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Chem. Mater. 2002, 14, 4104-4110
Variable Temperature Powder Neutron Diffraction Study of SmNiO3 through Its M-I Transition Using a Combination of Samarium and Nickel Isotopic Substitution Paul F. Henry,† Mark T. Weller,*,† and Chick C. Wilson‡ Department of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom, and ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom Received April 22, 2002. Revised Manuscript Received July 25, 2002
Neutron powder diffraction studies of 154Sm58NiO3, 154Sm60NiO3, and 154Sm62NiO3, at a range of temperatures through the M-I transition at approximately 128 °C, have been performed on the new general materials diffractometer (GEM) at ISIS, RAL. With combined data-set Rietveld analysis, using samples containing different nickel isotopes with contrasting scattering lengths, it has been found that extremely high quality structural parameters can be determined, even though total data collection times are more than an order of magnitude shorter than those previously used for this system. Rietveld analysis shows that the evolution of the structural parameters over the temperature range are smooth and that no symmetry change or abrupt structural transition occurs at the M-I transition. This is consistent with evolution of the high-temperature metallic material within the low-temperature insulating phase over the temperature range 108-128 °C. The key effects of thermal motion on the M-I transition have been extracted from the data and are discussed.
Introduction Members of the RNiO3 perovskite family, where R is a rare earth, are some of the few oxide materials that can exhibit metallic conductivity. The series was originally described by Demazeau et al.1 in 1971 but these compounds have enjoyed a renaissance of interest since the discovery of high-temperature superconductivity (copper-based) and giant magnetoresistance (manganesebased) in related perovskite systems. Detailed studies of such materials allow the relationship between the structural and the physical properties to be obtained and understood. Resistivity measurements have shown the existence of a sharp metal-insulator (M-I) transition in the compounds where R * La.2,3 The temperature evolution of the transition temperature, TM-I, as a function of lanthanide is correlated with the degree of deviation of the material from the ideal perovskite structure, which increases as the size of the rare-earth ion decreases. The degree of deviation can be described either by the tolerance factor, t,4 or by the octahedral tilt angle, ω,5 both of which measure the degree of rotation of the NiO6 octahedra that occurs to fill the extra interstitial space as a result of the decrease in size * To whom correspondence should be addressed. E-mail: Mtw@ soton.ac.uk. † University of Southampton. ‡ Rutherford Appleton Laboratory. (1) Demazeau, G.; Marbeuf, A.; Pouchard, M.; Hagenmuller, P. J. Solid State Chem. 1971, 3, 582. (2) Lacorre, P.; Torrance, J. B.; Pannetier, J.; Nazzal, A. I.; Wang, P. W.; Huang, T. C. J. Solid State Chem. 1991, 91, 225. (3) Torrance, J. B.; Lacorre, P.; Nazzal, A. I.; Ansaldo, E. J.; Niedermeyer, C. Phys. Rev. B 1992, 45, 8209. (4) Goldschmidt, V. M. Skr. Nor. Vidensk. Akad. Mater. Naturvidensk. Kl. 1926, 2, 1.
of the rare earth across the series. The M-I transition is also accompanied by a small decrease in the unit cell volume.6 It has also been noted that for the larger rareearth elements (Pr and Nd) ordering of the nickel magnetic moments also accompanies the M-I transition.7 For the smaller rare-earth elements, the onset of nickel magnetic ordering takes place at a temperature, TN, considerably lower than TM-I.7 Although the RNiO3 series was first prepared in 1971,1 there was no further publication in this area until 1989.8 The reason for this apparent lack of interest partly stemmed from the fact that extreme experimental conditions were believed to be required for their synthesis. Whereas NiII oxides are easily synthesized under low oxygen pressures (
