Rotationally ordered ammonium cations in the crystal structure of (NH 4 ) 2 OsBr 6

Rotational 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), (NH 4 ) 2 OsBr 6 , the analysis of XRPD data reveals a slight preference for a structure model with fully ordered ammonium cations ((ND 4 ) 2 PdCl 6 type (filled K 2 PtCl 6 type), Fm 3 ‾ m , a =1037.954(9) pm at 293(1) K, Br atoms on 24 e with x (Br)=0.2385(1), H atoms on 32 f with x (H)=0.191(2), Os atoms on 4 a , N atoms on 8 c ). 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. NH 4 Br and K 2 OsBr 6 . 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   T 2  pm/K 2 for 313 K≤ T ≤653 K. (NH 4 ) 2 OsBr 6 shows beginning sublimation at 680 K. We therefore conclude that (NH 4 ) 2 OsBr 6 adopts the cubic (ND 4 ) 2 PdCl 6 type with fully ordered NH 4 + cations from room temperature up to the temperature of its decomposition.