Synthesis and characterization of AlN-like Li3AlN2

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Journal of Crystal Growth 275 (2005) e395–e399 www.elsevier.com/locate/jcrysgro

Synthesis and characterization of AlN-like Li3AlN2 K. Kuriyamaa,, Y. Kanekoa, K. Kushidab a

College of Engineering and Research Center of Ion Beam Technology, Hosei University, Koganei, Tokyo 184-8584, Japan b Department of Arts and Sciences, Osaka Kyoiku University, Kashiwara, Osaka 582-8582, Japan Available online 8 December 2004

Abstract Li3AlN2 can be viewed as the assemblage of eight hypothetical zincblende-like sublattices (Li0.5Al0.5N)
partially filled with He-like Li+ interstitials at the empty tetrahedral sites. Li3AlN2 is synthesized by direct reaction between Li3N (powder, 99.5% pure) and Al (wire, 99.999% pure) with the molar ratio Li3N:Al of 1:1. The reaction is performed under N2 pressure of 700 Torr after the evacuation to 10
3 Torr. Typical reaction temperature and time are 1023 K and 5 h, respectively. The synthesized compounds are confirmed to be a single phase of Li3AlN2 (space group: Ia3) with lattice parameter 9.427 A˚ by a powder X-ray diffraction method. Twelve Raman peaks are observed, although the factor group analysis for Li3AlN2 allows eight peaks (Raman active modes: Ag+2E1g+2E2g+3Fg), indicating that the rest four peaks originate from the decrease in the lattice symmetry due to the distortion between Al and N bonds. With the reaction temperature above 1273 K, wurtzite-AlN is synthesized instead of Li3AlN2, which results from the extreme vaporization of lithium and nitrogen from Li–Al–N matrix. The band gap of Li3AlN2 evaluated using optical absorption and photoacoustic spectroscopy methods is 4.4 eV. r 2004 Elsevier B.V. All rights reserved. PACS: 81.10.Jt; 81.05.Je; 81.70.Cv; 78.40.
q Keywords: A1. Crystal structure; A1. Characterization; A1. X-ray diffraction; B1. Nitrides; B3. Semiconducting ternary compounds

1. Introduction Cubic nitride semiconductors such as III–V nitride-like LiXN (X ¼ Zn, Mg) [1–3] have been investigated as promising alternatives of wurtzite III–V nitrides such as AlN and GaN. LiZnN (or Corresponding

author. Tel.: +81 42 387 6185; fax: +81 42 387 6122. E-mail address: [email protected] (K. Kuriyama).

LiMgN) can be viewed as a zincblende GaN-like (ZnN)
lattice (or a zincblende AlN-like (MgN)
lattice) partially filled with Li+ at the tetrahedral sites, leading to the filled tetrahedral structure [1–3] (the space group F4-3m), although it belongs to a Nowotny-Juza compound with antiflourite structure [4,5]. In the previous studies [1–3], both materials were synthesized by a direct reaction between NH3 (or N2) and LiX alloys and the band gap values were reported to be 1.91 and 3.2 eV,

0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.009

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respectively. Particularly, the band gap nature of LiZnN, which has been predicted by an interstitial insertion rule [6], was confirmed to be direct. These results would induce an interest in other cubic nitrides such as Li3GaN2 and Li3AlN2, which were first reported by Juza et al. [4,5]. On the other hand, there has been much interest in the bulk synthesis of nitride materials such as GaN and AlN recently [7–10], since bulk nitrides are ideal substrates for homoepitaxial growth of highquality nitride films. However, it was generally difficult to obtain large size of bulk nitrides, because it decomposes at the temperature much lower than its melting point. In spite of this, Song et al. [10] prepared wurtzite GaN platelet crystals with a size of 1–4 mm by a Li-based flux method. Their synthesis method is based on the following two-step chemical reactions: 2Li3N+Ga ¼ Li3GaN2+3Li and Li3GaN2+Ga ¼ 2GaN+3Li. This suggests that AlN can be obtained via Li–Al–N matrix. In the present study, we describe the synthesis and characterization of AlN-like Li3AlN2 and demonstrate the synthesis of AlN via Li3AlN2.

2. Crystal structure and synthesis Li3AlN2 is a Nowotony–Juza compound and the crystal structure of Li3AlN2 depicted by Juza et al. is shown in Fig. 1(a) [4,5]. White and black circles are N and Al atoms, respectively. Li atoms are not shown in the figure for clearly understanding the allocation of each atom. Eight small cubic lattices (drawn by dotted lines) with two Al and six Li atoms at the respective corners are inside the unit cell. All atoms in the unit cell of Li3AlN2 are placed in the manner of preserving the point symmetry operation concerned with the body centered N atom. As a result, the two Al atoms occupy diagonal sites of the small cubic lattice as shown in Fig. 1(a). According to Juza et al., the Al–N bond distances are about 12% smaller than the distances from the N atoms to the other tetrahedral sites [4,5]. On the other hand, Li3AlN2 has an alternative representation of the unit cell for Li3AlN2. The bold lined cubic depicted in Fig. 1(a) is a 18 sublattice of an

Fig. 1. (a) Crystal structure of Li3AlN2 (the space group: Ia3) depicted by Juza et al. (Refs. [4,5]). White and black circles represent N and Al atoms, respectively. Li atoms are not shown in the figure, although they occupy the corners of the small cubes drawn by dotted lines. The bold lined cubic depicted in the unit cell is a 18 sublattice of an alternative unit cell, which corresponds to Fig. 1(b). The Al–N bond distances are about 12% smaller than the distances from the N atoms to the other tetrahedral sites. (b) A hypothetical zincblende-like (Li0.5Al0.5N)
filled with He-like Li+ at the empty tetrahedral sites next to anions, which is the 18 sublattice (solid lines) of the translated unit cell of Li3AlN2.

alternative unit cell of Li3AlN2, which corresponds to Fig. 1(b). As shown in Fig. 1(b), it is considered that in each sublattice, 50% of Al atoms in zincblende AlN are substituted with Li, leading to a zincblende AlN-like (Li0.5Al0.5N)
lattice.

ARTICLE IN PRESS K. Kuriyama et al. / Journal of Crystal Growth 275 (2005) e395–e399

Therefore, each sublattice can be viewed as zincblende AlN-like (Li0.5Al0.5N)
filled with Li+ at the empty tetrahedral sites next to anions, although two Al atoms are placed at the corners on an only one side in the small cubes in the new unit cell. In the new unit cell of Li3AlN2, all the atoms possess the 1801 rotation symmetry around the z-axis to preserve highly symmetric distribution of Al atoms as shown in Fig. 1(b), resulting in space group Ia3 of Li3AlN2 instead of F4-3 m. Li3AlN2 is synthesized by direct reaction between Li3N (powder, 99.5% pure) and Al (wire, 99.999% pure). A Ta crucible was used as a container because of the tolerance for Li attacking and was charged with these two raw materials with the molar ratio Li3N:Al of 1:1. The stoichiometry of the raw materials was severely required for the preparation of Li3AlN2. The deviation from the stoichiometry may cause the precipitation of Li complexes during synthesis. The error in weighing the raw materials was kept within 73 at% in the present study. The crucible was set into a longitudinal resistance furnace inside a stainless chamber with a gas inlet on the sidewall. After the chamber was evacuated to 10
3 Torr, N2 was introduced from the inlet until the inside gas pressure becomes 700 Torr. The N2 pressure affects the synthesis of Li3AlN2, which would be related to the decomposition temperature of Li3N. If the N2 pressure is lower than 700 Torr, the synthesized compounds contain metallic Al and Li–Al complex materials, which suggests that Li3N decomposes to Li and N2 before the reaction among Li, Al and N occurs. Typical reaction temperatures were 1023 K for Li3AlN2. With the reaction above 1273 K, wurzite-AlN tends to be synthesized. Each temperature was kept for 5 h. The synthesized compounds were polycrystalline and white in color. The compounds synthesized below 973 K contain a large amount of Li–Al complex alloy.

3. Structural identification The structural identification of the synthesized compounds was performed by an X-ray diffraction (XRD) analysis and a Raman spectroscopy. Fig. 2

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Fig. 2. X-ray powder diffraction patterns of Li3AlN2 (synthesized at lower temperature) and AlN (higher temperature).

shows XRD patterns of the compounds synthesized at (a) 1023 K and (b) 1323 K for 5 h, which were analyzed using a diffractometer (Rigaku Gigar-Flex Rad-C, CuKa radiations). The 1023 K synthesized compounds were confirmed to be a single phase of Li3AlN2 (space group Ia3) ( The value with the lattice parameter, a ¼ 9:427 A: is close to the reported one [4]. This result suggests that the synthesis at 1023 K is based on the chemical reaction between Li3N and Al as follows: 2Li3N+2Al+N2 ¼ 2Li3AlN2. However, the XRD patterns for the compounds synthesized above 1323 K showed a remarkable change as shown in Fig. 2(b), indicating a critical transformation of the crystal structure. Indeed, from Fig. 2(b), the compounds synthesized at 1323 K were confirmed ( to be a single phase of wurtzite AlN (a ¼ 3:135 A; ( c ¼ 4:979 A) instead of Li3AlN2. This structural transformation would be caused by rearrangement of Al and N atoms due to the extreme vaporization of Li and N from Li–Al–N matrix via the chemical reaction: 2Li3AlN2 ¼ 6Li+2AlN+N2.

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Fig. 3 shows the Raman spectra of the compounds synthesized at (a) 1023 K and (b) 1323 K. 12 Raman peaks (234.5, 254.0, 278.5, 321.0, 394.0, 415.5, 454.0, 482.5, 517.5, 536.5, 576.0 and 610.0 cm
1) were observed for the 1023 K synthesized compounds, although the factor group analysis for Li3AlN2 allows eight peaks (Raman active modes: Ag+2E1g+2E2g+3Fg), indicating that the rest four peaks originate from the decrease in the lattice symmetry due to the distortion between Al and N bonds [4,5]. However, we could not make more detailed assignment of each peak in the present study, since the samples were polycrystalline. For the 1323 K synthesized compound, the Raman peaks decreased to four lines, which are assigned to the E2-low at 246.0 cm
1, A1(TO) at 613.0 cm
1, E2-high at 648.0 cm
1, and E1(TO) phonon mode at 664.5 cm
1 of wurtzite AlN [12]. These results suggest that the bonding character of the synthesized compounds is changed from a zincblende-

Fig. 3. Raman scattering spectra of Li3AlN2 (synthesized at lower temperature) and AlN (higher temperature).

like to a wurtzite-like one. The Raman scattering data are consistent with the XRD results.

4. Band gap evaluation by optical absorption and photoacoustic spectroscopies Fig. 4 shows a typical transmission spectrum of Li3AlN2 (solid line), taken by a scanning spectrophotometer (Shimadzu UV-3101 PC) at room temperature. This instrument was used for determining the band gap value of LiZnN [1,2] and LiMgN [3]. The transmission curve showed an abrupt change at 400 nm (3.10 eV) and decreased down to 275 nm (4.51 eV), suggesting that the absorption edge of Li3AlN2 lies between 3.10 and 4.51 eV. The inset shows the photon energy (E) dependence of the optical absorption coefficient (a). When the linear portion of the plot is extrapolated to a ¼ 0; the band gap Eg is estimated

Fig. 4. Transmission spectrum (depicted by solid line) of Li3AlN2. The inset shows the photon energy dependence of the optical absorption coefficient. Photoacoustic spectrum of Li3AlN2 (dashed line) was observed for confirming the band gap value obtained by the optical absorption.

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to be about 4.4 eV. The details of the band gap nature for Li3AlN2 will be reported elsewhere. Photoacoustic spectrum of Li3AlN2 (dashed line) was observed for confirming the band gap value obtained by the optical absorption. Photoacoustic spectroscopy (PAS) is a useful tool for studying optical absorption spectra of light scattering materials such as powders and polycrystalline samples [2,3]. PAS is also expected to be applicable to the study of nonradiative process in excited solids, since photoacoustic (PA) signal seems to be related to the nonradiative part of de-excitation or recombination. Although a light intensity of xenon lamp is relatively weak in the energy range between 4.2 eV and 5.0 eV, the PAS system with a 700 W xenon lamp as a light source was used to detect the appearance of the energy gap state. The PA signal picked up with a microphone was detected through a lock-in amplifier at room temperature. All the spectra were normalized against a carbon black standard. The PA spectra were recorded at 270 Hz. Since PAS is concerned with the amount of absorbed light instead of the reflected or transmitted light, the influence of light scattering effects is greatly reduced [11]. Therefore, the samples for the PAS measurement were obtained by powdering the 1023 K synthesized Li3AlN2. The PA signal of Li3AlN2 began to increase at 400 nm (3.1 eV) and abruptly enhanced at 290 nm (4.4 eV). This abrupt enhancement would be mainly attributed to the nonradiative process of excited carriers arising from the absorption of photon energy. This result supports the absorption data.

5. Conclusion In conclusion, we have synthesised Li3AlN2 ( space group: Ia3) (lattice parameter a ¼ 9:427 A; by the direct reaction between Li3N and Al with the molar ratio Li3N:Al of 1:1 at 1023 K. Wurtzite ( c ¼ 4:979 A) ( was obtained AlN (a ¼ 3:135 A; by the synthesis above 1273 K because of the

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rearrangement of Al and N atoms due to the extreme vaporization of Li and N. Twelve Raman peaks were observed for Li3AlN2, although eight peaks (Raman active modes: Ag+2E1g+2E2g+3Fg) are allowed, indicating that the rest four peaks originate from the decrease in the lattice symmetry due to the distortion between Al and N bonds. The band gap of Li3AlN2 evaluated using optical absorption and photoacoustic spectroscopy methods is 4.4 eV.

Acknowledgements The present work was supported in part by a Grant-in-Aid for Scientific Research on Basic Areas from the Japanese Ministry of Education, Culture, Sports, Science and Technology (No.15360014) and also supported by a Yazaki Memorial Foundation for Science and Technology.

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