La(Mg 1/2 Ti 1/2 )O 3 –La 2/3 TiO 3 microwave dielectric ceramics A.N. Salak*, M.P. Seabra, V.M. Ferreira Department of Ceramics and Glass Engineering/CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Abstract Structure and microwave dielectric properties were studied in the (1x)La(Mg 1/2 Ti 1/2 )O 3 –xLa 2/3 TiO 3 system. Ceramics with this composition in the 04x40.5 range were processed from powders obtained by a citrate-based chemical route. Structure of these perovskite solid solutions changed from orthorhombic for x=0.1 and 0.3 to pseudocubic for x=0.5. Microwave and radio fre- quency measurements revealed increase in permittivity and temperature coefficient of the resonant frequency  f with increasing of La 2/3 TiO 3 content. Close to zero  f value was found near to x=0.5 composition of (1x)La(Mg 1/2 Ti 1/2 )O 3 –x La 2/3 TiO 3 . # 2003 Elsevier Ltd. All rights reserved. Keywords: Dielectric properties; Perovskites; Resonator; Sintering 1. Introduction A considerable group of materials for microwave applications is based on solid solutions with the per- ovskite structure. Wide possibilities of atomic sub- stitutions and/or formation of cation and anion vacancies mean it is possible to obtain complex per- ovskite system with desired properties. Important dielectric characteristics in the microwave range, like permittivity (" r ), quality factor (Q) and thermal coeffi- cient of the resonant frequency ( f ) can be adjusted by choosing the appropriate end members and varying the composition. On the other hand, the  f , " r and Q dependences in solid solutions are often non-linear and even non-monotonic. 1 3 Correlations between structure and microwave dielectric characteristics of perovskite compounds (e.g.  f ) have been revealed only for some particular compositions 4,5 and are not common. Therefore, the compositional behaviour of the earlier parameters in many solid solutions is diffi- cult to predict. La 2 O 3 –TiO 2 -based perovskite ceramics have been found to be of great interest as prospective materials for production of the microwave resonators. 6 9 In this family La(Mg 0.5 Ti 0.5 )O 3 (LMT) is characterized by a monoclinic perovskite structure due to oxygen octahe- dra tilting, La displacement, as well as Mg/Ti ordering. 6 8 The LMT ceramics were found to give " r =27.4, Q=10 500 at 7 GHz and  f 80 ppm/  C. 8,10 The orthorhombic structure of La 2/3 TiO 3 (LT) is con- sidered as ‘‘double perovskite’’ because its parameters are a  a 0 , b  a 0 and c  2a 0 . 11 The c-axis length is dou- bled due to the ordered arrangement of the La 3+ cations and vacancies in the A-site of the perovskite lattice. Owing to the high vacancy content, the per- ovskite phase of pure LT is unstable and therefore its dielectric properties have been studied in the solid solu- tions with other oxide compounds. 11,12 For the compo- sition x=0.96 of the (1x)CaTiO 3 xLT solid solutions, the dielectric resonator characteristics were " r =90, Q=2700 (at 10 GHz) and  f =190 ppm/  C. 11 These parameters were found to change monotonically with x. In the (1x)LaAlO 3 –xLT system, in which orthorhombic LaAlO 3 has a negative thermal coeffi- cient of the resonant frequency, the monotonic depen- dence of both permittivity and  f were also observed. 12 High " r and Q values together with  f close to zero were obtained at microwave frequencies in composi- tions near x=0.6. This work aims to study the structure and microwave dielectric properties of perovskite ceramics with (1x)La(Mg 0.5 Ti 0.5 )O 3 –xLa 2/3 TiO 3 composition. This system is of a direct practical interest because the end members have  f with opposite signs. Regarding micro- wave properties, La(Mg 0.5 Ti 0.5 )O 3 is close to LaAlO 3 and therefore one can expect a similar behaviour of the dielectric parameters in the solid solutions with LT, namely monotonic dependence of " r and  f on compo- sition as well as  f  0 for a suitable composition. 0955-2219/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0955-2219(03)00130-4 Journal of the European Ceramic Society 23 (2003) 2409–2412 www.elsevier.com/locate/jeurceramsoc * Corresponding author. E-mail address: salak@cv.ua.pt (A.N. Salak).