Journal of Electroceramics, 13, 305–309, 2004 C  2004 Kluwer Academic Publishers. Manufactured in The Netherlands. Electrical Properties of 6H-BaTi 0.95 M 0.05 O 3−δ Ceramics where M = Mn, Fe, Co and Ni GILLIAN M. KEITH, 1 KUMARAVINOTHAN SARMA, 2 NEIL MCN. ALFORD 2 & DEREK C. SINCLAIR 1 1 Department of Engineering Materials, University of Sheffield, Sheffield, S1 3JD, UK 2 School of EEIE, South Bank University, London, SE1 0AA, UK Submitted March 31, 2003; Revised February 27, 2004; Accepted June 24, 2004 Abstract. Hexagonal BaTiO 3 materials have been stabilised at room temperature according to the formula BaTi 0.95 M 0.05 O 3−δ where M = Mn, Fe, Co and Ni. Dense ceramics (>96% of the theoretical X-ray density) were sintered at 1450 ◦ C in flowing O 2 gas from calcined powders prepared by the mixed oxide route at 1300 ◦ C. All samples were single-phase and the bulk conductivity, σ b , measured by Impedance Spectroscopy and Q.f measured by microwave dielectric resonance methods showed a strong dependence on the type of dopant. σ b at 300 ◦ C was 10 −7 , 10 −5.5 , 10 −5.5 and 10 −4 Scm −1 for M = Mn, Fe, Ni and Co, respectively and Q.f at ∼ 5 GHz was 7790, 6670, 2442 and 1291 GHz, for M = Mn, Fe, Ni and Co, respectively. The correlation between σ b and Q.f is attributed to the presence of oxygen vacancies and/or mixed valency of the dopant ions. Keywords: BaTiO 3 , perovskites, microwave dielectric resonators, impedance spectroscopy Introduction BaTiO 3 has various polymorphs, all of which are based on the perovskite structure (ABO 3 ); however, due to their high permittivity, ferroelectric (tetragonal) BaTiO 3 -based materials have received the most atten- tion. In this form, the structure consists of pseudo-cubic close packed BaO 3 layers with corner sharing between the TiO 6 units and this type of structure is often re- ferred to as a 3C-type perovskite (Fig. 1(a)). Surpris- ingly, very little is known about the electrical properties and defect chemistry of the high temperature hexago- nal (6H) polymorph which is stable above ∼1430 ◦ C in ‘undoped’ BaTiO 3 [1]. Stoichiometric 6H-BaTiO 3 crystallises in the space group P6 3 /mmc [2] and can be described as pseudo-close packed BaO 3 layers, with a [cch] 2 sequence where c and h refer to cubic and hexagonal stacking, respectively. The Titaniums Ti(1) and Ti(2) occupy corner- and face-sharing octahedra, respectively (Fig. 1(b)). Stoichiometric 6H-BaTiO 3 un- dergoes structural phase transitions on cooling at 220 and ∼70 K [3] to give polymorphs with C222 1 (or- thorhombic) and P2 1 (monoclinic) space groups, re- spectively [4]. To date, 6H-BaTiO 3 has been stabilised at room tem- perature by several methods including; rapid quenching of undoped BaTiO 3 from >1500 ◦ C, partial reduction of Ti(+IV) to Ti(+III) by heating 3C-type BaTiO 3 un- der low pO 2 at >1300 ◦ C [5] and by partial substitution of Ti by various cations, eg Mg, Mn, Ru and Pt [6]. Recently we have undertaken a systematic study of the structure-composition-electrical property relationships in a range of 6H-BaTiO 3 materials based on various Ti-site dopants, eg Mg, Al, Ga, In, Mn, Fe, Co and Ni. To date, we have shown that Ga-doped 6H-BaTiO 3 ceramics have room temperature permittivity, ε ′ 25 of ∼70–80 and resonate at microwave frequencies with a quality factor, Q.f of ∼7000–8000 GHz at ∼5 GHz [7]. In addition, we have used impedance spectroscopy under various oxygen partial pressures to establish the high temperature conductivity behaviour of Ga-doped ceramics [8] which show the presence of a p-n tran- sition with slopes of ∼−1/4 and +1/4 in the n- and p-type regions indicating that the conductivity obeys the extrinsic model proposed by Smyth and co-workers [9, 10] for undoped and acceptor-doped 3C-BaTiO 3 type materials. Here we report and discuss the mi- crowave dielectric properties and high temperature