Regular Article

Effect of bromine deficiency on large elastic moduli of alpha-phase diisopropyl ammonium bromide (α-DIPAB) molecular crystals

¹ Department of Physical Sciences, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
² Department of Physics, The Hashemite University, P.O. Box 330127, Zarqa 13133, Jordan
³ Department of Physics, University of Nebraska at Omaha, Omaha, NE 68182, USA
⁴ Department of Physics, Khalifa University of Science and Technology, P.O. Box 127788, Abu Dhabi, UAE

^a e-mail: alsaad11@just.edu.jo

Received: 9 May 2019
Received in final form: 11 September 2019
Published online: 13 January 2020

Abstract

Elastic stiffness moduli were studied using dispersion-corrected density functional theory. The elastic stiffness moduli of α-DIPAB molecular crystals are found to be strongly anisotropic, with exceptionally high values of ~55 GPa. The magnitude of elastic stiffness modulus is strongly correlated with the relative orientation between the underlying hydrogen-bonding networks of DIPA molecules (“stitched” together by Br ions). These values of elastic stiffness modulus are remarkably high and suggest the design of hydrogen bond networks as a route for rational design of ultra-stiff molecular solids. Furthermore, Young’s modulus of α-DIPAB was found to attain extremely large value of as large as 50 GPa along certain crystallographic directions, while Br-deficient DIPAB has reduced Young’s modulus ( ~18 GPa). Anisotropy of Young’s modulus in α-DIPAB is very large with values below 40% of its maximum along specific spatial directions (and even lower in Br-deficient crystals). α-DIPAB and Br-deficient DIPAB show very different directionality of Young’s modulus due to the change in H-Br bond network upon Br deficiency. Additionally, Poisson’s ratio is strongly anisotropic as well with values ranging between a maximum of 0.4 for certain crystallographic directions and about 0.25 for other directions indicating the directionality of bonding in α-DIPAB. DIPAB systems are brittle based on the ratio between bulk and sheer elastic constants. Thus, DIPAB should be used as an element of composite materials to be used in thin-film flexible electronic application.