Difference between revisions of "Peak shape"

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The [[peak width]] observed in x-ray scattering can be related to the grain size of the ordered structure giving rise to the scattering peak. More generally, the peak shape also encodes information about the sample order. Thus, peak shape analysis can be used to extract higher-order information.
 
The [[peak width]] observed in x-ray scattering can be related to the grain size of the ordered structure giving rise to the scattering peak. More generally, the peak shape also encodes information about the sample order. Thus, peak shape analysis can be used to extract higher-order information.
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One interpretation of peak shape is that encodes the average grain ''shape''. That is, the peak shape function is the [[Fourier transform]] of the grain shape. Specific examples:
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* Gaussian peak in [[reciprocal-space]] implies a Gaussian-like decorrelation in [[realspace]] (i.e. that the ''average'' of grains/correlation-volumes decays with a Gaussian profile).
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* Lorentzian peak in reciprocal-space implies an exponential decorrelation in realspace.
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* Sinc function in reciprocal-space implies a top-hat function in realspace (sharp domain boundaries, with little to no variation in size of domains).
  
 
Note also that [[instrumental resolution]] contributes to peak width, and also to peak shape. Scattering peaks are thus sometimes fit using functions that include two contributes (e.g. a Gaussian, representing material [[grain size]], plus a Lorentzian, representing instrumental resolution).
 
Note also that [[instrumental resolution]] contributes to peak width, and also to peak shape. Scattering peaks are thus sometimes fit using functions that include two contributes (e.g. a Gaussian, representing material [[grain size]], plus a Lorentzian, representing instrumental resolution).

Latest revision as of 09:15, 12 October 2016

Peak.png

The peak width observed in x-ray scattering can be related to the grain size of the ordered structure giving rise to the scattering peak. More generally, the peak shape also encodes information about the sample order. Thus, peak shape analysis can be used to extract higher-order information.

One interpretation of peak shape is that encodes the average grain shape. That is, the peak shape function is the Fourier transform of the grain shape. Specific examples:

  • Gaussian peak in reciprocal-space implies a Gaussian-like decorrelation in realspace (i.e. that the average of grains/correlation-volumes decays with a Gaussian profile).
  • Lorentzian peak in reciprocal-space implies an exponential decorrelation in realspace.
  • Sinc function in reciprocal-space implies a top-hat function in realspace (sharp domain boundaries, with little to no variation in size of domains).

Note also that instrumental resolution contributes to peak width, and also to peak shape. Scattering peaks are thus sometimes fit using functions that include two contributes (e.g. a Gaussian, representing material grain size, plus a Lorentzian, representing instrumental resolution).

Generalized Peak Shape

A generalized peak shape can be computed using:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} L_{hkl}(q) & = \frac{2}{\pi\delta} \prod_{n=0}^{\infty}{\left( 1 + \frac{\gamma_{\nu}^2}{(n+\nu/2)^2} \frac{4 q_s^2}{\pi^2\delta^2} \right)^{-1}} \\ & = \frac{2}{\pi\delta} \left| \frac{ \Gamma\left[\nu/2 + i\gamma_{\nu}(4q_s^2/\pi^2\delta^2)^2\right] }{ \Gamma\left[\nu/2\right] } \right|^2 \end{alignat} }

Where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle q_s=(q-q_{hkl})} , Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \delta} describes the peak width, and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \nu} describes the peak shape. The parameter Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \gamma_{\nu}} is a ratio of gamma functions:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \gamma_{\nu} = \sqrt{\pi}\frac{\Gamma\left[ (\nu+1)/2 \right]}{\Gamma\left[ \nu+/2 \right]} }

The limiting cases for peak shape are:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle L_{hkl}(q_s) = \left\{ \begin{array}{c l l} \frac{\delta/2\pi}{q_s^2+(\delta/2)^2} & \mathrm{for} \,\, \nu\to0 & \mathrm{(Lorentzian)} \\ \frac{2}{\pi\delta}\exp\left[ -\frac{4q_s^2}{\pi\delta^2} \right] & \mathrm{for} \,\, \nu\to\infty & \mathrm{(Gaussian)} \\ \end{array} \right. }

Thus the parameter Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \nu} allows one to vary continuously between a Lorentzian peak shape and a Gaussian peak shape. Note that for Lorentzian, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \delta} describes the full-width at half-maximum (FWHM):

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \delta_{\mathrm{lorentz}} = \mathrm{fwhm_{\mathrm{lorentz}}} }

The Gaussian form can be written a few different ways:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} L_{hkl,\mathrm{gauss}}(q_s) & = \frac{2}{\pi\delta}\exp\left[ -\frac{4q_s^2}{\pi\delta^2} \right] \\ & = \frac{1}{\sqrt{2\pi}\sigma}\exp\left[ -\frac{q_s^2}{2\sigma^2} \right] \\ & = \sqrt{\frac{\ln{2}}{\pi}}\frac{1}{\mathrm{fwhm}} \exp\left[ -\frac{4 \ln{2} q_s^2}{\mathrm{fwhm}^2} \right] \\ \end{alignat} }

where the width is described by:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \delta_{\mathrm{gauss}} = \sqrt{\frac{8}{\pi}}\sigma_{\mathrm{gauss}} = \frac{\mathrm{fwhm}_{\mathrm{gauss}}}{\sqrt{\pi\ln{2} }}}
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma_{\mathrm{gauss}} = \sqrt{\frac{\pi}{8}}\delta_{\mathrm{gauss}} = \frac{\mathrm{fwhm}_{\mathrm{gauss}}}{2\sqrt{2 \ln{2} }}}
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathrm{fwhm}_{\mathrm{gauss}} = 2\sqrt{2 \ln{2} } \sigma_{\mathrm{gauss}} = \sqrt{\pi\ln{2} } \delta_{\mathrm{gauss}}}

And note that Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 2\sqrt{2 \ln{2} } \approx } 2.35482004503...

Source

Literature Examples

Warren/Averbach paracrystal

Williamson/Hall

Grain Size Distribution

Fourier Analysis

Maximum Entropy

Other

See Also

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