On the relation between experimental results for SASD type crystals and the two-sublattice model

On a correlation between EPR data for SASeD doped with Cr^{3+} and soft modes

“…This temperature is about 5 K lower than T C determined from dielectric measurements. The reasons for the existence of the other two anomalies can be explained in the way it was done for sodium ammonium sulphate dehydrate (SASD) in [3]. The role of soft modes reflects in the temperature behaviour of D ZZ below 175 K. According to Owens [5], the softening of the phonon mode for the systems exhibiting the first order phase transition and lacking a centre of inversion should lead to the following temperature dependence of D ZZ :…”

“…The following designations were used in (5)- (8): i is the lattice cell number; f is the number of sublattice (in every cell); a is the number of the branch of lattice vibration; Γ is the tunnelling constant for motion of SeO 4 group through a potential barrier; ∆ f is the height of this barrier; p 0 is the dipole momentum of the active groups; E is the external electric field; J ij , K ij are constants of interaction for particles from the same sublattice (J ij ) and from different sublattices (K ij ), K = K 11 = K 22 , ω a ( k) is the cyclic frequency of a-branch of lattice vibration with a fixed wave vector k; S x f i , S z f i are spin operators, β ka , β + ka are phonon operators. The potential barrier ∆ f for the crystal structure of SASeD can be taken in the form (see [3,7]):…”

“…The transition from paraelectric phase (with a space group P 2 1 2 1 2 1 ) to ferroelectric phase (with a space group P 2 1 ) is of the first order. It was also recognized that molecular rotations of two inequivalent groups (SeO 4 (I)) and (SeO 4 (II)) to a great extent contribute to the appearance of spontaneous polarization P S [2,3]. However the effect of soft modes in the phase transition of SASeD has not been described so far.…”

Experimental and Theoretical Studies of SASeD Doped with Cr3+

“…Over the last few years we have extensively studied yet another representative of the crystal belonging to the same isomorphic group, namely, sodium ammonium sulphate dihydrate SASD [3,4]. Analysing both the experimental and theoretical results we found that the proposed in the literature model for phase transition should be revised.…”

“…we constructed a theoretical model (discussed extensively in [3,4]) in which SO 4 (I) (SeO 4 (I)) and SO 4 (II) (SeO 4 (II)) groups were assumed to form a cluster. Considering short-and long-range interactions between the clusters characterized by phenomenological parameters J andK respectively we were able to explain the existence of three anomalies for SASD.…”