Insights into the structure of the stable and metastable(GeTe)m(Sb2Te

Abstract

Using first-principles calculations, we identify the mechanisms that lead to the lowest energy structures for the stable and metastable ͑GeTe͒ m ͑Sb 2 Te 3 ͒ n ͑GST͒ compounds, namely, strain energy release by the formation of superlattice structures along of the hexagonal ͓0001͔ direction and by maximizing the number of Te atoms surrounded by three Ge and three Sb atoms ͑3Ge-Te-3Sb rule͒ and Peierls-type bond dimerization. The intrinsic vacancies form ordered planes perpendicular to the stacking direction in both phases, which separate the GST building blocks. The 3Ge-Te-3Sb rule leads to the intermixing of Ge and Sb atoms in the ͑0001͒ planes for Ge 3 Sb 2 Te 6 and Ge 2 Sb 2 Te 5 , while only single atomic species in the ͑0001͒ planes satisfy this rule for the GeSb 2 Te 4 and GeSb 4 Te 7 compositions. Furthermore, we explain the volume expansion of the metastable phase with respect to the stable phase as a consequence of the different stacking sequence of the Te atoms in the stable and metastable phases, which leads to a smaller Coulomb repulsion in the stable phase. The calculated equilibrium lattice parameters are in excellent agreement with experimental results and differ by less than 1% from the lattice parameters derived from a combination of the GeTe and Sb 2 Te 3 parent compounds.

Atomistic origins of the phase transition mechanism in Ge2Sb2Te5

“…Despite the seeming simplicity, the structural details of the m-GST structure are still under intense debate. Firstprinciples calculations have found that the intrinsic vacancies form ordered ͑111͒ planes, 32,40 which confirm previous experimental studies based on HRTEM. 38 However, there is also strong evidence for a random distribution of the intrinsic vacancies at high energy configurations, 34 which might depend on the cooling rate from amorphous to crystalline.…”

“…The GST compound, as well as other compositions, crystallizes in two well known phases, namely, the stable hexagonal [23][24][25][26][27][28][29][30][31] ͑s-GST͒ and metastable rocksalt ͑RS͒ ͑m-GST͒ phases. Experimental studies indicate that only the m-GST phase takes part in optical storage applications; 3,5,[10][11][12][13][14] however, both structures have very similar structure features as discussed by Da Silva et al 32 employing first-principles calculations. The intrinsic vacancies, which originate from Sb 2 Te 3 , are present in both phases; however, the intrinsic vacancies occupy specific lattice sites only in the RS m-GST structure, while it only separate the building GST blocks in the s-GST phase.…”

“…In both phases, the Ge and Sb atoms are distributed in such a way that maximizes the number of Te atoms surrounded by three Ge and three Sb atoms ͑3Ge-Te3Sb rule͒, which helps decrease the crystal energy. 32 In this work, we will restrict ourselves to the m-GST phase due to the key role that it plays in optical storage applications. 3,5,[10][11][12][13][14] Experimental techniques such as high-resolution transmission electron microscopy ͑HRTEM͒, x-ray diffraction study ͑XRD͒, extended x-ray absorption fine-structure spectroscopy ͑EXAFS͒, and theoretical studies for the m-GST [27][28][29][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] phase have reported that the Te atoms occupy the anion sites and Ge, Sb, and the intrinsic vacancies ͑20% in GST͒ occupy the cation sites in the RS structure.…”

Structure of amorphousGe8Sb2Te11:<

“…6 2 3 , respectively͔͒ suggest that the stability of these compounds is enhanced by maximizing the number of Te atoms surrounded by three Ge and three Sb atoms. 7 The amorphous structures of chalcogenide alloys remain a challenging problem for experimentalists and theorists. Scattering methods provide much less structural information than in crystals with long-range order, although extended x-ray absorption fine structure ͑EXAFS͒ provides details of the local coordination and bond lengths.…”

Origin of the failed ensemble average rule for the band gaps of disordered nonisovalent semiconductor alloys

“…They also have many different properties, even different chemical trends. For example, the ground states of free-standing isovalent alloys are generally phase separated, due to the positive mixing enthalpy, but those of the nonisovalent alloys are the ordered alloy structures satisfying the octahedral rule 14,15 . Their electronic structures can also be very different.…”