Reinvestigations of the Li 2 O–Al 2 O 3 system. Part I: LiAlO 2 and Li 3 AlO 3

Reinvestigations of the Li 2 O–Al 2 O 3 system focused on the synthesis and properties of LiAlO 2 and Li 3 AlO 3 phases have been performed with the help of XRD and IR measuring techniques and Li 2 CO 3 , LiOH.H 2 O, Al 2 O 3 -sl., α -Al 2 O 3 , Al(NO 3 ) 3 .9H 2 O and boehmite as reactants. Results of investigations have shown the formation of α -, β -, and γ - polymorphs of LiAlO 2 . It was found that only the use of LiOH.H 2 O as a reactant yields to β -LiAlO 2 as a reaction product. On the other hand, it was proved that Li 3 AlO 3 does not form in the Li 2 O–Al 2 O 3 system. A new method for the synthesis of α -LiAlO 2 was developed, consisting in grinding the mixture of Li 2 CO 3 and Al(NO 3 ) 3 .9H 2 O and heating the obtained paste at the temperature range of 400–600 o C. The IR spectroscopy was used to characterize obtained phases.


INTRODUCTION
Compounds containing lithium have been the subject of comprehensive research for many years due to many different industrial applications, including the production of glass and heat-resistant ceramics 1-3 , luminescent ionizing radiation detectors 4, 5 , carbonate fuel cell components 6-9 , carbon dioxide absorbents 10 and solid electrolytes used for the production of lithium-ion batteries 11, 12 . Lithium aluminates are active catalysts for the hydrophosphinization of alkynes, alkenes and carbodiimides 13 . Lithium-based ceramics have been identifi ed as the most important material for obtaining tritium in Test Modules (TBMs) of the International Thermonuclear Experimental Reactor Project, ITER 14- 18 .
The literature review has shown that 5 compounds are formed in the two-component system of Li 2  Despite numerous studies on the Li 2 O-Al 2 O 3 system, there is still controversy about the number and type of phases formed in it, methods of their preparation and properties 19-55 . Therefore, our work aimed to verify the literature data on the Li 2 O-Al 2 O 3 system. The fi rst part of our investigations was focused on the LiAlO 2 and Li 3 AlO 3 phases.
The LiAlO 2 compound has four polymorphic modifi cations 26-46 : hexagonal α 26-29 , orthorhombic β 30, 31 , tetragonal γ 32 and the δ-LiAlO 2 formed at pressures above 9 GPa 33 . High-pressure studies carried out by Lei et al. 34 showed that the monoclinic form of β'-LiAlO 2 obtained by Cheng 35 under the pressure of 1.8 GPa is in fact the orthorhombic modifi cation of β-LiAlO 2 and it can be obtained already at the pressure of 0.8 GPa and tem-perature 623 K. On the other hand, the cubic form of LiAlO 2 described by Debray and Hardy 36 is in fact the teragonal γ-LiAlO 2 . Table 1 presents the basic crystallographic data of polymorphic modifi cations of LiAlO 2 .
The tetragonal γ-LiAlO 2 is considered to be the most thermodynamically stable polymorphic modifi cation of LiAlO 2 and it is considered to be a potential material for obtaining tritium for the purposes of nuclear fusion, substrates for epitaxial growth of II-V semiconductors such as GaN, components for the production of liquid carbonate fuel cells or radiation dosimeters 9, 12, 15-17, 37-39 . In recent years, however, attention has been paid to the hexagonal form of α-LiAlO 2 38-44 . It has been shown that at the operating temperature of the fuel cell equal to 650 o C, the alpha variety is more stable than the gamma variety 41 . The α-LiAlO 2 polymorph is also considered as a component for the production of electrode protective layers in lithium batteries 40-42 . A necessary condition for the use of α-LiAlO 2 , however, is to obtain a product containing nanometric grain size.
The literature review shows that the α-LiAlO 2 formed at temperatures not exceeding 600 o C is nanocrystalline, however, it is most often contaminated with substrates or by-products of the synthesis reaction 45, 46 . The large broadening of the diffraction refl ections of the α-LiAlO 2 obtained in such conditions is related to the presence of crystallites with dimensions of the order of 7-15 nm and a strong structure defect. SEM and TEM microscopic studies revealed the presence of dislocations and inclusions of spinel-like fragments or amorphous areas in the α-LiAlO 2 samples tested 6 . On the other hand, at temperatures above 650 o C, a slowly progressing phase transition under these conditions begins, leading to the tetragonal γ-LiAlO 2 27, 40 . The conducted literature review showed that the authors of the studies disagreed as to the temperature of phase transitions and the thermal stability of the LiAlO 2 polymorphs. Lejus 20, 47 , found that at 900 o C, α-LiAlO 2 undergoes a reversible transformation to the hightemperature γ polymorph however, the transformation from γ to α is very slow. LiAlO 2 melts at 1700 o C, but at temperatures higher than 1300 o C, it decomposes into LiAl 5  transformation of α→γ-LiAlO 2 occurs at temperatures above 600 o C. Hummel and co-workers 48 claim that the α phase undergoes a rapid phase transition at the temperature of 1200-1300 o C, and the melting point of LiAlO 2 is equal to 1610 ± 15 o C. Isupov et al. 49 investigated the effect of the gaseous atmosphere on the type of LiAlO 2 modifi cation produced using gibbsite and Li 2 CO 3 mixture. They showed that during synthesis at 800 o C in air with typical partial water pressure of 1300 Pa forms α-LiAlO 2 contaminated with small amounts of γ-LiAlO 2 . Synthesis in helium with water partial pressure not exceeding 4 Pa form both modifi cations in similar amounts but in vacuum with water pressure of 0.1 Pa mostly γ-LiAlO 2 is formed.
The structure of the α-LiAlO 2

33
and δ-LiAlO2 34 is known. The crystal lattice of the hexagonal layered α-LiAlO 2 and the high-pressure tetragonal δ-LiAlO 2 are deformed variants of the NaCl structure with ordered Li + and Al 3+ ions in the octahedral sites.
In the structure of γ-LiAlO 2 33 LiO 4 and AlO 4 tetrahedra connected by common corners form layers that connect to adjacent layers by common edges. In turn, the β-LiAlO 2 crystal lattice with a deformed wurtzite structure is built of LiO 4 and AlO 4 tetrahedrons connected via common corners 31 .
La Ginestra and co-workers 53 , as a result of heating at 400 o C for 500 hours of the mixture of γ-Al 2 O 3 and Li 2 O 2 obtained the Li 3 AlO 3 phase and presented the powder diffraction pattern of this compound. The authors did not manage to obtain single-phase Li 3 AlO 3 sample, but only a mixture containing about 40% of unreacted reagents. According to the researchers, the Li 3 AlO 3 is metastable and above 420 o C it decomposes with the release of α-LiAlO 2 and Li 5 AlO 4 53 . The authors of the study failed to obtain the Li 3 AlO 3 phase with the use of Li 2 CO 3 , LiNO 3 and Li 2 O 53 . The existence of the Li 3 AlO 3 compound was also postulated by Kroger and Fingars 54 and Fedorov and Shamari 55 , but the compound was not characterized by them.

EXPERIMENTAL
The following materials were used for the research: In the frames of this work new method of LiAlO 2 synthesis was developed. Lithium carbonate and aluminum nitrate(V) nonahydrate weighed in stochiometric proportions were ground in a mortar until the release of CO 2 bubbles ceases. The semi-fi nished product thus obtained was in the form of a paste. Subsequently, the paste obtained was heated in an air atmosphere in the temperature range of 400-600 o C, then, after taking it out of the furnace, it was cooled to room temperature in desiccator, ground in a mortar and subjected to Xray investigations.
The phase composition of samples was investigated by using XRD method and identifi ed by powder diffraction patterns of obtained samples recorded with the aid of the diffractometer EMPYREAN II, (PANalytical, The Nederlands) using the CuKa radiation with a graphite monochromator with the help of Highscore + software and PDF4+ICDD database. The powder diffraction patterns of selected phases were indexed using the RE-

RESULTS AND DISCUSSION
Transition modifi cations of alumina such as γ-, η-, δ-and θ-Al 2 O 3 obtained in the temperature range Table 1. Basic crystallographic data of α-LiAlO 2 , β-LiAlO 2 , γ-LiAlO 2 and δ-LiAlO 2 phases, where CS-crystal system: O -orthorhombic, T -tetragonal, H -hexagonal HP(GPa)-modifi cation obtain under high pressure equal to (GPa), SG (no.) -space group and its number; D -distorted structure, TW -this work by a very large number of crystallization water molecules contained in the crystal lattice of aluminum nitrate(V) nanohydrate, which, released during intense grinding, enables the aluminum nitrate(V) hydrolysis reaction leading to strong acidifi cation of the reaction medium and initiates the decomposition of Li 2 CO 3 . The mechanism of this process is currently being researched and the results will be presented in the next paper. The paste obtained after the evolution of CO 2 bubbles had ceased was then heated in a furnace under an air atmosphere in the temperature range of 400-600 o C. Figure 2 shows the diffractograms recorded after the successive stages of heating the obtained paste.
400-1000 o C have a defective spinel structure based on a cubic close packed lattice of oxide ions 60, 61 . For this reason, the powder diffraction patterns of individual modifi cations reported in the literature are similar to each other (similar d hkl values). It is very diffi cult to clearly identify these transition alumina. In this work, when writing about this type of phases, we will use the common symbol Al 2 O 3 -sl (spinel like).
In the fi rst stage of investigations synthesis of LiAlO 2 was carried out using Li 2 CO 3 and α-Al 2 O 3 , Al 2 O 3 -sl and boehmite as aluminum precursors. Figures 1A and 1B show fragments of powder diffractograms of the reaction mixtures prepared with the use of Al 2 O 3 -sl and Li 2 CO 3 (Fig. 1A) or of α-Al 2 O 3 and Li 2 CO 3 (Fig. 1B) with the compositions corresponding to the LiAlO 2 phase, and samples recorded after successive heating stages in the temperature range of 450-1000 o C. During the heating stage at 450 o C, almost all boehmite used in the synthesis decomposed to form Al 2 O 3 -sl, and the further synthesis process was carried out in this sample with the use of in situ formed precursor. A single-phase sample containing α-LiAlO 2 was obtained using boehmite and Al 2 O 3 -sl after the heating stage at the temperature of 700 o C. In both cases, the α-LiAlO 2 modifi cation appeared in reaction mixtures after a heating stage at 500 o C. Pure α-LiAlO 2 obtained after sintering at 700 o C was stable up to the temperature of 900 o C, at which the slow phase change leading to γ-LiAlO 2 began. However, a single-phase sample of γ-LiAlO 2 was obtained only after the heating stage at 1000 °C. The reaction of LiAlO 2 synthesis with the use of corundum was much slower. During it, the α-LiAlO 2 modifi cation appeared in the reaction mixture after a heating stage at 550 o C, but we failed to obtain a single-phase sample of α-LiAlO 2 . On the other hand small amounts of γ-LiAlO 2 were detected after the heating stage at 650 o C while the pure γ-LiAlO 2 was obtained after the heating stage at 950 o C (Fig. 1A and 1B).
In the frames of this work new method of LiAlO 2 synthesis was developed using a mixture of aluminum nitrate(V) and lithium carbonate as reactants. Grinding of the Li 2 CO 3 and Al(NO 3 ) 3 . 9H 2 O solids initiates the reaction between them, as evidenced by CO 2 gas bubbles intensively emitted during the grinding of the reagents in the mortar. The reaction is probably favored Single-phase sample containing α-LiAlO 2 was obtained after 30 minutes of heating at 600 o C, while the synthesis with Al 2 O 3 -sl and Li 2 CO 3 required heating the reactants at 700 o C. Lithium nitrate(V) melts at 255 o C, and boils and decomposes at 600 o C. The presence of LiNO 3 refl ections (PDF 04-010-5519) on the diffractogram of the reaction mixture after the heating step at 400 o C for 30 minutes shows that even molten LiNO 3 slowly reacts with the components of the reaction mixture. The α-LiALO 2 obtained at the temperature of 600 o C x 30 min was characterized by strongly broadened diffraction refl ections, and the average size of crystallites in this preparation determined by the Scherrer method was equal to 75Å. This value is consistent with the results of the research presented in 6 , where the effect of calcination time of the α-LiAlO 2 sample at 600 o C on the size of crystallites was analyzed. The reason for the signifi cant broadening of diffraction refl ections is, inter alia, a high concentration of defects in the crystal lattice of α-LiAlO 2 obtained at low temperatures 6 . It should be mentioned, however, that regardless of the type of metal precursors used in the synthesis of α-LiAlO 2 , the refl exes of this phase were considerable broadened. The crystallite size determined by the Scherrer method during the synthesis of α-LiAlO 2 with the use of Li 2 CO 3 and boehmite increased gradually with the increase of temperature from 101 Å (600 o C x 24 h) through 389 Å (700 o C x 24 h) to 406 Å (850 o C x 24 h).
The literature review showed that the Li 3 AlO 3 phase obtained by La Ginestra et al. 53 is relatively poorly contained a mixture of α-and β-LiAO 2 , and the intensity of refl ections characteristic of α-LiAO 2 increased. The sample after the heating step at 700 o C for 24 h contained γ-LiAlO 2 as the main component, accompanied by lower amounts of α-and β-LiAlO 2 .
studied. Taking into account the comments of the authors of the work 53 , an attempt was made to obtain the Li 3 AlO 3 phase by heating a mixture of LiOH . H 2 O and Al 2 O 3 -sl with a composition corresponding to the Li 3 AlO 3 phase in the temperature range of 400-500 o C. The diffractograms recorded after the fi rst and second heating steps at 400 o C for 72 h resembled that of the Li 3 AlO 3 phase presented by Ginestera. However, X-ray phase analysis showed that the samples obtained at 400 o C were not single-phase and contained a mixture of LiOH (PDF 00-032-0564), Li 2 CO 3 and β-LiAlO 2 (PDF 00-033-0785) (Fig. 3).  Figure 4 shows the diffractograms of the sample with the composition LiAlO 2 after subsequent stages of heating. The analysis of the XRD test results showed that the reaction mixture after the fi rst stage of heating at 500 o C for 72 h contained β-LiAlO 2 as the main component, accompanied by Li 2 CO 3 and α-LiAlO 2 in much smaller amounts. After the second stage of heating at 650 o C for 24 hours, the obtained product The conducted research indicates that the use of LiOH . H 2 O as a lithium precursor promotes the formation of β-LiAlO 2 . However, while striving to eliminate lithium carbonate from the reaction mixture by increasing the reaction temperature, the content of the α-LiAlO 2 is simultaneously increased, and above 650 o C β-LiAlO 2 undergoes a phase transition to γ-LiAlO 2 . Currently, research is conducted to obtain a single-phase β-LiAlO 2 sample and the results will be published soon.
The powder diffractograms of the β-LiAlO 2 , γ-LiAlO 2 and α-LiAlO 2 phases were indexed using the Refi nement program. The calculated values of the unit cell parameters are shown in Table 1. In the case of β-LiAO 2 , the results of the powder diffractogram pattern indexing are p resented in Table 2. To know better properties of obtained phases IR spectra of γ-LiAlO 2 , β-LiAlO 2 and α-LiAlO 2 were recorded. Analysis of the number and positions of absorption bands recorded in their IR spectra has shown good agreement with literature data 32, 47, 50-52 .  Figure 5 shows the IR spectra of γ-LiAlO 2 (curve a), β-LiAlO 2 (curve b) and α-LiAlO 2 (curve c). The literature survey has shown that crystal lattices of γ-LiAlO 2 and β-LiAlO 2 are built up of LiO 4 and AlO 4 tertahedra, when α-LiAlO 2 of LiO 6 and AlO 6 octahedra 32, 47, 50-52 . The presence of LiO 4 and AlO 4 tetrahedra in crystal lattices of γ-LiAlO 2 (Fig. 5, curve a) and β-LiAlO 2 (Fig. 5, curve b) with Li-O and Al-O bonds shorter than in the case of LiO 6 and AlO 6 octahedra is responsible for the shift of absorption bands in their spectra towards higher wavenumbers in comparison with the position of IR bands in the spectrum of α-LiAlO 2 . Moreover, good agreement of the number and positions of absorption bands in IR spectrum of β-LiAlO 2 obtained in our laboratory with data in paper 35 corroborates that polymorph of LiAlO 2 obtained by Chang crystallizes in an orthorhombic system.
A new method of LiAlO 2 synthesis was developed consisting in grinding in mortar mixture of lithium carbonate and aluminum nitrate(V) nonahydrate until the release of CO 2 bubbles ceases and subsequent heating of obtained paste in the temperature range of 400-600 o C.
As a result of the reaction of LiOH H 2 O with Al 2 O 3sl, β-LiAlO 2 was obtained, contaminated with a small amount of α-LiAO 2 .
It has been shown that the powder diffractogram of the Li 3 AO 3 phase is a set of diffraction refl ections that can be attributed to the mixture of LiOH, Li 2 CO 3 and β-LiAlO 2 .
The results of XRD and IR investigations showed that β-LiAlO 2 crystallizes in an orthorhombic system.