Glossary of terms used in MTDATA
The definitions given
below are intended to explain the meaning of words as used in MTDATA
are not necessarily suitable for general use.
"Substance" is a
general word denoting
any quantity of material, ranging from an atom or ion upwards,
having a definite identity. In MTDATA it is
to mean "pure substance" which is restricted to elements,
stoichiometric compounds, molecules and ions.
State of matter
For most purposes the
states of matter are solid,
liquid or gas. Solids are normally crystalline but may also be
amorphous or glassy with only short range order. A fourth state
of matter, plasma, ie ionised gases, is not treated differently
in MTDATA from unionised gases. On the other hand
phase is modelled in a rather distinct way from other types of
A phase is chemically
and structurally homogeneous
and is distinguishable from other phases by its name, structure
and properties (mechanical, physical and chemical). Crystalline
phases have a definite periodic structure, often with sublattices.
In MTDATA two sets of substances with identical
but radically different chemistry, eg MgO and NiO on the one hand
and NaCl and KCl on the other, can be prevented from entering
the same phase by tagging their constituents with different names,
eg <halite1> and <halite2>. The angular
used to denote phase names. Substances that mix must be described
by the same model, qv.
A single phase can
unmix into two or more phases
with the same name. For example an oxide melt can separate into
silica-rich and silica-poor regions, both described by the same
name and mathematical model.
An element cannot be
broken down to a simpler chemical
form by non-nuclear processes. The phase is not defined and the
atoms of the element may be present individually or grouped into
molecules, eg O2 and As4.
the electron is treated for most
purposes as an element with the symbol /-. Positive charge is
represented by /+ to avoid negative stoichiometry numbers.
Isotopes of the same
element have the same atomic
number but differing atomic weights. For a discussion of their
representation within MTDATA see information about
the ACCESS module.
An atom is the
smallest possible state of division
of an element
A chemically bonded
group of similar or different
Isomers are molecules
with the same formula but different
structures. In the THERMOTAB and UTILITY modules, in order to
distinguish isomers, a tag is appended after the phase name
eg <g,cis>. However, when retrieving data from databases
and viewing the list of substances,
a different convention is used, whereby the tag is appended to the
formula by an underscore character
eg C2H2Cl2_1,1<g>, C2H2Cl2_trans<g> or
An ion is an atom or
molecule carrying an electric
A species is an atom,
ion or molecule. The word is
normally used to describe identifiable constituents of phases
for example a gaseous molecule or an ion occupying a sublattice.
A compound is composed
of at least two different
elements. The phase is not defined. A compound in crystalline
form may be made up of individual molecules or it may have extended
ionic, covalent or metallic bonding. For example the overall
of rock salt is governed by the fact that there are equal numbers
of Na+ and Cl- ions on
The use of the word compound normally implies a stoichiometric
composition (ie the amounts of the elements are in simple ratio).
A chemical system is
defined by a set of chemical
entities known as components. In the simplest case these are the
elements comprising the system. However, they may also be compounds
of these elements, in which case they may be fewer or greater
in number than the number of elements. For the example of the
three elements C, H and O, the number of components might be 1,
ethanol; 2, ethanol-water; 3, C-H-O; or 4,
A common implication
of defining a system in terms
of components other than the elements is that the only substances
present are the named components but this is not the case in MTDATA
unless other substances are removed. For example naming a system
as 'C2H5OH,H2O' would allow data to be retrieved not only for
ethanol and water but also for ethene, C2H4,
and diethyl ether, (C2H5)2O,
since these can be formed by linear combination of C2H5OH
and H2O. On the other hand ethane, C2H6,
cannot be formed by a combination of ethanol and water and would
therefore be excluded.
The Na-K-Cl system
could also be defined as the NaCl-KCl-Cl2
system, the same set of substances would be retrieved from the
database. However, substances and overall compositions containing
more Na+K than Cl would have negative amounts of Cl2.
Negative amounts of components are thus allowed but not negative
amounts of elements in the overall composition of the system.
If the system were
defined as KCl-NaCl this would
impose the constraint that the total amounts of Na+K and Cl were
equal. It would also imply that no substances were present containing
more or less Na+K than Cl. If the data for KCl and NaCl were modelled
by taking account of the existence of ions, it would be necessary
to specify the system as 'NaCl,KCl,Cl/-' to allow data for
ions, alone or in neutral combination, to be retrieved.
Systems may be closed
or open. In closed systems
the total amount of the components is fixed, whereas in open systems
the composition can adjust to meet some external constraint. In
closed systems the amount of electric charge must be zero and
hence charge is not a component of the system. Open systems on
the other hand are not realistic. For example it is common practice
to consider systems in which the pH is low but where there is no
anion such as ClO4- to
balance the charge. This convention
is implicit in many diagrams drawn using COPLOT and can also be
adopted in MULTIPHASE.
The meaning of
"component" is implied in
the definition of a system given above. Different phases in the
same system cannot have an independent set of components.
The word "unary" is
used to describe the
constituents of a phase to which data can be assigned. For example in a
liquid phase the unaries
might be H2O<liquid> and C2H5OH<liquid>.
The data for these unaries are those of the pure liquids. Unaries
are not necessarily experimentally accessible. For example
is unstable but data for it are required to model the solution
of nickel in the bcc phase of steels. Moreover, to meet the
of models for ionic phases with sublattices a unary may carry
charge. For example the formation of an inverse spinel AB2O4
could be modeled by the mixing of the two unaries A3+(A3+)2O4<spinel>
which respectively have charges of +1 and -1 and therefore have
no independent existence (see Reference states).
A solution is a
homogeneous mixture within a single
phase. Local ordering may be present. A phase that includes a
variable proportion of unoccupied sites is also a solution but
the vacancies do not constitute a component. The data for solutions
are defined by reference to the unaries from which they are made
using the model and data describing the ideal and non-ideal mixing
between these unaries.
The word "model", as
it applies in MTDATA,
is given to any mathematical description of the properties of
a substance as a function of one or more variables such as temperature,
pressure and composition. Within a single phase the same model
must be used to describe the mixing between all binary and, if
necessary, higher order combinations of unaries. This entails
that care must be taken when developing data for a multicomponent
system that models and reference states are consistent.
The models used in MTDATA
in a modular way in the software, making it feasible to add new
models. "Model" may, under some circumstances, also have a more
definition in which the parameters in the mathematical description
are given definite numerical values.
All data normally
supplied with MTDATA for
unary substances are referred to the enthalpy of the elements
from which they are composed at 298.15 K. The symbol for
the Gibbs energy is (G-Hser).
Data for charged
unaries must be referred to a defined
state which is unique for the phase. In the well established case
of aqueous solutions, the reference is H+<aqueous>,
which by convention is given zero values for G, H and S. This
has the effect of defining a notional aqueous electron with the
same data as 1/2 H2<gas>.
The phase rule
The phase rule is P +
F = C + 2, where P is the number
of phases, F is the number of properties of the system that can
be varied independently and C is the number of components. Thus
in the unary system H2O, at the triple point
ice, liquid water and gaseous water coexist, the intensive properties,
namely the pressure and temperature, of the system are completely
defined. Only the amounts of the phases and other extensive quantities
In MULTIPHASE the
phase rule is not invoked explicitly.
The criterion that the system is at a minimum Gibbs energy is
sufficient to ensure that the rule is obeyed.