*.strfiles can be prepared easily by using the program PowderCell, which is able to import several structure data files (e.g. ICSD, SHELX), to calculate the powder pattern and to save the structure in the
The plaster consists mainly of three phases: quartz, calcite and gypsum.
Lets have a look at the simple structure of quartz, which is provided in
the free-format file
PHASE=Quartz SpacegroupNo=154 // PARAM=A=0.4913_0.49^0.4935 PARAM=C=0.5404_0.538^0.545 // RP=3 PARAM=B1=0_0^0.01 GOAL=GrainSize(1,0,1) // PARAM=GEWICHT=0_0 GOAL:quartz=GEWICHT E=SI+4 Wyckoff=a TDS=0.002 x=0.47 E=O-2 Wyckoff=c TDS=0.0015 x=0.415 y=0.268 z=0.786The explanation of the items is easy: You should provide a phase name and you must provide a space group number or a Hermann-Mauguin symbol. A list of all symbols is included in the file
Then the lattice constants follow (
with their starting values, lower (
_) and upper (
limits. The text
PARAM= means that the following value
is to be refined. All these parameters can have lower and/or upper limits.
Lattice constants with fixed values of 90 or 120 deg or if
need no explicit entries in the structure file. They are automatically
generated by the program based on the space group information.
The lattice constants and thermal displacement parameters are given in nm units.
If one wants to use Å the entry
UNIT=ANGSTROEM is necessary.
Nearly every phase has a size or microstrain broadening. With
RP=3 PARAM=B1=... a Lorentzian crystallite size dependent
broadening will be refined. The entry
the crystallite size in the given lattice direction. If you want to refine
additional microstrain broadening you have to write:
RP=4 PARAM=B1=0_0^0.01 PARAM=k1=0_0^1 PARAM=k2=0_0
k2are width parameters of a Gaussian-like broadening function.
k1is a measure of the width of the size distribution and
k2is the mean squared strain.
The weighting factor
GEWICHT determines the share of the phase
in the sample. It is comparable with the Rietveld scale factor, but it
includes the X-ray density. Thats why the mass proportions of the phases
later can easily be calculated from this parameter. Therefore the additional
GOAL:quartz=GEWICHT is needed.
For elderly BGMN versions, the first three lines must have a
double-letter at the end to tell the program that no atomic positions follow.
Starting with level 3.3.1, this
// delimiters are fully
optional and may be written at the end of every line (at the end of the atomic
position lines, too). They may be followed by any comment. Therefore, you may
write the above example as follows:
PHASE=Quartz SpacegroupNo=154 PARAM=A=0.4913_0.49^0.4935 PARAM=C=0.5404_0.538^0.545 RP=3 PARAM=B1=0_0^0.01 GOAL=GrainSize(1,0,1) PARAM=GEWICHT=0_0 GOAL:quartz=GEWICHT E=SI+4 Wyckoff=a TDS=0.002 x=0.47 // Silicon position // follows oxygen position E=O-2 Wyckoff=c TDS=0.0015 x=0.415 y=0.268 z=0.786
E= atomic positions are defined. Elements or Ions have to be
written in capital letters. A list of the available symbols is found in the
A Wyckoff position is needed from which the program determines the free
coordinates. These coordinates must be provided as constants as in the
example or as refineable parameters with the
TDS is the isotropic thermal displacement value for the atomic
position (in nm^2). It can also be a refineable parameter.
An additional explanation of the variables possible is available.