Origin of Microwave Dielectric Loss in ZnNb 2 O 6 –TiO 2 Dong-Wan Kim, Kyung Hyun Ko,* ,† Do-Kyun Kwon, and Kug Sun Hong School of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul, Korea (1x)ZnNb 2 O 6 xTiO 2 ceramics were prepared using both anatase and rutile forms of TiO 2 . At a composition of x  0.58, a mixture region of ixiolite (ZnTiNb 2 O 8 ) and rutile was observed and the temperature coefficient of resonant fre- quency ( f ) was 0 ppm/°C. We found that although  r and  f were comparable, the quality factor (Q  f, Q  1/ tan , f  resonant frequency) of 0.42ZnNb 2 O 6 0.58TiO 2 prepared from anatase and rutile was 6000 and 29 000, respectively. The origin of the difference in Q  f of both samples was investigated by measuring electrical conductivity and by anal- ysis of the anatase–rutile phase transition. The anatase-derived sample had higher conductivity, which was related to the reduction of Ti 4 . It is suggested that the increase of dielectric loss originates from an increase in Ti 3 and oxygen vacancies due to an anatase–rutile phase transition. I. Introduction M ICROWAVE and millimeter-wave applications have made re- markable progress in the development of advanced commu- nication systems, including mobile and satellite communications. Passive microwave devices, including dielectric resonant filters, require small sizes, low power losses, and excellent temperature stabilities. Dielectric materials for resonators should thus have high dielectric constants (ε r ), low dielectric losses (tan ), and small temperature coefficients of resonant frequency ( f ). In our previous study, the structure–property relationships of ZnNb 2 O 6 –TiO 2 were investigated. 1 The ε r and  f of ZnNb 2 O 6 – TiO 2 ceramics were compositionally tuned in a narrow mixture region of ixiolite (ZnTiNb 2 O 8 ) and rutile. TiO 2 in the form of anatase was used. TiO 2 exists in three forms: rutile, anatase, and brookite. Anatase and brookite are metastable and convert exo- thermically and irreversibly to rutile at high temperatures. 2 The transformation temperature and rate are largely dependent on particle size, morphology, degree of agglomeration, and impuri- ties. 3–5 The complexity of this transition is typically attributed to the reconstructive nature of the transition. 6 Shannon described the anatase–rutile phase transition in terms of the defect structure of TiO 2 . 7,8 He suggested that using either a reducing atmosphere or dopants with a valence less than 4 caused the acceleration of oxygen vacancy formation and resultant transition to rutile. The microwave dielectric loss of titanium oxide has been reported by Templeton et al. 9 High-Q TiO 2 was produced through additions of divalent and trivalent ions with ionic radii similar to that of Ti, and demonstrated that reduction of the Ti 4+ ion was prevented by a favorable compensation mechanism. It was also found that the dielectric loss was related to the reduction of Ti 4+ in titanium-containing ceramics. 10 –13 The goal of the present work is to compare the quality factor of 0.42ZnNb 2 O 6 0.58TiO 2 prepared from anatase versus rutile, hav- ing different concentrations of Ti 3+ and oxygen vacancies. We also investigated the anatase–rutile phase transition associated with reduction of Ti 4+ . II. Experimental Procedure The starting materials used were ZnO (Cerac, Japan), Nb 2 O 5 (High Purity Chemical Laboratory, Japan), anatase TiO 2 (Merck, Germany), and rutile TiO 2 (High Purity Chemical Laboratory, Japan), 99.9% pure powders. ZnNb 2 O 6 powders were prepared using conventional mixed-oxide methods and calcined at 1000°C for 2 h. Mixtures of ZnNb 2 O 6 and TiO 2 powders of varying compositions were ball-milled in a polyethylene bottle with ZrO 2 media for 24 h using distilled water. The milled powders were then dried, granulated, and pressed at 1000 kg/cm 2 to form pellets 8 mm in diameter and 3 mm thick. The pellets were sintered in air at 1250°C at a heating rate of 5°C/min. The crystal structure of sintered samples was investigated using X-ray powder diffraction (Model M18XHF, MacScience Instru- ments, Japan) in the 2 range of 20° to 60°. Samples were quenched in air after reaching a preset temperature to observe the anatase–rutile phase transition. Microwave dielectric properties of sintered samples were mea- sured using a network analyzer (Model HP8720C, Hewlett- Packard, USA) in the frequency range of 5–11 GHz. The quality factor was measured by the transmission cavity method using a Cu cavity and a Teflon supporter. 14 The relative dielectric constant (ε r ) was measured using the postresonator method 15 and the temperature coefficient of the resonant frequency ( f ) was mea- sured using an Invar cavity in the temperature range of 20° to 80°C. 16 To determine the dielectric properties in the frequency range from 1 kHz to 10 MHz, sintered samples were electroded with silver. The capacitance and tan  were measured using an imped- ance/gain phase analyzer (Model HP 4194A, Hewlett-Packard, USA). The electrical resistivity of the samples was measured at 800° to 1000°C using the dc four-point probe method. Samples used were bars with dimensions of 2 mm  2 mm  15 mm and both a dc current source (Keithley, Model 224) and a nanovoltmeter (Keithley, Model 181) were used. III. Results (1) Crystal Structures Observed in the ZnNb 2 O 6 –TiO 2 System In the (1-x)ZnNb 2 O 6 xTiO 2 system, the interrelation of the columbite and rutile structures lead to various structural transitions and a broad range of solid solutions as a function of composi- tion. 1,17 The crystal structure of sintered samples was investigated using X-ray powder diffraction. Four phase regions were observed with increasing TiO 2 content. The upper limit of the first region, a solid solution based on the columbite structure, was observed up to 50 mol% TiO 2 . At higher TiO 2 content only the ixiolite phase, ZnTiNb 2 O 8 , appeared. The ixiolite structure has a statistical P. K. Davies—contributing editor Manuscript No. 187695. Received June 25, 2001; approved December 10, 2001. This work was supported in part by the Ministry of Information and Communi- cation of Korea (“Support Project of University Foundation Research 2000” supervised by the Institute of Information Technology Assessment). *Member, American Ceramic Society. † Department of Materials Science and Engineering, Ajou University, Suwon, Korea. J. Am. Ceram. Soc., 85 [5] 1169 –72 (2002) 1169 journal