Structures and phases transition in hexylenediammonium pentachlorobismuthate (III) [NH3(CH2)6NH3]BiCl5 crystal

The crystal structure of [NH3(CH2)6NH3]BiCl5 was determined at: 223 K [P212121 (Z=4), a=7.788(1), b=13.886(2), c=13.972(2) Å], 308 K [P21/n (Z=8), a=19.972(3), b=7.772(2), c=20.166(3) Å, β=92.32(1)°] and 378 K [Pnma (Z=4), a=13911(2), b=7.834(7), c=14.457(2) Å]. It was consisted of isolated (BiCl52−)n anionic chains composed by distorted octahedra BiCl63− sharing two corners and +NH3(CH2)6NH3+ cations placed in the free cavities between anionic chains. In the β phase, there are two crystallographically inequivalent cations and two one-dimensional anionic chains (BiCl52−)n in which BiCl63− octahedra was doubly tilted and simply tilted. Two structural phase transitions at low and high temperatures α (P212121, 223 K)↔β (P21/n, 308 K)↔γ (Pnma, 373 K) are observed and discussed. It was crystallographically showed that both anionic and cationic entities contribute to phase transitions mechanisms. The BiCl63− octahedra were found to posses significant distortions on decreasing temperature and became more distorted in α (223 K) phase. It is argued that these deformations are caused by weak to moderate NH···Cl hydrogen bonding.

Projection of the crystal structure of [NH3(CH2)6NH3]BiCl5 down the a axis at 208 K.Figure optionsDownload as PowerPoint slideHighlights
► The crystal shows two phase transitions: α(223 K)↔β(308 K)↔γ(373 K).
► A discontinuous transition may be occurred between α and β phases.
► The α↔β and β↔γ phase transitions are of first order.
► Both anionic and cationic motions contribute to phase transition mechanisms.
► The BiCl63− octahedra showed significant distortions on decreasing temperature.