Dynamical effects and ergodicity in the dipolar glass phase:evidence from time-domain EPR and phase memory time studies ofAsO44− inRb1−x(NH4)xH2PO4 (x = 0,0.5,1)

Abstract

Three-pulse electron spin echo envelope modulation (ESEEM), hyperfinesublevel correlation spectroscopy (HYSCORE) investigations andtwo-pulse electron spin echo (ESE) measurements of phase memory timeTM, were carried out, in the 20–200 K temperature range, on anAsO44− paramagneticprobe stabilized in RbH2PO4 (RDP), NH4H2PO4 (ADP),and dipolar glass Rb0.5(NH4)0.5H2PO4 (RADP). The results obtained on ADP revealed hyperfine interaction of the probe ion with the14N of the ammonium ion, the coupling constant satisfying the condition of ‘cancellation’ at afield 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. Theseresults were confirmed by HYSCORE spectral measurements. The fast Fouriertransform (FFT) spectra of three-pulse ESEEM decays have clearly revealed theinteraction with protons in the bond both in ADP and RDP; and in RADP with an averaged couplingconstant. The phase memory times in RADP exhibited strong temperaturedependence and were found to be dependent on the nuclear spin quantum numbermI of75As. The temperaturedependence of TM exhibited a well-defined maximum around 90 K, coinciding with the temperature of onset of ‘freezing’in Rb0.5(NH4)0.5H2PO4. This is symptomatic of dynamic fluctuations in the dipolar glass phase, with onset around150 K, going through a maximum around 90 K and slowing down on further cooling. Theseresults suggest that in RADP, a dynamical mechanism with progressive slowing downbelow 90 K is operative in the glass formation. This implies that the RADP system, withx = 0.5, exists in an ergodic relaxor (R)-state in the 20–200 K temperature range wherein everyfluctuating monodomain can be viewed as statistically representative of the whole sample.