# Homework Help: Condensed Matter: Thermal Expansion coeff. using diffraction angles

1. May 9, 2013

### themongrelcat

1. The problem statement, all variables and given/known data

At a temperature of 18°C a beam of diffracted monochromatic x-rays is observed at an angle of 150.8° to the incident beam after being diffracted by a crystal with cubic structure. At a temperature of 318°C the corresponding beam makes an angle of 141.6° with the incident beam. What is the mean coefficient of linear expansion of the crystal in the given temperature range?

2. Relevant equations

α=Δl/lΔT

possibly nλ=2dsinθ

3. The attempt at a solution

I already have ΔT and the equation for thermal expansion, but I have no idea how to get l and Δl. I know it must have something to do with the diffraction angles, maybe Bragg's Law? I've scoured the entire internet looking for an equation to relate the two, and I just cant find it. Even just an equation relating those would be helpful. Thank you!

Last edited by a moderator: May 10, 2013
2. May 10, 2013

### TSny

Hello, themongrelcat. Welcome to PF!

See if you can find an expression for Δd/d.