ESR Study of a Cr⁵⁺ Centre in Ferroelectric Ammonium Sulphate

Abstract

X-band electron spin resonance (ESR) studies of (Cr04)2-doped, X-irradiated single crystais of ferroelectric ammonium sulphate ((NH,),SO,, Tc = 223 K) at 300 and 208 K are reported. The paramagnetic centre responsible for the ESR spectrum is identified to be W+. Superhyperfine interaction of the unpaired electron with two equivalent protons is observed. The spin-Hamiltonian parameters which are nearly axial a t 300 K, with 911 < g 1 indicating a dra+ orbital ground state, acquired rhombic character below Tc indicating a distortion of the sulphate tetrahedron. An increase in the value of the proton superhyperfine constant in the ferroelectric phase is indicative of stronger hydrogen bonding.

Phase transition and temperature dependence of the molecular distortion of ions in ammonium sulphate

“…Badr and Awad (1984) and Abe and Shibata (1977) have inferred that the SO:is responsible for the spontaneous polarisation and it plays a vital role in the process of transition. Manjunath and Srinivasan (1978) reiterate exactly what has been concluded by Jain and Bist (1974a, b). Zinenko et a1 (1980) believe that the unusual behaviour of P, in AS may be due to the fact that the phase transition is not associated with the ordering of NH; ions as suggested in their model but with the ordering of the SO',ion.…”

“…Ammonium sulphate (AS) undergoes a peculiar ferroelectric phase transition (PT) at 223 K. The tetra-molecular unit cell of AS belongs to the Dii space group in the paraelectric phase and to CT, in the ferroelectric; it contains two crystallographically different ammonium ions: NH: (I) and NH: (11) (Wyckoff 1964, Schlemper andHamilton 1966). This transition has been the subject of a large number of studies: structural (Schlemper and Hamilton 1966), twin plane motion (Makita et a1 1976), dielectric (Ikeda et (Blinc and Levstek 1960), EPR (Abe and Shibata 1977, Barb et a1 1978, Manjunath and Srinivasan 1978, Ramnathan and Srinivasan 1978, Hirabayashi and Abe 1980, Narayana and Kevan 1381, DTA (Badr and Awad 1984) as well as IR absorption and Raman scattering (Torrie et a1 1972, Jain et a1 1973, Jain 1974, Petzelt et a1 1974, Venkateswarlu et a1 1975, Iqbal and Christoe 1976. Several semi-theoretical and theoretical studies of the transition are also available (Sawada et a1 1973, Jain andBist (1974a, b), Dvoiak and Ishibashi 1976, Onodera et a1 1978, Zinenko et a1 1980.…”