Librational and reorientational specific heats of NH+4 in NH4ZnF3 and NH4CoF3

Abstract

Specific heat data on NH 4 CoF 3 and NH 4 ZnF 3 are presented. At room temperature these compounds crystallize in the cubic perovskite structure. At T c = 124.5 ± 0.1K and 115.1 ± 0.1 K, respectively, structural changes are found to occur, the main distortion being a tetragonal elongation of about 1.8%. For the Co 2+ compound it is accompanied by the onset of antiferromagnetic order. After subtracting the estimated lattice-phonon contribution and, for NH 4 CoF 3 also the magnetic contribution, we obtain the part of the specific heat associated with the NH + 4 movements. We find a nonanomalous contribution having a broad maximum near T ≌ 200 K ( C max R ≌ 2.3 ) which is ascribed to hindered rotations of the NH + 4 ions in the electrostatic potential of the surrounding ions. From a quantum-mechanical calculation of the rotational-energy spectrum for a rigid NH 4 tetrahedron in a potential appropriate for the cubic NH 4 ZnF 3 perovskite, we obtain a prediction that agrees quite well with the experimental specific heat. The fit yields a maximum energy barrier that is in good agreement with the calculated electrostatic barrier of 1857 K. In addition we find extremely high and sharp specific heat peaks at T c for both compounds, which are ascribed to a reorientational transition for the NH + 4 ions, accompanying the structural change. The associated transitional entropies are found to amount to R ln 3, implying a reduction by a factor of three in the number of orientational degrees of freedom.