Journal of Alloys and Compounds 488 (2009) 524–527 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Phase transition induced by pressure in TbCrO 4 oxide: Relationship structure–properties E. Climent a , J.M. Gallardo a , J. Romero de Paz b , N. Taira a , R. Sáez Puche a,∗ a Dpto. Quimica Inorgánica, Facultad Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria, E-28040 Madrid, Spain b CAI Técnicas Físicas, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain article info Article history: Received 30 May 2008 Received in revised form 9 October 2008 Accepted 14 October 2008 Available online 2 December 2008 Keywords: Dimorphism High-pressure synthesis Antiferromagnetism Ferromagnetism Metamagnetism abstract Terbium chromate TbCrO 4 , which has been described as belonging to the zircon-type structure show- ing tetragonal symmetry, space group I4 1 /amd, has been prepared as a dimorphic compound. When the zircon-type TbCrO 4 polymorph is treated at 40 kbar and 833 K takes place a phase transition to the tetrag- onal scheelite-type TbCrO 4 polymorph, space group I4 1 /a, with lattice parameters a = 5.0315(10) Å and c = 11.3740(2) Å. Magnetic susceptibility measurements reveal dramatic changes concerning the magnetic behavior of these two polymorphs. In this sense, the zircon-type TbCrO 4 polymorph behaves as ferromag- netic with a Curie temperature of 22 K, while the scheelite-type TbCrO 4 polymorph is antiferromagnetic with T N = 29 K. M vs. H plots at different temperatures show the presence of a metamagnetic transition for the scheelite-type TbCrO 4 polymorph with a critical field of 2.6T. The change of the sign of the magnetic interaction has been explained taking into account the differences found in the distances and bond angles of the superexchange Tb–O–Cr pathway through these interactions take place in both zircon and scheelite polymorphs. © 2008 Elsevier B.V. All rights reserved. 1. Introduction RCrO 4 oxides (R = rare earth) form a family of compounds with very interesting magnetic properties [1] and structural behavior [2]. The first member LaCrO 4 crystallizes with monazite-type structure, space group P2 1 /n [3]. In the case of the remaining rare earths show the zircon-type structure with tetragonal symmetry, space group I4 1 /amd [4]. Recently we have found that PrCrO 4 is a dimorphic oxide showing the mentioned zircon or monazite-type structure depending on the synthesis conditions [5]. Although phase transitions induced by pressure from zircon- to scheelite-type structure have been studied in some detail for the vanadate and arsenate families RXO 4 (X = V and As) [6,7], however only few studies have been recently reported for the analogous HoCrO 4 [8] and YCrO 4 [9] oxides. This could be due to the diffi- culty to prepare these oxides as pure phases because of the trouble to achieve the stabilization of the unusual 5+ oxidation state of chromium present in these phases. In fact, these high pressure scheelite phases show minor impurities of the perovskite-type RCrO 3 oxides [8,9]. Accompanying this structural phase transition from zircon to scheelite the magnetic properties change dramat- ∗ Corresponding author. Tel.: +34 913944353; fax: +34 913944352. E-mail address: rsp92@quim.ucm.es (R.S. Puche). ically. The coexistence of two paramagnetic cations, namely Cr 5+ and R 3+ , in these oxides constitutes a very interesting scenario to study 3d–4f magnetic interactions and the predominant role that the rare earth anisotropy plays in the resulting magnetic properties of these two forms of the RCrO 4 oxides. In this sense, the most of the zircon-type RCrO 4 oxides being ferromagnetic [10] become anti- ferromagnetic for the corresponding known scheelite polymorph [8,9]. The aims of this work are the determination of the specific synthesis conditions such as pressure and temperature necessary to prepare the scheelite-type TbCrO 4 polymorph and the subse- quent structural characterization using X-ray diffraction. Magnetic properties have been also studied from magnetic susceptibility and magnetization measurements at different temperatures and mag- netic field strengths. 2. Experimental details The zircon-type TbCrO4 polymorph was prepared by using a precursor method based on the decomposition of stoichiometric amounts of Tb(NO3)3 6H2O and Cr(NO3)3 9H2O according to the experimental procedure described elsewhere [2]. Then this zircon TbCrO4 oxide was placed within a platinum capsule and treated in a Belt-type apparatus at different high pressures, temperatures and reaction times. In order to eliminate the perovskite TbCrO3 impurity different synthesis conditions were explored and 40 kbar and 833 K for 30 min were inferred as the optimized synthesis conditions. X-ray powder diffraction data were collected on a Philips Panalytical X’Pert MPD diffractometer using the monochromatic Cu K1 radiation ( = 1.54056 Å). X-ray 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.060