CRYSTAL SRUKTUR OF CALCIUM TITANATE (CaTiO3) PHOSPHOR DOPED WITH PRASEODYMIUM AND ALUMINIUM IONS

STRUKTUR KRISTAL FOSFOR KALSIUM TITANIA DIDOPKAN DENGAN ION PRASEODYMIUM DAN ALUMINIUM IONS

Siti Aishah Ahmad Fuzi1* and Rosli Hussin2

1 Material Technology Group, Industrial Technology Division, Malaysian Nuclear Agency, Bangi, 43000 Kajang, Selangor Darul Ehsan, Malaysia.

2 Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 Skudai, Johor

1*[email protected], [email protected]

Abstract

The past three decades have witnessed rapid growth in research and development of luminescence phenomenon because of their diversity in applications. In this paper, Calcium Titanate (CaTiO3) was studied to find a new host material with desirable structural properties for luminescence-based applications. Solid state reactions o 3+ methods were used to synthesis CaTiO3 at 1000 C for 6 hours. Crystal structure of CaTiO3 co-doped with Pr 3+ and Al were investigated using X-Ray Diffraction (XRD) method. Optimum percentage to synthesis CaTiO3 was 3+ obtained at 40 mol%CaO-60 mol%TiO2 with a single doping of 1 mol%Pr . However, a crystal structure of 4 mol% of Al3+ co-doped with Pr3+ was determined as an optimum parameter which suitable for display imaging.

Keywords: calcium titanate, anatase, rutile

Abstrak

Semenjak tiga dekad yang lalu telah menunjukkan peningkatan yang ketara bagi kajian dan pembangunan dalam bidang fotolumiscen. Peningkatan ini berkembang dengan meluas disebabkan oleh kebolehannya untuk diaplikasikan dalam pelbagai kegunaan harian. Dalam manuskrip ini, kalsium titania (CaTiO3) telah dikaji untuk mencari bahan perumah dengan sifat struktur yang bersesuaian bagi aplikasi luminescen. Tindak balas keadaan o pepejal telah digunakan bagi mensintesis CaTiO3 pada suhu 1000 C selama 6 jam. Struktur kimia bagi CaTiO3 yang didopkan dengan Pr3+ dan Al3+ telah dikaji dengan meggunakan kaedah difraktogram X-Ray (XRD). Peratus optimum bagi sintesis CaTiO3 ditentukan pada 40 mol%CaO-60 mol%TiO2 dengan yang didopkan dengan 1 3+ 3+ 3+ mol%Pr . Walaubagaimanapun, struktur kristal bagi 4 mol% of Al didopkan bersama Pr ditentukan sebagai parameter optimum yang sesuai digunakan untuk aplikasi imej paparan.

Kata kunci: kalsium titania, anatase, rutil

INTRODUCTION

Calcium Titanate, CaTiO3 is one of the fundamentally important oxides in solid-state physics that has been the centre of various studies involving structural and physical properties. Commonly, calcium titanate used in photocatalytic, biomedical, flat screen full-color displays, amplifiers for fiber-optic communications, optoelectronic devices, and solid-state laser display imaging. CaTiO3 are known as the type of ABO3 materials which have the so-called perovskite structure. Structural CaTiO3 are similar to structural of SrTiO3, BaTiO3, BaSnO3 and others. In most symmetrical form, this structure is cubic and contains one formula unit per cell. Choosing A ions at the cube corners for convenience, then B ions occur at body centres and O ions at face-centres (Bailey et al. 1957). However, CaTiO3 is slightly distorted from the cubic structure and has an orthorhombic crystal structure at room temperature (Roushown Ali and M. Y. 2005). These materials are important not only for their technical applications but also for fundamental research. Their simple crystal structure combined with the variety of structural phase transitions which they display; make them suitable for experimental study and for testing theoretical models. In this work, the synthesis and characterization of CaTiO3 doped with praseodymium and aluminium ions are investigated using X-Ray diffraction (XRD). Praseodymium and aluminium ions are co-doped into CaTiO3 in order to enhance their structural and

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luminescence properties for displays imaging. Optimum composition of synthesis CaTiO3 are determined in order to apply for devices.

METHODOLOGY

Calcium oxide (99% CaO: Riedel-de Haen), titanium dioxide (99% TiO2: Fluka Alrich were employed as the raw materials. Praseodymium oxide (99% P203: Acros) was incorporated as a doped material in appropriate amounts for charge compensation purposes. The calcium titanate phosphor doped with praseodymium and aluminium are synthesized using solid-state reaction method using high temperature. Starting material were mixed and homogenized through the grinding process using ball milling for 1hour. Then, the samples were sintered at 1000°C for 6 hours. Lastly, the furnace shall be set to room temperature to let the mixture to cool down. After the reactions, the samples were characterized by X-Ray Diffraction (XRD) method with the 2θ range from 20° to 80°.

RESULT AND DISCUSSION

The effect of percentage Ca on host structure

The XRD patterns of calcium titanate, CaTiO3 after sintering at 1000°C for 6 hours with different percentage of CaO are shown in Figure 1. The amounts of the starting material in these samples were controlled by their percentage. The phase of CaTiO3 along with anatase TiO2 and rutile TiO2 were determined for those samples. Therefore, the synthesis of CaTiO3 are successfully prepared in different percentage of compositions, but under similar temperature and time duration by solid-state reaction process. At room temperature, the perovskite compound CaTiO3 could be indexed as orthorhombic unit cells with the space group of Pbnm. When sample contained 10 mol% of CaO, the CaTiO3 phase appear along with rutile phase. This feature indicated that at low Ca concentration, CaTiO3 crystal would be produce by solid state reaction method.

Peaks shown in the patterns were sharps and well defined, indicating that all samples were crystallized. For the samples of x=10 and 20 mol%, only rutile phase was detected in this samples. At this percentage, rutile phase became a major phase for this sample. In contrast, while x=30 and 40 mol%, the minor phase of anatase and rutile were formed in this samples. Hence, we believe that the modification percentage of modifier (CaO) could influence the phase transformations of anatase and rutile.

When the percentage of CaO increased, the peak intensities of CaTiO3 phase increased at the (1 1 2) plane diffraction (2θ = 33.1°). When Ca contents was above 10 mol %, the CaTiO3 phase started to appear and CaTiO3 phase eventually became the major phase for x=40 mol%. As the x increased, intensity of diffraction peaks of CaTiO3 phase at 2θ=33˚ (1 1 2) were gradually increased, exhibiting the fact that more CaTiO3 are formed in the samples (Lian Shixun et. al. 2006, and T.N. Lin et. al. 2005]. In addition, the diffraction patterns of the crystalline phases formed were identified as crystalline anatase TiO2 (ICDD 86-1157), rutile TiO2 (ICDD 86-0148) and CaTiO3 (ICDD 82-0228), as summarized in Table 1. This result was reasonable because Ca deficiencies in the CaTiO3 compounds would lead to the excess of Ti oxides. It is interesting to note that rutile instead of anatase TiO2 phase presented in the Ca deficient CaTiO3 powders, due to the fact that the powders were sintered at 1000 ◦C, and rutile was stable under such high temperature (Tao Li 2009). There diffraction patterns of CaTiO3 are in coincidence with the reported data in literature (Hidekazu Tanaka 2006).

Moreover, there was no diffraction peaks from Pr compounds such as Pr2(Ti2O7). It was considered that Pr ion was not excluded from host material, and it was replaced to Ca site in the crystal. In other word, Pr3+ ions were 3+ 2+ incorporated into the CaTiO3 lattice due to the closes ionic radius between Pr (99 pm) and Ca (100 pm), so it could be considered that Pr3+ substituted Ca2+ site in the samples (S.B. Zhang 2006). This is corresponding to the research done by Hiroki Kominami et al. which claimed that by adding lanthanide group, no changes was observed in XRD patterns (Hiroko Kominami 2006).

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3+ Figure 1. XRD pattern for samples of xCaO-(100-x)TiO2 doped with 1 mol% Pr

Table 1. Composition of the samples xCaO-(100-x)TiO2 and identified crystalline phase

Composition Phase CaO TiO2

10 90 Rutile TiO2 (m), CaTiO3 (mn)

20 80 Rutile TiO2 (m), CaTiO3 (mn)

30 70 Anatase TiO2 (mn), Rutile TiO2 (m), CaTiO3 (mn)

40 60 Anatase TiO2 (mn), Rutile TiO2 (mn), CaTiO3 (m)

3+ 3+ Crystalline phase study of CaTiO3 doped with 1 mol% Pr and different concentration of Al

Further investigation of has been done using optimum percentage in section 3.1. Aluminum are co-doped into a 3+ 3+ system of 40CaO-60TiO2:Pr (mol%) with different Al concentration. Effect of aluminum doping into crystal 3+ 3+ structure are study in detail. Figure 2 shows the XRD pattern of 40CaO-60TiO2-1Pr , xAl (mol%) where x in range of 1≤ x mol % ≤ 4. The analysis of XRD pattern of the samples indicated that this material possessed single phase CaTiO3 and could be completely indexed on the basis of an orthorhombic ICDD cards no. 22-0153 (Pbnm). In its crystal structure, the Ca atom occupied the 12-coordinated sites in CaTiO3 lattice.

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3+ 3+ 3+ Figure 2. XRD pattern for samples of 40CaO-60TiO2:Pr , xAl (mol%) with various Al concentration

The samples were sintered at 1000°C for 6 h shown very sharp and well-resolved X-ray diffraction peaks at 2θ=~33° due to CaTiO3 as main phase. Tsuneo Matsui et al. reported the formation of CaO and TiO2 also contributed to CaTiO3 phase (Tsuneo Matsui 1997). Besides the predominant peaks of CaTiO3, others diffraction peaks were observed at the same time. A certain amount of tetragonal anatase and rutile phase of TiO2 were also detected as the residual phase in the solid reaction process. The anatase phase of titania was seen to transform to rutile phase but the transformation was still incomplete.

3+ When Al ions increased, the main phase was same as CaTiO3 and no obvious transformation of phase were detected. The main phases remained the same to all samples due to small percentage of doping with Al3+ and Pr3+ ions. But, weak diffraction peaks of calcium aluminium oxide (Ca9(Al6O18)) were observed at the same time. It means that when Al3+ was added in the sample, it would react in the sample and change the crystal structure of 3+ CaTiO3 phase. Meanwhile, the line width of diffraction peaks in the XRD was much narrow when Al was added. Therefore, the cystallinity of CaTiO3 improved with Al addition.

As conclusion, most of added Al were not incorporated into host lattice due to small percentage of dopand, but the crystallinity of the host lattices improved by Al addition (Shinji Okamoto 2003). 4 mol% of Al3+ added into 3+ 40CaO-60TiO2:Pr (mol%) are most suitable percentage for applications purposes due to higher crystallinity obtained.

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CONCLUSION

The synthesis of CaTiO3 doped with Pr and Al using solid state reaction method were successfully obtained. Solid state reaction method is one of the easy and low cost method to synthesis CaTiO3. The composition of 40 3+ mol%CaO-60 mol% TiO2 doped with 1 mol% Pr were the optimum percentage to synthesis CaTiO3 due to higher degree of crystallinity obtained. In addition, in this percentage, CaTiO3 act as a major phase as compared 3+ to other compositions. By addition of praseodymium, Pr ion were only incorporated into the CaTiO3 lattice. Co- 3+ 3+ doping effect of different Al and 1 mol% Pr into 40 mol%CaO-60 mol% TiO2 shows the crystallite phase of calcium titanate obtained as main phase. However, the anatase and rutile phase as a residual phase has been obtained for all samples. The doping effect are not too much significant to the crystallinity of the samples due to 3+ 3+ no obvious transformation of phase. Therefore, 4 mol% of Al added into 40CaO-60TiO2:Pr (mol %) are the optimum percentage for applications purposes.

ACKNOWLEDGEMENT

We would like to thank Ministry of Higher Education, Universiti Teknologi Malaysia and Ministry of Energy, Technology, Science, Climate Change and Environment for the financial support.

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