First-principles study of organically modified muscovite mica with   ammonium (NH$_4^+$) or methylammonium (CH$_3$NH$_3^+$) ion
  Using density functional theory calculations, we have investigated the interlayer cation exchange phenomena in muscovite mica, which is motivated by a necessity to develop flexible high-insulating covering materials. The crystalline structures, chemical bonding properties, energetics, and electronic properties of muscovites before and after exchange of interlayer K$^+$ cation with ammonium (NH$_4^+$) or methylammonium (CH$_3$NH$_3^+$) ion were calculated. It was found that the unit cell volume changes are negligibly small upon exchange with NH$_4^+$ ion, while the unit cells are expanded with about 4 \% relative rate when replacing the interlayer K$^+$ cation with CH$_3$NH$_3^+$ ion. The energy band gap of pre-exchanged muscovite was calculated to be about 5 eV, which hardly changes upon interlayer cation exchange with either NH$_4^+$ or CH$_3$NH$_3^+$ ion, indicating the preservation of high insulating property of muscovite. The exchange energies were found to be about -100 kJ/mol for NH$_4^+$ and about -50 kJ/mol for CH$_3$NH$_3^+$ exchange, indicating that the exchange reactions are exothermic. A detailed analysis of atomic resolved density of states and electron density redistribution was provided. 
Chol-Jun Yu, Song-Hyok Choe, Yong-Man Jang, Gwang-Hyok Jang, and   Yong-Hyon Pae
cond-mat.mtrl-sci