научная статья по теме LUMINESCENCE STUDIES OF PEROVSKITE STRUCTURED TITANATES: A REVIEW Физика

Текст научной статьи на тему «LUMINESCENCE STUDIES OF PEROVSKITE STRUCTURED TITANATES: A REVIEW»

ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 6, с. 933-948

^ СПЕКТРОСКОПИЯ

КОНДЕНСИРОВАННОГО СОСТОЯНИЯ

УДК 543.42

LUMINESCENCE STUDIES OF PEROVSKITE STRUCTURED TITANATES:

A REVIEW

© 2015 г. G. Nag Bhargavi and Ayush Khare

Department of Physics, National Institute of Technology, Raipur-492010, India E-mail: akhare.phy@nitrr.ac.in Received March 15, 2014

Apart from widely known dielectric and ferroelectric properties, the perovskite type materials also constitute a class of materials, which are recently investigated for their optical properties. These materials are being used for fabrication of various microelectronics and optoelectronic devices. Photoluminescence (PL), mechano-luminescence (ML) and thermoluminescence (Tl) are such phenomena offering numerous applications in different fields like electro-optics, flat panel displays, LED technology, sensors, dynamic visualization etc. This paper briefly reviews the status and new progress in luminescence studies of ferroelectric materials like barium titanate (BT), barium zirconate titanate (BZT), calcium titanate (CT), calcium zirconate titanate (CZT), lead titanate (PT), lead zirconate titanate (PZT), etc., prepared through various methods.

DOI: 10.7868/S003040341506015X

INTRODUCTION

The advancement of research in the field of physics results in the development of novel materials. Among number of materials, there is considerable attention on phosphors due to their novel optical/luminescent properties. Luminescence is a phenomenon, which results as energy in the form of light, even though the input may be in the form of light, heat, pressure or by any other means. The materials exhibiting this phenomenon are known as luminescent materials. Depending on the time taken by the luminescent material to produce output, one can categorize the phenomenon as fluorescence and phosphorescence. The materials, which produce output within 10-8 s of input are known as fluorescent materials while others, which take more time as many hours are known as phosphorescent materials. Depending on the imperfections present in the material the recombination time and frequency may alter. There are so many organic and inorganic materials having good phosphor properties. In this sequence, we can quote examples of so many oxides, sulfides, sulfates, vanadates, phosphates, alu-minates etc. Apart from these widely known materials, there are certain materials like titanates, zirconates, zirconate titanate etc., which are of great interest.

HOST MATRIX

Great interest has been shown in the study of the active optical properties such as PL or non-linear optics of amorphous materials at room temperature, of special interest have been the perovskite type titanates [1]. Perovskite type oxides having general formula

ABO3 are some of the most fascinating and technologically important class of materials in condensed material research. Most of the useful ferroelectrics, such as barium titanate (BT), lead titanate (PbTiO3), lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), and potassium niobate (KNbO3) have the perovskite structure. Perovskite is the mineral name of calcium titanate (CaTiO3). Its simplest structure is cubic, which is the high temperature form of many mixed oxides of the ABO3 type [2]. These materials have been of interest for their optical and ferroelectric properties. These materials have great use in the fabrication of various microelectronic and optoelectronic components like capacitors, multilayer ceramics, thermistors etc., including ferroelectric random access memories, microsensors, microactuators and electro-optic modulators [3—5]. The development of flat panel displays (FPDs) has seen great

growth in the last decade. Titanate (TiO^ ) based ceramic systems has unique optical, thermal, electrical and mechanical properties, which can be exploited to fabricate promising phosphor materials. Recently, oxide based phosphors are known to be as a potential choice for FPDs due to the advantage of high chemical stability compared to sulphide phosphors when impacted by electrons [6]. Among them titanates

(TiO2- ) based oxides have attracted interest. The luminescence properties of titanate perovskites doped with trivalent praseodymium have been increasingly investigated since the mid 1990s for red emitting phosphors for display applications [7, 8].

Fig. 1. Cubic structure of BaTiO3 (perovskite structure).

In the unit cell of perovskite (ABO3) structure, the A atoms are at (0,0,0); B atoms are at (1/2,1/2,1/2) and O atoms are at (0,1/2,1/2), (1/2,0,1/2), (1/2,1/2,0) positions [9]. Figure 1 shows the per-ovskite structured unit cell of BaTiO3. The perovskite family (ABO3) is a network of oxygen octahedra inside which there lays a B site atom known as network former. In between eight oxygen octahedrons, i.e., in the dodecahedral sites, there exists an A site atom known as network modifier [10] (Fig. 2). Depending upon the valencies of the metal cations (A, B), perovskite materials are subdivided into two main groups, described by the formula A2+B4+O3 (e.g. BaTiO3, PbTiO3) and A3+B3+O3 (e.g. NdAlO3, LaAlO3), in both groups the crystal structure is usually depicted in a perovskite form [11]. The stability of the perovskite compounds

p .? ' /

B ™ B

B B

U Y Jf^&jj

B - * B * -j&jr' B j_ B t jT' y 1 / •

* *

Fig. 2. The 3D network of perovskite structured compounds (ABO3).

arises mainly of the electrostatic charge of the ions perfectly integrated [9].

BaTiO3 (BT) is a responsible member of perovskite (ABO3) family. Since its discovery in 1940's it has been one of the most popular dielectric materials. It displays a cubical structure and is an important ferroelectric material too. Depending upon the temperature of phase transition, BaTiO3 ceramic exhibits five different structures (orthorhombic, rhombohedral, tetragonal, hexagonal and cubic). The Rhombohedral structure is stable at —90°C. However, increasing the temperature by ~5°C it exhibits orthorhombic structure. When temperature is between 25 and 120° C it shows tetragonal structure. Above 120°C the structure is cubic and finally hexagonal structure exists only at elevated temperatures > 1430°C [12]. The effect of substitution of Ti by Zr is predominant because Zr is chemically more stable than Ti [13]. A small change in the concentration of substituent results in different Bravais lattices, which makes it possible to tune some basic physical properties like optical and electrical. Recently, barium zirconate titanate (BZT) has been utilized as an alternative material to barium strontium titanate (BST) in preparation of thin film or ceramic bulk. BZT is formed by solid solution of barium titanate and barium zirconate due to the substitution of Ti4+ ions by Zr4+ ions [11].

The tetragonal phase of BaTiO3 is relatively very important for fabricating electronic and optical devices. Apart from electrical properties, some of the research papers reported the optical properties of per-ovskite structured materials. The optical properties here mainly refer to the luminescence properties. This paper reports few researches concerning the luminescence properties (PL, ML and TL) of perovskite structured materials. In this review, it is reported that the luminescent properties of perovskite structured materials are related to external stress, temperature, irradiation, stimulation, etc. The width of luminescence band usually observed at room temperature in per-ovskite type crystals is associated with the presence of imperfections or defects in the form of oxygen vacancies. Nowadays, the rare earth ions are added to a wide variety of metallic oxides in order to improve electric, electronic, magnetic and optical properties [9]. The luminescent properties also get enhanced with addition of some rare earth elements. It is well known that the addition of rare earth elements creates new energy states vicinity to the conduction band which modifies this phenomenon. The defects produced by small amount of dopant concentration at A or B site of the structure could result in the emission of blue, green and red light which is commercially of great interest. The chemistry of lanthanides is different from main group elements and transition elements due to the presence of 4f shell. The structure of perovskite itself is also one of the important reasons behind the luminescence phenomenon. In perovskite crystals, a broad lu-

Normalised intensity

Temperature, K

Fig. 3. The TL glow curves of BaTiO3 at different irradiation test doses: 2.27 (7), 4.54 (2), 9.08 kGy (3).

Response

Fig. 4. The effect of gamma dose on the TL response of BaTiO3: y = (1.23348X - 1) x 10-10, R = 0.98995.

Normalised intensity

Temperature, K

Fig. 5. The dependence of glow curves on preheating temperatures (see text for explanation).

minescence band is usually observed at low temperatures and this behavior is associated with the presence of imperfections or defects into the band gap [13]. The most plausible origin of visible luminescence in ABO3 perovskite is attributed to the oxygen octahedra [BO6]. In general, in case of BaTiO3 there exists TiO6 octahedron and TiO5 pyramid structures in BT lattice and luminescence property is possibly attributed to the difference between two kinds of the structures. Electrons escape from O-2p6 band to Ti-3d2 band, but the energy level of Ti-4s2 is little lower than Ti-3d2, so the electrons continue going to the Ti-4s2, combine with the holes in the band and excite the photons [11].

LUMINESCENCE STUDIES OF SOME TITANATES

Thermoluminescence TL studies of BaTiO3 ceramics under X-ray irradiation were first reported in 1961. The X-ray irradiated barium titanate ceramic was found to show TL emission in the temperature region from 180 to 400°C with a peak around 330°C. Barium titanate has not been extensively studied for dosimetry applications despite the fact that the crystal possesses excellent ferroelectric a

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