Ionic rearrangements and domain growth near antiferroelectric transitions: EPR study of NH₄H₂AsO₄, ND₄D₂AsO₄, NH₄H₂PO₄, and ND₄D₂PO₄

Abstract

This paper is dedicated to Professor CharlesUsing electron paramagnetic resonance spectroscopy we have measured the temperature dependence of ionic rearrangements and domain nucleation around the antiferroelectric transition temperature Tc of NH4H2As04, ND4D2As04, NH4H2P04, and ND4D2P04. Cr043-radicals, produced by y-irradiation of K2Cr04-doped samples, were used as microscopic probes because their charge, size, and symmetry match those of the host anions. The use of high deuteration level and "monodomain" samples prepared by temperature cycling was found to be crucial in the signal assignment and line-shape analysis. The results show that as a sample approaches Tc from T > T,, the hydrogen-bonded protons slow down anomalously, much more slowly than expected for Arrhenius behaviour. Simultaneously, the lattices develop "clusters" of antiferroelectrically ordered molecules.These clusters eventually (T << T,) expand to occupy the whole lattice and thus complete the lattice ordering. The probe selection and sample preparation methodology discussed and the microscopic details of the ionic rearrangements reported appear to be highly significant for further studies of these phase transitions. Utilisant la spectroscopie en resonance paramagnttique tlectronique, on a mesurt les rtarrangements ioniques et la nucliation de domaine en fonction de la temperature qui se produit prks de la temptrature de transition antiferroilectrique, T,, du NH4H2As04, du ND4D2As04, du NH4H2P04 et du ND4D2P04. Des radicaux Cr043-, produits par l'action d'irradiations-y sur des tchantillons dopts au K 2 C d 4 , ont t t t utilists c o m e sondes microscopiques parce que leur charge, leur grosseur et leur symttrie sont comparables i celles des anions h6tes. Pour l'attribution des signaux et pour I'analyse de la forme des raies, il est crucial d'utiliser des tchantillons i <<domaines uniquesn qui ont t t t prtparts par des cycles de temptrature et qui contiennent des niveaux tlevts de deuttration. Lorsque la temperature d'un tchantillon approche de T,, on observe que les protons lits par des ponts hydrogtnes sont anormalement ralentis, beaucoup plus que ce que l'kquation d'Arrhenius nous laisserait prtvoir. En mime temps, les rtseaux foment des <(agrtgatsn de moltcules qui sont ordonnks d'un point de vue antiferrotlectrique. Lorsque T << T c , ces agrtgats prennent de l'expansion et completent sa mise en ordre. La mkthode de stlection de la sonde ainsi que la mtthodologie de prtparation d'tchantillons qui est discutke et les dktails microscopiques des rtarrangements ioniques rapportts semblent trks significatifs pour des ttudes ulttrieures de ces transitions de phase.[Traduit par la revue]

Coexistence of Order–Disorder and Displacive Behavior of KH2PO4 and Analogs from Electron Paramagnetic Resonance

“…Thus an anomalous variation in δ iso would indicate a change in the electronic structure of the local environment of the site being probed, and hence a direct evidence for the existence of a displacive component in the transition mechanism. Similar conclusions have been reported also from 13 C high resolution NMR studies of squaric acid (H 2 C 4 O 4 ), a related hydrogen-bonded antiferroelectric compound [55][56][57][58][59][60]. These observations prompted us to re-examine the earlier spin-probe EPR studies on the KDP family, which have long been interpreted in terms of an order-disorder behavior [30,[51][52][53][54].…”

“…Figure 4 shows a typical EPR spectrum of the CrO 3center, respectively [12,13,30]. Direct evidence for the order-disorder dynamics can be noticed from the temperature dependence of the proton superhyperfine structure for H along the c-axis, as shown for KDP in Fig.…”

“…Also, notice that the center of the high temperature spectrum is not at the average position of the two low-temperature peaks, thereby pointing to the existence of the "displacive" component in the phase transition mechanism. See text for details [12,13,30] Coexistence of Order-Disorder and Displacive Behavior in KDP 93 Fig. 5 Left panel: Temperature dependence of the proton superhyperfine splitting on the EPR spectrum of the CrO 4 3center ion KDP.…”

“…The present undertaking discusses the utilization of electron paramagnetic resonance (EPR) spectroscopy for investigating dielectric solids that are diamagnetic in their native state, but can be made slightly paramagnetic by lightly doping them with transition ions, such as Fe 3+ , as pioneered by Müller [2][3][4][5][6][7], or by using molecular probes such as the CrO 4 3-(Cr 5+ ) ion [1,[8][9][10][11][12][13], SeO 4 3radical [14][15][16][17][18][19][20][21][22] and the AsO 4 4center [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. While Müller's studies using Fe 3+ as a probe in SrTiO 3 and LaAlO 3 are considered as some of the classic examples of the power of EPR technique in probing microscopic mechanisms of phase transitions in the perovskites [2][3]…”

“…Also, notice that the center of the high temperature spectrum is not at the average position of the two low-temperature peaks, thereby pointing to the existence of the "displacive" component in the phase transition mechanism. See text for details[12,13,30]…”