On my inorganic exam last week, I wrote a question asking the students to construct a Born-Haber cycle to calculate the heat of reaction for the thermite reaction (nicely timed as our lab staff had just done the demo outside of our classroom for the Gen Chem students). The data that I provided were the first three ionization energies for Fe and Al, the heats of formation for Fe and Al in the gas phase, and the lattice energies for Al2O3 and Fe2O3.
Now, here is where things got interesting and surprising to me. I found a thermochemical lattice energy for Fe2O3 online (WebElements) and in the CRC Handbook. But I could not find a thermochemical lattice energy for Al2O3 anywhere. The best I could do was a calculated value. I turned that into a "learning experience" and asked the students as a follow-up how their answer might have changed if I had found a thermochemical lattice energy for Al2O3. But here is my question: Does anyone know why it is difficult to find a thermochemical lattice energy for Al2O3? It seems like it should not be that much harder to do than Fe2O3.