Description:
<jats:title>Abstract</jats:title><jats:p>The successive phase transitions of (NH<jats:sub>4</jats:sub>)<jats:sub>3</jats:sub>H(SO<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub> crystal and its deuterized analog are studied from 90 to 420 K using VO<jats:sup>2+</jats:sup> ion and the SeO<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-2.gif" xlink:title="urn:x-wiley:03701972:media:PSSB2221230103:tex2gif-stack-2" /> radical as EPR probes. On the basis of EPR experiments with ferroelastic domain switching in phase II, a new model of the VO<jats:sup>2+</jats:sup> complex in the crystal is proposed, with VO<jats:sup>2+</jats:sup> occupying an NH<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-3.gif" xlink:title="urn:x-wiley:03701972:media:PSSB2221230103:tex2gif-stack-3" /> (I) vacancy and able, at high temperatures, to reorient very extensively coordinating in three ways with four of the nearest six oxygen atoms. Gradual hindering of NH<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-4.gif" xlink:title="urn:x-wiley:03701972:media:PSSB2221230103:tex2gif-stack-4" /> reorientation with decreasing temperature enables reorientations in the sublattice of SO<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-5.gif" xlink:title="urn:x-wiley:03701972:media:PSSB2221230103:tex2gif-stack-5" /> ions and is invoked as being the reason of all the transitions occurring below room temperature. Gesi's crystal model of two oppositely polarized NH<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/tex2gif-stack-6.gif" xlink:title="urn:x-wiley:03701972:media:PSSB2221230103:tex2gif-stack-6" /> sublattices in phase III, as well as the ferrielectric nature of the spontaneous polarisation of phases VI and VII, are confirmed. The VO<jats:sup>2+</jats:sup> complex line splitting in phase VI of the deuterized crystal behaves similarly as the anomalous temperature dependence of spontaneous polarisation observed by Osaka et al.</jats:p>