Order of diffraction pattern confusion in XRD experiment

Question

Somewhere I came across this phrase:

"expected diffraction angle for first order reflection."

With respect to XRD do I take this as a rule of thumb that the diffraction will always be a first order one? I will quote some relatable lines:

"The Bragg equation can be used for determining the lattice parameters of cubic crystals. Let us first consider the value that n should be assigned. A second order reflection from (100) planes should satisfy the following Bragg equation. $2\lambda=2d_{110}\sinθ$ or $\lambda=d_{100}\sin \theta$. Similarly a first order reflection from (200) planes should satisfy the following condition $\lambda=2d_{200}\sin \theta $.

What do I consider the value of n as when this is given to me?

Answer 1

In Bragg's law, $n\lambda=2d\sin(\theta)$. Here, $n$ is the order of the reflection, and corresponds to the path length difference between X-rays diffracted from two different layers of atoms, in terms of the number of wavelengths. So if the path lengths differ by exactly one wavelength, it is a first order reflection.

By convention, we treat all reflections as first order, and introduce a different "effective" lattice spacing. This avoids having to deal with reflection order explicitly. As an example from the comments on the original question, if you consider a second order reflection from (100) planes, the equation is $2\lambda=2d_{100}\sin(\theta)$ and a first order reflection from (200) planes is satisfied by $\lambda=2d_{200}\sin(\theta)$. But we know that $2d_{200}=d_{100}$, so these two conditions both correspond to the same diffraction angle in the material. Again, we just consider all reflections as first order by convention.