization and
the formation of a carbonate form of aluminum lithꢀ
ium hydroxide. After milling for 10 min, the double
hydroxide is Xꢀray amorphous. Like in an earlier study,
the XRD pattern of the milling products shows broadꢀ
ened reflections from lithium carbonate, whereas the
reflections from aluminum hydroxide are extremely
weak (Fig. 1). The 27Al NMR spectrum of the mechanꢀ
ically activated mixture (Fig. 2) shows a band at a
chemical shift of 7 ppm, characteristic of aluminum
cations in octahedral oxygen coordination [11]. The
octahedral coordination of the aluminum in the
mechanically activated mixture lends support to the
assumption that milling converts some of the alumiꢀ
num hydroxide to an Xꢀray amorphous state, whereas
the rest is incorporated into the carbonate form of aluꢀ
α,
β,
γ
α
ꢀLiAlO2 (Fig. 4). The position of the first reflection
coincides with that of the 101 reflection from ꢀLiAlO2
and differs little from those of the two closely spaced,
strong reflections from ꢀLiAlO2: 110 and 011 (Fig. 4).
The second reflection is identical in position to the 102
reflection from ꢀLiAlO2 and the 120 reflection from
ꢀLiAlO2. At the same time, the third reflection differs
rather significantly in position from its nearest neighꢀ
bors: 200 and 002 of ꢀLiAlO2 and 200 of ꢀLiAlO2
γ
β
γ
β
β
γ
.
The 27Al NMR spectrum of the sample prepared at
650 shows an increased intensity of the band due to
°С
tetrahedrally coordinated aluminum.
The 6Li NMR spectrum shows a marked reduction
in the intensity of the band at a chemical shift of
–0.9 ppm and a sharp rise in the intensity of the band
at –0.3 ppm. Thus, the NMR data indicates that the
formation of an intermediate phase increases the perꢀ
centages of aluminum and lithium in tetrahedral oxyꢀ
gen coordination.
minum lithium hydroxide, [LiAl2(OH)6]2CO3
(Li–Al–CO3)
Heating the mechanically activated mixture of aluꢀ
minum hydroxide and lithium carbonate to 200
⋅
nH2O
.
°С
removes the water from the Xꢀray amorphous alumiꢀ
num hydroxide and Li–Al–CO3, without significant
changes in XRD patterns (Fig. 1) or 27Al or 6Li NMR
At heatꢀtreatment temperatures above 650 С, the
°
first two peaks in the XRD patterns narrow down,
without changes in their position. The third peak shifts
to smaller angles with increasing temperature. In addiꢀ
spectra (Figs. 2, 3).
Subsequently raising the heatꢀtreatment temperaꢀ tion, a new reflection emerges, identical in position to
ture to above 200 leads to a gradual decrease in the the 200 reflection from ꢀLiAlO2, and its intensity
°С
γ
intensity of reflections from lithium carbonate, which
are essentially missing at a heatꢀtreatment temperaꢀ
gradually increases. These changes, along with the
emergence of three peaks close in position to the 110,
111, and 211 reflections from gamma lithium alumiꢀ
nate, indicate the formation of gamma lithium alumiꢀ
ture of 550
tant gradually decreases. At heatꢀtreatment temperaꢀ
tures above 550 , the reaction product is essentially
Xꢀray amorphous.
°С, indicating that the amount of this reacꢀ
°С
nate, which is well seen at 850 С. Note that the XRD
°
patterns show, in addition to the reflections from
gamma lithium aluminate, weak reflections from
With increasing temperature, the 27Al NMR band
at a chemical shift of 68–70 ppm, arising from tetraheꢀ
drally coordinated aluminum [11], gradually grows.
As a result, the bands due to octahedrally and tetraheꢀ
drally coordinated aluminum in the spectrum of the
Xꢀray amorphous material are comparable in intensity.
Heating changes the shape of the bands in the 6Li
NMR spectrum of the mechanically activated mixꢀ
ture, which points to changes in the coordination of
αꢀLiAlO2, indicating that, under the experimental
conditions of this study, this phase is present as an
impurity.
27Al and 6Li NMR data lend support to the above
XRD data. In particular, the 27Al and 6Li NMR spectra
of the sample prepared at 850°С are dominated by
bands of aluminum and lithium in tetrahedral oxygen
coordination (Fig. 2), characteristic of
γ
ꢀLiAlO2
.
INORGANIC MATERIALS Vol. 47
No. 7
2011
Isupov
