HYDROGEN CYANIDE, CRYSTAL STRUCTURE AND PHASE TRANSITION 

Göran Svensson 

Department of Inorganic Chemistry, Chalmers  University of Technology, SE-412 96 Göteborg, Sweden,
E-mail: gorans@inoc.chalmers.se 

Keywords: Hydrogen cyanide, HCN, DCN, neutron, synchrotron

Two crystal structures of solid HCN are known with a phase transition at about -100 ^oC. The structure transforms from the orthorhombic space group Imm2 to the tetragonal I4mm, according to ref 1. The structure of HCN has been studied by several groups using various theoretical methods. These calculations often results in a structure where the HCN are arranged in parallel chains [2]. Parallel chains was also suggested from a single crystal structure determination [1].  The energy difference between the parallel and anti-parallel models are, however, only a few kcal. In order to experimentally clarify if the chains are parallel or not we have collected data from powder samples using neutron and synchrotron radiation. Data for both phases have been collected and the phase transition have been studied by synchrotron radiation. Neutron data set for hydrogen cyanide at 140 and 193 K and deuterium cyanide at 10 and 193 K were collected at the neutron reactor at Studsvik, Sweden. Synchrotron data have been collected with a image plate system at NSLS at Brookhaven National Laboratory, USA, and with a INEL CPS120 detector at MAX 2 in Lund, Sweden.

Analysis of the neutron data sets did not give a clear answer to the question if the structure contains parallel or anti-parallel chains. Both models could be fitted well to the data sets. These result as well as results from the synchrotron experiments will be presented.  

1.W. J. Dulmage & W. N. Lipscomb, Acta Cryst. 4 (1951) 330-334 
2. I. Panas, Chem Phys Lett. 194 (1992) 239-246