ISSN 0021-3640, JETP Letters, 2008, Vol. 88, No. 8, pp. 524–530. © Pleiades Publishing, Ltd., 2008. Original Russian Text © I.S. Lyubutin, A.G. Gavriliuk, V.V. Struzhkin, 2008, published in Pis’ma v Zhurnal Éksperimental’noœ i Teoreticheskoœ Fiziki, 2008, Vol. 88, No. 8, pp. 601–607. 524 1. INTRODUCTION The BiFeO 3 crystal is a ferroelectric–antiferromag- net and exhibits a comparatively large magnetoelectric effect [1, 2]. Among known multiferroics, BiFeO 3 has the highest Neél temperature T N = 643 K and highest ferroelectric Curie temperature T C = 1083 K [3, 4] and attracts great fundamental and applied attention. In the perovskite-like crystal structure of BiFeO 3 (space group R3c), the Fe 3+ ions are in distorted oxygen octa- hedra, and the Bi ions occupy the dodecahedral posi- tions and are strongly shifted from the central position towards one of the iron ions [5, 6]. The antiferromag- netic order in BiFeO 3 is inhomogeneous and is repre- sented by a complex spatially modulated cycloid-type spin structure whose modulation period is incommen- surate with the crystal lattice period [7–10]. A magnetic transition in which the magnetic moment of the iron ions collapses and the antiferromagnet passed to a non- magnetic state was recently observed in the BiFeO 3 crystal at high pressures near P = 50 GPa [11]. The structural phase transition with a change in the com- pressibility of the crystal was discovered in the same pressure region [12]. The behavior of the optical absorption edge indicates that the optical gap vanishes at 45–55 GPa, indicating the insulator–metal phase transition [13]. The metallization effect was more recently confirmed by the direct measurements of the electric resistance [14]. In this work, the spin crossover effect associated with the transition of the Fe 3+ ions from the high-spin (HS) state to the low-spin (LS) state is observed in BiFeO 3 at high pressures in diamond-anvil cells. This effect is studied by means of two independent experi- mental methods using synchrotron radiation: Fe K β high-resolution X-ray emission spectroscopy (XES) and 57 Fe nuclear resonant forward scattering of syn- chrotron radiation (NFS or synchrotron Mössbauer spectroscopy). 2. EXPERIMENTAL PROCEDURE AND RESULTS 2.1. High-Pressure Measurements A polycrystalline BiFeO 3 sample where iron was enriched in the 57 Fe isotope up to 96% was synthesized by ceramic technology. For high-pressure measure- ments, a plate 5–10 µm in thickness was prepared from a powder by preliminarily pressing the powder between the diamond anvils in the high-pressure cell. In an opti- cal microscope, the plate was transparent and had a dark red color. For XES and NFS investigations at high pressures, the 90 × 90 µm 2 Bi 57 FeO 3 plate was placed in the working volume of the cell. The diameter of the working area of the diamond anvils was about 300 µm, High-Spin–Low-Spin Transition and the Sequence of the Phase Transformations in the BiFeO 3 Crystal at High Pressures I. S. Lyubutin a , A. G. Gavriliuk a, b , and V. V. Struzhkin c a Shubnikov Institute of Crystallography, Russian Academy of Sciences, Moscow, 119333 Russia e-mail: lyubutin@ns.crys.ras.ru b Institute of High Pressure Physics, Russian Academy of Sciences, Troitsk, Moscow region, 142190 Russia c Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA Received August 26, 2008 The transition of Fe 3+ ions from the high-spin (HS) state (S = 5/2) to the low-spin (LS) state (S = 1/2) has been observed in the BiFeO 3 multiferroic crystal at high pressures in the range 45–55 GPa. This effect has been stud- ied in high-pressure diamond-anvil cells by means of two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS or synchrotron Mössbauer spectroscopy) and Fe K β high-resolution X-ray emission spectroscopy (XES). The HS–LS transition correlates with anomalies in the magnetic, optical, transport, and structural properties of the crystal. At room temperature, the transition is not stepwise, but is extended in a pressure range of about 10 GPa due to thermal fluctuations between the high-spin and low-spin states. It has been found that the transition of the BiFeO 3 insulator to the metal occurs only in the low-spin phase and the cause of all phase transitions is the HS–LS crossover. PACS numbers: 74.62.Fj, 75.50.-y, 78.70.En, 81.40.Rs DOI: 10.1134/S0021364008200125