AmericanChemicalSociety.com

Progress in each of the three main thrust areas is given below:

Aim 1: Explore conductive pyrochlores 

Research in this area began with the investigation of pyrochlores compounds which are expected to have good acid stability – namely those with Ti, Nb, and/or Ta as the B-site cation.  The first set of test samples combined these B-site cations with Bi and/or Ce on the A-site, as these two cations have been previously observed to improve the conductivity of other pyrochlore systems.  Neither of these strategies were successful, as all preparations resulted in samples with resistances exceeding MW values.  Work then turned to mixing Ti, Nb, and Ta with stable divalent or trivalent cations such as Ca or Y.  These compounds were then subjected to strongly reducing conditions to improve their conductivity.  Judging by the observed color changes, these strategies are promising and more work is being done to fully structurally characterize reaction products.

Aim 2: Explore hollandite compounds

Synthetic efforts in this area have focused on the synthesis of Ti and Ru containing hollandites.  A set of conductive compounds have been synthesized, and their structural characterization is in progress.  Electrochemical characterization tools have been purchased and installed; the initial characterization of these materials will begin in the coming months.

Aim 3: Produce large single crystals

Our optical floating zone has been recently installed.  We have successfully carried out the growth of a reduced (+3.8) titanate ternary compound using fully oxidized (+4) titanium precursors, demonstrating that forming gas can be effectively used to access lower valence states of early 3d transition metals.  The first crystals were about 0.5 cm in diameter and 1.0 cm in length; scale-up to larger sample dimensions is planned.  We will next be exploring some mixed Ti/Nb pyrochlores in this manner.

Additional crystal growths of some ternary niobate materials have been carried out by what appears to be vapor transport or grain growth at very high temperatures (~1600 C) under reducing conditions.  We have discovered two new reduced niobate compounds which are black in color and are anticipated to have good electronic conductivities.  Single crystal diffraction data has permitted the full structural refinement of these two new phases.