On the phase-transition in anthracene induced by high pressure Roland Resel a, * , Martin Oehzelt a , Katsuya Shimizu b , Atsuko Nakayama c , Kenichi Takemura d a Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria b Division of Materials Physics, Graduate School of Engineering Science and Research Center for Materials Science at Extreme Conditions, Osaka 560-8531, Japan c Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8565, Japan d National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan Received 2 September 2003; accepted 17 September 2003 by J.F. Sadoc Abstract The crystallographic structure of anthracene is studied under high pressure up to 27 GPa using diamond anvil cells. Different pressure-transmitting media (helium, a methanol – ethanol mixture, a normal- and iso-pentane mixture and no medium) were used up to specific critical pressures where the hydrostatic conditions are lost. It is clearly observed that the loss of hydrostaticity is associated with a crystallographic phase-transition in anthracene. The phase-transition is observed as reversible, and it is accompanied by a large hysteresis. These observations clarify conflicting reports in the literature on the high-pressure phase-transition of anthracene. q 2003 Elsevier Ltd. All rights reserved. PACS: 62.50. þ p; 61.50.Ks; 81.30. 2 t; 61.10.Nz Keywords: A. Organic crystals; C. X-ray scattering; D. Phase transitions; E. High pressure 1. Introduction The aromatic molecule anthracene (C 14 H 10 ) is since many decades a model molecule for conjugated organic semiconductors. Early studies on anthracene reveal several typical semiconducting effects like photoconduc- tivity, the photovoltaic effect, electroluminescence, a.o. Due to the high purity and quality of anthracene single crystals the charge carrier mobilities are still one of the highest observed in molecular crystals; values up to 1 cm 2 /V s are determined. The structural properties of anthracene are well charac- terised by various scattering techniques. Anthracene is a rigid molecule which crystallizes at atmospheric pressure and room temperature in the monoclinic spacegroup P2 1 =a with lattice constants of a ¼ 8:554  A; b ¼ 6:016  A; c ¼ 11:174  A and b ¼ 124:608 [1]. Within the crystal structure the molecules are arranged in a layered herringbone pattern. Temperature dependent single crystal studies reveal some changes in the molecular packing of anthracene and weak changes in the conformation of the individual molecules [2]. However, no temperature-induced phase-transition is observed down to 94 K. High pressure studies of anthracene are quite conflicting: a pressure-induced phase is reported at pressures between 1 and 3 GPa [3–9]. However, some reports could not observe any phase-transitions [10–17]. This paper is a detailed study about the appearance of the pressure-induced phase in anthracene as a consequence of non-hydrostatic conditions. The reasons for the conflicting literature data on that topic are clarified. 0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2003.09.019 Solid State Communications 129 (2004) 103–106 www.elsevier.com/locate/ssc * Corresponding author. Tel.: þ43-316-873-8476; fax: þ 43-316- 873-8466. E-mail address: roland.resel@tugraz.at (R. Resel).