Evidence from 900 MHz 1H MAS NMR of Displacive Behavior of the Model Order–Disorder Antiferroelectric NH4H2AsO4

Abstract

NH4H2AsO4 (ADA) is a model compound for understanding the mechanism of phase transitions in the KH2PO4 (KDP) family of ferroelectrics. ADA exhibits a paraelectric (PE) to antiferroelectric (AFE) phase transition at TN ∼ 216 K whose mechanism remains unclear. With the view of probing the role of the various protons in the transition mechanism, we have employed the high-resolution technique of magic angle spinning at the high Zeeman field of 21.1 T (1H resonance at 900 MHz). We measured the temperature dependence of the isotropic chemical shift and spin–lattice relaxation time, T1, of the O–H···O and NH4+ protons through the TN. As T → TN, NMR peaks from the PE and AFE phases are seen to coexist over a temperature range of about 3 K, showing formation of nearly static (lifetime > milliseconds) pretransitional clusters in this lattice as it approaches its TN, consistent with the near first-order nature of the phase transition. The isotropic chemical shift of the O–H···O protons exhibited a steplike anomaly at TN, providing direct evidence of displacive character in this lattice commonly thought of as an order–disorder type. No such anomaly was noticeable for the NH4+ protons. Both sets of protons exhibited order–disorder characteristics in their T1 data, as analyzed in terms of the standard Bloembergen, Purcell, and Pound (BPP) model. These data suggest that the traditionally employed classification of equilibrium phase transitions into order–disorder and displacive ones, should rather be “order–disorder cum displacive” type.