title: Nature of phase transitions in crystalline and amorphous GeTe-Sb2Te3 phase change materials.
abstract: The thermodynamic nature of phase stabilities and transformations are investigated in crystalline and amorphous Ge(1)Sb(2)Te(4) (GST124) phase change materials as a function of pressure and temperature using high-resolution synchrotron x-ray diffraction in a diamond anvil cell. The phase transformation sequences upon compression, for cubic and hexagonal GST124 phases are found to be: cubic → amorphous → orthorhombic → bcc and hexagonal → orthorhombic → bcc. The Clapeyron slopes for melting of the hexagonal and bcc phases are negative and positive, respectively, resulting in a pressure dependent minimum in the liquidus. When taken together, the phase equilibria relations are consistent with the presence of polyamorphism in this system with the as-deposited amorphous GST phase being the low entropy low-density amorphous phase and the laser melt-quenched and high-pressure amorphized GST being the high entropy high-density amorphous phase. The metastable phase boundary between these two polyamorphic phases is expected to have a negative Clapeyron slope.
authors.affiliation: Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley, California 94720, USA.
authors.forename: B
authors.initials: B
authors.lastname: Kalkan
authors.forename: S
authors.initials: S
authors.lastname: Sen
authors.forename: S M
authors.initials: SM
authors.lastname: Clark