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Ab initio study on the structural and electronic properties of Li3GaP2 compared with Li3GaN2
Solid State Sciences 12 (2010) 1080e1083
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Ab initio study on the structural and electronic properties of Li3GaP2 compared with Li3GaN2 H.Y. Liu a, C.H. Hu b, S.Q. Wu b, Z.Z. Zhu b, * a b
School of Science, Jimei University, Xiamen 361021, China Department of Physics, Institute of Theoretical Physics and Astrophysics, Xiamen University, Xiamen 361005, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2010 Received in revised form 15 April 2010 Accepted 23 April 2010 Available online 21 May 2010
Structural and electronic properties of Li3GaP2 and Li3GaN2 have been investigated by the first-principles calculations within the density functional theory. The calculated lattice parameters of the two compounds are in excellent agreement with the available experimental data. Both Li3GaP2 and Li3GaN2 are direct band gap semiconductors with the band gaps of 1.26 eV and 2.37 eV, respectively. The GaeP (GaeN) and LieP bonds consist of a mixture of ionic character and covalent nature, while the LieN bond exhibits almost ionic. The bonds in the Li3GaP2 are shown to have stronger covalency and weaker ionicity as compared to the corresponding ones in the Li3GaN2. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Li3GaP2 Li3GaN2 Electronic structures First-principles calculation
1. Introduction Electronic band structures of semiconductors play a key role in determining their properties and applications in various devices such as lasers, detectors, integrated circuits, etc. The direct band gap nature of semiconductors could be more advantageous for various potential applications such as light emitting devices. Since the valence band maximum (VBM) and conduction band minimum (CBM) lie at different point in the k-space for indirect band gap semiconductors, it must accompany a phonon absorbability and emission during electron transitions. This would hinder the probability of light absorbability and light emission comparing with the direct band gap semiconductors. A lot of studies have provided a feasible method for altering a zinc-blende indirect gap semiconductor into a direct gap material, by inserting small atoms such as He or He like Liþ ions at their tetrahedral interstitial sites [1e4]. The band gaps of the IIIeV semiconductors AlN [5] and GaP [6] for the zinc-blende phase are indirect, with the VBM lies at point G and CBM occurs at point X. However, the Li3AlN2 has been confirmed to alter a wide direct band gap semiconductor with a gap value of 4.40 eV [7,8] by inserting Liþ ions at the tetrahedral interstitial sites of its original zinc-blende AlN. It induces interest in the related compound Li3GaP2, which is also belong to the class of filled
* Corresponding author. Tel.: þ86 592 2182248; fax: þ86 592 2189426. E-mail address: [email protected] (Z.Z. Zhu). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.04.023
tetrahedral compound [7e10] and has been synthesized [9]. It is expected that Li3GaP2 could fellow the same behavior (i.e., with a direct gap semiconductor). However, to our best knowledge, the intrinsic properties of Li3GaP2, especially the electronic structure, are still poor understood. An in-depth study of Li3GaP2 based on first-principles calculations would provide very useful information for its potential applications. In this work, we employed the firstprinciples method to predict the lattice parameters, to investigate the band gap nature and the bonding characteristics of Li3GaP2. Furthermore, these properties of Li3GaP2 are also compared with those of Li3GaN2. 2. Computational details All calculations on Li3GaP2 and Li3GaN2 are carried out by using the projector augmented wave (PAW) scheme [11]. The exchangecorrelation energy functional is treated within the generalized gradient approximation (GGA) [12], as implemented in the VASP code [13, 14]. Wave functions are expanded by the plane waves up to a cutoff kinetic energy of 500 eV. Brillouin-zone integrations are approximated by using the special k-point sampling of MonkhorstePack scheme [15] with a 7 7 7 grid. The convergence of the total energy with respect to both k-point sampling and planewave cutoff energy is carefully examined, so that accuracy of the total energy could be at the level of 0.001 eV. And the above calculation setup is found to provide good precision in the present study. For the convergence condition of the electronic degree of
H.Y. Liu et al. / Solid State Sciences 12 (2010) 1080e1083
freedom, the total energy change and the band energy change are at the level of 105 eV. The crystallographic cell parameters and internal atomic coordinates of Li3GaP2 and Li3GaN2 are fully relaxed until the force on each atom is less than 0.005 eV/ A. 3. Results and discussion Li3GaP2, which is isostructural with Li3GaN2 [9,10], crystallizes in cubic symmetry with the space group Ia3, as shown in Fig. 1(a). All atoms in the unit cell of Li3GaP2 are placed in the manner of preserving the inversion symmetry operation concerned with the body-centered P atom. The unit cell includes eight small sublattices with one of which shown in Fig. 1(b). It is considered that in each sublattice of Li3GaP2 (see Fig. 1(b)), 50% of Ga atoms in zinc-blende GaP are substituted with Li. Therefore, each sublattice can be viewed as a zinc-blende-like (Li0.5Ga0.5P) filled with Liþ at the empty tetrahedral sites next to P atoms. Accordingly, Li3GaP2 consisting of eight hypothetical zinc-blende sublattices leads to a filled tetrahedral semiconductor. The structural parameters of Li3GaP2 and Li3GaN2 have been optimized, using a primitive cell with 48 atoms in a body center cube structure with the space group of Ia3. The Wyckoff position of Li, Ga, P1 (N1), and P2 (N2) atoms are 48e (x, y, z), 16c (u, u, u), 8a (0, 0, 0), and 24d (v, 0,1/4), respectively. The calculated lattice constants of Li3GaP2 and Li3GaN2 together with
Fig. 1. (a) Crystal structure of Li3GaP2; (b) A sublattice in the alternative unit cell.
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Table 1 The calculated equilibrium lattice constant a, together with their available experimental and other theoretical values of Li3GaP2 and Li3GaN2. Compounds
Li3GaP2 Li3GaN2
a ( A) Present
Expt.
Theo.
11.68 9.695
11.87 [9] 9.605 [16] 9.59[9]
9.52 [10]
the available experimental and other theoretical results are listed in Table 1. Our results are in excellent agreement with the available experiment values [9,16] and other theoretical results [10]. The deviations of the predicted lattice constants of both Li3GaP2 and Li3GaN2 are less than 2% as compared to the experimental data. Furthermore, the lattice parameter of Li3GaP2 is larger than that of Li3GaN2, which can be understood mainly from two facts. One of the facts is that the electronegativity of N is larger P, leading to a stronger interaction of N atoms to their neighbors. Another fact is that the atomic radius of P is larger than that of N [17]. For both the Li3GaP2 and Li3GaN2 at their equilibrium lattice constants, the calculated electronic band structures along the high symmetry directions in the Brillouin zone are shown in Fig. 2. In addition, the total and partial density of states (DOS) for Li3GaP2 and Li3GaN2 are presented in Fig. 3. The Fermi level is set to zero energy and marked by a dot line. The calculated band gaps and other theoretical and experimental results together with those for zinc-blende GaP and GaN are listed in Table 2. As shown in Figs. 2 and 3, the valence bands of Li3GaP2 and Li3GaN2 are split into two groups. The lower one (11 eV to 9 eV for Li3GaP2; 14 eV to 12 eV for Li3GaN2) comes mainly from P 3s electrons of Li3GaP2 (the P1, N1 and P2, N2 in Fig. 3 represent the phosphor and nitrogen atoms of different positions in the sublattice, see Fig. 1(b)) and N 2s electrons of Li3GaN2 together with a minor part of Ga 3s and 3p states and negligible contribution of Li 2s and 2p states. The bands from 7 eV to the EF are mainly contributed by P 3p electrons for Li3GaP2 and N 2p electrons for Li3GaN2 and Ga 3s and 3p electrons, which exhibit strong hybridizations between P, N and Ga atoms. The value and type of band gaps for Li3GaP2 and Li3GaN2 are listed in Table 2, also listed are the available experimental and other theoretical results. The VBM and CBM occur both at the G-point for both the compounds, making them typical direct gap semiconductors. The gap value is around 1.26 eV and 2.37 eV for Li3GaP2 and Li3GaN2, respectively. The fact that the calculated gap value is remarkably less than the experimental one (4.15 eV for Li3GaN2
Fig. 2. Band structures of (a) Li3GaP2 and (b) Li3GaN2.
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H.Y. Liu et al. / Solid State Sciences 12 (2010) 1080e1083
decrease in going from Li3GaP2 to Li3GaN2 (the widths of the upper valence bands are 6.947 and 6.607 eV, the lower ones are 1.981 and 1.755 eV for Li3GaP2 and Li3GaN2, respectively). This is due to the less hybridization between the cations (Liþ and Ga3þ) and the anion (N5) for Li3GaN2, as well as the stronger ionicity of LieN and GaeN bonding in Li3GaN2, as compared to those of Li3GaP2. To further explore the bonding nature in Li3GaP2 and Li3GaN2, ! we calculated the deformation charge density, Drð r Þ, which is ! defined as the difference between the total charge density rð r Þ in the solid and the superposition of independent atomic charge densities placed at the atomic sites of the same solid, i.e.,
! Drð! r Þ ¼ rð r Þ
N X
m¼1
! ratom ! r Rm
! where R m are the atomic positions. The contour plots of the deformation charge density in the (110) plane of 1/8 sublattice (Fig. 1(b)) for Li3GaP2 and Li3GaN2 are shown in Fig. 4. The electron accumulation is depicted by solid contours lines, while the electron
Fig. 3. Total and partial density of states (DOS) for (a) Li3GaP2 and (b) Li3GaN2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[16]) is expected to be due to the underestimation from the standard DFT calculations. It is worthwhile to note that the GaP with a zinc-blende structure is an indirect band gap semiconductor, with the VBM lies at G-point and the CBM lie at X-point of the Brillouin zone [6]. It suggests that the insertion of Liþ ions into the empty tetrahedral sites next to the anions P in the corresponding hypothetical IIIeV zinc-blende-like structure causes an upward shift of the conduction band at X-point, which converts an indirect band gap material into a direct one. On the other hand, the electronic structure of Li3GaP2 is similar to that of Li3GaN2. The band gap increases (1.26 eV and 2.37 eV) and the widths of valence bands Table 2 The calculated band gaps in comparison with the available experimental and theoretical values. Compound
Eg (eV)
Eg type
Li3GaP2
Present 1.26 (GeG)
Direct
Li3GaN2
Present 2.37 (GeG) Theo. 2.57 (GeG) [10] Expt. 4.15 (GeG) [16]
Direct Direct Direct
GaP Theo. (zinc-blende)
Expt. GaN Theo. (zinc-blende) Expt.
Notes
1.4058 (GeX) [6] Indirect Using the theoretical lattice constants [6] 1.5872 (GeX) [6] Indirect Using the experimental lattice constants [6] 2.35 (GeX) [18] Indirect 1.747 (GeG) [5] 2.348 (GeG) [5] 3.29 (GeG) [18]
Direct Direct Direct
Using GGA [5] Using GGA-EV [5]
Fig. 4. Contour plots of the deformation charge density (interval 0.02 e/ A3) in the (110) plane of 1/8 sublattice (see Fig. 1(b)) for (a) Li3GaP2 and (b) Li3GaN2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
H.Y. Liu et al. / Solid State Sciences 12 (2010) 1080e1083
depletion is represented by dashed contour lines. It can be observed (see Fig. 4(a)) that for Li3GaP2 the charge is accumulated in the intermediate region between Ga and P atoms, indicating the strong hybridization between Ga and P atoms. Considering the fact that the accumulated charge is closer to P atoms rather than Ga, it can be concluded that the GaeP bonding in Li3GaP2 exhibits a strong covalent characteristic together with a weak ionic nature. For LieP bonding, although the charge is transferred mainly from Li to P, the directional character of the charge distribution is obvious. Therefore, the LieP bond consists of an ionic character and also a weak covalency. While for Li3GaN2 (Fig. 4(b)), a stronger ionic character and a relatively weaker covalent nature of GaeN bonding can be found, as compared to those for GaeP in Li3GaP2. In addition, the LieN bonding in Li3GaN2 is mainly ionic, which is again different from that in Li3GaP2. The differences between GaeP and GaeN as well as LieP and LieN bonding can be attributed to the differences in the electronegativity of Li, Ga, P and N atoms (the electronegativity increases in the order of Li, Ga, P, N). The more the difference in the electronegativity between the atoms of a bond, the stronger the ionicity of the bonding. The present study on Li3GaP2 compared with Li3GaN2 shows that the stronger the ionicity of the chemical bond is, the smaller the hybridization between the atoms shows. Furthermore, the band gaps increase and the widths of valence bands decrease for the corresponding semiconductors (in going from Li3GaP2 to Li3GaN2). 4. Conclusion In summary, we have employed the first-principles calculations within the density functional theory to study the structural and electronic properties of Li3GaP2 and Li3GaN2. Our calculated lattice constants are in good agreement with the experimental values. The Li3GaP2 and Li3GaN2 are shown to be direct band gap semiconductors with band gaps of 1.26 eV and 2.37 eV, respectively. The
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direct band gap features of the Li3GaP2 and Li3GaN2 should have important significance in the technological applications. Furthermore, the GaeN, GaeP and LieP bonds for the two compounds, respectively, are shown to be consisted of a mixture of both ionic and covalent characters, while the LieN bond is almost ionic. The differences between the GaeP and GaeN (LieP and LieN) bondings as well as the band gaps can be attributed to the differences in the electronegativity for the atoms in the corresponding bond. Acknowledgments The present work was supported by the National Natural Science Foundation (Grant No. 10774124) and Natural Science Foundation of Fujian Province (Grant No. 2008J04018) of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18]
D.M. Wood, A. Zunger, R. de Groot, Phys. Rev. B 31 (1985) 2570. A.E. Carlsson, A. Zunger, D.M. Wood, Phys. Rev. B 32 (1985) 1386. L.H. Yu, K.L. Yao, Z.L. Liu, Physica B 353 (2004) 278. S. Djeroud, L. Kalarasse, B. Bennecer, H. Salmi, F. Kalarasse, J. Phys. Chem. Solids 70 (2009) 26. Rashid Ahmed, H. Akbarzadeh, Fazal-e-Aleem, Physica B 370 (2005) 52. S. Zh. Karazhanov, L.C. Lew Yan Voon, Semiconductors 39 (2005) 161. K. Kuriyama, Y. Kaneko, K. Kushida, J. Cryst. Growth 275 (2005) e395. K. Kushida, Y. Kaneko, K. Kuriyama, Phys. Rev. B 70 (2004) 233303. R. Juza, K. Langer, K. Von Benda, Angew. Chem. Int. Ed. 7 (1968) 360. F. Kalarasse, B. Bennecer, L. Kalarasse, Phys. Lett. A 372 (2008) 1324. G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15. G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169. H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. K. Kuriyama, T. Ishikawa, K. Kushida, Proceedings of the 28th International Conference on Physics of Semiconductors AIP Conf. Proc. No. 1479. AIP, New York, 2007. J.C. Slater, J. Chem. Phys. 41 (1964) 3199. M.P. Thompson, G.W. Auner, T.S. Zheleva, K.A. Jones, S.J. Simko, J.N. Hilfiker, J. Appl. Phys. 89 (2001) 3331.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Ab initio study on the structural and electronic properties of Li3GaP2 compared with Li3GaN2 H.Y. Liu a, C.H. Hu b, S.Q. Wu b, Z.Z. Zhu b, * a b
School of Science, Jimei University, Xiamen 361021, China Department of Physics, Institute of Theoretical Physics and Astrophysics, Xiamen University, Xiamen 361005, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2010 Received in revised form 15 April 2010 Accepted 23 April 2010 Available online 21 May 2010
Structural and electronic properties of Li3GaP2 and Li3GaN2 have been investigated by the first-principles calculations within the density functional theory. The calculated lattice parameters of the two compounds are in excellent agreement with the available experimental data. Both Li3GaP2 and Li3GaN2 are direct band gap semiconductors with the band gaps of 1.26 eV and 2.37 eV, respectively. The GaeP (GaeN) and LieP bonds consist of a mixture of ionic character and covalent nature, while the LieN bond exhibits almost ionic. The bonds in the Li3GaP2 are shown to have stronger covalency and weaker ionicity as compared to the corresponding ones in the Li3GaN2. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Li3GaP2 Li3GaN2 Electronic structures First-principles calculation
1. Introduction Electronic band structures of semiconductors play a key role in determining their properties and applications in various devices such as lasers, detectors, integrated circuits, etc. The direct band gap nature of semiconductors could be more advantageous for various potential applications such as light emitting devices. Since the valence band maximum (VBM) and conduction band minimum (CBM) lie at different point in the k-space for indirect band gap semiconductors, it must accompany a phonon absorbability and emission during electron transitions. This would hinder the probability of light absorbability and light emission comparing with the direct band gap semiconductors. A lot of studies have provided a feasible method for altering a zinc-blende indirect gap semiconductor into a direct gap material, by inserting small atoms such as He or He like Liþ ions at their tetrahedral interstitial sites [1e4]. The band gaps of the IIIeV semiconductors AlN [5] and GaP [6] for the zinc-blende phase are indirect, with the VBM lies at point G and CBM occurs at point X. However, the Li3AlN2 has been confirmed to alter a wide direct band gap semiconductor with a gap value of 4.40 eV [7,8] by inserting Liþ ions at the tetrahedral interstitial sites of its original zinc-blende AlN. It induces interest in the related compound Li3GaP2, which is also belong to the class of filled
* Corresponding author. Tel.: þ86 592 2182248; fax: þ86 592 2189426. E-mail address: [email protected] (Z.Z. Zhu). 1293-2558/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2010.04.023
tetrahedral compound [7e10] and has been synthesized [9]. It is expected that Li3GaP2 could fellow the same behavior (i.e., with a direct gap semiconductor). However, to our best knowledge, the intrinsic properties of Li3GaP2, especially the electronic structure, are still poor understood. An in-depth study of Li3GaP2 based on first-principles calculations would provide very useful information for its potential applications. In this work, we employed the firstprinciples method to predict the lattice parameters, to investigate the band gap nature and the bonding characteristics of Li3GaP2. Furthermore, these properties of Li3GaP2 are also compared with those of Li3GaN2. 2. Computational details All calculations on Li3GaP2 and Li3GaN2 are carried out by using the projector augmented wave (PAW) scheme [11]. The exchangecorrelation energy functional is treated within the generalized gradient approximation (GGA) [12], as implemented in the VASP code [13, 14]. Wave functions are expanded by the plane waves up to a cutoff kinetic energy of 500 eV. Brillouin-zone integrations are approximated by using the special k-point sampling of MonkhorstePack scheme [15] with a 7 7 7 grid. The convergence of the total energy with respect to both k-point sampling and planewave cutoff energy is carefully examined, so that accuracy of the total energy could be at the level of 0.001 eV. And the above calculation setup is found to provide good precision in the present study. For the convergence condition of the electronic degree of
H.Y. Liu et al. / Solid State Sciences 12 (2010) 1080e1083
freedom, the total energy change and the band energy change are at the level of 105 eV. The crystallographic cell parameters and internal atomic coordinates of Li3GaP2 and Li3GaN2 are fully relaxed until the force on each atom is less than 0.005 eV/ A. 3. Results and discussion Li3GaP2, which is isostructural with Li3GaN2 [9,10], crystallizes in cubic symmetry with the space group Ia3, as shown in Fig. 1(a). All atoms in the unit cell of Li3GaP2 are placed in the manner of preserving the inversion symmetry operation concerned with the body-centered P atom. The unit cell includes eight small sublattices with one of which shown in Fig. 1(b). It is considered that in each sublattice of Li3GaP2 (see Fig. 1(b)), 50% of Ga atoms in zinc-blende GaP are substituted with Li. Therefore, each sublattice can be viewed as a zinc-blende-like (Li0.5Ga0.5P) filled with Liþ at the empty tetrahedral sites next to P atoms. Accordingly, Li3GaP2 consisting of eight hypothetical zinc-blende sublattices leads to a filled tetrahedral semiconductor. The structural parameters of Li3GaP2 and Li3GaN2 have been optimized, using a primitive cell with 48 atoms in a body center cube structure with the space group of Ia3. The Wyckoff position of Li, Ga, P1 (N1), and P2 (N2) atoms are 48e (x, y, z), 16c (u, u, u), 8a (0, 0, 0), and 24d (v, 0,1/4), respectively. The calculated lattice constants of Li3GaP2 and Li3GaN2 together with
Fig. 1. (a) Crystal structure of Li3GaP2; (b) A sublattice in the alternative unit cell.
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Table 1 The calculated equilibrium lattice constant a, together with their available experimental and other theoretical values of Li3GaP2 and Li3GaN2. Compounds
Li3GaP2 Li3GaN2
a ( A) Present
Expt.
Theo.
11.68 9.695
11.87 [9] 9.605 [16] 9.59[9]
9.52 [10]
the available experimental and other theoretical results are listed in Table 1. Our results are in excellent agreement with the available experiment values [9,16] and other theoretical results [10]. The deviations of the predicted lattice constants of both Li3GaP2 and Li3GaN2 are less than 2% as compared to the experimental data. Furthermore, the lattice parameter of Li3GaP2 is larger than that of Li3GaN2, which can be understood mainly from two facts. One of the facts is that the electronegativity of N is larger P, leading to a stronger interaction of N atoms to their neighbors. Another fact is that the atomic radius of P is larger than that of N [17]. For both the Li3GaP2 and Li3GaN2 at their equilibrium lattice constants, the calculated electronic band structures along the high symmetry directions in the Brillouin zone are shown in Fig. 2. In addition, the total and partial density of states (DOS) for Li3GaP2 and Li3GaN2 are presented in Fig. 3. The Fermi level is set to zero energy and marked by a dot line. The calculated band gaps and other theoretical and experimental results together with those for zinc-blende GaP and GaN are listed in Table 2. As shown in Figs. 2 and 3, the valence bands of Li3GaP2 and Li3GaN2 are split into two groups. The lower one (11 eV to 9 eV for Li3GaP2; 14 eV to 12 eV for Li3GaN2) comes mainly from P 3s electrons of Li3GaP2 (the P1, N1 and P2, N2 in Fig. 3 represent the phosphor and nitrogen atoms of different positions in the sublattice, see Fig. 1(b)) and N 2s electrons of Li3GaN2 together with a minor part of Ga 3s and 3p states and negligible contribution of Li 2s and 2p states. The bands from 7 eV to the EF are mainly contributed by P 3p electrons for Li3GaP2 and N 2p electrons for Li3GaN2 and Ga 3s and 3p electrons, which exhibit strong hybridizations between P, N and Ga atoms. The value and type of band gaps for Li3GaP2 and Li3GaN2 are listed in Table 2, also listed are the available experimental and other theoretical results. The VBM and CBM occur both at the G-point for both the compounds, making them typical direct gap semiconductors. The gap value is around 1.26 eV and 2.37 eV for Li3GaP2 and Li3GaN2, respectively. The fact that the calculated gap value is remarkably less than the experimental one (4.15 eV for Li3GaN2
Fig. 2. Band structures of (a) Li3GaP2 and (b) Li3GaN2.
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H.Y. Liu et al. / Solid State Sciences 12 (2010) 1080e1083
decrease in going from Li3GaP2 to Li3GaN2 (the widths of the upper valence bands are 6.947 and 6.607 eV, the lower ones are 1.981 and 1.755 eV for Li3GaP2 and Li3GaN2, respectively). This is due to the less hybridization between the cations (Liþ and Ga3þ) and the anion (N5) for Li3GaN2, as well as the stronger ionicity of LieN and GaeN bonding in Li3GaN2, as compared to those of Li3GaP2. To further explore the bonding nature in Li3GaP2 and Li3GaN2, ! we calculated the deformation charge density, Drð r Þ, which is ! defined as the difference between the total charge density rð r Þ in the solid and the superposition of independent atomic charge densities placed at the atomic sites of the same solid, i.e.,
! Drð! r Þ ¼ rð r Þ
N X
m¼1
! ratom ! r Rm
! where R m are the atomic positions. The contour plots of the deformation charge density in the (110) plane of 1/8 sublattice (Fig. 1(b)) for Li3GaP2 and Li3GaN2 are shown in Fig. 4. The electron accumulation is depicted by solid contours lines, while the electron
Fig. 3. Total and partial density of states (DOS) for (a) Li3GaP2 and (b) Li3GaN2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[16]) is expected to be due to the underestimation from the standard DFT calculations. It is worthwhile to note that the GaP with a zinc-blende structure is an indirect band gap semiconductor, with the VBM lies at G-point and the CBM lie at X-point of the Brillouin zone [6]. It suggests that the insertion of Liþ ions into the empty tetrahedral sites next to the anions P in the corresponding hypothetical IIIeV zinc-blende-like structure causes an upward shift of the conduction band at X-point, which converts an indirect band gap material into a direct one. On the other hand, the electronic structure of Li3GaP2 is similar to that of Li3GaN2. The band gap increases (1.26 eV and 2.37 eV) and the widths of valence bands Table 2 The calculated band gaps in comparison with the available experimental and theoretical values. Compound
Eg (eV)
Eg type
Li3GaP2
Present 1.26 (GeG)
Direct
Li3GaN2
Present 2.37 (GeG) Theo. 2.57 (GeG) [10] Expt. 4.15 (GeG) [16]
Direct Direct Direct
GaP Theo. (zinc-blende)
Expt. GaN Theo. (zinc-blende) Expt.
Notes
1.4058 (GeX) [6] Indirect Using the theoretical lattice constants [6] 1.5872 (GeX) [6] Indirect Using the experimental lattice constants [6] 2.35 (GeX) [18] Indirect 1.747 (GeG) [5] 2.348 (GeG) [5] 3.29 (GeG) [18]
Direct Direct Direct
Using GGA [5] Using GGA-EV [5]
Fig. 4. Contour plots of the deformation charge density (interval 0.02 e/ A3) in the (110) plane of 1/8 sublattice (see Fig. 1(b)) for (a) Li3GaP2 and (b) Li3GaN2. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
H.Y. Liu et al. / Solid State Sciences 12 (2010) 1080e1083
depletion is represented by dashed contour lines. It can be observed (see Fig. 4(a)) that for Li3GaP2 the charge is accumulated in the intermediate region between Ga and P atoms, indicating the strong hybridization between Ga and P atoms. Considering the fact that the accumulated charge is closer to P atoms rather than Ga, it can be concluded that the GaeP bonding in Li3GaP2 exhibits a strong covalent characteristic together with a weak ionic nature. For LieP bonding, although the charge is transferred mainly from Li to P, the directional character of the charge distribution is obvious. Therefore, the LieP bond consists of an ionic character and also a weak covalency. While for Li3GaN2 (Fig. 4(b)), a stronger ionic character and a relatively weaker covalent nature of GaeN bonding can be found, as compared to those for GaeP in Li3GaP2. In addition, the LieN bonding in Li3GaN2 is mainly ionic, which is again different from that in Li3GaP2. The differences between GaeP and GaeN as well as LieP and LieN bonding can be attributed to the differences in the electronegativity of Li, Ga, P and N atoms (the electronegativity increases in the order of Li, Ga, P, N). The more the difference in the electronegativity between the atoms of a bond, the stronger the ionicity of the bonding. The present study on Li3GaP2 compared with Li3GaN2 shows that the stronger the ionicity of the chemical bond is, the smaller the hybridization between the atoms shows. Furthermore, the band gaps increase and the widths of valence bands decrease for the corresponding semiconductors (in going from Li3GaP2 to Li3GaN2). 4. Conclusion In summary, we have employed the first-principles calculations within the density functional theory to study the structural and electronic properties of Li3GaP2 and Li3GaN2. Our calculated lattice constants are in good agreement with the experimental values. The Li3GaP2 and Li3GaN2 are shown to be direct band gap semiconductors with band gaps of 1.26 eV and 2.37 eV, respectively. The
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direct band gap features of the Li3GaP2 and Li3GaN2 should have important significance in the technological applications. Furthermore, the GaeN, GaeP and LieP bonds for the two compounds, respectively, are shown to be consisted of a mixture of both ionic and covalent characters, while the LieN bond is almost ionic. The differences between the GaeP and GaeN (LieP and LieN) bondings as well as the band gaps can be attributed to the differences in the electronegativity for the atoms in the corresponding bond. Acknowledgments The present work was supported by the National Natural Science Foundation (Grant No. 10774124) and Natural Science Foundation of Fujian Province (Grant No. 2008J04018) of China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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