Characterization and Defect Structure of Anion-Excess Fluorite-Related Phases in LnOF-LnF

Journal Article · Thu Feb 15 00:00:00 EST 2001 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1006/jssc.2000.9064· OSTI ID:783962

Laval, J P; Taoudi, A; Abaouz, A

The phases LnO{sub 1-x}F{sub 1+2x}, derived from the fluorite-type structure by anionic excess, form, for medium size Ln cations (Ln=Nd, Sm, Eu, Gd), a homogeneous series of orthorhombic symmetry (space group Pmmn, a{approx}b{approx}a{sub F}{radical}2/2; c=a{sub F}). Structural comparison of these phases by X-ray powder diffraction allows us to make a better determination of their defect structure: O/F order in sheets perpendicular to the [001] axis and formation of 1:0:2 clusters which can be associated in dimer units. For Nd, an orthorhombic-tetragonal transition is explained by a change from 1:0:2 to denser 1:0:3 clusters. Comparison of the structural features of these phases with ''vernier'' modulated phases, isolated for Sm-Lu cations, allows a general classification of LnO{sub 1-x}F{sub 1+2x} phases at low temperature to be proposed. All are characterized by the preservation of an anionic O/F order in sheets perpendicular to the [001]{sub F} axis and by a progressive loss of freedom in the accommodation of the anionic excess (nature and orientation of the clusters, tendency to long-range ordering) from La to Lu, caused by the decrease in Ln size and therefore in Ln coordination.

• Research Organization: Science des Procedes Ceramiques et de Traitements de Surface, Limoges (FR); Laboratoire de Chimie Minerale Appliquee, Faculty de Sciences, Meknes (MA)
• Sponsoring Organization: None (US)
• OSTI ID: 783962
• Journal Information: Journal of Solid State Chemistry, Vol. 157, Issue 1; Other Information: PBD: 15 Feb 2001; ISSN 0022-4596
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CATIONS
CLASSIFICATION
DEFECTS
DIFFRACTION
DIMERS
ORIENTATION
PRESERVATION
SYMMETRY
NEODYMIUM COMPOUNDS
NEODYMIUM FLUORIDES
NEODYMIUM OXIDES
SAMARIUM COMPOUNDS
SAMARIUM FLUORIDES
SAMARIUM OXIDES
EUROPIUM COMPOUNDS
EUROPIUM FLUORIDES
EUROPIUM OXIDES
GADOLINIUM COMPOUNDS
GADOLINIUM FLUORIDES
GADOLINIUM OXIDES
OXYFLUORIDES
ORTHORHOMBIC LATTICES
RARE EARTHS
ANIONS
SPACE GROUPS
X-RAY DIFFRACTION
CRYSTAL DEFECTS
CRYSTAL PHASE TRANSFORMATIONS
TETRAGONAL LATTICES
EXPERIMENTAL DATA