CaCu 3 Ti 4 O 12 ceramics with a range of resistivities have been prepared using both conventional sintering and spark plasma sintering. For all cases, the high effective permittivity is associated primarily with an internal barrier layer capacitor mechanism. Additional polarization associated with the electrode-sample interface may appear but its visibility depends on the grain boundary resistivity (Rgb) of the ceramic. If the Rgb is large, the electrode polarization is obscured by sample-related effects; if the Rgb is small, a separate impedance associated with the electrode polarization may be seen. Discrepancies in the literature regarding the magnitude and origin of the high effective permittivity are attributed to a combination of differences in processing conditions, electrode contact material and measuring frequency range.

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A new nitrogen-rich phase has been observed to form in rare-earth-metal stabilised a-sialon preparations containing Sm and Nd. The X-ray powder pattern of this phase overlapped to a great extent that of a-sialon. Large quantities of this phase were also found in samples of the overall composition Ln2AI4SillNl8O4 with Ln = Ce and La. The La compound was selected for crystal structure determination, and the obtained X-ray pattern could be indexed with an orthorhombic unit cell; a = 9.4303( 7), b = 9.7689(8), c = 8.9386(6) A. A structural model was refined in the space group Pbcn using the Rietveld method. Based on 267 theoretical reflections in the 28 range 10-82", the following R-values were obtained for the composition LaAI(Si6-zAI,)N,o-zOz with z=1; R, , =1. 8, R,=6.6 and RF=5.l0/~. The Al atoms and the (Si,AI) atoms are tetrahedrally coordinated by (N,O) atoms, yielding an AI(Si,AI),(N,O),:-network. The La atoms are found in tunnels extending along the [OOl] direction and are irregularly coordinated by seven (N,O) atoms. The structure of this phase contains structural fragments which show strong similarities with some present in LaSi ,N,; their compositional relationship is evident if their chemical formulae are expressed as LaN-AIN-2(Si,AI),(N,O), and 2LaN-2Si,N4.Recent studies by us and others concerning the thermal stability of the x-sialon phase in various Me-Si-Al-0-N systems have shown that the a-sialon phase is formed at elevated temperatures but decomposes at lower temperatures, yielding an Me-rich intergranular phase. '-' In connection with these studies we have prepared a great number of samples along the tie-line between Si3N4 and Ln203.9A1N, with Ln= Yb, Y, Dy, Sm and Nd, of the overall composition L n ~s i ~~-~, ~x A l ~, ~x o ~~~x N ~~-~~5 ~ and with 0.25 < x < 1.0. In order to reveal the true phase relationships at high temperatures it was found necessary to quench the samples rapidly, i.e. by applying cooling rates exceeding 400°C min-l. In the systems with Ln=Nd and Sm and for x>O.6 we observed a new intergranular phase (termed the JEM phase) and elemental analysis indicated that the Ln : Si : A1 atomic ratio was in the range 13 : 65 : 22.3,4 The JEM phase was formed in conjunction with a-sialon in comparatively large amounts in the Nd system and in minor amounts in the Sm system. The JEM phase is, however, not formed in the Y, Dy and Yb systems, indicating that it is stable only for Ln ions with ionic radii larger than that of dysprosium. Note that the X-ray powder pattern of the JEM phase resembles and overlaps that of a-sialon.Microstructural investigation in a scanning electron microscope showed that this phase was not evenly distributed in the matrix but confined to certain parts. The 'flower-like' morphology of these areas resembled to a great extent that previously found in p-sialon materials prepared with La203 as sintering aid.'.' In the latter system very few areas of this type appeared, and their size was too small to allow a reliable determination of the elemental content; however, ...