JOURNAL OF MATERIALS SCIENCE: MATERIALS IN ELECTRONICS 16 (2 0 0 5 ) 663 – 666 Improved dielectric properties in pyrochlore type oxides: Ca 3 Sm 3−x Bi x Ti 7 Nb 2 O 26. 5 (x = 1.0, 2.0 or 3.0) by Bi substitution P. PRABHAKAR RAO 1, ∗ , K. RAVINDRAN NAIR 1 , P. DIVYA 1 , PETER KOSHY 1 V. K. VAIDYAN 2 1 Regional Research Laboratory (CSIR), Thiruvananthapuram – 695 019, India E-mail: padala rao@yahoo.com 2 Department of Physics, Kerala University, Thiruvananthapuram – 695 581, India Improved dielectric properties are observed in pyrochlore type oxides, Ca 3 Sm 3−x Bi x Ti 7 Nb 2 O 26.5 (x = 1.0, 2.0 or 3.0) by Bi substitution. The dielectric constant increased with increasing Bi concentration. The dielectric constant obtained for Ca 3 Bi 3 Ti 7 Nb 2 O 26.5 is 110, whereas, without Bi (Ca 3 Sm 3 Ti 7 Nb 2 O 26.5 ) it is 62 at 100 kHz. The Powder X-ray diffraction analysis reveals that cubic pyrochlore type phase is formed for all the compositions. The experimental results further show that the formation of Bi substituted compounds is complete at a lower temperature than the compounds without Bi. Microstructure studies reveal that the grains formed are acicular when Bi is present in large amounts compared to cuboid grains in samples having no Bi. C  2005 Springer Science + Business Media, Inc. 1. Introduction A-R-Ti-Nb-O systems (A = alkaline metals, R = rare earth) have been extensively investigated for their high dielectric constant and low dielectric loss [1–5]. Their structure could be usually tungsten bronze type, and they have useful dielectric and ferroelectric properties which make them potential candidates for applications such as high dielectric constant capacitors both discrete and multilayer (MLC) and low loss substrates for mi- crowave integrated circuits and sensors. The high di- electric constant materials are also very important in advanced microelectronic technologies and microwave communication systems for the size reduction and cost effectiveness of the microwave components used in fil- ters and oscillators. In most of these applications, ma- terials with high dielectric constant and low dielectric loss are required. There is a demand for new materials with advancement in microwave communication and cellular phones for miniaturisation of components. During our investigations on these materials, we found that apart from the above mentioned tungsten bronze type oxides, new pyrochlore type oxides could also be formed in Ca-R-Ti-Nb-O system [6]. Cu- bic pyrochlore type compounds having the formula Ca 3 R 3 Ti 7 Nb 2 O 26.5 (R = rare earth) have been studied previously for their structure and dielectric properties. These compounds have been found to have dielectric constant in the range 60–80. It is well known that cer- tain complex oxides containing Bi have high dielectric constant values and sometimes they even show ferro- electric properties [7–10]. Some of these Bi compounds ∗ Author to whom correspondence should be addressed. also have pyrochlore structure. Because of this, we were interested in studying the effects of incorporation of Bi in the pyrochlore type oxides Ca 3 R 3 Ti 7 Nb 2 O 26.5 (R = rare earth) which we prepared earlier [6] on their di- electric properties. We found that substitution of Bi for rare earth in these compounds increases their dielec- tric constant and results are reported here. The aspects of phase constitution and microstructure are also in- vestigated and the relation between compositions and properties is discussed. 2. Experimental procedure Samples of Ca 3 Sm 3−x Bi x Ti 7 Nb 2 O 26.5 (x = 1.0, 2.0, 3.0) were prepared by the conventional solid state reac- tion technique. The starting materials were high purity chemicals CaCO 3 (99.9%, Acros Organics), Sm 2 O 3 (99.9%, Acros Organics), TiO 2 (99.9% Merck) and Nb 2 O 5 (99.9%, Aldrich Chemicals). These materials were taken in stoichiometric amounts and then mixed thoroughly in an agate mortar and ground until a fine powder is obtained. Acetone is added to the powder for proper mixing. The mixture was then dried by keeping it in an air oven at 100 ◦ C. The mixed product was cal- cined in a platinum crucible by heating in air at 1200 ◦ C for 3 h. For fully Bi substituted compound, calcination at 1100 ◦ C for three hours, is enough for completion of reaction. The calcination is repeated thrice to com- plete the reaction. The calcined powder after cooling to room temperature was transferred to an agate mor- tar and ground well until a fine powder is obtained. To 0957–4522 C  2005 Springer Science + Business Media, Inc. 663