Journal of the European Ceramic Society 26 (2006) 1845–1851 Low-temperature microwave and THz dielectric response in novel microwave ceramics S. Kamba a,∗ , D. Noujni a , A. Pashkin a , J. Petzelt a , R.C. Pullar b , A.-K. Axelsson b , N. McN Alford b a Institute of Physics, ASCR, Na Slovance 2, 182 21 Prague 8, Czech Republic b London South Bank University, 103 Borough Road, London SE1 0AA, UK Available online 19 October 2005 Abstract Low-temperature dielectric properties of BaZn 1/3 Nb 2/3 O 3 -based ceramics, CeO 2 -based ceramics and Ruddlesden–Popper Sr n+1 Ti n O 3n+1 (n = 1–4) ceramics has been studied in microwave, THz and infrared frequency range down to 10K. Extrinsic dielectric losses originating probably from diffusion of charged defects are observed in two families of compounds by a minimum in the temperature dependence of microwave quality Q. The rise of microwave permittivity and dielectric losses at low temperatures in Sr n+1 Ti n O 3n+1 (n = 2–4) ceramics was explained by softening of an optical polar mode in SrTiO 3 , which is in the Sr n+1 Ti n O 3n+1 (n = 3, 4) ceramics contained as a second phase. © 2005 Elsevier Ltd. All rights reserved. Keywords: Dielectric properties; Spectroscopy; Perovskites; Microwave ceramics 1. Introduction Modern communication systems have moved to the microwave (MW) frequency region, where advanced dielectric ceramics are frequently used in resonators and filters. Miniatur- ization requires high relative permittivity (ε ′ ) materials (since the size of resonators is inversely proportional to √ ε ′ ) with a small or zero temperature coefficient of resonance frequency τ f (|τ f | < 10 ppm K -1 ). Furthermore, ceramics with low dielec- tric loss ε ′′ (often described in terms of high dielectric quality, Q = ε ′ /ε ′′ ) are needed for the high selectivity and optimized bandwidth of the filters. Within the last 20 years many new suitable ceramics with high ε ′ and low τ f were described. How- ever, it was shown that although the two former parameters are not very dependent on the method of sample preparation, the quality Q is extremely sensitive on conditions of sample preparation. It is not rare that in some cases the Q value varies within 2 orders of magnitude depending on sintering tempera- ture, cooling rate, atmosphere of annealing, etc. Technologists spent much time with improvement of sample processing to obtain losses as low as possible (highest Q), but the methods ∗ Corresponding author. Tel.: +420 2 6605 2957; fax: +420 2 8689 0527. E-mail address: kamba@fzu.cz (S. Kamba). used were purely empirical, without knowing the lowest limit of the losses (intrinsic losses) with an origin in multi-phonon absorption. Almost 20 years ago, Wakino et al. 1,2 proposed infrared (IR) reflectivity spectroscopy as a tool for investigating the intrin- sic MW dielectric properties of dielectric resonators. It is well known that the main infrared dispersion of the permittivity is given by the sum of polar phonon contributions ε ∗ (ω) = ε ′ (ω) - iω ′′ (ω) = n  j=1 ε j ω 2 j ω 2 j - ω 2 + iωγ j + ε ∞ (1) where ω j and γ j are the frequency and damping of the jth polar phonon, respectively; ε j denotes the mode contribution to the static permittivity ε ′ (0) and ε ∞ denotes the electronic part of the permittivity. We note that this formula is as a rule valid in dielectrics with ε ′ (0) ≤ 100. In higher permittivity materials (e.g. ferroelectrics) an additional dispersion occurs below polar phonon frequencies, in the simplest case modelled by a Debye relaxation ε r ω r /(ω r +iω)(ε r and ω r are the dielectric strength and relaxation frequency, respectively), which should be added into Eq. (1). Wakino et al. 1,2 mentioned that extrapolation of Eq. (1) from IR down to MW range, i.e. 2–3 orders of magnitude below ω j ’s 0955-2219/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2005.09.010