title: Dynamical effects and ergodicity in the dipolar glass phase: evidence from time-domain EPR and phase memory time studies of AsO(4)(4-) in Rb(1-x)(NH(4))(x)H(2)PO(4) (x = 0,0.5,1).
abstract: Three-pulse electron spin echo envelope modulation (ESEEM), hyperfine sublevel correlation spectroscopy (HYSCORE) investigations and two-pulse electron spin echo (ESE) measurements of phase memory time T(M), were carried out, in the 20-200 K temperature range, on an AsO(4)(4-) paramagnetic probe stabilized in RbH(2)PO(4) (RDP), NH(4)H(2)PO(4) (ADP), and dipolar glass Rb(0.5)(NH(4))(0.5)H(2)PO(4) (RADP). The results obtained on ADP revealed hyperfine interaction of the probe ion with the (14)N of the ammonium ion, the coupling constant satisfying the condition of 'cancellation' at a field of 480 mT. The ammonium ion was found to be in two different sites in ADP, which became indistinguishable on the formation of dipolar glass RADP. These results were confirmed by HYSCORE spectral measurements. The fast Fourier transform (FFT) spectra of three-pulse ESEEM decays have clearly revealed the interaction with protons in the [Formula: see text] bond both in ADP and RDP; and in RADP with an averaged coupling constant. The phase memory times in RADP exhibited strong temperature dependence and were found to be dependent on the nuclear spin quantum number m(I) of (75)As. The temperature dependence of T(M) exhibited a well-defined maximum around 90 K, coinciding with the temperature of onset of 'freezing' in Rb(0.5)(NH(4))(0.5)H(2)PO(4). This is symptomatic of dynamic fluctuations in the dipolar glass phase, with onset around 150 K, going through a maximum around 90 K and slowing down on further cooling. These results suggest that in RADP, a dynamical mechanism with progressive slowing down below 90 K is operative in the glass formation. This implies that the RADP system, with x = 0.5, exists in an ergodic relaxor (R)-state in the 20-200 K temperature range wherein every fluctuating monodomain can be viewed as statistically representative of the whole sample.
authors.affiliation: Department of Physics and Measurement Technology, Linköping University, S-581 83 Linköping, Sweden.
authors.forename: M D
authors.initials: MD
authors.lastname: Sastry
authors.forename: H
authors.initials: H
authors.lastname: Gustafsson
authors.forename: M
authors.initials: M
authors.lastname: Danilczuk
authors.forename: A
authors.initials: A
authors.lastname: Lund