An ESR Study of Structural Phase Transitions of(NH4)2Pb[Cu(NO2)6]

Abstract

The structural changes of (NH4)2Pb[Cu(NO2)6] crystals during the phase transitions at 316, 287, and 95 K were studied by means of a single crystal ESR technique. Anisotropic g values [g∥ > g⊥) were obtained in the lowest temperature phase IV, whereas the ESR spectra recorded in the intermediate phases III and II, in which the crystal is known to be tetragonally compressed along the c axis, could be interpreted by use of effective g values [g(∥c) < g(⊥c)]. In the highest temperature phase I an isotropic g value was obtained. The phases I, II, and III o f the present complex can be assumed to be very similar to the corresponding phases observed above ~ 250 K for the compounds R2Pb[Cu(NO2)6] with R = K, Rb, Cs, and Tl. However, the lowest temperature phase transition transforms the phase III with an antiferrodistortive order of the elongated [Cu(NO2)6]4- octahedra into the phase IV with a ferrodistortive one. This occurs only for the complex with R = NH4 and is very unusual. The results of the 14N NQR experiments already reported for the phases III and IV could be well explained by the present newly proposed model.