How to Interpret and Understand Villars Distinct Phase Labels
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by P. Villars, H. Okamoto, and K. Cenzual
Distinct Phase Labels Concept
A phase label is defined by the chemical system and the crystal structure and has
been given a unique name by the combination of its formula and its
This unique name has been used through the “ASM Alloy Phase Diagrams Database” as phase labels in the phase diagrams. In addition, each crystal data entry has been grouped to such a phase label. These distinct phase labels make it possible to link directly any phases in the phase diagrams to its crystallographic data, if any are available.
The crystal structure is defined referring to a prototype,
if known. For not yet (fully) investigated structures, partial
structural information is given if available, e.g. the complete Pearson
symbol may be replaced by t** (tetragonal) or cI* (cubic body centered).
Information about colloquial names and stability with respect to
temperature, pressure, or composition, has been used to assign a phase
label to entries with no structural data.
Phases that crystallize with the same prototype, but are separated by a
two-phase region in phase diagrams, have been distinguished. The same is true for
temperature- or pressure-induced isostructural phase transitions where a
discontinuity in the cell parameters is reported. On the contrary, the terminal
solid solution of (Fe) with bcc structure has been considered as a single phase,
since in some systems there exists a continuous solid solution ranging from
α-Fe to δ-Fe.
Structures with different degrees of ordering have in some cases been considered
separately, in others not, depending on the possibility of assigning
unambiguously one or the other modification to the data sets. More ”detailed”
structure refinements, considering for instance split atom positions, have
often been grouped under the simplest type.
Structure proposals stated to be incorrect in later literature have been
grouped under a phase identifier in agreement with more recent reports. A
Structure data set reporting a hexagonal cell may in such a case, for instance,
be grouped under an orthorhombic phase.
Terminal solid solutions include also solid solutions of interstitial character.
The definition of a prototype applied here makes that a continuous solid
solution may smoothly shift from one type to another. A typical case is the
progressive transition of a phase AxB from a
NiAs type to a Ni2In-type structure
by filling progressively first one A site, then a second one. Refinements
considering one or the other type have here been grouped together.
It follows that there is a certain amount of subjectivity when assigning a phase
identifier, and there may be errors in the assignment. We believe, however, that
this approach represents a substantial advantage for the user.
Chemical Formulae and Phase Label
The chemical formulae have been standardized so that the chemical elements are
always written in the same order, an order that corresponds roughly to the order
of the groups in the periodic system. Chemical units, such as water molecules,
azide ions, etc., are distinguished and written within square brackets.
Deuterium and tritium are considered as distinct chemical elements.
Whenever a prototype has been assigned to the published data, the chemical
formula is written so that the number of formula units per cell is the same as
for the type-defining compound. A phase containing 50 at.% A and 50 at.% B, for
instance, will be called A0.50B0.50 if the structure type is Cu,cF4,Fm3m (Z = 4),
but AB if it is CuAu,tP2,P4/mmm (Z = 1) and A2B2 if it is Cu3Au,cP4,Pm3m (Z = 1).
Such conventions imply a certain hypothesis on the atom distribution in case of
off-stoichiometric formulae. In particular it is necessary to choose between a
formula assuming a structure with vacancies and one with mixed occupation, e.g
between A0.9B and A0.95B1.05. Adding to this the uncertainty on the chemical
composition itself, especially when the authors did not recognize the crystal
structure, this must be taken as a formal way of writing and no claims are made
on its correctness.
Each phase is assigned a phase label, which, in the general case, is a representative
chemical formula, written as described above. A chemical element in parentheses
indicates a limited terminal solid solution, whereas for complete solid
solutions the two chemical elements are written within the parentheses,
separated by comma. Whenever several phases are known for the same chemical
composition, a short code specifying the modification is added. Preference is
given to terms such as rt (room-temperature), ht (high-temperature), lt
(low-temperature) or hp (high-pressure), possibly followed by a digit when
several temperature- or pressure induced phase transitions are known. If only
one modification, stable at room temperature, is known, the field modification
is left blank. The term ht is in principle added for phases that are only
stable above room temperature, and by analogy, the term lt for phases that are
only stable below room temperature. The specification rt means that a phase
stable under ambient conditions decomposes or undergoes a structural
transition before reaching the melting point, or on cooling below room
temperature. In cases where contradictory information is found in the
literature, a specification like cub (cubic), rhom (rhombohedral), orth
(orthorhombic), etc., may have been preferred. Ramsdell notations are used for
polytypic compounds such as CdI. Mineral names are abbreviated to the first
three letters, when several minerals with the same chemical composition are
known. A chemical element followed by a plus sign means that the phase
probably contains more of that element than indicated in the chemical
formula; an additional chemical element written within parentheses has a
The prototype is a well-known concept in inorganic chemistry where
often a large number of compounds crystallize with very similar atom
arrangements. The compilation Strukturbericht started to catalogue crystal
structures into types, named by codes such as A1, B1, or A15. These notations
are still in use; however, nowadays prototypes are generally referred
to by the name of the compound for which this particular kind of atom
arrangement was first identified, i.e. for the types enumerated above:
Cu, NaCl, Cr3Si. This product uses a longer notation, which includes also
the Pearson symbol and the space group number: Cu,cF4,Fm3m, NaCl,cF8,Fm3m,
Cr3Si,cP8,Pm3n. In a few cases several prototypes correspond to the
same code, e.g. several polytypes of CdI have the same notation. A similar
situation occurs for the old, wrong, and the correct structure proposals
for FeB, which have the same Pearson code and space group. In these cases
a letter is added after the type-defining compound, assigned in
chronological order, e.g. the correct FeB type will be referred to as
All data sets with published coordinates are classified into prototypes,
following criteria defined in TYPIX.* According to this definition,
isotypic compounds must crystallize in the same space group, have similar
cell parameter ratios, and occupy the same Wyckoff positions in the
standardized description with the same or similar values of the atom
coordinates. If all these criteria are fulfilled, the atomic environments
should be similar. No distinction is made between structures with fully
and partly occupied atom sites.
When possible, a prototype has been assigned also to data sets
without atom coordinates. The prototype is often stated in the
publication, in other cases it has been assigned. The assigned type may
in some cases be an approximation of the real structure, ignoring for
instance a certain disorder. The exact space group setting to which the
cell parameters refer has been added when this was not published.
The term filled-up is used for unrefined structures that are either partly or
fully filled versions of refined structures. These are treated as unfilled structures.
For example, Ru2Sm has a crystal structure of the MgZn2 prototype. H5Ru2Sm has similar
cell parameters and belongs to the same space group, but the hydride belongs surely to a
different prototype. Therefore to show its structural relation, we classify it as filled-up
* E. Parthé, L. Gelato, B. Chabot, M. Penzo, K. Cenzual and R. Gladyshevskii,
Gmelin Handbook of Inorganic and Organometallic Chemistry, 8th Ed.
TYPIX - Standardized Data and Crystal Chemical Characterization of Inorganic
Structure Types, 4 volumes, Heidelberg: Springer 1993, 1994.
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