Dr. Terrell A. Vanderah

Research Interests

• Phase equilibrium relationships in technically important ceramic oxide systems for communications, energy, medical, and optoelectronic applications
• Inorganic non-molecular solid-state chemistry with emphasis on crystal chemistry and the interdependence of crystal structure, chemical composition, and physical properties

Figure 1:  Structures of the family of four pyrochlore-related phases found in the Bi2O3:Fe2O3:Nb2O5 system (labelled respectively by Bi:Fe:Nb molar ratios).  The structures shown in (b,c,d) are derived from the parent pyrochlore-type phase (a) by chemical twinning, and are unit-cell intergrowths of slabs of the pyrochlore (PC) and hexagonal-tungsten-bronze-type (HTB) structures.  Like the parent phase in a), the derivative phases exhibit dielectric relaxation which is attributed to a shared structural feature:  displacive disorder in the Bi-O networks of all of the phases.  (J. Solid State Chem. 181(3), 499-507 (2008))

 
Figure 2(left):  Phase equilibrium relations in the CaO:TiO2:Nb2O5 system indicating the region of pyrochlore formation.  The observed compositional range indicates that approximately 0.5 mol Ti4+ mixes on the A sites with Ca2+.  This unlikely crystal-chemical substitution is made possible by the displacive disorder on this shared site, and is analogous to that observed for pyrochlores with Bi3+ instead of Ca2+.  (J. Solid State Chem. 181(3), 406-414 (2008)); Figure 3(right):  Relative dielectric permittivity (e´) and dielectric loss (e´´) for a CaO:TiO2:Nb2O5 pyrochlore specimen, measured at frequencies from 1 kHz to 1MHz.  Below 300 K the dielectric relaxation characteristic of bismuth based pyrochlores is clearly observed:  with increasing measuring frequency the peak in the dielectric loss shifts towards higher temperatures and the width of the dielectric loss peak increases.  Observation of dielectric relaxation in this system, which does not contain Bi or Pb, demonstrates that it arises from the displacive disorder and not from the presence of polarizable lone-pair cations such as Bi3+ or Pb2+.  (J. Solid State Chem. 181(3), 406-414 (2008))

Postdoctoral Research Opportunities

Knowledge of phase relationships and the structures of phases is essential for developing structure-property relationships of advanced materials and for optimizing the processing of materials to minimize cost while controlling useful properties.  This research opportunity involves experimental measurement of phase equilibrium relationships in complex oxide systems of interest for electronic or other functional properties.  Studies focus on ceramic systems pertinent to critically enabling materials for technologies such as communications, energy, medical, and optoelectronic.  The technical approach includes synthesis and characterization using X-ray and other diffraction methods.  For more information...

Awards and Honors

• Fellow, the American Ceramic Society, 2009
  
• Judson C. French Award, NIST, 2007
• Spriggs Phase Equilibria Award, The American Ceramic Society, 2006
• Equal Opportunity/Diversity Award, NIST, 2003
• Alan Berman Research Publication Award, Naval Research Laboratory, 1995
• The Technical Director’s Award for Outstanding Technical Accomplishment, Naval Weapons Center, 1988
• Phi Beta Kappa