Reverse Group-Subgroup Relation at the Ferroelastic Phase Transition in [(C2H5)4N][(CH3)4N]MnBr4

Exceptional physical properties have been revealed in a newly synthesized hybrid organic-inorganic compound, [(C2H5)(4)N][(CH3)(4)N]MnBr4. At elevated temperatures, above 460 K, the crystal exhibits two-dimensional negative thermal expansion. These properties occur as a consequence of the fast temperature-induced motions of the disordered methylammonium and ethylammonium cations, and the layered crystal structure. Moreover, the crystal undergoes two successive first-order phase transitions at 484 and 565 K. The mechanism of the transition at 484 K involves an order-disorder contribution coupled with a rearrangement of the anionic framework, but the most unusual is the symmetry lowering on heating the crystal. The low-temperature tetragonal phase III of space group P (4) over bar2(1)m transforms into the orthorhombic P2(1)2(1)2-symmetric phase II, which satisfies the temperature-reversed group-subgroup relation. Ferroelastic domains consistent with Aizu's species (4) over bar 2mF222 have been evidenced in the crystal phase II. Another unique property is the increase of the order parameter with increasing temperature, above the transition point in phase II.