Local structure of liquid GeTe via neutron scattering and<i>ab initio</i>simulations

Raty, Jean‐Yves; Godlevsky, Vitaliy; Gaspard, Jean-Pierre; Bichara, Christophe; Bionducci, M.; Bellissent, R.; Céolin, R.; Chelikowsky, James R.; Ghosez, Philippe

doi:10.1103/physrevb.65.115205

Cited by 49 publications

(32 citation statements)

References 40 publications

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“…Earlier DF simulations for liquid GeTe reported diffusion constants that are significantly lower ͑1.5ϫ 10 −5 and 0.4ϫ 10 −5 for Ge and Te, respectively͒. 39 This difference from our result may arise from the shorter ͑1.6 ps͒ and smaller ͑64 atoms in the cell͒ simulation. Our calculated viscosity ranges between 1.1 and 1.2 cP for both alloys, depending on the particle radius chosen in the Stokes-Einstein relation.…”

Section: F Diffusioncontrasting

confidence: 57%

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

2007

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Phase-change materials are of immense importance for optical recording and computer memory, but the structure of the amorphous phases and the nature of the phase transition in the nanoscale bits pose continuing challenges. Massively parallel density functional simulations have been used to characterize the amorphous structure of the prototype materials Ge 2 Sb 2 Te 5 and GeTe. In both, there is long-ranged order among Te atoms and the crucial structural motif is a four-membered ring with alternating atoms of types A ͑Ge and Sb͒ and B ͑Te͒, an "ABAB square." The rapid amorphous-to-crystalline phase change is a reorientation of disordered ABAB squares to form an ordered lattice. There are deviations from the "8 − N rule" for coordination numbers, with Te having near threefold coordination. Ge atoms are predominantly fourfold coordinated, but-contrary to recent speculation-tetrahedral coordination is found in only approximately one-third of the Ge atoms. The average coordination number of Sb atoms is 3.7, and the local environment of Ge and Sb is usually "distorted octahedral" with AB separations from 3.2 to 4 Å in the first coordination shell. The number of A -A bonds is significantly greater in GeTe than in Ge 2 Sb 2 Te 5 . Vacancies ͑voids͒ in the disordered phases of these materials provide the necessary space for the phase transitions to take place. The vacancy concentration in Ge 2 Sb 2 Te 5 ͑11.8%͒ is greater than in GeTe ͑6.4%͒, which is consistent with the better phase-change performance of the former.

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Section: F Diffusioncontrasting

confidence: 57%

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

2007

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“…This is caused by the presence of Ti 3s,3p semicore states in the LDA valence charge (these states are in the core for the SIC calcula- Now we present our results for GeTe. 38,39 This is an interesting ferroelectric since it possesses extremely anomalous BEC 11 and shows electronic properties markedly different from those of the most common ferroelectric prerovskites. At high-temperature it has the rocksalt structure, and below T c ∼ 650 K 38 it undergoes a transition to a ferroelectric rhombohedral structure, with polarization parallel to the [111] axis.…”

Section: B Batio3 and Getementioning

confidence: 99%

Strong-correlation effects in Born effective charges

Filippetti

Spaldin

2003

Phys. Rev. B

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Large values of Born effective charges are generally considered as reliable indicators of the genuine tendency of an insulator towards ferroelectric instability. However, these quantities can be very much influenced by strong electron correlation and metallic behavior, which are not exclusive properties of ferroelectric materials. In this paper we compare the Born effective charges of some prototypical ferroelectrics with those of magnetic, non-ferroelectric compounds using a novel, self-interaction free methodology that improves on the local-density approximation description of the electronic properties. We show that the inclusion of strong-correlation effects systermatically reduces the size of the Born effective charges and the electron localization lengths. Furthermore we give an interpretation of the Born effective charges in terms of band energy structure and orbital occupations which can be used as a guideline to rationalize their values in the general case.

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“…Experimental work on binary Ge x Te 1−x alloys [8][9][10] and GST-225 ͑Refs. 11-14͒ has given rise to speculations concerning their structural phase changes.…”

Section: Introductionmentioning

confidence: 99%