Reports: ND1052309-ND10: Exploratory Synthesis and Characterization of Pyrochlore Oxynitrides for Solid State Lighting

Robin T. Macaluso, PhD, University of Northern Colorado

Mas Subramanian, PhD, Oregon State University

Experimental Design

            We have developed synthesis setups at both Oregon State University (OSU) and University of Northern Colorado (UNC).  Ammonolysis requires a relatively simple set-up with a tube furnace connected to an NH3 gas cylinder. Below is a picture of one of our recently installed ammonlysis stations.


The thermal ammonolysis station consists of a tube furnace with a gas inlet at one end and an outlet at the other end that leads to a flask containing dilute HCl that serves as a trap for residual ammonia gas. The gas inlet line is branched to allow for either nitrogen (N2) gas or ammonia (NH3) gas tank flow through the tube furnace.  The furnace tube was fitted with stainless steel end-caps and both the gas inlet and outlet tubes were made of stainless steel to prevent any reaction with the ammonia gas.

Upon heating, ammonia either reacts with the samples and/or decomposes (to H2O and N2). The decomposition reduces the flow rate at higher temperatures, therefore gas flow is monitored regularly throughout the heating cycle. Upon furnace calibration of the furnace and gas flow based on methods outlined by Brophy et al., the optimal sample position was determined to be at the center of the tube furnace.1

We unfortunately encountered an unexpected delay in ordering NH3 gas cylinders. Although we are not aware of the specific attributes of new regulations on NH3 gas cylinders, new transportation policies have been recently changed that make it more difficult and more expensive to purchase NH3 cylinders. We were not able to successfully obtain a NH3 cylinder at either UNC or OSU until March 2013. Because the NH3 cylinders are critical to our synthesis procedure, the progress of our experimental results has been seriously impeded.

Experimental Progress

Research was performed individually and collaboratively between UNC and OSU. The PI and her graduate student, Molly Anderson, performed field work at OSU during the summer of 2013. Both groups at UNC and OSU have been screening oxides as potential precursors to oxynitride synthesis. We strategically chose compounds with structure types that are known to tolerate nitrogen substitution.2

            Various compounds have been synthesized by traditional solid-state reaction. Thus far, we have synthesized CeZrO4, CeTiO4, Eu2Zr2O7, Eu2Ti2O7, and Sm2Ru2O7. Initial ammonloysis experiments were performed on Eu2Zr2O7, and Eu2Ti2O7.  No reaction occurred upon ammonolysis of Eu2Zr­2O7 at 900 °C for 6 hours.  XRD confirmed that the pyrochlore phase remained after reaction as the color and weight did not change under the reaction conditions.  XRD experiments show the conversion of Eu2Ti2O7, which possesses the pyrochlore structure type, to EuTiO3, which possesses the perovskite structure type. Elemental analyses are currently underway. An initial ammonolysis of Sm2Ru2O7-xNx was also attempted. Although lower temperatures were used to prevent the reduction of RuO2 to Ru metal, the sample decomposed to Sm2O3 and RuO2.

Attempts to prepare the pyrochlore oxides of Y2TiNbO7 and Y2WAlO7 as precursors for ammonolysis were unsuccessful; therefore a direct route to the oxynitrides was investigated.  Synthesis of Y2TiNbO7-x­Nx and Y2WAlO7-xNx were attempted using the metal oxide starting materials well mixed and loosely packed in alumina boats.  The materials were heated two times at 900 °C for 6 hours.  The resulting materials were mixed phases of pyrochlores and other ternary metal oxides.  There was no indication that nitrogen was incorporated into the phases that were present.

Successful synthesis of GaN and Ta2N5 was performed at 900 °C for 6 hours under ammonia and with N2 gas flow upon cooling.  These materials were used as starting materials in an attempt to synthesize the delafossite oxynitrides, AgGa0.5Ta0.5ON and CuGa0.5Ta0.5ON.  Ag2O or Cu2O were mixed in stoichiometric proportions with the prepared GaN and Ta2N5.  These mixtures were heated in sealed fused silica tubes.  Two different synthesis temperatures – 500 °C and 1000 °C for 12 hours – were attempted, but both resulted in mixtures of Ag or Cu metal with the other metal oxides.

Future Plans

We plan to:

1) Continue ammonolysis of Ce and Eu-oxide precursors. We are currently performing ammonlysis experiments of CeTiO4 and CeZrO4, which we have already synthesized. Two new undergraduate students have recently joined PI Macaluso's group and will explore other Ce and Eu oxide precursors and ammonolysis.

2) Characterize in more detail the oxygen and nitrogen content of materials of products from ammonlysis reactions on Eu2Ti2O7. We will utilize neutron scattering opportunities and explore combustion analysis.

3) Explore other synthetic routes towards oxynitrides. An undergraduate and graduate student in Macaluso's research group are exploring solid-state methods towards the formation of oxynitrides.

4) Study potential solid solutions for A and B site substitution in ABO2N compounds such as Ba1-xAxTa1-xMxO2N, where A = Sr and/or B = Ti, Nb.

5) Investigate co-doping on the oxygen site with N3- and F- which has been shown by Yoon et. al.3 to increase the amount of N doped into the structure for enhanced band gap engineering.

 ADDIN ZOTERO_BIBL {"custom":[]} CSL_BIBLIOGRAPHY [1] Brophy, M. R.; Pilgrim, S. M.; Schulze, W. A. J. Am. Ceram. Soc. 94 ,2011, 4263.

[2] Tessier, F.; Marchand, R. J. Solid State Chem. 171, 2003, 143.

[3] Yoon, S. et al. J. Solid State Chem. 2013, 2013, 226.