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 <i>T</i>_N ∼ 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, <i>T</i>₁, of the O–H···O and NH₄⁺ protons through the <i>T</i>_N. As <i>T</i> → <i>T</i>_N, 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 <i>T</i>_N, 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 <i>T</i>_N, 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 <i>T</i>₁ 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 <i>order–disorder</i> and <i>displacive</i> ones, should rather be “<i>order–disorder cum displacive</i>” type