Understanding an Order–Disorder Phase Transition in Ionothermally Synthesized Gallium Phosphates

Two new organically templated gallium phosphates were synthesized under ionothermal conditions. Single crystals were grown from a mixture of Ga(NO3)3·H2O, H3PO4, and either 1-methylpiperazine or 2-methylpiperazine, which were allowed to react in 1-ethyl-3-methyl-imidazolium bromide or 1-butyl-3-methyl-imidazolium bromide at 150 °C for 4 days. The use of 1-methylpiperazine and 2-methylpiperazine resulted in compounds that both contain [Ga(HPO4)2/2(PO4)2/2]n2n– chains and extensive hydrogen-bonding networks. Solvent effects associated with ionothermal versus hydrothermal conditions in this system result in marked differences in structure and stoichiometry. [1-MethylpiperazineH2][Ga(HPO4)(PO4)] exhibits a phase transition from an ordered monoclinic structure at 100 K to a disordered, averaged structure at 298 K, as determined using both single crystal X-ray diffraction and density-functional calculations of the total energy.