Order of diffractionformula

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.

"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 isorder of diffractionin Bragg's law

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.

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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: