Neutron diffraction study of crystal structure and temperature driven molecular reorientation in solid α-CO

Lijuan Wang,Jianzhong Zhang,Hongwu Xu,Wei Guo,Yusheng Zhao,Jinlong Zhu,Chuli Sun

doi:10.1063/1.5121337

Abstract

Neutron diffraction studies have been carried out on carbon monoxide (CO) in the temperature range of 14–68 K. From the data of the temperature dependent crystal parameters, the volume thermal expansion β (T), C–O bond length, and intermolecular distance of CO have been determined and compared with calculations. The volume evolution of cubic CO solids indicates a normal thermal expansion. However, the bond length of CO molecules contracts with the increase in temperature. Correspondingly, the distance between CO molecules increases much faster with an increase in the temperature. We find that intermolecular and intramolecular interactions account for these abnormal temperature behaviors of CO molecular crystals. The abnormal change observed in the curve β (T), evolution of C–O bond length, and distance between two CO molecules indicate an order–disorder phase transition induced by head-to-tail reorientations of CO dipoles.

Highlights

Carbon monoxide (CO) is the second most abundant gas after H2 in the universe and is a common solid molecule found in the interstellar medium and solar system ice.1 It has been reported that more than half of the atmospheric CO came from anthropogenic sources.2 Disturbance in the CO amount will perturb tropospheric chemistry, resulting in global warming and other climatic changes.3 Recently, the Planck space telescope made available the first-ever allsky map of carbon monoxide in the cosmos, which can help spot star-forming regions where carbon monoxide glows brightly
The abnormal change observed in the curve β (T), evolution of C–O bond length, and distance between two CO molecules indicate an order–disorder phase transition induced by head-to-tail reorientations of CO dipoles
The structure of solid α-CO has been identified as P213, which differs from the Pa3 structure in which CO has a molecular dipole compared with N2 molecules

Summary

Introduction

Carbon monoxide (CO) is the second most abundant gas after H2 in the universe and is a common solid molecule found in the interstellar medium and solar system ice. It has been reported that more than half of the atmospheric CO came from anthropogenic sources. Disturbance in the CO amount will perturb tropospheric chemistry, resulting in global warming and other climatic changes. Recently, the Planck space telescope made available the first-ever allsky map of carbon monoxide in the cosmos, which can help spot star-forming regions where carbon monoxide glows brightly. The CO molecule is isoelectronic with N2, in which C and O atoms are connected by a triple bond. It consists of two regular covalent bonds and one dative covalent bond, which is known as a dipolar bond or coordinate bond. 7–9, solid α-CO occupies centrosymmetric positions and has been assigned a disordered structure in the space group Pa3 (analogous to the structure of α-N2). Its molecular axes are aligned parallel to the cell diagonals These authors reject the space group P213 of a lower symmetry in which the molecular axes are directed along the four cube body diagonals and α-CO only slightly shifts from the Pa3 arrangement..

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Conclusion