pubs.acs.org/IC Published on Web 02/15/2010 r 2010 American Chemical Society Inorg. Chem. 2010, 49, 2827–2833 2827 DOI: 10.1021/ic902228h Spinel to CaFe 2 O 4 Transformation: Mechanism and Properties of β-CdCr 2 O 4  Angel M. Ar  evalo-L  opez,* ,† Antonio J. Dos santos-Garcı ´a, †,‡ Elizabeth Castillo-Martı ´nez, †,§ Alejandro Dur  an, †,^ and Miguel  A. Alario-Franco † † Departamento de Quı´mica Inorg  anica, Facultad de Quı´micas, Universidad Complutense de Madrid, 28040 Madrid, Spain, ‡ Parque Cientı´fico y Tecnol  ogico de Albacete, Instituto de Investigaci  on en Energı´as Renovables, Fuel Cell Department, Universidad de Castilla;La Mancha, Paseo de la Innovaci  on 1, 02006 Albacete, Spain, and ^ Centro de Nanociencias y Nanotecnologı´a, Universidad Nacional Aut  onoma de M exico, Apartado Postal 2681, C.P. 22800, Ensenada, B.C. M exico. § Present address: NanoTech Institue, University of Texas at Dallas, 800 Campbell Road BE26, Richardson, Texas 75252. Received November 11, 2009 The CdCr 2 O 4 spinel transforms to a 10.6% denser new polymorph of the CaFe 2 O 4 -type structure at 10 GPa and 1100 °C. This new polymorph has a honeycomb-like structure because of double rutile-type chains formed by [Cr-O 6 ] edge-shared octehedra. This crystal structure is prone to be magnetically frustrated and presents low-dimensional antiferromagnetism at 25 K < T < 150 K, accompanied by more complex interactions as the temperature decreases. These transitions are evidenced by magnetic susceptibility and heat capacity measurements. We also discuss a possible structural mechanism for the transformation. Introduction Structural transformations are a very fertile research area where physicochemical, ceramic, and earth sciences overlap to investigate basic aspects of the materials and possible applications. In this sense, besides the numerous applications of, for example, spinel-like magnetic ferrites, 1 the spinel structure is also important to both material sciences and geosciences. For instance, the discontinuities at 410 and 660 km in the earth’s mantle are associated with spinel transformations: the first an olivine to spinel transition, a “simple” change in stacking that could be relevant to earth- quakes; the second a spinel to perovskite plus ferropericlase (magnesiow€ ustite) decomposition. 2 In laboratory experiments under pressure, the AB 2 X 4 spinel undergoes interesting phase transformations to the CaFe 2 O 4 -type (CF) and CaTi 2 O 4 -type (CT) structures. 3 This process, which has also been discovered naturally, as it was first observed in the Suizhou meteorite, 4 has been reproduced over a large number of oxides; e.g., MgAl 2 O 4 , MgCr 2 O 4 , ZnCr 2 O 4 , ZnFe 2 O 4 , CoFe 2 O 4 , LiMn 2 O 4 , and CuRh 2 O 4 have been found to present this spinel-CF-type transformation under pressure and temperature. 5-11 It is interesting to note that one of the qualitative limiting criteria for the CF-type structure to appear is the ratio r B /r A , where r B and r A are the B and A ionic radii, respectively. No CF oxide with r B /r A < 0.53 has ever been found. 12 On the other hand, CT-type structure compounds have been less investigated mainly because of their difficult syntheses. 13 They require substantially higher pressures! CdCr 2 O 4 crystallizes in the cubic spinel structure, similar to MgCr 2 O 4 and ZnCr 2 O 4 . 14 Other chromites(III) crystallize either in a SrCr 2 O 4 -type structure (like SrCr 2 O 4 itself, BaCr 2 O 4 , or R-CaCr 2 O 4 15 ) or in the CaFe 2 O 4 -type structure (such as the β-CaCr 2 O 4 polymorph 16 ). In each of these compounds, the [Cr-O 6 ] octahedra are connected in different ways, and this contributes to the differences in their physical properties. For instance, the CdCr 2 O 4 magnetism is quite complex, with a spiral spin structure. 17 Taking into account that, in CdCr 2 O 4 , *To whom correspondence should be addressed. E-mail: gozdriov@ gmail.com. (1) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. (2) Helffrich, G. R.; Wood, B. J. 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