Journal of Magnetism and Magnetic Materials 316 (2007) e728–e730 The low-temperature phase separation in Pr 0.5 Ca 0.5 CoO 3 A. Chichev a,b , J. Hejtma´nek a , Z. Jira´k a,Ã , K. Knı´zˇek a , M. Marysˇko a , M. Dlouha´ b , S. Vratislav b a Institute of Physics ASCR, Cukrovarnicka´ 10, Prague 6, Czech republic b Faculty of Nuclear Science and Physics Engineering, Brˇehova´ 7, Prague 1, Czech Republic Available online 14 March 2007 Abstract The perovskite cobaltite Pr 0.5 Ca 0.5 CoO 3 has been characterized by the magnetic and electric transport measurements, and structurally studied by neutron diffraction. The sample is a metallic-like conductor in a broad temperature range. On cooling, it undergoes at 75 K an incomplete transition to the ferromagnetic phase of enhanced metallic character. The residual paramagnetic regions exhibit at about 30 K a metal–insulator (M–I) transition, which is interpreted as a change from the t 2g 5 s *0.5 cobalt states with collective e g electrons to a mixture of localized Co 3+ (t 2g 6 ) and Co 4+ (t 2g 5 ) low-spin states. The new insulating phase is of the same orthoperovskite type but distinguished by about 2% smaller unit cell volume compared to the ferromagnetic phase. The lattice contraction is caused by significant shortening of Pr,Ca–O bonding distances while the Co–O bond lengths remain nearly constant. r 2007 Elsevier B.V. All rights reserved. PACS: 74.25.Ha; 75.30.Wx; 61.12.Ld Keywords: Transition metal oxide; Spin-state transition; Neutron diffraction 1. Introduction In the studies of ‘half-doped’ perovskite cobaltites there are two topics that attract particular attention—the microscopic description of the ferromagnetic and para- magnetic metallic phases (typical example is La 0.5 Sr 0.5- CoO 3 ), and the occurrence of the low-temperature metal–insulator (M–I) transition that was firstly reported for Pr 0.5 Ca 0.5 CoO 3 by Tsubouchi et al. [1] and later re- investigated in a broader class of compositions by Fujita et al. [2]. The insulating phase can be stabilized using high external pressures for compositions far from the ideal Co 3+ /Co 4+ (1:1) ratio. This shows that the M–I transition is not of ‘‘charge-ordering’’ origin. The conducting and insulating phases represent rather two incompatible elec- tronic ground states that are close in energy and can coexist due to the quenched disorder (see e.g. [3]). Such possibility is demonstrated in the present paper where conducting Pr 0.5 Ca 0.5 CoO 3 sample of a single perovskite structure at room temperatures (RT) develops below 75 K an incom- plete transition to the ferromagnetic state. The residual paramagnetic regions are transformed at 30 K to a new phase of insulating paramagnetic character and distinct lattice parameters. 2. Experimental Ceramic sample of Pr 0.5 Ca 0.5 CoO 3 has been prepared by a similar method as described in Ref. [2]. Homogenized mixtures of Pr 6 O 11 , CaCO 3 and Co(NO 3 ) 2  6H 2 O have been calcined twice at 600 1C for 10 h and at 800 1C for 24 h, pressed into tablets and fired at 1200 1C for 24 h in an oxygen atmosphere. The resulting material was proved by X-ray diffraction analysis to be of a single-perovskite phase of orthorhombic Pbnm symmetry. The observed lattice parameters a ¼ 5.337 A ˚ , b ¼ 5.339 A ˚ , c ¼ 7.548 A ˚ and unit cell volume V ¼ 215.1 A ˚ 3 are very close to those reported in earlier works [1,2]. The transport properties have been measured up to 1000 K. The resistivity varies only little with temperature. ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.03.078 Ã Corresponding author. E-mail address: jirak@fzu.cz (Z. Jira´k).