Secondary Ferroics Research Articles

Partial Ferroelastics

As it follows from the symmetry consideration of nonmagnetic crystals there are only 11 possible nonferroelectric species for partial ferroelastics, which are characterized by the absence of full primary ferroic properties. It has been suggested that these partial ferroelastics should be full secondary ferroics (ferroelastoelectrics or/and ferrobielastics). To prove this statement the crystallophysical method was considered. It has been shown that partial ferroelastics may be (i) full ferrobielastic and full ferroelastoelectric (one species); (ii) full ferrobielastic only (two species); (iii) full ferroelastoelectric and partial ferrobielastic (the rest 8 species). Primary and secondary ferroic properties of cristobalite (SiO2) and paratellurite (TeO2) crystals are discussed.

Size effects in ferroics

Ferroic crystals have domain walls which can be moved by electric field (ferroelectrics), magnetic fields (ferromagnets), mechanical stress (ferroelastics) or a higher order phenomenon involving a combination of the three (secondary ferroics). Because of the large contribution movable domain walls make to piezoelectricity, permittivity, permeability, and to elastic compliance, these materials are used as transducers, capacitors, transformers, sensors and actuators. A basic understanding of size effects is critical to the development of these devices, which are being made in smaller and smaller sizes in the form of thin films, fiber-filled composites, and multilayer ceramics with submicron grain sizes. Four regimes are observed when the particle size is made smaller: (1) crystals or large grains in ceramics contain many domain walls, but the number of walls, and the number of types of walls, grows smaller as grain size is reduced. (2) When the size is decreased sufficiently, the particles become single domain. The properties of single domain particles are quite different from those of multi-domain particles. (3) At yet smaller grain size, the particles exhibit diffuse phase transformations, nonlinear physical properties, and glass-like behavior. (4) The fourth regime is in the nanometer range where ferroic behavior disappears as surface effects begin to dominate.

A new (C2H5NH3)2ZnCl4 crystal with a pure Pnma-P212121 ferroelastic phase transition

Optical, calorimetric, and X-ray measurements performed on (C2H5NH3)2ZnCl4 (C2ZnCl) together with a group theoretical analysis have revealed that this crystal shows a pure first-order ferroelastoelectric phase transition at 243.3 K. In the low-temperature phase the crystal shows two explicit forms of ferrocity: gyrotropy and piezoelectricity, which is a common fact in this class of secondary ferroics. The experimental data show that the corresponding phase transition is similar to that observed in (C5H11NH3)2ZnCl4 (C5ZnCl) at 249 K. The thermodynamic function values for this phase transition in both compounds and the X-ray results confirm that the conformational changes must be excluded from the physical mechanisms involved in the onset of the ferroelastoelectric properties. The microscopic origin of this phase transition should be related to the freezing of the dynamical disorder of the N-H...Cl hydrogen bonding and with the corresponding tilting of the hydrocarbon chains. The experimental data seem to be in agreement with the Landau phenomenological theory except near Tc, where the experimental specific heat is well described by an empirical power law. Finally, as found in some other crystals of this family, C2ZnCl crystals also exhibit an isotropic point around 25 K. The isotropic point temperature shows a very strong dependence on n (chain length) for n<or=5 and an asymptotic behaviour for n>or=12.

Studies of quartz as a secondary ferroic

Abstract The theory of secondary ferroics is applied to quartz. Our studies of quartz are summarized, and the recent work on twin wall orientations and switching dynamics is detailed.

Polycrystalline secondary ferroics

Domain walls in secondary ferroics can be displaced by electric fields, magnetic fields, mechanical stresses, or by some combination of the three. The expected physical properties of fully “poled” secondary ferroics are calculated and compared with those of the unpoled polycrystalline material, and with available experimental evidence. The effects of poling are most obvious for null properties such as piezoelectricity and magnetoelectricity, which depend entirely on domain alignment. Experiments on NH 4Cl provide the first demonstration of ferroelastoelectric “poling” in a polycrystalline material; piezoelectricity appears after annealing at −30°C in parallel electric fields and mechanical stress. Other examples of “poled” polycrystalline secondary ferroics include ferromagnetoelectric Cr 2O 3, ferrobielastic SiO 2, and ferrobimagnetic MnO.

Secondary ferroics and domain-divided piezoelectrics

Abstract Ferroic crystals contain domain states which can be switched by suitably chosen external forces. The domains in a primary ferroic differ in spontaneous polarization (ferroelectricity) spontaneous magnetization (ferromagnetism), or spontaneous strain (ferroelasticity), while those in a secondary ferroic differ in induced strain, induced magnetization, or induced polarization. Examples of ferrobielastics, ferroelastoelectrics and other secondary ferroics are discussed, together with their applications as multi-domain piezoelectric resonators. Twinning is often considered a nuisance, but it can be used to enhance certain modes of motion, or to eliminate others, by making use of differences in the piezoelectric coefficients. Multi-domain crystals and ceramics can also be used to produce “forbidden” harmonics, or as high-frequency resonators, and as focused acoustic transducers.

Symmetry of secondary ferroics. II

In an earlier paper (Part I), the symmetry requirements for the secondary ferroic phenomena of ferrobielectricity, ferrobimagnetism and ferromagnetoelasticity and ferromagnetoelectricity were considered, and examples of materials illustrating the behavior discussed. In this second paper, the phenomena of ferrobielasticity and ferroelastoelectricity are considered. It is shown that there are 5 ferrobielastics, 15 ferroelastoelectric and 10 ferrobielastic-ferroelastoelectric species in which the primary ferroic effects are forbidden by symmetry. For α-quartz and other secondary ferroics it is suggested that the orientation of twin or domain walls which effect state switching under stress are dictated by strain compatibility between the contiguous domain states involved.

Symmetry of secondary ferroics. I

Although at present there are only a few well documented examples, secondary ferroic phenomena may prove to be rather common, since the symmetry requirements are not especially stringent. In this paper, the conditions for ferrobielectricity, ferrobimagnetism, ferromagnetoelasticity and ferromagnetoelectricity are discussed. It is shown that ferrobielectricity can be expected in antiferroelectric ferroelastic classes with substantial dielectric anisotropy. Nickel oxide and other antiferromagnetic ferroelastics with anisotropic magnetic susceptibility show ferrobimagnetism. Evidence for ferromagnetoelastic domains is found in CoF 2 and FeCO 3, while Cr 2O 3 shows ferromagnetoelectric switching. Of the 90 magnetic point groups, 35 are potentially ferromagnetoelastic and 40 potentially ferromagnetoelectric. In a companion paper (Part II) to be published, symmetry restrictions for ferrobielasticity and ferroelastoelectricity will be considered, and the conditions which determine the orientation of domain walls in secondary ferroics examined.