Source code for pymatgen.analysis.diffraction.xrd

# coding: utf-8
# Copyright (c) Pymatgen Development Team.
# Distributed under the terms of the MIT License.

"""
This module implements an XRD pattern calculator.
"""

import os
import json
from math import sin, cos, asin, pi, degrees, radians

import numpy as np

from pymatgen.symmetry.analyzer import SpacegroupAnalyzer

from .core import DiffractionPattern, AbstractDiffractionPatternCalculator, \
    get_unique_families

__author__ = "Shyue Ping Ong"
__copyright__ = "Copyright 2012, The Materials Project"
__version__ = "0.1"
__maintainer__ = "Shyue Ping Ong"
__email__ = "ongsp@ucsd.edu"
__date__ = "5/22/14"


# XRD wavelengths in angstroms
WAVELENGTHS = {
    "CuKa": 1.54184,
    "CuKa2": 1.54439,
    "CuKa1": 1.54056,
    "CuKb1": 1.39222,
    "MoKa": 0.71073,
    "MoKa2": 0.71359,
    "MoKa1": 0.70930,
    "MoKb1": 0.63229,
    "CrKa": 2.29100,
    "CrKa2": 2.29361,
    "CrKa1": 2.28970,
    "CrKb1": 2.08487,
    "FeKa": 1.93735,
    "FeKa2": 1.93998,
    "FeKa1": 1.93604,
    "FeKb1": 1.75661,
    "CoKa": 1.79026,
    "CoKa2": 1.79285,
    "CoKa1": 1.78896,
    "CoKb1": 1.63079,
    "AgKa": 0.560885,
    "AgKa2": 0.563813,
    "AgKa1": 0.559421,
    "AgKb1": 0.497082,
}

with open(os.path.join(os.path.dirname(__file__),
                       "atomic_scattering_params.json")) as f:
    ATOMIC_SCATTERING_PARAMS = json.load(f)


[docs]class XRDCalculator(AbstractDiffractionPatternCalculator): r""" Computes the XRD pattern of a crystal structure. This code is implemented by Shyue Ping Ong as part of UCSD's NANO106 - Crystallography of Materials. The formalism for this code is based on that given in Chapters 11 and 12 of Structure of Materials by Marc De Graef and Michael E. McHenry. This takes into account the atomic scattering factors and the Lorentz polarization factor, but not the Debye-Waller (temperature) factor (for which data is typically not available). Note that the multiplicity correction is not needed since this code simply goes through all reciprocal points within the limiting sphere, which includes all symmetrically equivalent facets. The algorithm is as follows 1. Calculate reciprocal lattice of structure. Find all reciprocal points within the limiting sphere given by :math:`\\frac{2}{\\lambda}`. 2. For each reciprocal point :math:`\\mathbf{g_{hkl}}` corresponding to lattice plane :math:`(hkl)`, compute the Bragg condition :math:`\\sin(\\theta) = \\frac{\\lambda}{2d_{hkl}}` 3. Compute the structure factor as the sum of the atomic scattering factors. The atomic scattering factors are given by .. math:: f(s) = Z - 41.78214 \\times s^2 \\times \\sum\\limits_{i=1}^n a_i \ \\exp(-b_is^2) where :math:`s = \\frac{\\sin(\\theta)}{\\lambda}` and :math:`a_i` and :math:`b_i` are the fitted parameters for each element. The structure factor is then given by .. math:: F_{hkl} = \\sum\\limits_{j=1}^N f_j \\exp(2\\pi i \\mathbf{g_{hkl}} \\cdot \\mathbf{r}) 4. The intensity is then given by the modulus square of the structure factor. .. math:: I_{hkl} = F_{hkl}F_{hkl}^* 5. Finally, the Lorentz polarization correction factor is applied. This factor is given by: .. math:: P(\\theta) = \\frac{1 + \\cos^2(2\\theta)} {\\sin^2(\\theta)\\cos(\\theta)} """ # Tuple of available radiation keywords. AVAILABLE_RADIATION = tuple(WAVELENGTHS.keys()) def __init__(self, wavelength="CuKa", symprec=0, debye_waller_factors=None): """ Initializes the XRD calculator with a given radiation. Args: wavelength (str/float): The wavelength can be specified as either a float or a string. If it is a string, it must be one of the supported definitions in the AVAILABLE_RADIATION class variable, which provides useful commonly used wavelengths. If it is a float, it is interpreted as a wavelength in angstroms. Defaults to "CuKa", i.e, Cu K_alpha radiation. symprec (float): Symmetry precision for structure refinement. If set to 0, no refinement is done. Otherwise, refinement is performed using spglib with provided precision. debye_waller_factors ({element symbol: float}): Allows the specification of Debye-Waller factors. Note that these factors are temperature dependent. """ if isinstance(wavelength, float): self.wavelength = wavelength else: self.radiation = wavelength self.wavelength = WAVELENGTHS[wavelength] self.symprec = symprec self.debye_waller_factors = debye_waller_factors or {}
[docs] def get_pattern(self, structure, scaled=True, two_theta_range=(0, 90)): """ Calculates the diffraction pattern for a structure. Args: structure (Structure): Input structure scaled (bool): Whether to return scaled intensities. The maximum peak is set to a value of 100. Defaults to True. Use False if you need the absolute values to combine XRD plots. two_theta_range ([float of length 2]): Tuple for range of two_thetas to calculate in degrees. Defaults to (0, 90). Set to None if you want all diffracted beams within the limiting sphere of radius 2 / wavelength. Returns: (XRDPattern) """ if self.symprec: finder = SpacegroupAnalyzer(structure, symprec=self.symprec) structure = finder.get_refined_structure() wavelength = self.wavelength latt = structure.lattice is_hex = latt.is_hexagonal() # Obtained from Bragg condition. Note that reciprocal lattice # vector length is 1 / d_hkl. min_r, max_r = (0, 2 / wavelength) if two_theta_range is None else \ [2 * sin(radians(t / 2)) / wavelength for t in two_theta_range] # Obtain crystallographic reciprocal lattice points within range recip_latt = latt.reciprocal_lattice_crystallographic recip_pts = recip_latt.get_points_in_sphere( [[0, 0, 0]], [0, 0, 0], max_r) if min_r: recip_pts = [pt for pt in recip_pts if pt[1] >= min_r] # Create a flattened array of zs, coeffs, fcoords and occus. This is # used to perform vectorized computation of atomic scattering factors # later. Note that these are not necessarily the same size as the # structure as each partially occupied specie occupies its own # position in the flattened array. zs = [] coeffs = [] fcoords = [] occus = [] dwfactors = [] for site in structure: for sp, occu in site.species.items(): zs.append(sp.Z) try: c = ATOMIC_SCATTERING_PARAMS[sp.symbol] except KeyError: raise ValueError("Unable to calculate XRD pattern as " "there is no scattering coefficients for" " %s." % sp.symbol) coeffs.append(c) dwfactors.append(self.debye_waller_factors.get(sp.symbol, 0)) fcoords.append(site.frac_coords) occus.append(occu) zs = np.array(zs) coeffs = np.array(coeffs) fcoords = np.array(fcoords) occus = np.array(occus) dwfactors = np.array(dwfactors) peaks = {} two_thetas = [] for hkl, g_hkl, ind, _ in sorted( recip_pts, key=lambda i: (i[1], -i[0][0], -i[0][1], -i[0][2])): # Force miller indices to be integers. hkl = [int(round(i)) for i in hkl] if g_hkl != 0: d_hkl = 1 / g_hkl # Bragg condition theta = asin(wavelength * g_hkl / 2) # s = sin(theta) / wavelength = 1 / 2d = |ghkl| / 2 (d = # 1/|ghkl|) s = g_hkl / 2 # Store s^2 since we are using it a few times. s2 = s ** 2 # Vectorized computation of g.r for all fractional coords and # hkl. g_dot_r = np.dot(fcoords, np.transpose([hkl])).T[0] # Highly vectorized computation of atomic scattering factors. # Equivalent non-vectorized code is:: # # for site in structure: # el = site.specie # coeff = ATOMIC_SCATTERING_PARAMS[el.symbol] # fs = el.Z - 41.78214 * s2 * sum( # [d[0] * exp(-d[1] * s2) for d in coeff]) fs = zs - 41.78214 * s2 * np.sum( coeffs[:, :, 0] * np.exp(-coeffs[:, :, 1] * s2), axis=1) dw_correction = np.exp(-dwfactors * s2) # Structure factor = sum of atomic scattering factors (with # position factor exp(2j * pi * g.r and occupancies). # Vectorized computation. f_hkl = np.sum(fs * occus * np.exp(2j * pi * g_dot_r) * dw_correction) # Lorentz polarization correction for hkl lorentz_factor = (1 + cos(2 * theta) ** 2) / \ (sin(theta) ** 2 * cos(theta)) # Intensity for hkl is modulus square of structure factor. i_hkl = (f_hkl * f_hkl.conjugate()).real two_theta = degrees(2 * theta) if is_hex: # Use Miller-Bravais indices for hexagonal lattices. hkl = (hkl[0], hkl[1], - hkl[0] - hkl[1], hkl[2]) # Deal with floating point precision issues. ind = np.where(np.abs(np.subtract(two_thetas, two_theta)) < AbstractDiffractionPatternCalculator.TWO_THETA_TOL) if len(ind[0]) > 0: peaks[two_thetas[ind[0][0]]][0] += i_hkl * lorentz_factor peaks[two_thetas[ind[0][0]]][1].append(tuple(hkl)) else: peaks[two_theta] = [i_hkl * lorentz_factor, [tuple(hkl)], d_hkl] two_thetas.append(two_theta) # Scale intensities so that the max intensity is 100. max_intensity = max([v[0] for v in peaks.values()]) x = [] y = [] hkls = [] d_hkls = [] for k in sorted(peaks.keys()): v = peaks[k] fam = get_unique_families(v[1]) if v[0] / max_intensity * 100 > AbstractDiffractionPatternCalculator.SCALED_INTENSITY_TOL: x.append(k) y.append(v[0]) hkls.append([{"hkl": hkl, "multiplicity": mult} for hkl, mult in fam.items()]) d_hkls.append(v[2]) xrd = DiffractionPattern(x, y, hkls, d_hkls) if scaled: xrd.normalize(mode="max", value=100) return xrd