Physica B 369 (2005) 287–292 Raman scattering studies on mercuric iodide at high pressures and at low temperatures S. Karmakar à , S.K. Deb, Surinder M. Sharma Synchrotron Radiation Section, Bhabha Atomic Research Centre, Mumbai 400085, India Received 22 July 2005; received in revised form 20 August 2005; accepted 20 August 2005 Abstract Raman scattering measurements have been performed on single crystal red mercuric iodide under high-pressures up to 5.5GPa and at low-temperatures down to 40K. Our low-temperature measurements help clarify the issues related to the accidental degeneracy of the E 1 g and B 2 1g modes in the tetragonal phase. Raman modes stiffen and their widths reduce with the lowering of the temperature. All the Raman active mode frequencies increase with pressure up to 1.3GPa, beyond which the modes of the tetragonal phase disappear and new modes appear supporting the earlier reportedred - high-pressureyellowstructuraltransitionatthispressure.Knowledgeofbothtemperatureandpressure dependences has allowed us to separate, for all the Raman active modes, the explicit phonon-excitation contribution and the implicit volume-driven contribution to the observed temperature coefficient do/dT for both the phases. r 2005 Elsevier B.V. All rights reserved. PACS: 61.50.Ks; 78.30.j Keywords: Mercuric iodide; Raman scattering; High pressure; Low-temperature 1. Introduction Mercuric iodide (HgI 2 ) is an important material due to its potential use in opto-electronic devices [1] and g-ray spectrometry [2]. The temperature– pressure phase diagram of HgI 2 shows that it can exist in several tetrahedral-framework semicon- ductor and molecular structures. Crystallographic structure identification of some of these phases, as a function of temperature and pressure, has been carried out by Hostettler et al. [3]. Depending on the method of growth, they have identified three phases of HgI 2 at ambient conditions, which can be grossly identified by their color—red, yellow and orange phases. Of these, yellow and orange phases are found to be unstable under even small mechanical stresses. Under ambient condition ARTICLE IN PRESS www.elsevier.com/locate/physb 0921-4526/$-see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.08.028 à Corresponding author. Tel.: +912225591312; fax: +912225505151. E-mail address: sdak@magnum.barc.ernet.in (S. Karmakar).