Stability and equation of state of a nanocrystalline Ga-Ge mullite in a vitroceramic composite: A synchrotron x-ray diffraction study

Abstract

Synchrotron x-ray diffraction and diamond anvil cell techniques were used to characterize the phase transformations and to evaluate the structural stability at elevated pressures of a developed nanocrystalline composite. The optically transparent material was built of a germanium oxide-based amorphous host matrix with homogeneously dispersed $13\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ Ga-Ge mullite-type nanocrystals, which had a structure similar to the conventional ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2}$ mullite. The equation of state of the nanocrystals and the overall structural integrity of the nanocomposite were investigated in quasihydrostatic conditions on compression to $36\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and on the following decompression to ambient conditions. The overall pressure-induced changes of x-ray diffraction patterns evidenced that the structural integrity of the material is well preserved up to about $14--16\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The nanocomposite decompressed from $36\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ to ambient pressure showed a very limited reversibility of the pressure-driven changes. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero-pressure bulk modulus, ${K}_{0}$, for the nanocrystalline phase of $229(15)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ which makes this material potentially interesting for structural applications at elevated pressures.