# Diffraction condition - Kittel's Intro to Solid State Physics 8th ed.

1. Apr 19, 2014

### DiogenesTorch

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

How to derive equation (22) on page 31 of Kittel's Intro to Solid State Physics 8th edition.

The equation is: $$2\vec{k}\cdot\vec{G}+G^2=0$$

2. Relevant equations

The diffraction condition is given by $\Delta\vec{k}=\vec{G}$ which from what I can surmise is the starting point for the derivation

Here are some other relevant equations/definitions
\begin{align*} \vec{k} & & \text{ incident wave vector} \\ \vec{k'} & & \text{ outgoing\reflected wave vector} \\ \Delta \vec{k'} = \vec{k'}-\vec{k} & & \text{ scattering vector} \\ \vec{G} & & \text{ reciprocal lattice vector} \\ \end{align*}

3. The attempt at a solution

Starting with the diffraction condition $\Delta\vec{k}=\vec{G}$

\begin{align*} \Delta\vec{k} &= \vec{G} & \\ \vec{k'}-\vec{k} &= \vec{G} & \\ \vec{k}+\vec{G} &= \vec{k'} & \\ |\vec{k}+\vec{G}|^2 &= |\vec{k'}|^2 & \text{If 2 vectors are equal, their squared magnitudes are equal}\\ |\vec{k}+\vec{G}|^2 &= |\vec{k}|^2 & \text{since magnitudes k and k' are equal}\\ \end{align*}

Using the law of cosines we now have

\begin{align} |\vec{k}+\vec{G}|^2 &= |\vec{k}|^2 & \nonumber \\ |\vec{k}|^2 + |\vec{G}|^2 -2|\vec{k}||\vec{G}|\cos\theta &= |\vec{k}|^2 & \text{(1)} \\ \end{align}

$\cos\theta$ is the cosine of the angle between the vectors $\vec{k}$ and $\vec{G}$ which is just

\begin{align*} \cos\theta=\frac{\vec{k}\cdot\vec{G}}{|\vec{k}||\vec{G}|} \end{align*}

Substituting the above into equation (1)

\begin{align*} |\vec{k}|^2 + |\vec{G}|^2 -2\vec{k}\cdot\vec{G} &= |\vec{k}|^2 \\ |\vec{G}|^2 -2\vec{k}\cdot\vec{G} &= 0 \\ G^2 -2\vec{k}\cdot\vec{G} &= 0 \\ \end{align*}

However the book is showing $2\vec{k}\cdot\vec{G} + G^2 = 0$

Why is Kittel losing the minus sign in front of the dot product? I have been scratching my head for hours and can't find my error. I know this is some bone-headed oversight on my part but can't seem to find my error.

If $\theta$ is the angle between $\vec{k}$ and $\vec{G}$ then the angle in the law of cosines should be $180^o-\theta$