Mechanisms of Atomic Motion Through Crystalline GeTe

Deringer, Volker L.; Lumeij, Marck; Stoffel, Ralf P.; Dronskowski, Richard

doi:10.1021/cm400316j

Cited by 41 publications

(50 citation statements)

References 61 publications

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“…1). The bond lengths are calculated to be 3.25Å and 2.82Å, in good agreement with other DFT results which are approximately 3.25Å and 2.86Å, 11,14,36 and slightly larger than experimental values 37 (3.15Å and 2.83Å). The deviation between the computational and experimental bond Fig.…”

Section: Resultssupporting

confidence: 89%

Structural disorder in the high-temperature cubic phase of GeTe

Lei

Yuan

et al. 2018

RSC Adv.

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In traditional materials science, structural disorder tends to break the symmetry of the lattice. In this work, however, we studied a case which may be opposite to this intuition. The prototypical phase change material, GeTe, undergoes the phase transition from the rhombohedral structure to a more symmetric cubic one at $625 K. Using ab initio molecular dynamics simulations, we demonstrated that even in the cubic phase, the lattice is constructed by random short and long bonds, instead of bonds with a uniform length.Such bifurcation of the bond lengths enabled by Peierls-like distortion persists in the entire temperature range (0-900 K), yet with different degrees of disorder, e.g., the atoms are distorted along a certain direction in the rhombohedral phase (i.e., structural order) but the distortion varies stochastically in terms of direction and amplitude at high T (i.e., structural disorder). A more symmetric lattice frame coexisting with severe local structural disorder is the signature of this cubic GeTe. Our simulations have provided a theoretical support on the disordered Peierls-like distortion in the high-T cubic phase discovered earlier by X-ray experiments. By modulating the physical properties that different degrees of disorder may induce, we are able to design better functional materials for various applications in electronic and photonic devices.

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Section: Resultssupporting

confidence: 89%

Structural disorder in the high-temperature cubic phase of GeTe

Lei

Yuan

et al. 2018

RSC Adv.

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“…Figure 4a shows that PD in c-GST is not as prominent as in crystalline GeTe: its near-collinear short/long bonds are distributed around 2.93/3.10 Å (Figure 4a), compared with crystalline GeTe whose short/long bonds are 2.85/3.28 Å. [49,50] Moreover the change of the PD in c-GST with P appears to be subtle, e.g., a medium P (≈7 GPa) compresses the lattice uniformly and shortens both short and long bonds into 2.88/3.00Å (Figure 4b). In contrast, at ambient P, PD is very pronounced in a-GST: the short bonds are distributed around 2.90 Å, while the "long bonds" are peaked at 3.28 Å and can extend to 4.50 Å (Figure 4c).…”

Section: Aimd Results and Discussionmentioning

confidence: 98%

Reversing the Resistivity Contrast in the Phase‐Change Memory Material GeSb₂Te₄ Using High Pressure

Wang

et al. 2015

Adv Elect Materials

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Phase‐change memory devices distinguish “1” and “0” states by the electrical contrast between the amorphous and the crystalline phases. Under ambient conditions, the amorphous phase normally exhibits a higher resistivity, exceeding its crystalline counterpart by 2–5 orders of magnitude. Here, however, it is demonstrated that such pronounced resistivity contrast is remarkably reduced and even reversed with increasing hydrostatic‐like pressure in the prototypical phase‐change material GeSb2Te4 (GST). This anomalous resistivity reversal originates from the differences in the pressure‐induced atomic rearrangement of these two phases, as revealed by ab initio molecular dynamics simulations. Specifically, a low to medium pressure (<7 GPa) primarily compresses the bonds in crystalline GST without significantly displacing the atoms and vacancies off the lattice sites. As a result, only relatively small changes in the band structure are induced. In contrast, in amorphous GST, the fraction of voids changes drastically with pressure and the Peierls‐like distortion is greatly reduced, yet the average bond length remains almost constant. These effects eventually turn the semiconducting glass into a metallic one. Our work reveals distinct behaviors of amorphous and crystalline phase‐change materials under stress, shedding light on the mechanisms of electronic transport in different phases, and thus may have important implications on the design of phase‐change memory devices.

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“…[24] that such movements could be important in the mechanism of domain wall movement. Mechanisms involving vacancies can probably be ruled out on the basis that activation energies for diffusion of Ge in GeTe are expected to have values ≥ ~1 eV [54]. Each Debye loss peak can also be fit using the expression [45,53,55,56] …”

Section: B Acoustic Loss Qmentioning

confidence: 99%

Strain coupling, microstructure dynamics, and acoustic mode softening in germanium telluride

et al. 2016

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GeTe is a material of intense topical interest due to its potential in the context of phase-change and nanowire memory devices, as a base for thermoelectric materials and as a ferroelectric. The combination of a soft optic mode and a Peierls distortion contributes large strains at the cubic -rhombohedral phase transition near 625 K and the role of these has been investigated through their influence on elastic and anelastic properties by resonant ultrasound spectroscopy. The underlying physics is revealed by softening of the elastic constants by ~30-45%, due to strong coupling of shear and volume strains with the driving order parameter and consistent with an improper ferroelastic transition which is weakly first order. The magnitude of the softening is permissive of the transition mechanism involving a significant order/disorder component. A Debye loss peak in the vicinity of 180 K is attributed to freezing of the motion of ferroelastic twin walls and the activation energy of ~0.07 eV is attributed to control by switching of the configuration of long and short Ge-Te bonds in the first coordination sphere around Ge. Precursor softening as the transition is approached from above can be described with a Vogel-Fulcher expression with a similar activation energy, which is attributed to coupling of acoustic modes with an unseen central mode that arises from dynamical clusters with local ordering of the Peierls distortion. The strain relaxation and ferroelastic behaviour of GeTe depend on both displacive and order/disorder effects but the dynamics of switching will be determined by changes in the configuration of distorted GeTe 6 octahedra, with a rather small activation energy barrier.

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Mechanisms of Atomic Motion Through Crystalline GeTe

Cited by 41 publications

References 61 publications

Structural disorder in the high-temperature cubic phase of GeTe

Structural disorder in the high-temperature cubic phase of GeTe

Reversing the Resistivity Contrast in the Phase‐Change Memory Material GeSb₂Te₄ Using High Pressure

Strain coupling, microstructure dynamics, and acoustic mode softening in germanium telluride

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Mechanisms of Atomic Motion Through Crystalline GeTe

Cited by 41 publications

References 61 publications

Structural disorder in the high-temperature cubic phase of GeTe

Structural disorder in the high-temperature cubic phase of GeTe

Reversing the Resistivity Contrast in the Phase‐Change Memory Material GeSb2Te4 Using High Pressure

Strain coupling, microstructure dynamics, and acoustic mode softening in germanium telluride

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Reversing the Resistivity Contrast in the Phase‐Change Memory Material GeSb₂Te₄ Using High Pressure