Mg 2 SnO 4 ceramics I. Synthesis±processing±microstructure correlation Abdul-Majeed Azad *, Liew Jing Min Department of Physics, University Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Received 6 June 2000; received in revised form 18 June 2000; accepted 19 July 2000 Abstract Phase stability of magnesium meta- and orthostannate has been examined in samples synthesized via the traditional solid-state reaction (SSR) and a novel self-heat-sustained (SHS) technique with two molar ratios of magnesium to tin (viz., 1:1 and 2:1). The powder mixtures were calcined over a wide temperature±time (T±t) span ranging from 600 to 1300  C and 3 to 72 h. The powders obtained from the two preparative methods have been processed and sintered under identical conditions. In the 2:1 molar mixtures, Mg 2 SnO 4 has been formed as a single phase up on calcination in both SRR and SHS methods. This phase remained the only compound in the sintered bodies as well. In the 1:1 composition, the ultimate reaction product was a mixture of Mg 2 SnO 4 and SnO 2 . Both SSR and SHS techniques with 2:1 molar mixture yielded a single phase Mg 2 SnO 4 in the sintered compacts. Compacts with near zero porosity could be achieved in SSR derived samples up on sintering up to 1600  C, while some signi®cant porosity was an interesting feature of the SHS derived samples. Systematic microstructural evolution with the variation of sintering conditions has been discussed. # 2001 Elsevier Science Ltd and Techna S.r.l. All rights reserved. Keywords: A. Sintering; B. Electron microscopy; B. X-ray methods; E. Capacitors 1. Introduction The alkaline±earth stannates having the general che- mical formula MSnO 3 (M=Ca, Sr and Ba), have recently been studied as potential electronic ceramics, such as, thermally stable capacitors with low permittiv- ity and small loss tangent [1±6]. Interestingly, even though magnesium is a member of the alkaline±earth metal group to which Ca, Sr and Ba belong, no reliable technical information on the electrical behavior of the compounds in the pseudobinary MgO±SnO 2 system appears to exist in the published literature. Limited amount of literature is available on the synthesis aspects of compounds in Mg±Sn±O system. Only P±T relation- ship has been reported in the literature; no reliable composition±temperature phase diagram exists [7]. On the other hand, corresponding titanates (MgTiO 3 , Mg 2 TiO 4 and MgTi 2 O 5 ) and silicates (MgSiO 3 , steatite and Mg 2 SiO 4 , forsterite) are commercially produced as low dielectric constant, high resistance and low tem- perature coecient of dielectric constant (TCK) com- ponents [8]. In addition, the behavior of magnesium metastannate (MgSnO 3 ) and orthostannate (Mg 2 SnO 4 ) is totally dierent from the corresponding Ca, Sr and Ba counterparts. Consequently, the literature deals with the thermo- dynamic stability of the orthostannate alone, albeit with large dierences in the computed values of the standard enthalpy of formation of the double oxide from one research to another [9±12]. In addition, the JCPDS ®les show the presence of another phase with nominal for- mula M 3 Sn 2 O 7 in all the MO±SnO 2 systems except for Mg. The meta- and orthostannates of calcium, stron- tium and barium are known to be stable independently up to very high temperatures without disproportiona- tion. MgSnO 3 is unstable and disproportionates into orthostannate and tin oxide upon heating above 700  C. In some cases, even the orthostannate cannot be synthesized as a single-phase [13]. No correlation has been established among key para- meters, such as, synthesis, processing, microstructure and electrical behavior of these materials; such a corre- 0272-8842/01/$20.00 # 2001 Elsevier Science Ltd and Techna S.r.l. All rights reserved. PII: S0272-8842(00)00085-7 Ceramics International 27 (2001) 325±334 www.elsevier.com/locate/ceramint * Corresponding author at current address: NexTech Materials, Ltd, 720-I Lakeview Plaza Blvd, Worthington, OH 43085, USA. Tel.: +1- 614-842-6606; fax: +1-614-842-6607. E-mail addresses: azad@nextechmaterials.com (A.-M. Azad).