A phenomenological theory of dielectric and mechanical properties of improper ferroelectric crystals

Abstract

A phenomenological theory has been developed for explaining the anomalous dielectric and electro-mechanical properties of some improper ferroelectrics. An ordering parameter 8, which plays the role of a trigger for the onset of the spontaneous polarization P is introduced into the thermodynamical potential function. The peculiar dielectric properties of (NH4)2S04-and Gd,(MoO,),-group crystals and the anomalous behaviour of the lattice strains of Fe-I-boracite can be well explained on the basis of this theory. A graphical investigation in a proper 8-P plane shows how the strength of the coupling between 8 and P characterizes the transition mechanisms of the improper ferroelectrics. As a result, it has become clear that the common origin of the peculiar physical properties of both crystal groups lies in the extreme weakness of the couplings. Experimental methods to disclose improper ferroelectric mechanisms are discussed.Eine ph%nomenologische Theorie wurde entwickelt, die die anomalen dielektrischen und elektromechanischen Eigenschaften einiger nichtidealer Ferroelektrika erklart. Ein Ordnungsparameter 8, der die Rolle eines Triggers fur den Einsatz der spontanen Polarisation P spielt, wird in die thermodynamische Potentialfunktion eingefuhrt. Die eigenartigen dielektrischen Eigenschaften von (NH,),SO,-und G$(Mo04)s-Kristallen und das anomale Verhalten der Gitterdeformation der Fe-I-Borazite lassen sich auf der Grundlage dieser Theorie gut erklaren. Eine graphische Untersuchung in einer geeigneten 8-PEbene zeigt, wie die Kopplungsstarke zwischen 8 und P die Ubergangsmechanismen nichtidealer Ferroelektrika charakterisiert. Als Ergebnis wird klar, daO der gemeinsame Ursprung der besonderen physikalischen Eigenschaften der beiden Kristallgruppen in der extrem geringen Kopplung liegt. Es werden experimentelle Methoden zur Aufdeckung der Mechanismen nichtidealer Ferroelektrika diskutiert.

Theory of the low-temperature phases in boracites: Latent antiferromagnetism, weak ferromagnetism, and improper magnetostructural couplings

“…Same notation as in Table IV. r2. 5qM, '+ M (M"+My +Mz ) -H. M r3~Y/ g 6gMy Fg(r)~r4):5'g Pz + E (P~+Py +Pz ) 8 P PoP 2Xo Fg ( rl~r4 ) -5/ ( 1 J e ] + A 2e2 +A 3e3 ) +(orthorhombic elastic energy ) -0e -( e &e I +e2e2 +e 3e 3 ) r2: FMs(r~~r4) =M"(+~el +aqeq+a3e3) (u =x, y, z) 2 y )gM, P, ME r, . y,~M"P +y,~P M r4 y $ @My P +y2X/M Py resentation of the crystallographic group Pca 2i.…”

“…9(a) and 9(b), the magnetic ions are represented in projection on the pseudocubic plane (001), and within the monoclinic cell, respectively. Having regard to their structural environment, the 12 independent nickel ions form three groups, denoted (1,2,3,4), (5,6,7,8), and (9,10,11,12) in Fig. 9, located inside mixed oxygen halogen octahedra, the axes of which are parallel or at 45' to the monoclinic plane.…”

Crystal structure of the salt 1,1′-(ethane-1,2-diyl)bis(1,4-diazabicyclo[2.2.2]octan-1-ium) diperchlorate, C14H28N4(ClO4)2

“…Salts, co-crystals, polymorphs, solvates and hydrates are of general interest. Furthermore, there is a research interest in organic salts because of their brilliant properties [8][9][10][11]. …”

Temperature-dependence of the lattice parameters of nickel-bromine boracite between 25 and 200°C

“…row of figures in parentheses gives the primitive unit-cell volumes of the crystal in terms of that of the cubic phase. Thermodynamic theories of the phase transitions have been attempted by Kobayashi, Enomoto and Sat0 (1972), Dvorak (1971Dvorak ( , 1973, Gufan and Sakhnenko (1973) and Levanyuk and Sannikov (1975); a useful summary can be found in the article by Ishibashi and Takagi (1976). The theory of Kobayashi, Enomoto and Sat0 (1972) will not be considered further here since it introduces an order parameter having the same symmetry as the spontaneous polarisation of the orthorhombic phase and would thus provide an example of a "pseudoproper" ferroelectric phase transition in which the unit-cell volume would not be altered (see Dvorak, 1974 for further discussion).…”