In addition to the numeric or character data values that a CIF data item may have, the special characters '?' and '.' may be used as data values irrespective of the data type assigned in a CIF dictionary. Note that this implies that a CIF reader must accept these special values for any data field, prior to type checking. Note also that they should appear in the CIF without surrounding quotation marks.
Both characters are used as placeholders, but with the following specific meanings.
Unknown value
The value '?' represents an unknown value of the quantity. It appears typically in template files to indicate data items whose value should be supplied by an application or user; or it may appear in the output from an application extracting information from a CIF in response to a request list.
Examples
Example 1: An extract from the Acta Cryst. C author's template (the full current version is available as https://www.iucr.org/__data/iucr/ftp/pub/form.cif).
##############################################################################
### CIF submission form for molecular structure report (Acta Cryst. C) ###
##############################################################################
# This is an electronic "form" for submitting a structural paper to Acta Cryst.
#
# Note that all fields should be numeric or character type EXCEPT those which
# are flagged as 'text' - free-form text of any length may be included in
# these latter fields provided the text block begins and ends with a semicolon
# as the first character of a new line. Note also that the query marks
# '?' are significant as placeholders, and should not be deleted where a data
# item is not given, UNLESS the accompanying data name is also deleted.
# Lines should not exceed 80 characters in length.
#==============================================================================
# CHEMICAL DATA
_chemical_name_common ?
_chemical_formula_moiety ?
_chemical_formula_sum ?
_chemical_formula_structural ?
_chemical_formula_analytical ?
_chemical_formula_iupac ?
_chemical_formula_weight ?
_chemical_melting_point ?
#==============================================================================
# CRYSTAL DATA
_symmetry_cell_setting ?
_symmetry_space_group_name_H-M ?
_symmetry_space_group_name_Hall ?
loop_
_symmetry_equiv_pos_as_xyz #<--must include 'x,y,z'
?
_cell_length_a ?
_cell_length_b ?
_cell_length_c ?
_cell_angle_alpha ?
_cell_angle_beta ?
_cell_angle_gamma ?
_cell_volume ?
Example 2: (a) is a portion of a defective CIF that does not list a complete set of unit-cell parameters. (b) is the output from a query intended to retrieve the unit-cell dimensions.
_symmetry_cell_setting monoclinic
_symmetry_space_group_name_H-M 'C 1 2 1'
loop_ _symmetry_equiv_pos_as_xyz
x,y,z
1/2+x,1/2+y,z
-x,y,-z
1/2-x,1/2+y,-z
_cell_length_a 16.734(5)
_cell_length_b 7.830(4)
_cell_length_c 12.028(5)
_cell_angle_beta 119.12(3)
_cell_volume 1376.8(10)
(a) The generating application has omitted the orthogonal angles of a monoclinic cell.
_symmetry_cell_setting monoclinic
_symmetry_space_group_name_H-M 'C 1 2 1'
loop_
_symmetry_equiv_pos_as_xyz
x,y,z
1/2+x,1/2+y,z
-x,y,-z
1/2-x,1/2+y,-z
_cell_length_a 16.734(5)
_cell_length_b 7.830(4)
_cell_length_c 12.028(5)
_cell_angle_alpha ? # requested item not present
_cell_angle_beta 119.12(3)
_cell_angle_gamma ? # requested item not present
_cell_volume 1376.8(10)
(b) Output from the program QUASAR, given a request list designed to extract the cell parameters. Note how the QUASAR application annotates the missing values.
Inapplicable value
The value '.' represents an inapplicable value of the quantity. It may be inappropriate or meaningless (as in the case of _refine_ls_hydrogen_treatment reported for an inorganic salt containing no hydrogens). It may represent a value omitted intentionally, for some good reason. Or, in some circumstances (described in the _definition portion of the appropriate CIF dictionary entry), it may indicate the use of a default value.