Temperature-Induced Density Anomaly in Te-Rich Liquid Germanium Tellurides: p versus sp 3 Bonding? Christophe Bichara CRMCN–CNRS and Aix-Marseille Universities, Campus de Luminy, Case 913, F13288 Marseille, France Mark Johnson Institut Laue Langevin, 6 rue Jules Horowitz, BP 156, Grenoble, France Jean Yves Raty Physique de la Matie `re Condense ´e, B5, Universite ´ de Lie `ge, B4000 Sart-Tilman, Belgium (Received 17 May 2005; published 23 December 2005) 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. DOI: 10.1103/PhysRevLett.95.267801 PACS numbers: 61.20.Ja, 71.15.Pd Density anomalies or negative thermal expansion coef- ficients in liquid or amorphous matter are commonly asso- ciated with tetrahedral bonding. A well-documented [1] example is liquid water that displays a density maximum at 277 K and atmospheric pressure. Another well-known example is amorphous silica [2]. In these two cases, direc- tional tetrahedral bonding plays a key role. p-bonded elements are another class of materials displaying such a density anomaly. For these, pure tellurium is a prominent example, as its density [3] is maximal at 733 K and decreases when the temperature is lowered below the melting temperature (723 K) in the undercooled regime. Alloys of germanium and tellurium (Ge x Te 1x ) around the eutectic composition (x  0:15) display an even sharper density anomaly in the stable liquid state [4]. In a recent study of the parent Ge 2 Sb 2 Te 5 (GST) system [5], Kolobov and co-workers proposed an ‘‘umbrella-flip’’ mechanism to account for the differences between crystal- line and amorphous phases. In this scheme, Ge atoms sur- rounded by Te change their local order from p-bonded, sixfold coordinated in the crystalline state to sp 3 , fourfold coordinated in the amorphous state. Density and electrical conductivity changes occurring during the amorphization/ recrystallization process as well as the bonding type [6] are key features for the use of GST alloys in the upcoming phase change random access memory devices. It is, there- fore, essential to validate the mechanisms involved in this transition. The structural change in liquid Ge x Te 1x is experimen- tally well-documented, and this system is best suited to theoretical investigation. Investigating this simpler system is particularly relevant since, according to Ref. [5], the changes in GST alloys are localized in the Te first neighbor shell of Ge atoms. Also, neglecting the small difference in the numbers of s and p electrons between these two sys- tems (that may play an important role [7]) and in a simple tight binding approximation with equal hopping integrals, adding Sb that is p-bonded with s and p atomic energy levels sitting between those of Ge and Te [8] should not change the hybridization state of Ge. In addition, Ge- Te is very different from Ge-X (X  O; S; Se) systems, which present a GeX 2 , tetrahedrally bonded compound, while there is no stable GeTe 2 compound in the phase diagram [10]. Owing to the relatively low temperature, accurate mea- surements of the density [4], the specific heat [11], and the electrical resistivity [12] were performed, leading to a consistent set of data indicating a structural change in the liquid state. Although numerous neutron and x-ray scatter- ing [13,14] and x-ray absorption fine structure [15] mea- surements have tried to uncover the structural changes associated with the sharp extremum of the thermodynamic response functions, no clear picture of the mechanisms involved emerges. At least two different mechanisms should be considered, assuming different types of bonding states for Ge. According to Kolobov et al. [5], a sp 3 hybridization state of Ge could be assumed, in which case the density anomaly would enter the classical scheme for tetrahedrally bonded systems, or purely p-bonding around Ge could be assumed, in which case an explanation for the density anomaly should be provided. With reference to the p-bonding scheme, it should be noted that, upon crystallization, the eutectic liquid decomposes into two p-bonded phases, namely, pure tellurium and the GeTe PRL 95, 267801 (2005) PHYSICAL REVIEW LETTERS week ending 31 DECEMBER 2005 0031-9007= 05=95(26)=267801(4)$23.00 267801-1  2005 The American Physical Society