
CIF dictionary browser
Expanded view
View CIF syntax
TWIN
CIF
Data items in the TWIN category record general details about the nature of the twinning in the sample. Terminology for twin dataname definitions was taken directly from: "International Union of Crystallography Commission on Mathematical and Theoretical Crystallography Research themes: Crystal twinning" by Massimo Nespolo, February 3, 2009. http://www.crystallography.fr/mathcryst/twins.htm .
_twin.dimensionality
CIF
The degree of overlap between the twin lattices. Most twin lattice symmetry (TLS) and twin lattice quasi-symmetry (TLQS) twins as defined by Donnay and Donnay will be triperiodic.
Reference: Donnay, G. & Donnay, J. D. H. (1974). Can. Mineral. 12, 422-425.
_twin.formation_mechanism
CIF
A description of the method of twin formation.
_twin.morphology
CIF
The physical relationship of the different twins to one another.
_twin.special_details
CIF
Information about twinning in the sample not contained in other data items.
Example:
Individuals 3 and 4 arise from form (I) following an enantiotropic phase transition by sudden cooling in a cryostat to 173K.
TWIN_INDIVIDUAL
CIF
Data items in the TWIN_INDIVIDUAL category describe properties of each twinned individual, and the symmetry relationships between the individuals.
_twin_individual.id
CIF
The unique identifier for this twin individual.
The twin for which _twin_individual.twin_matrix
is the identity matrix is called the reference twin.
It is recommended that the reference twin be labelled '1'.
_twin_individual.mass_fraction_refined
CIF
The refined mass fraction of this twin individual; the sum of all mass fractions must equal unity within the limits of experimental uncertainty.
_twin_individual.twin_lattice_type
CIF
Identification of the symmetry relationships between the twin lattices as described in International Tables for Crystallography (2004), Vol. C, Chapter 1.3. The twin with the identity matrix should be denoted as 'ref'.
_twin_individual.twin_matrix
CIF
The matrix U that multiplies the Miller indices h,k,l of the reference twin to give the Miller indices h',k',l', of the twin specified by _twin_individual_id
(h' k' l') = U (h k l )
The reference twin must have U = I, the identity matrix.
_twin_individual.twin_matrix_11
CIF
The [1,1] element of the matrix U in _twin_individual.twin_matrix.
_twin_individual.twin_matrix_12
CIF
The [1,2] element of the matrix U in _twin_individual.twin_matrix.
_twin_individual.twin_matrix_13
CIF
The [1,3] element of the matrix U in _twin_individual.twin_matrix.
_twin_individual.twin_matrix_21
CIF
The [2,1] element of the matrix U in _twin_individual.twin_matrix.
_twin_individual.twin_matrix_22
CIF
The [2,2] element of the matrix U in _twin_individual.twin_matrix.
_twin_individual.twin_matrix_23
CIF
The [3,2] element of the matrix U in _twin_individual.twin_matrix.
_twin_individual.twin_matrix_31
CIF
The [1,3] element of the matrix U in _twin_individual.twin_matrix.
_twin_individual.twin_matrix_32
CIF
The [2,3] element of the matrix U in _twin_individual.twin_matrix.
_twin_individual.twin_matrix_33
CIF
The [3,3] element of the matrix U in _twin_individual.twin_matrix.
TWIN_REFLN
CIF
Items in the TWIN_REFLN category tabulate the calculated values of F^2^ for each peak observed in the diffraction pattern, together with the measured value of F^2^. The calculated F^2^ for an observed peak is a weighted sum of the contribution of all the twin components. The calculated structure factor for each twin contribution can also be given.
_twin_refln.datum_id
CIF
This item serves to identify an observed diffraction peak. It may include contributions from one or more twin components and so may appear more than once in the list. The values of _twin_refln.F_squared_calc and _twin_refln.F_squared_meas indexed by _twin_refln.datum_id include the contributions from all the twins. The diffraction peak is indexed by _twin_refln.index_hkl referred to the cell of the twin component identified by _twin_refln.individual_id.
_twin_refln.F_squared_calc
CIF
Calculated value for the overall squared structure factors (in electrons squared for X-ray diffraction) arising from all the twin contributions to the peak defined by _twin_refln_datum_id. The total contribution from all twins to a single observed reflection is modelled according to the method of Pratt, Coyle and Ibers as follows:
_twin_refln.F_squared_calc = osf * Sum (mf * _reflnr._F_squared_calc)
where osf is the overall scale factor, mf is the mass fraction, and the sum is taken over all the twin components that contribute to this diffraction peak.
Reference: Pratt, C. S., Coyle, B. A. & Ibers, J. A. (1971). J. Chem. Soc. A, pp. 2146-2151.
_twin_refln.F_squared_calc_individual
CIF
Contribution of the twin identified by _twin_refln.individual_id to _twin_refln.F_squared_calc.
_twin_refln.F_squared_calc_individual = osf * mf * _refln.F_squared_calc
where osf is the overall scale factor, mf is the mass factor. For X-ray diffraction the units are electrons squared.
_twin_refln.F_squared_meas
CIF
The observed value F^2^ of the peak referenced by
_twin_refln.datum_id. This is typically the observed peak
intensity after Lorentz and polarization corrections.
_twin_refln.F_squared_meas_sigma
CIF
Standard uncertainty of _twin_refln_F_squared_meas
_twin_refln.include_status
CIF
A symbol indicating how the reflection is treated during refinement.
_twin_refln.index_h
CIF
Miller index h in the vector _twin_refln.index_hkl.
_twin_refln.index_hkl
CIF
Miller index vector of a reflection for the twin component
labelled by _twin_refln.individual_id.
The values of the Miller indices must correspond
to the cell defined for this twin component.
_twin_refln.index_k
CIF
Miller index k in the vector _twin_refln.index_hkl.
_twin_refln.index_l
CIF
Miller index l in the vector _twin_refln.index_hkl.
_twin_refln.individual_id
CIF
The unique identifier of the twin individual whose Miller indices are given in _twin_refln_index_hkl. The ID must match a _twin.individual_id in the TWIN_INDIVIDUAL category.
The list reference includes both _twin_refln.datum_id and _twin_refln.individual_id. The combination of these two items may only appear once in a list.