COEXISTENCE OF PARAELECTRIC/PROTON-GLASS AND FERROELECTRIC (ANTIFERROELECTRIC) ORDERS IN Rb_{1-x}(NH_{4})_{x}H_{2}AsO_{4} CRYSTALS
This paper reviews the results of dielectric studies of the proton glass state in a mixed crystal Rubidium Ammonium Dihydrogen Arsenate (RADA) The coexistence of paraelectric/proton glass and ferroelectric or antiferroelectric orders, confirmed by other studies, has been discussed in detail. The phase diagram of RADA is asymmetric. The proton glass state exists for ammonium concentration in the range of 0.1 < x < 0.5 . The phase diagram of deuterated DRADA is presented. The proton glass state region in DRADA, 0.2 < x < 0.35 is narrower than that for non-deuterated RADA. The effects of hydrostatic pressure on the dielectric properties in the proton glass state are presented. The glass temperature Tg decreases with pressure and is expected to vanish at 5 kbar. Low temperature behaviour is still an open question, since there is no experimental evidence of Tg(p) dependence below 4K for proton glass systems.
The ferroelectric and proton glass coexistence region in K0.77(NH4)0.23H2AsO4 detected by complex permittivity measurements
This paper reports results of the studies on complex permittivity epsilon *= epsilon '-i epsilon " in a mixed K0.77(NH4)0.23H2ASO4 crystal. Measurements along the crystallographic a axis have clearly indicated the coexistence of the ferroelectric state and proton glass state in this crystal. The temperature dependence of epsilon 'a shows a maximum at Tc=61 K, indicating a ferroelectric transition, while the dispersion of epsilon 'a and epsilon a" below T=40 K satisfies the Vogel-Fulcher equation with the parameters T0=8.3 K, Ec=183.5 K and nu c=1.65*1011 Hz, which proves the presence of the proton glass state. Analysis of the shape of the temperature dependence of dielectric losses epsilon c"( nu ,T) observed in the measurements along the ferroelectric c axis revealed the presence of Another process of relaxation apart from that related to the freezing of ferroelectric domain wall motion known for KDP type crystals. The two relaxation processes are also described by the Vogel-Fulcher equation with the parameters T01=34 K, Ecl=7.7 K and nu cl=6.8*105 Hz for one of them and for the other T02=20.5 K, Ec2=13.4 K and nu c2=5.6*106 Hz. Our results prove the coexistence of the ferroelectric and proton glass states in the K0.77(NH4)0.23H2AsO4 crystal, and the occurrence of different kinds of cluster of local ordering (proton glass and relaxer clusters) and ferroelectric domains.
MAS-NMR spectroscopy studies in the setting reaction of glass ionomer cements.
OBJECTIVES
The main objective is the characterisation of the setting reaction in glass ionomer cements based on experimental ionomer glasses with different fluorine content and a commercial glass ionomer cement liquid by using 13C CP/MAS-NMR, 29Si, 27Al and 31P MAS-NMR spectroscopy in order to receive information specifically about the cross-linking process.


METHODS
Different fluorine containing glass compositions based on 4.5SiO2-3Al2O3-1.5P2O5-(5-z)CaO-zCaF(2) where z=0-3, were mixed with a commercially available polymer liquid to form glass ionomer cements. The cements were subjected to 27Al, 13C CP/MAS, 29Si, and 31P MAS-NMR analysis.


RESULTS
The 27Al spectra showed clearly the formation of six-fold coordinate Al(VI), that may crosslink the carboxyl groups in the poly-acid molecules. A shift towards to more positive values of the carboxyl peak in the 13C CP/MAS-NMR spectra showed clearly the proton dissociation of the carboxyl groups. A shift towards more negative values was observed in the 29Si MAS-NMR spectra, suggesting formation of hydrated silica gel and consequently formation of additional Si-O-Si bonds. 31P MAS-NMR spectra also reflected changes in the coordination state around a PO4(3-) tetrahedron. Increasing the fluorine content of the glasses resulted generally in increased reactivity during setting, due to promoting cross-linking and repolymerisation of the silicate phase, followed by clear changes in the MAS-NMR spectra.


CONCLUSIONS
The cross-linking process during the setting reaction of glass ionomer cements can be followed by MAS-NMR spectroscopy observing the conversion of Al(IV) to Al(VI). The acid base setting reaction is completed in 1 day and no further significant changes in the MAS-NMR spectra can be observed. Further study is required in order to understand the role of phosphorus.