Elastic properties of complex transition metal oxides studied by Resonant Ultrasound Spectroscopy

Author

Yanbing LuanFollow

Date of Award

5-2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Veerle M. Keppens

Committee Members

George M. Pharr, Chuck L. Melcher, Adriana Moreo

Abstract

The elastic properties of novel transition metal oxides have been investigated, using a powerful technique known as Resonant Ultrasound Spectroscopy (RUS). Two sets of transition metal oxides have been studied. One is the ruthenate Ca_2-xSr_xRuO₄ series with a layered perovskite structure, a Mott transition system that connects the Mott insulator Ca₂RuO₄ with the unconventional superconductor Sr₂RuO₄. The other set contains geometrically frustrated materials, including vanadium spinels AV₂O₄ (A = Zn, Mn and Fe) and titanate pyrochlores A₂Ti₂O₇ (A= Y, Tb, Yb, Ho and Dy).

The elastic response of five Ca_2-xSr_xRuO₄ single crystals (x = 2.0, 1.9, 0.5, 0.3 and 0.2) has been measured. For 2.0 ≥ x ≥ 0.5, a dramatic softening over a wide temperature range is observed upon cooling, caused by the rotational instability of RuO₆ octahedra (for x = 2.0 and 1.9) or the static rotation of the octahedra (for x = 0.5). For the Ca-rich samples (x = 0.3 and 0.2), the softening occurs in a very narrow temperature range, corresponding to the structural phase transition from high-temperature-tetragonal to low-temperature-orthorhombic symmetry.

Elastic softening in ZnV₂O₄ is observed near the cubic-to-tetragonal structural phase transition at 50 K. The elastic response of MnV₂O₄ is quite unusual, displaying a softening over a wide temperature range with decreasing temperature. Upon cooling, C’ of FeV₂O₄ becomes so soft that it drops to almost zero around 140 K, where the cubic-to-tetragonal structural transition occurs.

For Y₂Ti₂O₇, all three elastic constants show normal “Varshni” behavior. For spin liquid Tb₂Ti₂O₇, all three elastic constants show a pronounced softening below 50 K, indicative of a possible Jahn-Teller, cubic-to-tetragonal transition at very low temperatures. It is also found that the application of a magnetic field suppresses the elastic softening in this compound. Another spin liquid Yb₂Ti₂O₇ shows no elastic softening. The elastic moduli of the spin-ice compounds, Ho₂Ti₂O₇ and Dy₂Ti₂O₇, show a broad “dip” around 100 K, which is believed to be caused by the strong crystal field effect in those two compounds.

Recommended Citation

Luan, Yanbing, "Elastic properties of complex transition metal oxides studied by Resonant Ultrasound Spectroscopy. " PhD diss., University of Tennessee, 2011.
https://trace.tennessee.edu/utk_graddiss/993