Hole and electron doping of R[sub 2]BaNiO[sub 5] (R = rare earths)

Journal Article · Fri Apr 01 04:00:00 EST 1994 · Journal of Solid State Chemistry; (United States)

DOI:https://doi.org/10.1006/jssc.1994.1098· OSTI ID:7018144

Alonso, J A; Rasines, I ^[1]; Rodriguez-Carvajal, J ^[2]; Torrance, J B ^[3]

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1. Instituto de Ciencia de Materiales de Madrid, Madrid (Spain)
2. CEN Saclay, Gif-Sur-Yvette (France) Institut Laue-Langevin, Grenoble (France)
3. Almaden Research Center, San Jose, CA (United States)

Black polycrystalline samples of the composition R[sub 2[minus]x]Ca[sub x]BaNiO[sub 5] (Ni-oxidized samples), with R = Y, Nd, Er, Lu, and 0.2 [le] x [le] 0.5, have been prepared in air by solid state reaction of R[sub 2]O[sub 3], CaCo[sub 3], BaO[sub 2], and NiO. They crystallized in the orthorhombic Nd[sub 2]BaNiO[sub 5] structure type, space; group Immm, that contains chains of flattened NiO[sub 6] octahedra sharing corners along the [1 0 0] direction. X-ray and neutron powder diffraction data show that Ca[sup 2+] replaces R[sup 3+] at random, whereas the oxygen positions remain fully occupied, which induces a proportional increase in the formal oxidation state of Ni with Ca content. The unit-cell volume decreases as a result of the shrinkage of the NiO[sub 6] octahedra. The electrical conductivity rises with Ca doping, showing a semi-conductor behavior. The linearity of the log [sigma] vs T[sup [minus]1/4] plot suggests a thermally activated electron hopping conduction mechanism, by intervalence transitions between adjacent Ni[sup 2+]-Ni[sup 3+] cations. The samples can be reduced in two steps, giving oxygen-deficient compounds in which the structure is basically unchanged. After the first step all the Ni[sup 3+] is reduced to Ni[sup 2+]. The second reduction process leads to phases, greenish in color, that are thought to contain Ni in both monovalent and divalent oxidation states. The presence of Ca[sup 2+] in the structure seems to be essential in the stabilization of Ni[sup +]. A neutron diffraction study of the compounds Er[sub 2[minus]x]Ca[sub x]BaNiO[sub 5[minus][delta]], prepared in a H[sub 2]/N[sub 2] flow at 500[degrees]C, shows that the O2 axial oxygens are lost during the reduction process. This breaks the continuity of the chains of octahedra, thus hindering the intervalence transitions along the chains. The electrical conductivity is, in fact, several orders of magnitude lower than that of the corresponding Ni-oxidized samples.

• OSTI ID: 7018144
• Journal Information: Journal of Solid State Chemistry; (United States), Journal Name: Journal of Solid State Chemistry; (United States) Vol. 109:2; ISSN 0022-4596; ISSN JSSCBI
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360202 -- Ceramics
Cermets
& Refractories-- Structure & Phase Studies
360204* -- Ceramics
Cermets
& Refractories-- Physical Properties
ALKALINE EARTH METAL COMPOUNDS
BARIUM COMPOUNDS
BARIUM OXIDES
CHALCOGENIDES
CHEMICAL REACTIONS
COHERENT SCATTERING
CRYSTAL DOPING
CRYSTAL LATTICES
CRYSTAL STRUCTURE
DIFFRACTION
ELECTRIC CONDUCTIVITY
ELECTRICAL PROPERTIES
ELECTRONS
ELEMENTARY PARTICLES
ERBIUM COMPOUNDS
ERBIUM OXIDES
FERMIONS
HOLES
LEPTONS
LUTETIUM COMPOUNDS
LUTETIUM OXIDES
NEODYMIUM COMPOUNDS
NEODYMIUM OXIDES
NEUTRON DIFFRACTION
NICKEL COMPOUNDS
NICKEL OXIDES
ORTHORHOMBIC LATTICES
OXIDES
OXYGEN COMPOUNDS
PHYSICAL PROPERTIES
RARE EARTH COMPOUNDS
REDUCTION
SCATTERING
STOICHIOMETRY
TEMPERATURE DEPENDENCE
TEMPERATURE RANGE
TEMPERATURE RANGE 0400-1000 K
TEMPERATURE RANGE 1000-4000 K
TRANSITION ELEMENT COMPOUNDS
VALENCE
X-RAY DIFFRACTION
YTTRIUM COMPOUNDS
YTTRIUM OXIDES