Neutron diffraction study of orientational freezing in TlNO2 and CsNO2

The low-temperature crystal structure and temperature dependence of lattice parameters and atomic mean-square displacements of thallium nitrite (TlNO2) and cesium nitrite (CsNO2) have been determined by neutron powder diffraction. Previously, based on specific heat and dielectric relaxation measurements, both compounds were reported to exhibit partial dynamic orientational disorder of the NO2 anions below the transition temperatures 282.4 and 209.2 K, respectively, from the plastic, disordered high-temperature phase I to the low-temperature phase II. A glass-transition-like freezing of the associated reorientational motions was reported to occur below 60 and 40 K, respectively. In the present study, we could not confirm the previous space-group assignments Cmmm and R3¯m for the low-temperature crystal structure of TlNO2 and CsNO2, respectively. Instead, we find that these compounds have space group P3121 in phase II. A structural model assuming that all atoms occupy unique sites and the two oxygen atoms of each NO2 anion are located on crystallographically equivalent sites gives excellent agreement with the observed data. This model leaves no large-angle orientational degrees of freedom for the NO2 anion anticipated by previous studies. The glasslike behavior apparent in specific heat and dielectric relaxation studies can only be reconciled with our results if the glasslike transition involves relatively small displacements or reorientations of the NO2 anion. This would be a novel type of glasslike transition.