Lattice dynamics and Born instability in yttrium aluminum garnet,Y3A15O12

Abstract

We report lattice dynamics calculations of various microscopic and macroscopicvibrational and thermodynamic properties of yttrium aluminum garnet (YAG),Y3Al5O12, as a function of pressure up to 100 GPa and temperature up to 1500 K. YAG is animportant solid-state laser material with several technological applications. Garnet has acomplex structure with several interconnected dodecahedra, octahedra and tetrahedra.Unlike other aluminosilicate garnets, there are no distinct features to distinguish betweenintramolecular and intermolecular vibrations of the crystal. At ambient pressure, lowenergy phonons involving mainly the vibrations of yttrium atoms play a primary role in themanifestations of elastic and thermodynamic behavior. The aluminum atoms in tetrahedraland octahedral coordination are found to be dynamically distinct. Garnet’s stability can bediscerned from the response of its phonon frequencies to increasing pressure. The dynamicsof both octahedral and tetrahedral aluminum atoms undergo radical changes undercompression which have an important bearing on their high pressure and temperatureproperties. At 100 GPa, YAG develops a large phonon bandgap (90–110 meV) andits microscopic and macroscopic physical properties are found to be profoundlydifferent from that at the ambient pressure phase. There are significant changesin the high pressure thermal expansion and specific heat. The mode Grüneisenparameters show significant changes in the low energy range with pressure. Ourstudies show that the YAG structure becomes mechanically unstable aroundP = 108 GPa due to the violation of the Born stability criteria. Although this does not rule outthermodynamic crossover to a lower free energy phase at lower pressure, this places an upper boundof P = 110 GPa for the mechanical stability of YAG.