In X-ray diffraction, the pseudo-Voigt model is a combination of Gaussian and Lorentzian distributions, and is often used to model peaks. The form of the peak is often described as
$V(x)$ = (1-$\alpha$)$G(x)$ + $\alpha$$L(x)$
G(x) and L(x) are the gaussian and Lorentzian parts.
What is the physical significance of $\alpha$?
Are the mechanisms that cause Lorentzian broadening different than those causing Gaussian?
I'm afraid $\alpha$ has no physical meaning here.
The line profile (i.e. the profile of the diffraction peak) is a convolution of the instrument and physical broadening. The latter means broadening caused by the microstructure of the sample, which is typically divided into the size and strain components. (keyword: size-strain analysis)
But all the components above can contribute to both Gaussian and Lorentzian parts, so it's not like in spectroscopy that you can assign physical meaning to each part. Here Pseudo-Voigt (or Voigt or Pearson VII) is just an empirical shape.
