Temperature- and Pressure-Induced Changes in the Crystal Structure of Sr(NH₃)₈Cl₂

ABSTRACT: The structural transformations occurring in the crystal structure of strontium chloride octamine, Sr(NH3)8Cl2, as a function of temperature and
pressure of ammonia gas were studied by detailed in situ X-ray powder diffraction (XRPD) and supported by density functional theory (DFT) calculations. Rietveld refinements were used to study the crystal structure of Sr(NH3)8Cl2 in details, and the potential presence of super symmetry is discussed. The Rietveld refinements show that the interatomic distance from the strontium ion to one of the ammonia molecules (Sr−N1) increases from 2.950(7) Å at 275 K to 3.50(6) Å at 322 K at P(NH3) = 2.0 bar. DFT calculations show that only half the energy is required to elongate the Sr−N1 bond from its equilibrium distance compared to the standard Sr−N bonds. The in situ XRPD data show that the a parameter of the unit cell increases relatively more than the b and c parameters during the heating, which is correlated to the crystallographic transformation. The in situ XRPD data show that increasing the heating rate pushes the structural transformation in the crystal structure to higher temperatures by a few kelvin. The in situ XRPD data show that the Sr(NH3)8Cl2 → Sr(NH3)2Cl2 + 6NH3(g) reaction has the lowest transformation temperature for all the studied ammonia pressures. The Sr(NH3)8Cl2 → Sr(NH3)Cl2 + 7NH3(g) reaction also plays a significant role at lower ammonia pressure. For the absorption of ammonia, Sr(NH3)Cl2 + 7NH3(g) → Sr(NH3)8Cl2 was the only observed reaction.