Powder X-ray Diffraction
Reading: The introduction and theory for this experiment are discussed in Ch. 34.1-34.2. Pay
particular attention to Examples 34.2.1 and 34.2.2.
Prelab:
 Look up the densities of NaCl and KCl, to four significant figures, in a standard reference
before coming to lab.
Introduction: The goal of this experiment is to determine the unit cell distance (i.e. the lattice
parameter) for NaCl and for KCl. Both NaCl and KCl crystallize in the halide lattice, which
consists of two interpenetrating FCC lattices, one FCC-lattice for the cations and one FCC-lattice
for the anions. However, because K+ and Cl– are isoelectronic, the ionic scattering factor is very
nearly the same for both K+ and Cl–. As a result the KCl lattice appears to be primitive cubic;
observe Figure 34.1.6 but ignore the differences between the cations and anions. FCC lattices
should have z = 4, that is four formula units per unit cell. You can check your unit cell distance
by using the literature densities of NaCl and KCl, the formula weights, and the calculated unit
cell distances to find the corresponding number of formula units per unit cell. You can consult
your General Chemistry text for help on this issue.1 However, you can also let the units of the
density guide you through the calculations.
Procedure: Your instructor will guide you through the use of the Bruker D2-Phaser powder
diffractometer to produce a listing of the d-spacings of the observed reflections.
Calculations and Report: Use the attached form to report the results of your calculations. Use
the sequence of M-values to verify the lattice type. Calculate the number of formula units per
unit cell, z. [Hint: both values of z should be 4 within experimental error – right?] The report
form is due at the end of the lab session (Monday afternoon). You don’t submit a formal lab
report.
References:
1. T. L. Brown, H. E. LeMay, Jr., B. E. Bursten, C. J. Murphy, P. M Woodward, M. W.
Stoltzfus, Chemistry the Central Science, 13th Ed., Pearson, New York, NY, Section 12.5 and
Sample Exercise 12.2, pp. 498-502.
Powder X-ray Diffraction
Name____________________________
d
1/d2
M=h2+k2+l2
exact M
a (Å) (from exact M)
Substance 1_____________
Substance 2________________
a _____________________cm
a _____________________cm
density_________________g/cm3
density_________________g/cm3
FW____________________g/mol
FW____________________g/mol
density_________________molecules/cm3
density_________________molecules/cm3
V unit cell______________cm3
V unit cell______________cm3
z______________________formula units
to 3-sig.figs.
per unit cell
z______________________formula units
to 3-sig.figs.
per unit cell
Structure: primitive, BCC, FCC
Structure: primitive, BCC, FCC