Temperature-Induced Density Anomaly in Te-Rich Liquid Germanium Tellurides:pversussp3Bonding?

Abstract

The density anomaly of liquid Ge(0.15)Te(0.85) measured between 633 and 733 K is investigated with ab initio molecular dynamics calculations at four temperatures and at the corresponding experimental densities. For box sizes ranging from 56 to 112 atoms, an 8 k-points sampling of the Brillouin zone is necessary to obtain reliable results. Contrary to other Ge chalcogenides, no sp(3) hybridization of the Ge bonding is observed. As a consequence, the negative thermal expansion of the liquid is not related to a tetrahedral bonding as in the case of water or silica. We show that it results from the symmetry recovery of the local environment of Ge atoms that is distorted at low temperature by a Peierls-like mechanism acting in the liquid state in the same way as in the parent solid phases.

Evidence for electronic gap-driven metal-semiconductor transition in phase-change materials

“…It is apparent that all basic experimental features of () in both the amorphous and crystalline phase are reproduced. Note that the technique has the precision to correctly obtain both the frequency and conductivity scales to be in close correspondence with experiment [the density functional approach is known to underestimate energy gaps (14)]. We also note that optical conductivities of the amorphous state obtained under thermal-and under volumequench conditions are essentially identical.…”

“…The Car-Parrinello ab initio MD (12) studies presented herein were performed by using Kohn-Sham Density Functional Theory in conjunction with the B-LYP approximate density functional (31), and superscalable software and methods (27) on IBM's Blue Gene/L supercomputer. The computations were validated via extensive comparisons to previously published results (13,14) and our own experimental datasets. The AIMD structure factors S(Q) for the crystalline and amorphous phases agree closely with experiments (see Figs.…”

“…Electronic driving mechanisms for the phase-change considered early on (2, 10) were never satisfactorily proved (11), whereas numerous ab initio molecular dynamics (MD) studies (12)(13)(14)(15) tended to emphasize local atomic rearrangements (16) while sidelining the relationship to electronic structure.…”

“…This-in addition to the isothermal means of reaching the amorphous state being simpler-significantly reduces the demand on computational resources (12,14) required to track the process by using ab initio MD (AIMD) and to test the MD results against our experimental data.…”

Communication: Van der Waals corrections for an improved structural description of telluride based materials

“…7,8 In order to characterize these basic phenomena and to understand the observed anomalies, a certain number of computational studies using First Principles Molecular Dynamics (FPMD) and density functional theory (DFT) have been reported on, e.g., elemental Te 9 and on tellurium based alloys. [10][11][12][13][14] Unfortunately, FPMD usually overestimates the bond distances and atomic neighbors in telluride systems so that computed standard structural signatures (pair distribution function g(r), structure factor S(k)) fail to agree [9][10][11][12][13][14] with corresponding experimental measurements from X-ray or neutron diffraction. [15][16][17] Among various flaws, one should note that in Ge 15 Te 85 the first peak of the pair distribution function g(r) corresponding to the Ge-Te bond distance is usually found at 2.78−2.85 Å, 10,14 to be compared with the measured value of 2.75 Å (at 673 K, Ref.…”