Band Structure and Electronic Properties of Lithium Phosphide Li 3 P
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BAND STRUCTURE AND ELECTRONIC PROPERTIES OF LITHIUM PHOSPHIDE Li 3P MAX SEEL AND RAVI PANDEY Department of Physics, Houghton, MI 49931-1295
Michigan
Technological
University,
ABSTRACT Ab initio Hartree-Fock band structure calculations have been performed to study the electronic structure of Li 3P in the hexagonal P6/mmm crystal structure. The total energy, band structure, density of states, and charge densities are computed. The band structure is very similar to that calculated for lithium nitride with a small indirect gap between I and K of 2 eV. However, the charge distribution in Li 3P is more anisotropic with a greater ionicity in the x-y plane compared to the c direction. This is also supported by a large calculated core level split of 6.5 eV of the Li ls core bands. INTRODUCTION Recently, lithium phosphide Li 3P has been introduced as a good lithium ion conductor /l/. For almost a decade most of the work in lithium ion conductors (for applications in solid state lithium batteries) has been focused on lithium nitride Li 3 N, both experimentally /2,3/ and theoretically /4-7/. The synthesis and experimental study of lithium phosphide Li 3P and lithium arsenide Li 3As /3/ opened up the field and allowed to study the transition from ionic to metallic conductor. Here, we present the first Hartree-Fock band structure calculations for lithium phosphide and discuss similarities and differences in various electronic properties when compared to lithium nitride. A similar study is underway for lithium arsenide. Detailed results for monitoring and comparing ionic and electronic properties of all three compounds will be published elsewhere /8/. COMPUTATIONAL DETAILS Li 3P crystallizes in the hexagonal P6/mmm space group. The experimental geometry as described in Ref. 3 is used, i.e., a = 4.271 and c=7.590 A. Li 2P layers - lithium atoms in these layers are labeled Li(2) in Fig. 1 - alternate with pure lithium layers (Li(l) atoms) along the c direction. The lattice is expanded anisotropically compared to Li 3N (a=3.648 c=3.873
A
A,
A)/3/.
The all-electron linear combination of atomic orbitals Hartree-Fock (LCAO HF) band structure calculations are performed employing the CRYSTAL88 program /9/. The program and the various truncation criteria for the infinite lattice sums are described in a recent monograph /10/. A series of calculations was performed to obtain a basis set of a total of 40 contracted Gaussians (5s, 4p, and Id polarization functions for P, 3s and lp for each Li atom) which are based on Huzinaga's basis for P/11/ and on the Li basis of Ref. 5. They represent a triple zeta Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
156
L i MlI
L i(2)
1. Hexagonal P6/mmm structure of lithium phosphide a=4.271 A and c=7.590 A. The in-plane Li(2)-P distance is 2.466 A, the perpendicular Li(l)-P distance is 3.795 A. Fiq.
with
valence basis with a set of polarization functions. The outermost exponents for the s and p functions on P are 0.8, 0.3, and 0.1, the exponent for the d polarization fun
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