Microwave Dielectric Properties of Co 2 La 4 Ti 3 Si 4 O 22 Ceramics Janardhanan Chameswary, Sherin Thomas, and Mailadil Thomas Sebastian w Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology, CSIR, Thiruvananthapuram 695019, Kerala, India Co 2 La 4 Ti 3 Si 4 O 22 ceramic was synthesized by the conventional solid-state ceramic route. The structure and microstructure of the sintered ceramics were characterized by X-ray diffrac- tion (XRD) and scanning electron microscopic techniques. XRD study showed the presence of SiO 2 as a secondary phase. Attempts were made to obtain phase-pure material by prepar- ing nonstoichiometric Co 2 La 4 Ti 3 Si (4x) O (22d) (where x 5 0.02, 0.05, 0.1, 0.25, and 0.5) ceramics. The secondary phase of SiO 2 could be effectively suppressed for x 5 0.1 with good microwave dielectric properties. The Co 2 La 4 Ti 3 Si 3.9 O (22d) has e r 5 17.4, Q u  f 5 48 700 GHz, and s f 5 155 ppm/1C. I. Introduction T HE recent rapid development in modern microwave com- munication systems has increased the demand for low-loss dielectric ceramics. The microwave dielectric ceramics for prac- tical application as a resonator must have a high relative per- mittivity (e r 410) for miniaturization, a high quality factor (Q u  f) for selectivity, and a near-zero temperature coefficient of frequency (t f ) for temperature stability. 1,2 Low relative per- mittivity materials have been playing a more prominent role because the carrier frequency of interest in communication sys- tems extends from GSM 900 MHz to ISM bands (2.4, 5.2, and 5.8 GHz) or even to the millimeter wave range. 3 The microelec- tronic industry is continuously reinvestigating novel ceramic materials for dielectric resonator applications. Still the search for new materials having the above-mentioned properties is in rapid progress owing to the drive for further system miniatur- ization and improved filtering capabilities. 2,4,5 The silicates have a predominantly covalent bonding, which restricts the rattling of the atoms 2 leading to low relative permittivity. The silicates such as Mg 2 SiO 4 , Li 2 MgSiO 4 , Sr 2 ZnSi 2 O 7 , etc. and rare earth-based silicates have been investigated as the potential candidates for dielectric resonator applications. 6–13 Although extensive work has been reported on several silicates, the dielectric properties of Co 2 La 4 Ti 3 Si 4 O 22 have not yet been reported. This material be- longs to the perrierite-type structure and was first reported by Bonatti and Gottardi from Italy. 14 It comes under the sorosil- icate (Si 2 O 7 ) 2 family of silicates. The general formula of per- rierite minerals may be represented as A 4 BC 2 Ti 2 O 8 (Si 2 O 7 ) 2 where A 5 rare earth ion, B 5 divalent cation, and C 5 trivalent cation. 12 This structure allows several substitutions such as the trivalent C cations may be partially replaced by divalent as well as tetravalent cations. Green and Pearson 15 reported the syn- thesis of rare earth-based perrierite materials at high pressure and temperature. Co 2 La 4 Ti 3 Si 4 O 22 has a monoclinic structure with space group P2 1 /a. It has been reported that introduction of a small amount of nonstoichiometry can improve the densifi- cation and microwave dielectric properties. 16–19 The present study investigates the synthesis, characterization, and dielectric proper- ties of nonstoichiometric composition Co 2 La 4 Ti 3 Si (4x) O (22d) where x 5 0.02, 0.05, 0.1, 0.25, and 0.5. II. Experimental Procedure Co 2 La 4 Ti 3 Si (4x) O (22d) (where x 5 0.0, 0.02, 0.05, 0.1, 0.25, and 0.5) ceramics were prepared by the conventional solid-state ceramic route. The oxide chemicals, Co 3 O 4 (99%, Sigma Ald- rich, Milwaukee, WI), TiO 2 (99.8%, Sigma Aldrich), SiO 2 (99.6%, Sigma Aldrich), and La 2 O 3 (99%, Indian Rare Earths Limited, Kerala, India) powders were mixed by ball milling for 24 h in distilled water medium using zirconia balls in plastic bottles. After drying, the powders were double calcined at 10501C for 6 h and then it was ground well and again double calcined at 11501C for 8 h. The calcined powders were reground and mixed with 4 wt% polyvinyl alcohol and shaped into disks of 20 mm diameter and 10 mm height by applying a pressure of 150 MPa. The sintering was carried out in the temperature range of 12001–13001C for 4 h. The sintered samples were pow- dered and used to analyze the crystal structure and phase purity by X-ray diffraction (XRD) method using CuKa radiation (Philips X’pert PRO MPD XRD; Philips, Eindhoven, the Neth- erlands). The morphology and microstructure were recorded from the polished and thermally etched surface of the sintered samples using a scanning electron microscope (SEM) (JEOL- SEM 5600LV, Tokyo, Japan). The bulk densities of the pol- ished samples were measured using the dimensional method. Sintered and polished samples were used for microwave di- electric property measurements using an Agilent Network An- alyzer (model 8753 ET, Agilent Technologies Inc., Palo Alto, CA). The relative permittivity (e r ) was measured by the post- resonator method of Hakki and Coleman 20 using the TE 01d mode of resonance coupled through E-field probes, as described by Courtney. 21 The unloaded quality factor (Q u ) of resonance was determined using a resonance cavity method proposed by Krupka et al. 22 The measurements were made in the frequency range of 4–6 GHz. The temperature coefficient of resonant fre- quency (t f ) was measured by noting the variation of resonant frequency of the TE 01d mode in the reflection configuration over the temperature range of 251–751C. t f ¼ 1 f  Df DT (1) where Df is the variation in the resonant frequency from room temperature and DT is the difference in temperature. III. Results and Discussion Co 2 La 4 Ti 3 Si 4 O 22 belongs to the mineral category of perrierite and the synthesis of a pure material at ordinary conditions is difficult. In order to synthesize phase-pure material, several steps H. T. Kim—contributing editor This study was supported by the Department of Science and Technology. w Author to whom correspondence should be addressed. e-mail: mailadils@yahoo.com Manuscript No. 26998. Received October 22, 2009; approved January 1, 2010. J ournal J. Am. Ceram. Soc., 93 [7] 1863–1865 (2010) DOI: 10.1111/j.1551-2916.2010.03639.x r 2010 The American Ceramic Society 1863