Advanced Materials Research Vol. 795 (2013) pp 658-663 Online available since 2013/Sep/04 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.795.658
Ferroelectric and Relaxor Ferroelectric to Paralectric Transition Based on Lead Magnesium Niobate (PMN) Materials Rozana A. M. Osman1, a, Mohd Sobri Idris2,b, Zul Azhar Zahid Jamal1,c, Sanna Taking1,d, Syarifah Norfaezah Sabki 1,e, Prabakaran A/L Poopalan1,f , Mohd Natashah Norizan1,g, Ili Salwani Mohamad1,h 1
School of Microelectronics Engineering, Universiti Malaysia Perlis, Kampus Pauh Putra, 02600 Arau, Perlis , Malaysia
2
Cluster of Sustainable Engineering, School of Materials Engineering, Universiti Malaysia Perlis, Taman Muhibbah, Jejawi, 02600 Arau, Perlis, Malaysia. Corresponding author:
[email protected],a,
[email protected] 1,b,
[email protected] 1,c ,
[email protected] 1,d,
[email protected] 1,e ,
[email protected] 1,f
[email protected] 1,g and
[email protected] 1,h
Keywords: Lead Magnesium Niobate (PMN), ferroelectric, relaxor ferroelectric and paraelectric.
Abstract. First ferroelectric materials were found in Rochelle salt was in a perovskite structure. Lead Magnesium Niobate (PMN) is a perovskites with a formula of PbMg1/3Nb2/3O3 (PMN) and are typical representatives for most of all ferroelectrics materials with relaxor characteristic. It posses high dielectric permittivity which nearly ~ 20,000[1] with a broad dielectric permittivity characteristic, known as relaxor ferroelectric below room temperature. Some of the researcher might think that the transition from relaxor ferroelectric to paraelectric is similar to the characteristic as observed from ferroelectric to paraelectric, but it is not necessary. The puzzling is how do we categorise them. How is the domain structure look like typically in ceramic materials. Introduction In this paper,a short review was written to help a new beginner to study and explore ferroelectric and relaxor ferroelectric materials. Data was presented by doing some research on the previous study and also some work during the study on Lead Magnesium Niobate materials. PMN with chemical composition (PbMg1/3Nb2/3O3) was first synthesised by Prof. Smolenskii and his group[2]. This material was reported to adopt a cubic perovskite crystal structure with unit cell a=4.0660(3)Å and space group Pm-3m[3]. PMN materials became of interest to study as they are generally well characterised previously as a ferroelectric but undergo a relaxation before they transfer from a proper ferroelectric state to a proper paraelectric state. Typical perovskite sturucture is always described by the structure of BaTiO3, Figure 1. In ferroeletric BaTiO3 above Tc, 120oC, the structure is cubic and it does not posses a net dipole moment and it behave as a normal dielectric. Below Tc, the structural distortion is occurs in the structure where the TiO6 octahedra is no longer at the position. Ti tend to displace off-centered and this gives rise to a splontaneous polarisation [6]. The effect is, the permittivity became higher.
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O2-
Ba2+
(a)
(b)
Ti4+
Fig. 1 : BaTiO3 unit cell a) Perovskite-type Barium Titanate (BaTiO3) unit cell in the symmetric cubic state above Tc b)Tetragonally distorted unit cell below Tc[2] Hysteresis Loop. The easiest way to recognize ferroelectricity is having a large dielectric permittivity and hysteresis loop which has the ability to retain some of the residual electrical polarisation after removal of the applied electric field. The reorientation of ferroelectric crystals/ceramics can be explained through a study of hysteresis, Figure 2. Ferroelectric hysteresis also explains domain-wall switching[5]. Start at point A, no net polarization occurs as there is no electric field applied. A linear polarization occurs, and but at this stage the field is not high enough to switch the domains. By increasing the applied field the polarization response in this segment becomes non-linear because the field is high enough to switch the polarization to the desired direction. At this stage all the domains is fully aligned, called saturation. If the applied field is being removed the field the polarization does not drop to zero because the relaxation occurs slowly and it take sometime to recover the original position. Negative applied field need to be applyto bring back the polarization to zero. When the field is further increased in the negative direction, new alignment of dipoles occurs. The field is then reduced to zero to complete the cycle.
Fig. 2 : Ferroelectric (P-E) hysteresis loop. Circles with arrows represent the polarization state of the material at the indicated field[5]
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Ferroelectric to Paraelectric Transformation. Usually, a normal ferrolectric are characterised by a sharp transition between ferroelectric state to paraelectric state, Figure 3. The Ferroelectric state is usually a low temperature effect since dipole moment movement is not disturbed by temperature [6]. As the temperature increases, there is sufficient energy to break down the common displacement of dipoles and destroy the domain structure. Normally, the permittivity will be decrease after the Curie Temperature, Tc.
Fig. 3 : Illustration of the changes in a ferroelectric material which transforms from a paraelectric cubic into a ferroelectric tetragonal phase with decreasing temperature[5]. Relaxor Ferroelectric to Paraelectric Transformation. In normal ferroelectrics, the sharp transition in dielectric constant also indicates a structural phase transition as the symmetry of the crystal changes. However, for relaxors, the broad peak in dielectric constant might not correspond to a structural change, eg: tetragonal to cubic structure. The broad peak indicates that the high dielectric constant is attainable within a wide temperature range. For a normal ferroelectric since it has its maximum dielectric constant well above room temperature, and the sharp temperature dependence makes it unsuitable for certain applications such as microwave devices, piezoelectric devices and capacitors[7].
Fig. 2 : Dielectric constant of relaxor PbMg1/3Nb2/3O3 as a function of temperature[2]
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Other researcher found that the relaxor have different behavior suc as, the static dielectric susceptibility or dielectric constant peak is sharp and narrow near Tc for ferroelectric materials. Relaxor ferroelectrics exhibit a very broad dielectric permittivity peak and strong frequency dispersion in the peak temperature, Tm, Figure 3.
(a)
(b)
Fig. 3 : Dielectric constant as a function of temperature for typical (a) ferroelectric (b) relaxor materials [2] A hysteresis loop with large remanent polarisation (PR) is a characteristic of ferroelectric materials. PR is a manifestation of the cooperative nature of ferroelectric phenomena. In a relaxor, a slim hysteresis loop gives a small PR. This is evidence for the presence of some degree of cooperative freezing of dipolar or nanodomain orientations, Figure 4.
(a)
(b)
Fig. 4 : Hysteresis loop for typical (a) ferroelectric (b) relaxor materials For ferroelectric materials, the polarisation, P, vanishes near Tc, which mean that no polar domains exist above Tc, , Figure 5a. In contrast, in relaxor materials, the polarisation decreases smoothly through the dynamic transition temperature Tm, Figure 5b. This is believed to be due to the fact that nano-size polar domains persist to well above Tm.
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(a)
(b)
Figure 5: Temperature dependence of spontaneous polarization of (a) ferroelectric (b) relaxor materials Summary In relaxors , it is recently found that mixing cations on the B-site of relaxor families of crystal structure causes breaking of the translational symmetry that leads to the formation of local movement of individual dipoles. As a consequence, polar nanodomains are formed and inhibit the domains to undergo a normal ferroelectric long-range ordered transformation. The relaxor state is also commonly described as a network of randomly interacting polar nanoregions (PNRs)[8]. If the materials is ferroelectric, the spontaneous polarization will be occur and no net polarization detected as they undergo paraelectric transition. The ferroelectric domains can be explained by uniformity of oriented spontaneous polarization. Usually, spontaneous polarization in a single crystal/grain in a ferroelectric ceramic is not uniformly aligned in the entire crystal along the preferred direction (eg: [001] in BaTiO3)[5]. Ferroelectric domains are formed within grains to reduce electrostatic energy. It was difficult to see in ceramics materials since the grains are randomly oriented compared to single crystal. The polar regions which are uniform are called ‘ferroelectric domains’ and the regions between two domains are called ‘domain walls’. The formation of domain walls usually is affected by local electric fields and mechanical stresses that ferroelectric materials are subjected to when cooled through the transition from paraelectric to ferroelectric phase[5]. Acknowledgements Authors would like to thank Prof. A.R West for his guidance and a big appreciation goes to Ministry of Higher Education Malaysia,University Malaysia Perlis, FRGS and RAGS grant for a funding. References [1]
L. E. Cross, Relaxor Feroelectrics, Ferroelectrics 76, 241-267 (1987).
[2]
W. Heywang, K. Lubitz and W. Wersing, Piezoelectricity: Evolution and Future of the Technology, Springer, (2008).
[3]
P. Bonneau, P. Garnier, G. Calvarin, E. Husson, J. R. Gavarri, A. W. Hewat and A. Morell, Xray and neutron diffraction studies of the diffuse phase transition in PbMg1/3Nb2/3O3 ceramics, Journal of Solid State Chemistry 92, 350-361 (1991).
[4]
Rozana A. M. Osman, Synthesis and Characterisation of Novel Electronic Ceramics, Unpublish Thesis, Sheffield University, (2011).
[5]
D. Damjanovic, Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics, Reports on Progress in Physics 61 (9), 1267 (1998).
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[6]
A. R. West, Basic Solid State Chemistry, Second Edition,Wiley, (1999).
[7]
C. J. Stinger, Structure-Property-Performance Relationship of New High Temperature Relaxor for Capacitor Applications, Unpublished thesis, The Pennsylvania State, 2006.
[8]
D. Viehland, J. F. Li, S. J. Jang, L. E. Cross and M. Wuttig, Glassy polarization behavior of relaxor ferroelectrics, Physical Review B 46 (13), 8013-8017 (1992).
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Ferroelectric and Relaxor Ferroelectric to Paralectric Transition Based on Lead Magnesium Niobate (PMN) Materials 10.4028/www.scientific.net/AMR.795.658
