Ammonium trifluoroberyllate. High temperature phases I and II

[NH4][BeF3], Msub(r)=84.05; phase I: orthorhombic, Pmnb, a=5.743 (5), b=4.643 (5), c=12.789 (2) A, V=341.0 (2) A3, Z=4, Dsub(m)=1.63 (2), Dsub(x)=1.637 (3) Mg m-3, lambda(Mo Kα)=0.7107 A, μ=0.94 mm-1, F(000)=168, T=386 K, final R=0.057 for 287 observed reflections; phase II: orthorhombic, P212121, a=5.661 (4), b=4.600 (5), c=12.990 (3) A, V=338.3 (4) A3, Z=4, Dsub(x)=1.650 (3) Mg m-3, lambda(Cu Kα)=1.5418 A, μ=2.04 mm-1, T=295 K, final R=0.051 for 419 observed reflections. Phase II is metastable at room temperature which allowed data collection at room temperature. Both high-temperature phases are closely related to the ferroelastic phase III. Rigid BeF4 groups preserve nearly the same shape through the phase transition. Temperature-induced motion of the rigid molecular units leads to pronounced changes in conformation of the (BeF4)sub(n) chains and through a system of hydrogen bonds of N-H...F type causes reorientation of NH4+ ions during phase transition. (orig./BHO)