 To whom corresponence should be addressed. Fax: #52-999-9812917. E-mail: pquint@mda.cinvestav.mx. Journal of Solid State Chemistry 166, 191 } 196 (2002) doi:10.1006/jssc.2002.9581, available online at http://www.idealibrary.com on Subsolidus Phase Equilibria in the System CaO+Al 2 O 3 +CoO and the Crystal Structure of Novel Ca 3 CoAl 4 O 10 B. Vazquez,* L. M. Torres-Martinez,* N. Alvarez,- J. F. Vente,? and P. Quintana? * Facultad de Ciencias Quimicas, Universidad Autonoma de Nuevo Leon, A.P. 1864, Monterrey, Nuevo Leon, Mexico; - Universidad de la Habana, Zapata y G. Plaza de la Revolucion, Cd. de la Habana, Cuba; ? Department of Applied Physics, CINVESTAV IPN-Unidad Merida, Km 6 Antigua Carretera a Progreso, Cordemex, C.P. 97310 Merida, Yucatan, Mexico Received November 28, 2001; in revised form February 25, 2002; accepted March 8, 2002 The subsolidus phase diagram, CaO+Al 2 O 3 +CoO, and its phase relations below 13003C have been studied in air. The stability regions of nine subsolidus compatibility triangles were established and a new ternary phase was found. The structure of this compound, Ca 3 CoAl 4 O 10 (orthorhombic, space group Pbc2 1 , a  5.1452(2) A  , b  16.7731(5)A  , c  10.7055(3) A  ), was deter- mined from X-ray di4raction data and found to be isostructural with Ca 3 ZnAl 4 O 10 . This is an open framework compound with three crystallographically di4erent channels, each with a dia- meter of &3.5 A  . The two end members of the binary CoO+CaO system are surrounded by small regions of solid solutions. Lab color parameters were measured in several com- positions. No ternary phases were found when Co was substituted by other divalent cations such as Sr, Ba, Mn, Ni, Cu, Cd, Sn and Pb. ^ 2002 Elsevier Science (USA) Key Words: phase diagram; cobalt calcium-aluminates; X-ray di4raction; Rietveld analysis; color measurements. INTRODUCTION Only a very limited number of studies has been performed on the ternary system CaO}Al  O  }MO. The driving force to study the system with M"Mg is related to its geological occurrence and to processes in the ceramics industry (1}6). Several ternary compounds have been reported as Ca  MgAl  O  , a metastable phase Ca  MgAl  O  (1, 5), and Ca  Mg  Al  O  and CaMg  Al  O  in the alumina- rich part (6}8). The system with M"Zn has also been studied to some extent (9), for its importance of stabilizing waste raw materials in cement. As ternary compounds, apart from Ca  ZnAl  O  , Ca  Zn  Al  O  and Ca  Zn  Al  O  , have also been reported (9, 10). As part of our ongoing research into the phase relations of concrete and cement-related materials and its possible application for the safe and stable disposal of raw-related materials, we have studied the system CaO}Al  O  }CoO. Mixtures of cobalt-rich oxides, with aluminum and/or sili- con oxides as additives are widely used for colored glazes in the ceramic industry. In the glass industry, it is applied to create blue hues and to mask the greenish tinge of glass and porcelain caused by iron impurities (11, 12). These com- pounds are characterized by having a high stability with respect to light, environment, high temperature and chem- icals (13, 14). The three limiting two-component diagrams in the ter- nary system CaO}Al  O  }CoO are well established and contain a number of binary phases. In the CaO}Al  O  system, several binary compounds have been reported (15, 16): Ca  Al  O  , Ca  Al  O  , CaAl  O  , CaAl  O  and CaAl  O  . However, the presence of moisture has a great in#uence on the formation and the stability of Ca  Al  O  (C12A7) (7, 16, 17). The only compound found in the system CoO}Al  O  is CoAl  O  (18). No compound with the gen- eral formula Ca  Co  O is known. However, CaO and CoO do react and form at least four di!erent compounds in which cobalt has a higher formal valence (19, 20). More recently, Ca  Co  O  has attracted some attention for its one- dimensional properties (21, 22) and Ca  Co  O  for its anisot- ropic giant magnetoresistance (23). However, the average formal valence of cobalt in these compounds is three. In this paper, the phase diagram of the ternary system CaO}Al  O  }CoO and a description of the new ternary phase Ca  CoAl  O  are presented. Unsuccessful attempts were made to synthesize other ternary compounds replacing CoO by MO, with M"Sr, Ba, Mn, Ni, Cu, Cd, Sn and Pb. EXPERIMENTAL In order to determine the phase diagram CaO}Al  O  }CoO, over 50 strategically chosen compositions were prepared by traditional solid-state techniques, using high- purity ( '99.99%), CaCO  , Al  O  and CoO. Appropriate amounts of the dried reagents were mixed in acetone until 191 0022-4596/02 $35.00  2002 Elsevier Science (USA) All rights reserved.