X-Ray Structural Study of Li 3 N at High Pressure
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ALLEN C. HO, MAURICE K. GRANGER, AND ARTHUR L. RUOFF Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853
ABSTRACT The equation of state (EOS) of Li3 N has been determined by energy-dispersive x-ray diffraction (EDXD) using synchrotron radiation up to 35 GPa at ambient temperature. Both the hexagonal D h (P63/mmc) and the hexagonal
Dlh (P6/mmm)
phases were present at ambient
pressure. The D6h -structure completely transforms into the D h-structure at modest pressure. The change in Gibb's free energy as a function of pressure for Li 3N was calculated using the experimental EOS. INTRODUCTION Solid state scientists study lithium nitride, Li 3N, for both fundamental and technological reasons.' Lithium nitride is potentially a superionic conductor.2 Fundamentally, lithium nitride provides an excellent opportunity for the study of the N 3 - in an anomalous state. 3'4 It is well known that the contribution from the intra-ionic polarizability of 02- in alkali earth oxides dominates the optic overtone modes in second-order Raman scattering. 5 Since N3 is the heavy ion, it is expected to make major contributions to the acoustic overtone modes. Lithium nitride crystallizes in the hexagonal
Dh
(P6/mmm) structure at room
temperature.6 A metastable phase was detected at 4.2 kbar in a nuclear magnetic resonance (NMR) experiment. 7 Subsequently, this new phase was confirmed by powder x-ray diffraction to have a hexagonal D4 (P6 3/mmc) structure at 10 kbar. 8 The lattice dynamic of Li3 N has been investigated by a number of groups using inelastic neutron scattering, 9 infrared and Raman spectroscopy,l1 and ab initio calculations." However, all of these studies involved the Dlh
-
structure at ambient pressure. Interestingly, lithium nitride has been used to produce nano-sized crystals of GaN by reaction with gallium chloride (GaCI 3) in a benzene solution.12 However, at higher temperatures, 133 Mat. Res. Soc. Symp. Proc. Vol. 499 01998 Materials Research Society
higher pressures are required to insure the stability of the GaN.13 To more fully understand the thermodynamics of GaN growth using Li 3N, the EOS for Li 3N was measured up to 35 GPa and compared with an ab initio calculation. The EOS was then used to calculate the change in Gibb's free energy as a function of pressure or normalized volume.
EXPERIMENT
Lithium nitride powder of 99.5% purity was purchased from Strem Chemical Inc. The powder had a dark red color and was opaque. A preliminary powder x-ray diffraction pattern indicated that both the hexagonal D6h (P6 3/mmc) and the hexagonal D'h (P6/mmm) structures were present at ambient pressure. Energy-dispersive x-ray diffraction (EDXD) measurements were performed at room temperature at the Cornell High Energy Synchrotron Source (CHESS). Static pressure was generated by the use of a diamond-anvil cell (DAC). Two separate experiments were carried out. A pair of 1/3-carat brilliant-cut diamond anvils with 300-pim flat culets was employed for this experiment. Sample handling was done
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