Size-Induced Reduction of Transition Pressure and Enhancement of Bulk Modulus of AlN Nanocrystals

An in situ X-ray-diffraction study of AlN nanocrystals under hydrostatic (or quasihydrostatic) conditions was performed to pressures of 36.9 GPa, using an energy dispersive synchrotron-radiation technique in a diamond-anvil cell (DAC). Hexagonal AlN nanocrystals have a particle size of 10 nm on average, and display an apparent volumetric expansion as compared to the bulk AlN polycrystals. Upon compression to 14.5 GPa, AlN nanocrystals start to transform to a rocksalt structure phase. This pressure is significantly lower than the transition pressure of 22.9 GPa observed from the bulk AlN by using the same technique. The nanosized wurtzite phase has a bulk modulus (B0) of 321 ± 19 GPa, larger than that of the bulk AlN crystals with a B0 of 208 GPa. There is a large volumetric decrease of 20.5% upon the wurtzite-to-rocksalt phase transformation. Combination of the size-induced volumetric expansion and resulting softening of the Poisson ratio and shear modulus may explain the reduction of transition pressure ...