Structural basis for the fast phase change of Ge2Sb2Te5: Ring statistics analogy between the crystal and amorphous states

Abstract

The three-dimensional atomic configuration of amorphous Ge 2 Sb 2 Te 5 and GeTe were derived by reverse Monte Carlo simulation with synchrotron-radiation x-ray diffraction data. The authors found that amorphous Ge 2 Sb 2 Te 5 can be regarded as "even-numbered ring structure," because the ring statistics is dominated by four-and six-fold rings analogous to the crystal phase. On the other hand, the formation of Ge-Ge homopolar bonds in amorphous GeTe constructs both odd-and even-numbered rings. They believe that the unusual ring statistics of amorphous Ge 2 Sb 2 Te 5 is the key for the fast crystallization speed of the material.

Structure of amorphousGe8Sb2Te11:<

“…14 Our earlier DF/MD simulations on GeTe and Ge 2 Sb 2 Te 5 ͑Refs. 15, 16, and 19͒ agreed very well with measured structure factors 12 and have been extended here to a melt-quench simulation of Ge 8 Sb 2 Te 11 . The sample size ͑630 atoms͒ reduces the effect of periodic boundary conditions and generates adequate statistics for atoms and cavities, and the simulation time ͑more than 0.4 ns͒ represents reliably the experimental procedure used to produce the amorphous phase and accommodates the structural relaxation ͑e.g., "wrong bonds"͒ that arises during rapid cooling.…”

“…In GST-225, for example, it has been proposed that a Ge atom flips between tetrahedral and octahedral sites. 11 Recent numerical simulations of the amorphous phases [15][16][17][18][19] have provided new information and a more consistent picture of both systems: ͑a͒ octahedral bond angles predominate, 12,15,17,19 ͑b͒ tetrahedrally coordinated Ge atoms do occur, 15,17,19 ͑c͒ evennumbered rings are characteristic, 12 particularly "ABAB squares" ͑A: Sb, Ge; B: Te͒, 15,18,19 and ͑d͒ Te atoms ͑and Sb in GST-225͒ are overcoordinated; i.e., they do not obey the "8 − N" rule, where N is the number of valence electrons. 13,15,17,19 One might speculate that GST-8,2,11 has similar features to GST-225, or that its GeTe-rich nature ͑x = 1 9 ͒ could lead to a close resemblance to GeTe ͑x =0͒.…”

Amorphous structures of Ge/Sb/Te alloys: Density functional simulations

“…The original DF calculations [16] agreed reasonably well with measured S(Q) and g(r) for GST [18], but several details were open to improvement. In particular, we had used the approximation of Perdew, Burke, and Ernzerhof (PBE) [20] for E xc , and the bond lengths that resulted were longer than those measured.…”

“…In particular, we had used the approximation of Perdew, Burke, and Ernzerhof (PBE) [20] for E xc , and the bond lengths that resulted were longer than those measured. A valuable collaboration between theory and experiment began when we discovered that the structure obtained in the original reverse Monte Carlo (RMC) fit to the X-ray diffraction (XRD) measurements [18] gave rise to a metallic density of states, i.e., no band gap at the Fermi energy. We combined the strengths of theory and experiment with the goal of finding a structure that: (i) reproduces the measured S(Q) and the total PDF g(r) for XRD and/or neutron diffraction, (ii) has bond lengths that are not artificially short and are consistent with EXAFS measurements, (iii) has an electronic structure with a band gap at the Fermi energy, (iv) has a DF total energy close to the minimum.…”

“…Liquid GST (900 K) is less ordered than the amorphous state. The similarities between ring statistics in a-and c-GST have been proposed as the basis for the structural PC in GST [18]. The statistics of all irreducible loops (rings) in a-and '-GST have been calculated using a bond cutoff of 3.2 Å and periodic boundary conditions (Fig.…”