CAREER: Fundamental Structure-Dielectric Property Relationships of Fluorite Related compounds

Dr. Juan C. Nino, MSE Dept., University of Florida

DMR-0449710

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Die

lect

ric C

onst

ant

Temperature (oC)

SH

G (arb. units)

Based on the crystal structure determination and refinement, the transition is mainly due to the Nb5+ ions shifting to off-center positions within the NbO6

octahedra, as well as Ln3+ shifts to off-center positions within the LnO8 cubes, in an anti-parallel fashion. See diagram on the left.

All the Ln3NbO7 compounds exhibit a dielectric relaxation. However, only Gd3NbO7 exhibits a distinct non-centrosymmetric to centrosymmetric transition at about 70oC based on second harmonic generation (SHG) measurements. See the plot on the left.

From previous measurements, it is observed that the dielectric relaxation temperature, at which the maxima of the dielectric constant occurs, matches well with the phase transition temperature.

This study focuses on the understanding of the origin of dielectric relaxation in the fluorite-related compounds (A2B2O7, A and B are cations), especially Ln3NbO7 (Ln3+ = La3+, Nd3+, and Gd3+).

Crystal structure of Ln3NbO7

LnO8

NbO6

Ln in LnO7

In the crystal structure of Ln3NbO7, there are three types of polyhedra: distorted LnO8 cubes, distorted NbO6 octahedra, and LnO7.

Gd3NbO7

OLn

Nb

Off-center distortion of LnO8 and NbO6 below 70oC, the dashed circles stand for the centers of polyhedra

CAREER: Fundamental Structure-Dielectric Property Relationships of Fluorite Related compounds

Dr. Juan C. Nino, MSE Dept., University of Florida

DMR-0449710Education and Outreach

During the last year, the main graduate student in this project, Lu Cai, has mentored two undergraduate students Alex Luis Arias and Donald Moore. After one or more semesters under Lu’s training, they both have their own projects now: Alex is working in the single crystal growth of TGS-TGSe and Don is working in dielectric/ferroelectric property measurements of oxyfluorides. To expand the awareness of science and engineering of ceramic materials as a professional field, during the summer, the NRG group has continued to be a host of the Student Science Training Program (SSTP). One high school student, Jonathan Oulton, from Daytona Beach, is participating in the group’s research and working on molten salt growth of Gd3NbO7 single crystals.

As part of the educational and outreach component of this project, the PI organized the second in the series of International Workshops on Electroceramics across the Bolivarian countries in South America. This second workshop took place at the Universidad Nacional de Ingeniería, in Lima, Peru, from August 20-22, 2008. The local host was Professor Abel Gutarra. There were 20 presentations from researchers across Perú and one from Chile. There were 45 student participants.

Lu

Donald

Alex

Jonathan

International Workshop on Electroceramics, Lima, PERÚ 2008