Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Two-dimensional ionic crystals: The cases of IA-VII alkali halides and
  IA-IB CsAu   The alkali halides, known as ionic crystals, have the NaCl-type or CsCl-type
structure as the ground state. We study the structural, vibrational, and
electronic properties of two-dimensional (2D) ionic crystals from
first-principles. Two potential structures that are hexagonal and tetragonal
are investigated as structural templates. Through phonon dispersion
calculations, 8 and 16 out of 20 alkali halides in the hexagonal and tetragonal
structures are dynamically stable, respectively. The electron energy gaps range
from 6.8 eV for LiF to 3.9 eV for RbI and CsI in the tetragonal structure
within the generalized gradient approximation. By considering the Madelung
energy and the core-core repulsion, we propose a hard sphere model that
accounts for the nearest-neighbor bond length and the cohesive energy of 2D
alkali halides. The 2D CsAu in the tetragonal structure is also predicted to be
stable as an ionic crystal including only metallic elements, showing a band gap
of 2.6 eV that is higher than that of the 3D counterparts.

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The categories of this paper are: math-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-th
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The categories of this paper are: alg-geom
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The categories of this paper are: cmp-lg
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The categories of this paper are: mtrl-th
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The categories of this paper are: dg-ga
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The categories of this paper are: comp-gas
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The categories of this paper are: q-fin
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The categories of this paper are: ao-sci
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The categories of this paper are: nucl-ex
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The categories of this paper are: adap-org
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The categories of this paper are: atom-ph
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The categories of this paper are: math
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The categories of this paper are: funct-an