Can Mono Domain Polar Molecular Crystals Exist?

• Published in: Crystal growth & design Vol. 12; no. 11; pp. 5211 - 5218
• Main Authors: Hulliger, J, Wüst, T, Brahimi, K, Martinez Garcia, J. C
• Format: Journal Article
• Language: English
• Published: Washington,DC American Chemical Society 07.11.2012
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Magnetic properties and materials; Materials science; Methods of crystal growth; physics of crystal growth; Other ferromagnetic metals and alloys; Physics; Studies of specific magnetic materials; Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation; External fields; Stochastic model; Molecular crystals; Theoretical study; Ferromagnetic materials; Crystal seeds; Intermolecular interaction; Monte Carlo methods; Onsager theory; Growth mechanism; Instability; Polarity; Square lattices
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/cg300422c

The growth of polar molecular crystals (near thermodynamic equilibrium) is investigated by a stochastic growth model. Two types of growth are considered: (i) layer by layer growth and (ii) a kink attachment model within a square lattice. An analytical treatment is in qualitative agreement with Monte Carlo simulations. Essentially, a polar seed progresses along the unique axis to form a minor number of orientational defects in one direction, whereas in the opposite direction a “global” reversal of dipolar building blocks is predicted. Polarity reversal can start from a single orientational defect, undergoing continuation and growth in consecutive layers/attachments (kink), leading finally to complete reversal of all molecular dipoles. Eventually, this behavior results in crystals featuring two sectors of opposite average polarity. The number of growth steps upon dipole reversal depends on the intermolecular interactions. It is shown that for lateral coupling close to or smaller than estimated by the Onsager theory of a ferromagnetic transition in 2D (external field H = 0), there is only a small number of attachments needed to obtain reversal, whereas for strong parallel couplings, the number of grown layers before dipole reversal increases exponentially. Our analysis shows that molecular crystals (made of dipolar building blocks) nucleating into a polar seed are subject to a fundamental growth instability which is adverse to the formation of mono domain polar crystals in general.