EQUATION OF STATE AND PRESSURE-INDUCED PHASE TRANSITION IN AlN: RESULTS FROM IN-SITU X-RAY AND RAMAN SCATTERING STUDIES. Murli H. Manghnani, L. C. Ming and V. Vijaykumar*, Mineral Physics Group, Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822 USA
Like the other nitrides of III-V group, AlN commonly crystallizes in the wurtzite structure at room temperature. Complementary in-situ high pressure X-ray diffraction (EDXRD), using synchrotron radiation, and Raman scattering studies have been carried out on AlN in a diamond-anvil cell at room temperature to 30 and 20 GPa, respectively. The main objectives were to determine as well as to characterize the phase transition from wurtzite (hexagonal) to rock salt (cubic) structure, and to establish the equation of state (pressure-volume relationship) for the two phases. Previous studies have reported the pressure of this transition to range from ~16.5 to 22.9 GPa. Our Raman results clearly show a continuous change of wurtzite structure to rock salt structure, beginning at pressure as low as 14 GPa, as evidenced by the splitting of A1(TO) and E2(2) modes. The pressure dependencies of all the six observed Raman modes are found to be a linear function of pressure.
The X-ray data on the other hand show the transition to occur at higher pressure (~23 GPa). The P-V relationships for the two phases are deduced. There is a good agreement between the Ko (bulk modulus) and Ko' values for the wurtzite phase deduced from the X-ray data and ultrasonic measurements on a polycrystalline specimen of AlN.
The above results are discussed in light of the previously reported shock wave results and theoretical studies.