VOLUME 83, NUMBER 25 PHYSICAL REVIEW LETTERS 20 DECEMBER 1999 High Pressure Behavior of Silicon Clathrates: A New Class of Low Compressibility Materials A. San-Miguel, 1 P. Kéghélian, 1 X. Blase, 1 P. Mélinon, 1 A. Perez, 1 J. P. Itié, 2 A. Polian, 2 E. Reny, 3 C. Cros, 3 and M. Pouchard 3 1 1 Département de Physique des Matériaux, Université Claude Bernard-Lyon 1 and CNRS (UMR 5586), Bâtiment 203, 43 Boulevard 11 Novembre 1918, 69622 Villeurbanne, France 2 Laboratoire de physique des milieux condensés, Université Paris VI, 4 Place Jussieu, T13, 75252 Paris Cedex 05, France 3 Institut de Chimie de la Matière Condensée de Bordeaux, Université Bordeaux I, 33608 Pessac, France (Received 19 May 1999) The high pressure evolution of silicon clathrates is studied at room temperature by x-ray diffraction up to 15 GPa. Remarkably, no transition towards the diamond structure is observed and the clathrate phase transforms directly into the b-tin metallic phase at 11 GPa. Further, the bulk modulus is found to be 90 6 5 GPa, that is, only 8 6 5% smaller than the one of the diamond phase. These results are in good agreement with ab initio calculations which predict further that carbon clathrates, if synthesized, should be less compressible than cubic BN. PACS numbers: 61.50. - f, 64.70.Kb, 71.15.Nc, 81.40.Vw There is a great experimental and industrial interest [1,2] in obtaining low-compressibility structures having large hardness [3]. Nature provides the best known example of such a material: the carbon diamond phase. Unfortunately, the precious gem is rare and the industrial production is quite limited. Consequently, much work has been devoted to predicting novel forms of low compressibility materials. For example, on the basis of empirical and theoretical considerations relating the bulk modulus of a material to its structural and electronic properties, a candidate b-C 3 N 4 carbon-nitride compound was proposed [3] in analogy with the existing b-Si 3 N 4 structure. Even though the synthesis of carbon-nitride materials has proven to be difficult, the idea of using known silicon-based compounds as a “template” to design novel carbon-based structures is a motivating line of work. In this paper, we present a joint experimental and theo- retical study of the stability and structural properties under pressure of silicon clathrates. Such low-density phases, recently synthesized [4] for the Si and Ge elements, are metastable at ambient conditions and have been predicted [5] to have a cohesive energy larger than that of the b-tin structure [6]. However, the equation of state of such com- pounds has never been studied experimentally, leaving the phase diagram of Si incomplete on the low-density side of the diamond phase. Beyond this fundamental motivation for the study of Si clathrates, our study reveals further that such compounds display unique properties under pressure. We show, in particular, that such a material is character- ized by a transition pressure towards denser phases equiva- lent or even larger than that of the corresponding diamond structure. In addition, the compressibility of clathrates is close to the one of the diamond phase. These two results indicate therefore a novel route towards the synthesis of low-compressibility materials, providing much incentive in attempting to synthesize carbon-based clathrates. Silicon clathrates are cagelike crystals that can be assimilated to the large family of zeolite-network sys- tems. The structure studied in this work, labeled Si-34 [7], is characterized by the 3D periodic arrangement of (Si 20 -I h ) and (Si 28 -T d ) cages having shared faces (see Fig. 1). All atoms are hybridized in a nearly sp 3 con- figuration without dangling bonds. This geometry leads to very large predominance of fivefold rings (87%) that induces a 0.7 eV opening of the band gap as compared to Si-2 [8]. Si clathrates are synthesized by thermal decom- position of NaSi precursor under vacuum or argon at high temperatures, leading to Na-“intercalated” Na x @Si-34 (0 , x , 24) clathrates [9]. The Na atoms, which are located inside the cages, can be subsequently removed to produce relatively pure Si-34 phases. The main crystallo- graphic data and some relevant properties of Si-34 com- pared with Si-2 can be found in Ref. [10]. Presently, both silicon or germanium clathrates are currently produced FIG. 1. Structure of silicon clathrate (Si-34). (a) Elementary building blocks of the structure. We note that we have omitted the “sticks” representing the bonding of each Si atom outside the fullerene-type cages, linking by shared faces these basic blocks. ( b) Projection of the structure following the (111) direction. The structure can be seen as a fcc staking of Si 28 building blocks. The contact between the Si 28 units gives rise to the Si fourfold coordination as in the diamond structure. The remaining voids of the fcc staking form Si 20 cages. 5290 0031-9007 99  83(25)  5290(4)$15.00 © 1999 The American Physical Society