Synchrotron and neutron powder diffraction study of phase transition in weberite-type Nd{sub 3}NbO{sub 7} and La{sub 3}NbO{sub 7}

Journal Article · Mon Aug 15 00:00:00 EDT 2011 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/j.jssc.2011.06.017· OSTI ID:21580229

Nino, Juan C ^[1]

---

1. Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 (United States)

La{sub 3}NbO{sub 7} and Nd{sub 3}NbO{sub 7} are insulating compounds that have an orthorhombic weberite-type crystal structure and undergo a phase transition at about 360 and 450 K, respectively. The nature of the phase transitions was investigated via heat capacity measurements, synchrotron X-ray and neutron diffraction experiments. It is here shown that above the phase transition temperature, the compounds possess a weberite-type structure described by space group Cmcm (No. 63). Below the phase transition, the high temperature phase transforms into a weberite-type structure with space group Pmcn (No. 62). The phase transformation primarily involves the off-center shifting of Nb{sup 5+} ions inside the NbO{sub 6} octahedra, combined with shifts of one third of the Ln{sup 3+} (Ln{sup 3+}=La{sup 3+} and Nd{sup 3+}) ions at the center of the LnO{sub 8} polyhedra towards off-center positions. The phase transition was also proven to have great impacts on the dielectric properties. - Graphical abstract: The phase transition of La{sub 3}NbO{sub 7} and Nd{sub 3}NbO{sub 7} upon cooling primarily involves the off-center shifting of Nb{sup 5+} ions inside the NbO{sub 6} octahedra, combined with shifts of one third of the Ln{sup 3+} (Ln{sup 3+}=La{sup 3+} and Nd{sup 3+}) ions at the center of the LnO{sub 8} polyhedra towards off-center positions. Above the phase transition temperature, Ln{sup 3+} and Nb{sup 5+} sit at a position with mirror symmetry and 2-fold axis. Below the phase transition temperature, Ln{sup 3+} and Nb{sup 5+} shift away from the 2-fold axis but still in the mirror plane. Highlights: > La{sub 3}NbO{sub 7} and Nd{sub 3}NbO{sub 7} exhibit a phase transition at 360 and 450 K, respectively. > The phase transformation primarily involves the off-center shifting of Nb{sup 5+} and Ln{sup 3+}. > The off-center ionic displacement decreases with increasing temperature. > Dielectric properties are greatly related to the off-center displacement.

• OSTI ID: 21580229
• Journal Information: Journal of Solid State Chemistry, Vol. 184, Issue 8; Other Information: DOI: 10.1016/j.jssc.2011.06.017; PII: S0022-4596(11)00343-4; Copyright (c) 2011 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; ISSN 0022-4596
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
DIELECTRIC PROPERTIES
LANTHANUM IONS
NEODYMIUM IONS
NEUTRON DIFFRACTION
NIOBIUM IONS
ORTHORHOMBIC LATTICES
PHASE TRANSFORMATIONS
SPACE GROUPS
SPECIFIC HEAT
SYNCHROTRONS
TEMPERATURE RANGE 0400-1000 K
TRANSITION TEMPERATURE
X-RAY DIFFRACTION
ACCELERATORS
CHARGED PARTICLES
COHERENT SCATTERING
CRYSTAL LATTICES
CRYSTAL STRUCTURE
CYCLIC ACCELERATORS
DIFFRACTION
ELECTRICAL PROPERTIES
IONS
PHYSICAL PROPERTIES
SCATTERING
SYMMETRY GROUPS
TEMPERATURE RANGE
THERMODYNAMIC PROPERTIES