Lecture #17, Wednesday, May 6th

After blasting Lil' Wayne and the Game's My Life I started lecture on solid state synthesis. I reiterated why we have to grind samples for extended periods of time and emphasized the fact that we have to PROVE the identity of the synthesized product. This is done by properly interpreting an XRD pattern.

We went through various scenarios of the attempt to synthesize BiVO₄ by reacting Bi₂O₃ with V₂O₅. At room temperature only the starting materials exist and we see two distinct structures in the XRD pattern. As we gradually heat the reactants we see the peaks originating from the starting materials begin to get weaker and peaks from BiVO₄ begin to form. As heating progresses, the BiVO₄becomes a pure homogeneous phase. Something happens though when we heat to higher temperatures. The XRD pattern reveals there are additional peaks, which tell us something is going on. What is happening is when the synthetic temperature gets over 475 degrees the BiVO₄ converts over to another structure type. Using X-ray diffraction we can monitor the temperature this transition occurs.

I also discussed the capabilities and limitations X-ray diffraction and UV-Vis spectroscopy. An X-ray diffractometer can tell us which crystal structure(s) is/are present, but it cannot tell us the color of a compound. A UV-Vis spectrometer can tell us which excitations are responsible for the color of a compound, but we cannot verify a structure using UV-Vis. When both of these instrumentation are combined together they can be extremely powerful in determining structure-property relationships, or how changing a crystal structure can have an impact on the color of a compound.

We went over several examples from class and showed how combining XRD and UV-Vis can give us very valuable information. I then discusses how the valence electron count on the cation has an impact on the band gap of a solid. On Friday we will discuss various ways to manipulate the band gap of a solid and go into the details how an extended solid differs from a discrete molecule. We will also discuss the impact of choosing a transition metal for the cation, and how its d orbitals will affect the color of a compound. I will see everyone in class on Friday, Dr. Fus