| MgO geometry input |
MgO crystallizes in a cubic cell with a
rock-salt structure. The crystal structure can be described as a fcc
lattice
of Mg ions with O ions occupying all the octahedral holes or vice
versa.
The rock-salt structure is the most common for MX compounds.
MgO is an important oxidic system in
minerals,
in defective systems as well as in adsorption phenomena. Therefore,
despite
its simplicity, MgO has been the subject of many research studies.
| COL
ICSD Collection Code 9863 DATE Recorded Jan 1, 1980; updated Jan 19, 1999 |
| NAME Magnesium oxide |
| MINR
Periclase FORM Mg O = Mg O TITL X-ray determination of electron-density distributions in oxides, Mg O, Mn O, Co O, and Ni O, and atomic scattering factors of their constituent atoms REF Proceedings of the Japan Academy PJACA 55 (1979) 43-48 AUT Sasaki S, FujinoK, TakeuchiY SYM x, y, z y, z, x z, x, y x, z, y y, x, z z, y, x x, -y, -z y, -z, -x z, -x, -y x, -z, -y y, -x, -z z, -y, -x -x, y, -z -y, z, -x -z, x, -y -x, z, -y -y, x, -z -z, y, -x -x, -y, z -y, -z, x -z, -x, y -x, -z, y -y, -x, z -z, -y, x -x, -y, -z -y, -z, -x -z, -x, -y -x, -z, -y -y, -x, -z -z, -y, -x -x, y, z -y, z, x -z, x, y -x, z, y -y, x, z -z, y, x x, -y, z y, -z, x z, -x, y x, -z, y y, -x, z z, -y, x x, y, -z y, z, -x z, x, -y x, z, -y y, x, -z z, y, -x |
| CELL a=4.217(1) b=4.217(1) c=4.217(1) alpha=90.0 beta=90.0 gamma=90.0 |
| V=75.0 D=3.56 Z=4 |
| SGR F m -3 m (225) - cubic |
| CLAS
m-3m (Hermann-Mauguin) - Oh (Schoenflies) PRS cF8 ANX AX PARM Atom__No OxStat Wyck -----X----- -----Y----- -----Z----- -SOF- |
|
Mg 1 2.000 4a
0.
0.
0. O 1 -2.000 4b 0.5 0.5 0.5 |
| WYCK
b a ITF Mg 1 B=0.312 ITF O 1 B=0.362 REM M PDF 43-1022 RVAL 0.013 |
The information you need to define the crystal structure is highlighted. Basically, the crystal structure is determined by the space group, by the shape and size of the unit cell and by the relative position of the atoms in the asymmetric unit.
MgO geometry input, derived from ICSD data, will be prepared and discussed, line by line.
1. Title section
MgO bulk: crystal structure from ICSD
The first line contains the title section. It can be useful to indicate the system in study and other relevant information about the job. The title section is printed in the output file, but it is not otherwise used by CRYSTAL.
2. Dimensionality of the system
CRYSTAL
The first record of the geometry
definition
must specify the dimensionality of the system.
CRYSTAL adopts four keywords: CRYSTAL,
SLAB, POLYMER
and MOLECULE, for 3D, 2D, 1D and 0D systems, respectively.
In this case the keyword to specify is
CRYSTAL.
3. Crystallographic information (for 3D systems only)
0 0 0
three integer numbers:
- convention
for the space group identification: sequential number (0) or
alphanumeric
code (1).
- type of cell
for rhombohedral groups: hexagonal (0) or rhombohedral (1).
- setting of the origin (see CRYSTAL
User's Manual for
further details).
4. Space group
225
It can be indicated either with its sequential number (0), as in this case, or by the Hermann-Mauguin alphanumeric code (1). In the ICSD file you can find both of them.
So, till now, the input file would look
something like this:
| MgO
bulk: crystal structure from ICSD CRYSTAL 0 0 0 225 |
MgO
bulk: crystal structure from ICSD CRYSTAL 1 0 0 F M 3 M |
according to the sequential number (on
the left) or the alphanumeric code (on the right).
Note: CRYSTAL adopts F M 3 M
instead
of F M -3 M for compatibility with a previous edition of the
International
Tables for Crystallography (see CRYSTAL User's Manual
for further
details).
5. Lattice parameters
4.21
The minimal set of crystallographic cell parameters is indicated (in Angstrom and degrees). For MgO, cubic system, the length of the edge of the cell fully defines shape and size of the conventional unit cell (note, however, that CRYSTAL works on the primitive cell).
6. Atomic position specification
2
12
0.0 0.0 0.0
8
0.5 0.5 0.5
The first line gives the number of atoms in the asymmetric unit. One line per atom in the asymmetric unit follows, to specify the conventional atomic number and the coordinates in fractional units of the crystallographic lattice vectors. These atoms are usually indicated as non-equivalent atoms, i.e. atoms not symmetry related. The whole structure of MgO is defined by 2 atoms.
7. Closing the geometry input section
END
This keyword ends the geometry input section. Before ending the section, you may specify optional keywords to modify the structure.
The completed input file looks
like:
| MgO
bulk CRYSTAL 0 0 0 225 4.21 2 12 0.0 0.0 0.0 8 0.5 0.5 0.5 |
1.
Title of the job 2. Dimensionality of the system 3. Crystallographic information (3D only) 4. Space Group number 5. Lattice parameters 6. Number of atoms in asymmetric unit Atomic position specification in fractionary coordinates |
|
| TESTGEOM | Optional keyword to stop execution after geometry input | |
| END | 7. end of the geometry input section |