Phase transition studies in trans ‐stilbene crystal using 2D‐ACAR

Abstract

PACS 64.70.Md, 78.70.Bj Two-dimensional electron momentum distributions (EMD) mapping in a (c-a) plane of molecular transstilbene crystal have been made, using the two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) at room temperature (RT), 210 K and 110 K. Further, we discuss the changes in static and dynamic electron momentum distribution and the positron behavior in molecular crystals at different temperatures to clarify their static and dynamic origin. An attempt has been made to visualize the electronic structure of organic molecular crystal trans-stilbene during phase transition. The mapped EMD is explained with the combination of disordered to ordered transition concept and molecular vibration freezing model.1 Introduction Trans-stilbene, one of the most useful organic crystal scintillators with high scintillation efficiency and a short decay time is used in the detection of particles by pulse shape discrimination method [1]. Stilbene is noted for its interesting electronic structure and liquid crystal features at room temperature (RT). The luminescence in organic scintillators generally depends on the local electronic environment. The efficiency in luminescence is associated with conjugated aromatic organic molecules with its π-electronic structure [2][3][4][5][6][7][8]. Trans-stilbene crystal belongs to the monoclinic space group P2 1 /c with four molecules in the unit cell [9][10][11][12][13][14].Crystallographic studies on trans-stilbene have shown high structural disorder at RT with one of the two independent sites showing orientational disorder, while it is perfectly ordered at low temperature [9][10][11][12][13]. A quasi continuous 'pseudo-second order' (215-225 K) and a second order (105-115 K) phase transitions were reported based on Raman and luminescence studies [14,15]. Positron life-time measurements have revealed that the delocalised electrons hinders of positronium (Ps) formation due to its aromatic structure at RT. These studies also revealed the phase transition at low temperature and a meager probability of Ps formation [16]. Further the 2D-EMD mapping at RT and the hindrance of Ps formation was reported for trans-stilbene by the authors for the first time [23]. Differential Scanning Callorimetry studies on these materials also exhibited a spreading of the phase transition and revealed the orderliness and disorderliness in its lattice structure over a wide temperature range [16]. The disorderliness is due to the rotational vibration of benzene rings around the normal axis to the two carbons of ethylene bond. The torsional vibrations of ethylene carbon and phenolic group change the orientation of ethylene group in crystal lattice [10]. Though phase transition studies have been carried out using Raman, X-ray and Positron lifetime spectroscopies, the understanding of the phase transition phenomenon is not clear at this