Journal of the European Ceramic Society 27 (2007) 2957–2961 The correlation between the structure and the dielectric properties of K x Ba 1-x Ga 2-x Ge 2+x O 8 ceramics Marjeta Macek Krzmanc a,∗ , Anton Meden b , Danilo Suvorov a a Advanced Materials Department, Joˇ zef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia b Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aˇ skerˇ ceva 5, 1000 Ljubljana, Slovenia Available online 12 December 2006 Abstract A study of K x Ba 1-x Ga 2-x Ge 2+x O 8 ceramics revealed that these solid solutions undergo a monoclinic-to-monoclinic P2 1 /a ⇔ C2/m phase transition. The temperature of this phase transition decreases with an increase in x, in a similar way to the sintering temperature, which decreases from 1100 ◦ C (x = 0) to 970 ◦ C(x = 1). The temperature of the P2 1 /a ⇔ C2/m phase transition is below the sintering temperatures of K x Ba 1-x Ga 2-x Ge 2+x O 8 (0.67 ≤ x ≤ 1) solid solutions, whereas the compositions at lower x (x = 0.4 and 0) remain in the P2 1 /a modification over a wide temperature range above the sintering temperature. Compared to the C2/m modification of K x Ba 1-x Ga 2-x Ge 2+x O 8 (0.67 ≤ x ≤ 1), with a permittivity of 6.2–6.9, the P2 1 /a modifications exhibit permittivities of 5.9–7.0 and three-to-eight times higher Q × f values of ∼100,000 GHz (at ∼12 GHz). The temperature coefficient of the resonant frequency is ∼-25 ppm/K, regardless of the composition. © 2006 Elsevier Ltd. All rights reserved. Keywords: Powders; X-ray methods; Dielectric properties 1. Introduction The continuing growth in mobile telecommunications has increased the need for new, low-permittivity, low-loss materi- als. Firstly, due to their potential use as a substrate material, and secondly, due to the expansion of the utilized frequency range from the microwave (MW) to the millimetre-wave region. Low-temperature cofired ceramic (LTCC) technology imposes the additional requirement of a low sintering temperature: lower than 960 ◦ C. The low sintering temperature of LTCC sub- strate materials can be achieved by recrystallization or by the addition of low-melting-point glass. Due to the presence of a glassy phase in both cases the dielectric losses are higher than in glass-free material. 1 However, this is to be expected, since the dielectric losses strongly depend on the structural order. The influence of ordering on Q-factor improvement in the perovskite-type Ba(Zn 1/3 Ta 2/3 )O 3 has been the subject of many studies. 2 In contrast, the correlations between structural order and dielectric properties in feldspars are not so well investigated, even though the commercially used dielectrics for LTCC systems are based on the feldspar MAl 2 Si 2 O 8 (M = Ca, Sr, Ba) In our previous studies we attempted to determine ∗ Corresponding author. Tel.: +386 1 477 3292; fax: +386 1 477 3875. E-mail address: Marjeta.Macek@ijs.si (M. Macek Krzmanc). some of these correlations for Na x Ca 1-x Al 2-x Si 2+x O 8 and Sr x Ba 1-x Al 2 Si 2 O 8 ceramics. 3,4 In an analogy with the solid solubility of Na x Ca 1-x Al 2-x Si 2+x O 8 we expected the exis- tence of a solid solubility (K x Ba 1-x Ga 2-x Ge 2+x O 8 ) between KGaGe 3 O 8 and BaGa 2 Ge 2 O 8 , which has been reported to crys- tallize in the monoclinic paracelsian structure type with the space group P2 1 /a. 5,6 The Ga- and Ge-containing feldspars could be interesting from the point of view of LTCC technol- ogy because of their sintering temperatures, which would be expected to be lower than the sintering temperature of alumi- nosilicate feldspars. In addition to this, the tetrahedral ordering, which strongly influences the dielectric losses in the MW fre- quency range, is known to be faster in Ge-containing feldspars. 7 These are the starting points for the synthesis and dielec- tric characterization of K x Ba 1-x Ga 2-x Ge 2+x O 8 solid solutions, the existence of which has not been reported yet. Our main focus was to study the P2 1 /a ⇔ C2/m phase transitions along the K x Ba 1-x Ga 2-x Ge 2+x O 8 solid-solubility range and correlate them with the dielectric properties. 2. Experimental The K x Ba 1-x Ga 2-x Ge 2+x O 8 (0 ≤ x ≤ 1) solid solutions were synthesized using the solid-state reaction technique. Stoichio- metric mixtures of reagent-grade oxides (Ga 2 O 3 , 99.99%, 0955-2219/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2006.11.021