Spin Glass Behavior of New Perovskites Ba 2 In 2x Co x O 5 (0.5 x 1.70) A. D. Lozano Gorrı´n,* P. Nu´n˜ez,* M. A. Lo´pez de la Torre,w J. Romero de Paz,z and R. Sa´ez Puchez ,1 *Departamento de Quı´mica Inorga ´nica, Universidad de La Laguna, 38206-La Laguna, Tenerife, Spain; wDepartamento de Fı´sica Aplicada, Universidad de Castilla-La Mancha, Campus Universitario, 13071 Ciudad Real, Spain; and zDepartamento de Quı´mica Inorga ´nica, Facultad de Ciencias Quı´micas, Universidad Complutense Madrid, 28040 Madrid, Spain Received October 4, 2001; in revised form December 26, 2002; accepted January 18, 2002; published online March 22, 2002 New oxides of general formula Ba 2 In 2x Co x O 5 (0.5 x 1.70) have been synthesized as polycrystalline materials and fully characterized by means of X-rays. They are all isostruc- tural and crystallize in a perovskite-type structure, showing cubic symmetry, space group Pm 3m. The magnetic properties of these oxides have been studied by v ac and v dc measurements as a function of magnetic field and temperature. In the case of x=0.5 (BaIn 1.50 Co 0.5 O 5 ) the sample remains paramagnetic from 200 to 2 K. However, the magnetic behavior of the oxides Ba 2 In 2x Co x O 5 (0.70x1.70) is rather different: v dc shows broad maxima which are shifted with the x cobalt content. Below this temperature a strong irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) curves has been detected, more pronounced as the cobalt content is higher. v ac measurements also show all the common features of spin glasses, fully confirmed from the analysis of the non linear part of the v dc . # 2002 Elsevier Science (USA) Key Words: In–Co perovskites; spin-glass; nonlinear magnetic susceptibility. INTRODUCTION Many transition oxides show the very versatile structure of perovskite. The rich variety of physical properties such as high-temperature superconductivity, giant magnetorre- sistence, and spin glasses observed in these compounds makes them very attractive for both academic and technological purposes. Compounds belonging to the BaO–In 2 O 3 ternary system have been the concern of different studies, including single- crystal X-ray diffraction analysis, which show that most of them are derived from the cubic perovskite structure (1–5). Ba 3 In 2 O 6 (6) and Ba 4 In 2 O 7 (7) can be described as layers of InO 5 square pyramids. The existence of Ba 2 In 2 O 5 crystal- lizing in an orthorhombic-perovskite subcell with para- meters a o =6.111(1) A ˚ , b o =5.992(1) A ˚ , c o =4.204(2) A ˚ has been reported (4). Even though a related phase, BaInO 2.5 , which was prepared at 14001C (6), has also been described as a cubic perovskite (a=4.219(2) A ˚ ), such a structure was not observed at temperatures up to 13001C (1). Several authors (1, 2) have found that Ba 2 In 2 O 5 , as well as Sr 2 In 2 O 5 (8), is isotypic with Ca 2 FeAlO 5 (brownmillerite) (9), where the In(III) ions are located in both tetrahedral and octahedral coordination. On the other hand, the solid solutions Ba 2 M 2x Cu x O 4+d , M=In, Sc, Lu, have also been synthesized and characterized, leading to a double perovskite structure (2, 10). More recently, we have prepared the compound Ba 2 ScCoO 5 , which has been described as a cubic oxygen- deficient single perovskite with the B-sites randomly occupied by Sc and Co atoms (11). Magnetic materials with triangle-based lattice with frustrate magnetic interactions have been the subject of many studies in the past 20 years (12–14). Different structural types have served as models to investigate their geometrical frustration. More recently, magnetic frustra- tion giving rise to spin-glass behavior has been reported for perovskite oxides of general formula A 2 BB 0 O 6 , where the B and B 0 ions are settled with either random or ordered arrangement (15). Recently, different cobalt oxide perovskites having a cubic structure have been reported, where the B site accommodates the random occupation of the Co and different diamagnetic cations. This produces a kind of magnetic dilution of the cobalt in the B sublattice that yields a spin-glass-like behavior transition at low tempera- tures (16, 17). Cobalt is well known to exhibit three different spin configurations for the Co 3+ ion in octahedral environment, denoted as high-spin (HS; S=2), intermediate-spin (IS; S=1), and low-spin (LS; S=0). These spin states are a function of the temperature and many studies have been reported concerning the spin transition in LaCoO 3 , but 1 To whom correspondence should be addressed. Fax: 33913944352. E-mail: RSP92@quim.ucm.es. Journal of Solid State Chemistry 165, 254–260 (2002) doi:10.1006/jssc.2002.9515, available online at http://www/idealibrary.com on 254 0022-4596/02 $35.00 r 2002 Elsevier Science (USA) All rights reserved.