Crystal Growth of RNiO 3 Perovskites Under High Oxygen Pressure and Hydrothermal Conditions.
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Y3.4.1
Crystal growth of RNiO3 perovskites under high oxygen pressure and hydrothermal conditions. J.A. Alonso, a M.J. Martínez-Lope,a M.T. Casais,a A. Muñoz, b A. Largeteau,c G. Demazeauc
a. Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Cantoblanco, E-28049 Madrid, Spain. b. Depto. de Física, EPS, Univ. Carlos III. Butarque 15, Leganés, E-28911 Madrid, Spain. c. Institut de Chimie de la Matière Condensée de Bordeaux – UPR-CNRS 9048 Université Bordeaux Sciences et Technologies, 87 Avenue du Dr. A. Schweitzer, 33608 PESSAC Cedex, France.
Abstract
Well-shaped single crystals of RNiO3 perovskites (R= Nd, Ho, Y) have been grown under high oxygen pressure conditions, in a piston-cylinder press at 2 GPa or in a belt-type press at 4 GPa. The reaction took place in sealed platinum capsules, in the presence of KClO3 as oxidizing agent. It seems that the choice of hydroxides of the involved cations as precursors is crucial for the success of the crystal growth, via water vapor transport reactions. The perovskites have been characterized by X-ray diffraction, scanning microscopy and DSC measurements, suitable to identify the metal insulator transition. The transition temperatures of the as-grown crystals show a significantly lower hysteresis in the heating and cooling runs than those reported for powder materials, suggesting a lower density of defects.
Y3.4.2
Introduction
The family of RNiO3 perovskites (R= rare earths) offers an excellent opportunity to study a metal-to-insulator (MI) transition in narrow σ* band oxides, in which the bandwidth can be significantly varied as the eg (Ni)- 2pσ(O) covalent mixing is progressively reduced from R= La to Lu due to the increasing structural distorsion. This appealing system has been, nevertheless, very little studied, given the difficulties of its preparation procedure, involving the use of high-oxygen pressure necessary to stabilize Ni3+ cations. The complete series of rare-earth nickelates RNiO3 (R= Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Lu) was first described by Demazeau et al. in 1971, who prepared these metastable materials under high pressure conditions of 6 GPa [1]. These materials were completely forgotten for 20 years, until the report by Lacorre et al. [2] of metal-to-insulator transitions for the less-distorted terms of the perovskite series with R= Pr, Nd,…Eu, as a function of temperature and the size of the rare-earth cation. These properties were measured in powder samples prepared under moderate O2 pressures of 20 MPa. They present orthorhombic symmetry (s.g. Pbnm) both above and below the MI transition. The subtle structural changes accompanying the metal-insulator transition [3] do not imply a symmetry change. Additionally, the insulating phase becomes antiferromagnetically ordered below TN’s ranging from 120 K (R= Lu) and 200 K (R= Sm), and the corresponding phase diagram as a function of the tolerance factor was mapped out [4,5]. Subsequent neutron diffraction studies demonstrated that the magnetic ordering of Ni below TN [6-8] is defined by an unexpect
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