Crystal Structures of Nitronium Tetranitratogallate and Its Reversible Solid-State Phase Transition Mediated by Nonmerohedral Twinning

Single-crystal X-ray crystallographic analyses of [NO2][Ga(NO3)4] reveal that it undergoes a reversible phase
transition without any apparent damage to the crystal during repeated temperature cyclings. The room-temperature,
noncentrosymmetric, body-centered tetragonal (I 4̄), polymorph 1 (a = 9.2774(3) Å, c = 6.1149(2) Å, Z = 2)
consists of well-separated nitronium and tetranitratogallate ions. The [Ga(NO3)4]- units exhibit a slightly squashed
tetrahedral geometry in which all of the ligands are monodentate. Below approximately 250 K, distortions lower
the symmetry to the chiral, body-centered monoclinic nonstandard space group I2. Both components (2a:  a =
9.5857(2) Å, b = 5.9399(1) Å, c = 8.9759(2) Å, β = 90.409(1)°, Z = 2. 2b:  a = 9.5898(2) Å, b = 5.9376(1)
Å, c = 8.9784(1) Å, β = 90.420(1)°, Z = 2) of the nonmerohedrally twinned structure are independently refined
and found to be enantiomeric with nearly identical distance and angle parameters. As in the high-temperature
polymorph, the cations and anions are well separated. The most notable change involves two of the nitrato ligands
in the [Ga(NO3)4]- ions that have become bidendate, causing the molecular structure to distort toward octahedral
geometry.