JOURNAL OF SOLID STATE CHEMISTRY 139, 238 — 247 (1998) ARTICLE NO. SC987836 Structures in the System CaTiO 3 /SrTiO 3 C. J. Ball, B. D. Begg, D. J. Cookson, G. J. Thorogood, and E. R. Vance Materials Division, ANSTO, Menai, New South Wales 2234, Australia Received November 12, 1997; accepted March 24, 1998 At room temperature the sequence of phases with increasing amounts of strontium in the system CaTiO 3 /SrTiO 3 is ortho- rhombic (Pnma), orthorhombic (Bmmb), tetragonal (I4/mcm), and cubic (Pm3m). All phase boundaries shift toward smaller strontium contents with increase of temperature. Volume cha- nges resulting from phase transformations are small for all compositions. Shape changes are greatest ( &0.3%) for the Bmmb/I4/mcm transition, but would probably be accommodated by microtwinning and so are unlikely to affect the mechanical integrity of a specimen.  1998 Academic Press INTRODUCTION Perovskite, CaTiO  , is one of the major phases of Synroc, a titanate ceramic designed for immobilization of high-level radioactive waste (1). It is the major host phase for stron- tium, an important radwaste element, and will also incor- porate significant amounts of the actinides. Obviously, the limits of solubility of strontium and other waste elements, and the phases formed when these limits are exceeded, are of interest. It is known that CaTiO  and SrTiO  are com- pletely miscible and cubic at high temperatures, but the temperatures of transformation to the less symmetrical low- temperature phases, and any volumetric and shape changes associated with such phase changes during cooling from the temperature of formation, are not known. There is even some uncertainty as to what the room temperature phases are. The ternary system BaTiO  /SrTiO  /CaTiO  is of con- siderable technological importance on account of the ferroelectric behavior of many of its compositions. While most attention has been given to Ba-rich compounds, the Ca  Sr  TiO  binary system has been studied by several groups of workers. Gra¨nicher and Jakits (2) reported the existence of orthorhombic, rhombohedral, ‘‘nearly cubic,’’ tetragonal, and cubic structures with increasing proportions of SrTiO  , at room temperature, but they did not measure To whom correspondence should be addressed. the cell parameters for any of these phases. McQuarrie (3) found only orthorhombic, tetragonal, and cubic phases. By extrapolation of the cell parameters of the orthorhombic phase he placed the orthorhombic/tetragonal phase bound- ary at x"0.55 at room temperature, which is close to the ‘‘nearly cubic’’/tetragonal boundary reported by Gra¨nicher and Jakits. McQuarrie also found that the phase boundaries shifted to lower strontium contents at higher temperatures, though at room temperature the rate of change of composi- tion with temperature for both phase boundaries was small. Mitsui and Westphal (4) investigated only the strontium- rich end of the phase diagram (x50.8). They confirmed the existence of the tetragonal phase, and put the tetrag- onal/cubic phase boundary at x"0.9 at room temperature. A sample with x"0.8 changed from tetragonal to a differ- ent structure, which appeared to be the ‘‘nearly cubic’’ structure of Gra¨ nicher and Jakits, near 110K. More re- cently Ceh et al. (5) questioned the existence of the tetrag- onal phase. They reported cell parameters for the orthorhombic phase for 04x40.9, though they also stated that for x"0.6 only lines characteristic of a cubic structure were present. Because of the importance of this system, and the lack of agreement between previous workers, we have reexamined the phases in the (Ca/Sr) TiO  system. In this we have been greatly helped by the excellent resolution obtainable with synchrotron radiation. EXPERIMENTAL Materials of composition Ca  Sr  TiO  , with x"0 and 0.10 up to 1.0 in steps of 0.05, were prepared by first hydrolyzing a known quantity of Ti-isopropoxide with an aqueous Sr-nitrate solution. An appropriate amount of CaCO  was slurried with water and converted to Ca-nitrate solution by addition of HNO  , before being added to the other precursors. The sample was then stir dried and cal- cined at 600°C for 1 h. The resulting powder was pelleted and sintered at 1400°C for 96 h in air. The sintered pellet was ground to (0.1 mm, repelleted, and refired at 1550°C for 85 h before final grinding. 238 0022-4596/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.