Rotationally ordered ammonium cations in the crystal structure of (NH4)2OsBr6

Abstract

AbstractRotational disorder is a common phenomenon in ammonium salts. The localization of hydrogen atoms and thus the distinction between rotationally ordered and disordered ammonium atoms by laboratory based X‐ray powder diffraction (XRPD) is a challenge, especially for compounds containing heavy atoms. For ammonium hexabromido osmate(IV), (NH4)2OsBr6, the analysis of XRPD data reveals a slight preference for a structure model with fully ordered ammonium cations ((ND4)2PdCl6 type (filled K2PtCl6 type), Fm m, a=1037.954(9) pm at 293(1) K, Br atoms on 24e with x(Br)=0.2385(1), H atoms on 32f with x(H)=0.191(2), Os atoms on 4a, N atoms on 8c). Interatomic distances (d(Os−Br): 6*247.6(1) pm, d(N−H): 4*106(2) pm, d(H−H): 3*173(3) pm) are consistent with those in comparable compounds, e. g. NH4Br and K2OsBr6. X‐ray diffraction (293 K≤T≤673 K) and thermal analysis (113 K≤T≤673 K) did not show any phase transition or any signs of disorder. The lattice parameters develop nearly linearly as described by the polynomial fit a=1024.38(16) pm+0.0410(7) T pm/K+8.8(7)×10−6 T2 pm/K2 for 313 K≤T≤653 K. (NH4)2OsBr6 shows beginning sublimation at 680 K. We therefore conclude that (NH4)2OsBr6 adopts the cubic (ND4)2PdCl6 type with fully ordered NH4+ cations from room temperature up to the temperature of its decomposition.