59th Annual Report on Research 2014

Both research groups at University of Northern Colorado (UNC) and Oregon State University (OSU) have continued syntheses and characterization of oxide precursors and oxynitrides. After screening several rare-earth oxides as candidates for ammonolysis to form oxynitrides, we focused on the application of CeZrO₄, CeTiO₄, Eu₂Zr₂O₇, and Eu₂Ti₂O₇ towards our synthesis efforts this past year.

Major accomplishments include the successful syntheses of EuTiO_3-xN_x and CeZrO_3-xN_x. Similar procedures were used to make both materials. Eu₂Ti₂O₇ and CeZrO₄ were synthesized and then used as precursor materials for ammonolysis (heat treatment under flowing NH₃ gas). The resulting oxynitride samples were then reoxidized by heating them under flowing O₂ gas. Figure 1 shows optical images of a) the CeZrO₄ precursor oxide, b) the CeZrO_3-xN_x oxynitride, c) Eu₂Ti₂O₇, and d) EuTiO_3-xN_x oxynitride.

CeZrO_3-xN_x

Molly Anderson, a graduate student in Macaluso's research group collected neutron diffraction data through the Mail-in Program using the POWGEN beamline at the Spallation Neutron Source, Oak Ridge National Laboratory. Data were collected from three samples – the precursor oxide, CeZrO₄; the oxynitride; and an oxynitride sample that was reoxidized under oxygen atmosphere. She collaborated with Dr. Jun Li, a postdoctoral researcher in Prof. Mas Subramanian's group, to analyze the data. LeBail refinements indicate a unit cell expansion of 4.3%, which suggests that nitrogen substitution of oxygen indeed took place during ammonolysis. Thermogravimetric analysis experiments on the oxynitride sample showed a 2.3% weight increase when the oxynitride sample was heated under flowing O₂. This weight increase corresponds to CeZrO_2.48N_0.32.

EuTiO_3-xN_x

Powder X-ray diffraction patterns collected from EuTiO₃ and EuTiO_3-xN_x samples are shown in Figure 2. The shift of the PXRD peaks from the EuTiO_3-xN_x sample indicates a unit cell expansion and suggests that nitrogen substitution of oxygen was successful. The sample can be reoxidized to form EuTiO₃ as suggested by the shift in smaller 2-theta of the characteristic diffraction peaks (shown as green). Thermogravimetric and combustion analyses also confirm the presence of nitrogen in the oxynitride, EuTiO_3-xN_x. Neutron diffraction experiments would provide further evidence for the successful synthesis of an oxynitride and further insight into the crystal structure. Powder neutron diffraction experiments of EuTiO_3-xN_x will be performed in the next academic year.

Figure 7. PXRD patterns of Eu₂Ti₂O₇ before and after ammonolysis, and the known perovskite EuTiO_3. Blue, red, and green lines represent Eu₂Ti₂O₇, EuTiO_3-xN_x, and EuTiO₃, respectively. Data for EuTiO₃ obtained from  (Brous, Fankuchen, & Banks, 1953).

Neither CeTiO₄ nor Eu₂Zr₂O₇ successfully yielded an oxynitride under ammonlysis conditions.

A UNC undergraduate student, Linda Nguyen, and Molly Anderson performed exploratory synthesis of oxynitrides using the citrate method, which involves the preparation of an amorphous oxide followed by ammonolysis. An amorphous sample of La₂Zr₂O₇ was successfully synthesized and heated under flowing NH₃ gas for six hours at 900 C. PXRD patterns of the sample showed that the sample progressively crystallized with increased heatings; however, the characteristic diffraction peaks did not shift in 2-theta. This suggests that nitrogen substitution of oxygen was not successful using the amorphous oxide.

In the next year, we will:

1) measure the optical properties of EuTiO_3-xN_x and CeZrO_3-xN_x

2) measure the electrical properties of EuTiO_3-xN_x and CeZrO_3-xN_x

3) collect neutron diffraction data on EuTiO_3-xN_x

4) prepare a manuscript for publication.

5) continue soft chemistry synthesis efforts. We will explore the effect of various ammonolysis parameters on the synthesis of oxynitrides. Some of the parameters that we will investigate include temperature, dwell time, and placement of the sample relative to the NH₃ inlet position.