High-Pressure-Induced Phase Transitions in Pentaerythritol: X-ray and Raman Studies Zbigniew A. Dreger* and Yogendra M. Gupta Institute for Shock Physics and Department of Physics, Washington State UniVersity, Pullman, Washington 99164-2816 Choong-Shik Yoo and Hyunchae Cynn Lawrence LiVermore National Laboratory, UniVersity of California, LiVermore, California 94551 ReceiVed: August 3, 2005 The high-pressure response of pentaerythritol crystals has been examined to 10 GPa in diamond-anvil cells using angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy. The results reveal two first- order phase transitions: one at 4.8 GPa from phase I, tetragonal I4h(S 4 2 ), to phase II, orthorhombic Pnn2(C 2V 10 ), with a small ∼0.5% volume change, and the other at 7.2 GPa to phase III with an unknown crystal structure. We found that phase I exhibits a large crystallographic anisotropy which rapidly decreases with increasing pressure: the ratio of linear compressibilities between two primary crystal axes decreases from  o ) 8.1 at 1 atm to  P ) 2.6 at 4 GPa. We suggest that this apparent decrease in crystal anisotropy is due to the disruption of hydrogen bonding in the (001) plane of phase I and eventually leads to an orthorhombic distortion from a quadrilateral network structure in phase I to a quasi one-dimensional structure in phase II. The crystal structure of phase III exhibits a disordered character, and it is likely a conformational variant of phase II. Introduction The application of high pressure strongly modifies the intermolecular interaction in molecular solids. Pentaerythritol (2,2-bis(hydroxymethyl)-1,3-propanediol, C(CH 2 OH) 4 or PE in short) is a simple solid polyalcohol and prototypical molecular crystal of scientific and practical importance. 1-3 The molecules of this crystal are held together both by weak van der Waals (vdW) and relatively strong hydrogen interactions. The crystal structure and stability of PE at high pressure is determined mainly by the balance between these two types of interactions. Therefore, high-pressure studies on PE are valuable for under- standing the relationship between these short-range inter- molecular interactions and the long-range crystal order of PE phases. Because of its relatively simple structure, PE can also be considered as a model system for studying phase stability of molecular crystals with hydrogen bonding. At ambient conditions, PE crystallizes into a body-centered tetragonal (bct) structure with two molecules per unit cell (I4h space group) as shown in Figure 1. 4-9 In this structure, PE molecules are arranged in layers parallel to the (001) plane. While relatively weak vdW interactions exist between the layers, PE molecules within the layers are held by relatively strong hydrogen bonds. In Figure 1, each PE molecule is connected with four adjacent molecules via eight symmetry equivalent hydrogen bonds (marked in blue lines), which form a quasi- planar quadrilateral network structure in the (001) plane. Our recent Raman studies 10 on PE single crystals revealed a number of abrupt spectral changes which indicated the onset of phase transitions at 4.6 ( 0.2 and 6.8 ( 0.3 GPa. However, little is known about the exact nature of crystal structures and chemical bonding in these high-pressure phases. Therefore, the objective of the present study was to determine the molecular/ crystal structural information about PE phases through syn- chrotron powder X-ray diffraction and Raman spectroscopy. Our results suggest that the crystal structure of phase II, Pnn2(C 2V 10 ), and the I f II transition are manifested by pressure-induced changes in hydrogen bonding in the ab-plane. In this paper, we also compare our results on the low-pressure phase I with previous results 11 obtained to 1.15 GPa. Experimental Methods Polycrystalline pentaerythritol (99+% purity, Sigma-Aldrich) was used without further purification in the present study. For both powder X-ray diffraction and Raman scattering, PE crystals were ground in a mortar to powders with a grain size less than a few micrometers. A diamond-anvil cell (DAC), furnished with 0.5 mm diamond anvils mounted on a Be seat on one side for X-ray diffraction and a WC slit on the other side for Raman spectroscopy, was used to generate high pressures. PE powder and ruby particles were loaded into a small hole (0.2 mm in diameter and 0.08 mm thick) drilled at the center of a preindented stainless steel gasket placed on the top of a diamond anvil. The entire DAC assembly with the sample was then immersed in liquid Ar which was used as a hydrostatic pressure- transmitting medium. Pressure was determined from the fre- quency shift of the ruby R 1 fluorescence line. 12-14 By monitoring the separation and widths of both R 1 and R 2 lines, we confirmed that hydrostatic conditions were maintained throughout these experiments. The precision in our pressure measurements was estimated to be around 0.05 GPa. Raman measurements were performed using a cw Ar-ion laser (514.5 nm line), a 0.6 m triple spectrometer, and a liquid nitrogen cooled charge couple detector (CCD). Pressure-induced shifts of overlapping Raman bands were analyzed by fitting the * To whom correspondence should be addressed. E-mail: dreger@wsu.edu. 22581 J. Phys. Chem. B 2005, 109, 22581-22587 10.1021/jp0582181 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/04/2005