Assistant Professor

Physics

M P College

 Mohania, Bihar, India 

Since the discovery of ferroelectricity in BaTiO there has been ever increasing amount of research and development rangingfrom most fundamental studies of the phenomena to awide device and system applications. Moreover, a large number of lemoelectric material of different structural family and chemical formulae have been synthesized and characterised in search of materials or compositions for device making phenomena and applications such ones as various electronic, electroopticcompiten and communication devices Ferroelectric materials have recently received considerable attention due to their immense importance in growing use of their electronic. electro-optical, otical microwave applications and other technical applications. A fairly large number organic, inorganic conductor, liquid crystals, biological and polymer materials have been examined in their simple or complex/composite forms for applications in computer memory & display. optoelectronic, electro-optical modulators ete as well.

It is now well established that the material properties or device parameters can be tailored or changed by suitable modifications in the material composition or by change in experimental conditions suscle ones as temperature, pressure, electrical and magnetic fields, radiation etc. The area of material science is fast emerging as one of the most important avenues for development in the domain of science and technology because one of the fascinating aspects of the field is its interdisciplinary nature. The field is on the threshold of being paradigm of studies in some of its leading and most competing areas like the studies of mixed perovskites lead based complex perovskits and the solid solution relaxor and regular ferroelectrics and even soure of the niobate materials with structure similar to tungsten Bronz (TB) with relaxor characteristics having enormous potential of diverse applications. Relaxor Ferroelectrics (RFE)/ferroelectric relaxor (FER) are of great practical interest, especially because of them high dielectric constant and electrostriction. Due to compeitional disorder properties of RFE differ from those of a normal ferroelectric material. More recently even in the branch of integrated optics which is the optical analogue of integrated electronics, it deals with the technology of integration of various optical components on a simple substrate with the aim of realizing compact, efficient and high performance optical circuits of optical signal for applications in telecommunication sensing etc. We are noticing the interesting uses of ferroelectric niobate materials like lithium niobate. Although many materials such as glass polymers and semiconductors are being studied widely, the most sophisticated and highest performance optical circuits have been produced in this ferroelectric material (i.e. lithium niobate, LiNbO₃This is primarily due to the very good transparency of lithium niobate in the wavelength range of interest, the large electrooptic and nonlinear optical coefficient availablity of large crystals of good optical quality and the possibility of fabrication of low loss optical waveguides All the samll and large integrated circuits acquire the dielectric materials in some forms. The requirement of variation in the dielectric properties an be easily brought out for the case of niobate materials with structure similar to tungsten broze (TB). Therefore, instead of taking veral dielectric materials variously doped/substituted niobate. Can be employed to meet the objective of their device application Yet other characteristics of niobate materials such ones as the present one under study are their great structural stability under varying ambient conditions. Thus ferroelectric devices have a widk field of application in today's technique. This is due to their special properties resulting from ferroelectricity and to the possibilities opened by the ceramo for material engineering. For example the high relative permittivity (e') of some ferroelectrics was the most obvious property for their use in electrotechnics. Moreover high permittivity ceramics make it possible to noticeably miniaturize various passive microwave devices The ferroelectric material is only used as dielectric with high permittivity for the storage capacitors of dynamic semiconductor memories (DRA M). Pronounced higher requirements have to be fulfilled by ferroelectric thin films in nonvolatile RAMS. Since here the storage condenser must be switched for reading & writing, the highest storage density can be achieved when the transistors of the storage cells are ferroelectric field effect transistors. In light of foregoing discussion and ever growing and pressing needs of ferroelectric materials, it is, therefore, proposed to undertake the synthesis and characterization of the ferroelectric ceramic Ca₁ Bi₂ Ti₄ Nh₆ O₃, (hereafter CBTN) in the present thesis.

Even the ferroelectric niobate materials having structure similar to tungsten bronze (TB) have attracted attention of researchers due to their interesting phase transition behaviour and aforesaid properties useful to technical applications. The general formula in this group AB₂ O₃, where A is a divalent metal atom like Ba, Ca Sr. etc and B is Nb or Ta atom which may be advantageously substituted by some tetravalent atoms like Ti. The ferroelectric compounds in this group contain five formulas per unit cell in which there are six sites which may be occupied by the A atoms. This means that five A-type atoms must be distributed over six sites. The number of A atoms per cell may be increased to a maximum of six for example by doping/ substitution of Ti or Zr by Nb or Ta or alternatively by introducing monovalent cation into the crystals It has been shown that such subtle differences among the structrures of these closely related tungsten bronze (TB) type compounds can lead to major differences in the resulting ferroelectric properties. [Huang et al (1975)].

Our aforementioned one niobate compound (CBTN) under) present study belongs to this group.The parent compound is CaNb₂ O₆, and it is known for its tungsten bronze (TB) structure. The crystal structure of such TB compound consists of distorted Nb₂ O₆ octahedra linked together at their corners in a complex way to three types of openings which are occupied by the A site cations. Out of the available six sites only five are filled by the A site (Ca²⁺) cations and one site, therefore, remains vacant. By proper choice of tetravalent substitution for A and B cations, it is possible to fill all six sites (as is seen to be in present case) by maintaining the overall charge neutrality The resulting system may, therefore, referred to as "a charge compensated disordered" tungsten-bronze (TB) structure of AB₂O₆ type materials. [Jaffe & Jaffe (1971)]. The charge shortage due to the presence of tetravalent ions (Ti⁴⁺) at B-site in our case is compensated duly by the same amount of the excess charge resulting from the presence of trivalent ions at the A-site. The current literature survey however suggests that no detailed studies regarding synthesis, structure & dielectric characteristics of our material CBTN [Sharma et al, (2002)] have been undertaken so far. Therefore, it was considered worthwhile to study the synthesis and characterization of this material to understand the different phenomena like phase transition behaviour diffuseness in dielectric constant(e'), and dielectric loss (tanδ) with respect to temperature and frequency both to ascertain whether the composition under investigation shows relaxor ferroelectric behaviour or regular ferroelectric characteristic or diffuse phase transition: (DPT) feature. In the present case we have however shown that there is an abrupt rise in the value of dielectric constant(e') with temperature beyond-60°C and its value finally peaks at-13°C. The peak value of higher frequency data (i.e. 10 kHz) is, however found to be less than the corresponding value for 1 kHz. This may be attributed to intrinsic dielectric response of the material for difference frequencies. The peak in tanδ versus temperature graph also occurs at -13°C in this case. This, therfore, suggests that the dielectric anomaly observed is due to a thermodynamic phase tranistion as expected in accordance with Kramer-Kronig relationship [Lines and Glass (1977)]. The rising trend in the tanδ-T graph beyond 80°C at both frequencies (i.e. 1kHz and 10 kHz) may, however, be attributed to an increase in the conductivity of the material possibly due to thermally activated processes. Although the variation of dielectric constant(e') and dielectric loss (tanδ) with temperature at two different frequecies (1 kHz and 10 kHz appears to be slightly diffuse, it is, however, not of the type usually reported for ordered relaxors [Shrout and Halliyal (1987)] In any relaxors the temperature at which tanδ peaks in invariably lower than the temperature corresponding to the peak of dielectric constant [Cross (1987) & Pandey (1995)].

The present thesis comprises six chapters. The chapter 1 to 3 contain among other things the aim and scope of present work, an account of different experimental methods and synthesis of our ferroelectric samples respectively. The chapter 4 to 5 attempt to present the structure and the dielectric characterization of our material CBTN. The last chapter (i.e. 6th one) finally gives the summary of work undertaken with suggestions for further studies.

The preparation of the materials (ie.CBTN) is done by using high temperature solid-state reaction (dry) route, the details of which are available elsewhere in our published work on the material CBTN [Sharma et al (2002)]. In the characterization, the lattice structure by the X-ray diffraction (XRD) studies, density and dielectric measurements (e' and tanδ 6) of the samples of the material have been obtained X-ray diffractrograms (XRD) patterns of our samples have been taken at room temperature with the help of Philips PW/710 (Holland) and Rigaku (Japan) diffractometer (e') and the dielectric constant and dielectric loss (tanδ) measured as a function of frequency (10² to 10⁴ Hz) & as a function of temperature from liquid N₂ (i.e.-180°C) to a considerably high temperature above the room temperature using GR-1610 AP capacitance measuring assembly. Main points of our findings resulting out of these observations are as follows:

(i) The X-ray diffractograms (XRD) of our samples CBTN synthesized contain single sharp peaks showing that this compound has been synthesized in its single phase. All the peaks of the XRD patterns of the material are indexed and their basic/preliminary structures have been determined. The complete absences of the prominent peaks of the constituent materials of our compound have well confirmed the formation of our desired material. The lattice parameters have been subsequently determined from least squares refinement method using a standard computer software whose detailed reference is available in the Chapter-4 of this thesis.

The nonavailability of any files in respect of the structure of our material in the existing updated JCPDS (Joint Commission on Powder Diffraction Source) files points towards the importance and utility of reporting its structural studies. The measured density of the sintered pellets have been found to be around 95% (or even higher) of its respective theoretical value .Using the d-values of 21 strong and moderate reflections of CBTN material the lattice parameters have been calculated with the help of a aforesaid standard software.

The cell parameters a = 6.771 (1) Å. b = 11.124 (1) Å and c=19.623 (1) Å can account for all the observed d-values. The good agreement between the calculated and observed d-values suggests the correctness of preliminary crystal system and cell configuration. With limited powder data, it has not been possible to determine the space group of the material. The scanning electron microscope (SEM) was employed to take the micrographs of the CBTN pellet at different magnification to analyse the surface morphology. The uniform distribution of grains and not much voids and islands in SEM of our material CBTN.

(ii) The dielectric measurements of the compounds indicate that all the samples of material (i.e. CBTN) show typical a normal ferroelectric behaviour [Bera and Choudhary (1995)]. The evaluation of dielectric behaviour including dielectric constant (e') and dielectric loss (tanδ) over a wide frequency range 500 Hz to 10kHz has given a clear indication of ferroelectric phase transition as evidenced from well occurring maxima in e' (T) and tanδ (T) graphs.

We have found that our samples of CBTN have the transition temperatures below room temperature (299K). The studies of the variation of dielectric constant (e') and dielectric loss (tanδ) with frequencies at room temperature of our material point towards a normal behaviour of a dielectric. We have also measured the variation of e and tang with temperature at two different frequencies (1 kHz and 10 kHz) from liquid N₂ (-180°c) upto a considerably higher temperature about 200°C in both cases. The detailed analysis of dielectric constant and dielectric loss (tanδ) as a function of temperature reveals dielectric anomaly and consequent structrual phase transition from ferroelectric phase to paraelectric one. The frequency dependence of e' and tanδ of ceramic CBTN at five different temperatures. -180°C, 40°C.- 10°C, 26 C (room temperature) and 75°C have also been done. It is apparent from the enclosed figures in (chap.-5) that at temperatures relatively far away from the phase transition temperature Le-180°c.20 and 75°c, there is practically no variation both in e' and tanδ values As one, however, approaches the transition temperature (-13°C both e' and tanδ & are seen to rise sharply at the low frequency side. This trends is more evident for data at 10. This simply implies that the system responds more sensitively at this frequency became each of different domain sizes transforms into the paraelectric phase above -13°C We have also shown the variation of 1/e' versus temperature above the peak temperature. The good quality of straight line fit which has been obtained by the least squares method to the observed data point justified the validity of Curie-Weiss law in the paraelectric region. The Curie-Weiss temperature (T₀) is found to be -115°C which seems to be well below the phase transition temperature. This, in turn, enables us to infer that phase transition in the CBTN material is of a first order nature (Lines ME and Glass AM 1977). The value of Curic constant C (=1.1x10⁴) is also in the expected range of ferroelectric materials.

To summarize, it may be said that the single phase compound (CBTN) has been sunthesized and characterized for its structure and dielectric characteristics. The compound has orthorhombic lattice structure in the puraelectric phase at room temperature (299K) having lattice parameters a = 6.771 (1) Å. b = 11.124 (1) Å and c = 19.623 (1) Å. A dielectric anomaly in the e versus T plot. characteristic of ferroelectric phase transition, has been observed. We have initially expected our material CBTN like other TB based nichates such one as lead barium niobate (P B N) to show relaxer ferroelectric transition because the dielectric (e' and tanδ) measurements with respect to frequency & temperature both exhibited slight divergence of peaks in tanδ-Tande'-7 graphs at two different frequencies (i.e. 1 kHz & 10 kHz). This divergence in the aforesaid tang-T and e-7 graphs do not indicate any frequency dispersion nor is the temperature T_m' smaller than T_m'' where temperatures T_m' and T_m'' refer to temperatures corresponding to the peaks in e' (T) and tanδ (T) graphs respectively. For a material to exhibit relator type phase transition, T must be invariably lower than T_m' (i.e. T_m'<T_m'') and T_m'  & T_m''both should shift to higher temperature side on increasing frequency. In the present case the material CBTN does not fulfil any one of these creteria of relaxor ferroelectric phase tranistions [Pandey (1995). Cross (1987), &Shrout et al (1987)]. It is therefore. settled beyond doubt that dielectric anomalies occurring in the present case CBT Nare due to the consequent structural normal ferroelectric phase transtions [Bera&Choudhary (1995)] to paraelectric phase and these transitions need not at all be attributed to the relaxor ferroelectric behaviour. The compound (CBTN) under study shows the formation of a single phase solid solution having orthorhombic crystal structure in the paraelectric phase. The dielectric constant(e') and dielectric loss (tanδ) as a function of temperature (-180°C to about 200°C) at 1 KHz and 10 KHz both & as a function of frequency (10²-Hz to 10⁴ Hz) at five different temperature [i.e..-180°C, -40°C. - 10°C, 26°C (room temperature) and 75°C ] have shown a dielectric anomaly of usual normal ferroelectric type phase transition at-13°C.

During the course of this thesis work we have published a paper entitled "Synthesis, structure and characterization of ceramic Ca, Bi. Ti NhOin the Bull Mater Sci 25 (2) 133 (2002) (Indian Academy of science, Bangalore, India).

References

1. Bera S. ChoudharyRNP Indian J Phys A 69 (3) 371 (1995)

2. Cross L. E Ferroelectrics 76 241 (1987)

3. Huang et al J Appl. Phys 77 (4) 1677 (1995)

4. Jaffe B Cook W R and Jaffe H Piezoelectric ceramics (London/ New York Academic Prop.) (1971)

5. Lines ME & Glass AM Principles & application of ferroelectric & related material (Oxford University Press) (1977)

6. Pandey D Key engineering materials 101, 177 (1995).

7. Sharma H.... &ChoudharyRNP Bull Mater Sci 25(2) 133 (2002).

8. Shrout T R and Halliyal A Am CermSoc Bull 66 704 (1987).