ISSN 0018-151X, High Temperature, 2018, Vol. 56, No. 1, pp. 38–43. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © M.I. Nikitin, E.V. Skokan, 2018, published in Teplofizika Vysokikh Temperatur, 2018, Vol. 56, No. 1, pp. 35–40.
The Thermochemistry of Crystalline M
and Gaseous MAlF
Fluoroaluminates of Alkali Metals, M
M. I. Nikitin* and E. V. Skokan
Department of Chemistry, Moscow State University, Moscow, 119991 Russia
Received September 6, 2016
Abstract—The formation enthalpies of crystalline M
and gaseous MAlF
are confirmed and refined and
the formation enthalpy of KAlF
(cryst.) is estimated for MF–AlF
systems (M is Li, Na, or K) with a mini-
mum in the total saturated vapor pressure.
The data on the thermodynamic properties of com-
pounds in the simplest systems, e.g., the alkali fluo-
ride–aluminum trifluoride system, are required for
the creation of theories that explain the formation of
aluminum ores and the ability of minerals to coexist in
them . At the same time, they are a basis for the
construction of the phase-state diagrams of electro-
lyte-composing systems in the production of alumi-
num and corresponding engineering calculations.
Despite a lengthy period of studies and a great volume
of accumulated information, some inherent contra-
dictions in it exist due to different approaches and
interpretations of initial experimental data.
Thus, for example, the existence of crystalline
was established  (1954), and its thermody-
namic properties that enable the incorporation of this
compound into the NaF–AlF
phase diagram were
estimated  (2008). On the other hand, its thermal
instability was revealed as early as in  and the
decomposition of this compound at room temperature
was also confirmed in  (1963). The thermodynamic
instability of NaAlF
at temperatures below the eutec-
tic temperature presented in the phase diagram  was
shown in  (1986).
According to the data [6, 7], the formation enthal-
pies of molecules of MAlF
(M is an alkali metal),
which are the major compounds of the gas phase of
these systems, appreciably differ from each other.
It seems advisable to consider the set of experimen-
tal data one again to reconcile the final results. In the
further calculations, the thermodynamic functions
and formation enthalpies of reaction participants from
the handbook  are used, unless otherwise specified.
1. CRYSTALLINE NaAlF
was synthesized by heating an
equimolar mixture of sodium and aluminum fluorides
or chiolite  and further fast condensation of the vapor
composed predominantly of NaAlF
A principally important condition of successful
synthesis is a high vapor cooling rate, which clearly
indicates the absence of equilibrium in the reaction
at temperatures below 750 K  and the thermody-
namic instability of NaAlF
The metastability of NaAlF
(cryst.) was most com-
pletely and reliably established by experiment in [4, 5, 9].
Let us cite the basic results of these works.
(cryst.) disproportionates by reaction (1) for
2–3 years at room temperature . The heating of
(cryst.) from 373 to 873 K leads to the irrever-
sible disappearance of the initial phase by reaction (1);
the phase composition was monitored by high-tem-
perature X-ray diffraction . The temperature of 780 K,
at which reaction (1) occurs at an appreciable rate, and
the enthalpy of this reaction = –66.9 ±
7 kJ/mol were determined by differential scanning
calorimetry (the phase composition was monitored by
high-temperature X-ray diffraction) [5, 9].
Hence, the rather long existence of NaAlF
at room temperature is caused only by the kinetic
retardation of reaction (1).
The thermodynamic calculations of stability for
(cryst.) in [3, 5, 9] lead to opposite conclu-
sions, first of all due to the estimative character of its
thermodynamic properties. It is possible to discuss the
reliability and precision of such estimates, but the con-
dition of change in the standard Gibbs energy of reac-
5NaA l F cr y s t .
Na Al F cryst. 2AlF cryst.=+