| Abstract
| - A reversible solid-state phase transition occurs in single crystals of [NO2][Ga(NO3)4] near 250 K. The low-temperature polar polymorph exhibits a nonmerohedrally twinned structure in which two of the originally monodentate nitrato ligands become bidentate.
- Single-crystal X-ray crystallographic analyses of [NO2][Ga(NO3)4] reveal that it undergoes a reversible phasetransition 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 squashedtetrahedral geometry in which all of the ligands are monodentate. Below approximately 250 K, distortions lowerthe 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 refinedand found to be enantiomeric with nearly identical distance and angle parameters. As in the high-temperaturepolymorph, the cations and anions are well separated. The most notable change involves two of the nitrato ligandsin the [Ga(NO3)4]- ions that have become bidendate, causing the molecular structure to distort toward octahedralgeometry.
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