# Why are powder XRD axes sometimes plotted in reverse?

Just plotting some powder XRD data and I wanted to know why the axes in some of the literature that I'm looking at is plotted in reverse. Is there a reason it starts at 60 Degrees (2 theta) and then goes down toward zero?

• In the days of old it was common to plot these data against $d$ rather that $\theta$, which would kinda explain the reverse direction. – Ivan Neretin Apr 20 '18 at 17:51

You can convert from "position-time" space, which is what we're comfortable with as humans, and describe the system in "momentum-energy" space, which is simpler from a physics point of view. Position is the reciprocal of momentum and energy is the reciprocal of time. If you convert the diffraction equations to momentum ($k$), which is proportional to $\frac{1}{sin(\theta)}$, then the diffraction equation $\lambda = 2dsin(\theta)$ is transposed to $k' = k + K$ where $K$ describes the peak in momentum space, $k$ describes the momentum of the incoming photon, and $k'$ describes the diffracted photon. This equation is linear and much easier to manipulate than the "real-space" form of $\lambda = 2dsin(\theta)$.