TABLE VIII: Band Assignments for the Species Produced in the Matrix Reactions of In and Ga with 16G2 frequency,0,6 cm"1 assignment6 In + 1 02 Ga + ,602 species mode 1084.2 1089.5 \ ( l'i(Ai) 332.2 380.0 > mo2(c2U) WA,). [277.7] 285.5 ) U(B.) 1015.9(N2) 1015.6(N2) 02M-M02(D2d) ,s(B2), 0-0 443.2(N2) 458.6(N2) M02M(D2h) M / \ Vs(B2u) [816.2] M-d zO[C2U] M M / \ MA.) 554 795.3(N2) 968.0 952.7(N2)<'(BU) 0 Values are for Ar matrices unless specified otherwise. 6 Tentative assignments are indicated by brackets.

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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 <

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