ORNL-1771 - Oak Ridge National Laboratory

x-ray diffraction unit io within 5°C of the melting
points. No transformation was observed in either
salt to within 2 or 3OC of the fusion temperature.
The volume changes for these solids from 25% to
the fusion point were 12% for CsBr and 11.2% for
Csi. Comparison of the molar volumesof the solids
with volumes extrapolated for these molten salts2'
down to their respective melting points shows that
an increase of approximately 27% tokes place on
melting. These data indicate that a structural
change, similar to the lowering of the ionic coordination
from 8 to 6 in solid CsCI, occurs on fusion.
NaF-ZrF,. It was desirable to determine the
magnitude of the volume change found in complex
salts on melting. Nickel powder was admixed to
a sample of a homogeneous phase having the com-
position of NaF-ZrF, (53-47 mole %)a The lattice
expansion was measured up to 491°C. The data
are listed in Table 5.10. The volume expansion
from 25°C to the melting point is 3.8%. The volume
is an almost linear function of temperature. The
thermal coefficient of volume expansion, CL,,, IS
7.75 x deg-'. Thus the molar volume of the
solid at the melting point of 52OOC gives a density
of 4.00 g/cm3. The latter value is based on a
molecular weight of 1287.5 g, corresponding to the
presence in the unit cell of 6.77 NaF i- 6 ZrF,
molecules26 for this composition, Om extrapolation
-
2Sfnfernatzonul Cntrcul 'rubles, Vol. 3, p 24.
26P. A. Agron and M. A. Bredig, ANP Quur. Prog. Rep.
June 10, 1954, ORNL-1729, p 47.
Run No.
II- 1
-4
-2
-7
-3
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-8
Extrapol ation
PERlQD ENDING SEPTEMBER 10, 1954
of the liquid densityz7 of NaF-ZrF4 (50-50 mole %)
measured at above 6W0C to 520°C, a value of
3.32 g/cm3 for the liquid is obtained. An adiusf-
ment of this to the composition of NaF-ZrF4 (53-47
mole %), by assuming an almost constant molar
volume, lowers the density to approximately 3.2
g/cm3. Thus a relatively large density and volume
chunge of 26% is indicated for this complex salt
on melting. The volume increase may be considered
as indicative of a decrease in ionic coordination
in the fused state similar to that observed in the
CsBr and csl soits.
The ionic coordination in the solid is assumed to
be similar to that in NaUF;,, for which Zachariasen
proposed a fluorite type of packing with equal
coordinations for both the Na' and Zr4+ of the
order of 8 or 9. It seems possible that the volume
increase on melting results from a lowering in the
coordination of the fluorines around Zr4 ', which
might also be considered as radical ion formation
around zirconium; this point requires further study.
Well-formed single crystals which originated from
the central portion of a large melt of NaF-ZrF4
(53-47 mole %) have become available. Single-
crystal x-ray analysis has been initiated in an
attempt to make a complete structure determination,
especially of the positions of the fluoride ions.
27S. I. Coheri md T. N. Jones, A Surnmusy of I)ensity
Measurements on Molten Fluoride Mixtures rmd n Come-
lotion Ilsejul zn Predictin Densities of Fluoride Mixtures
of Know Compositions, 6RNL-1702 (May 14, 1954).
TABLE 5.10. THERMAL EXPANSION OF NoF-ZrF, (53-47 mole X)
Hexagonal Cell
Temperature Dimensions (A) Mol or \bolumo Den si ty*
("C)
25
190
250
278
378
39 5
A9 1
5 20
_I_ ll__....__....-__l
n c
13.79 9.407 309.60
13.84 9.453 313.38
13.87 9.475 315.62
13.89 9.482 316.64
13.90 9.497 3 17.9Q
13.90 9.504 318.14
13.95 9.525 321.18
321.50
(cin 3 (g/cm3)
4.158
4.108
4.079
d.066
4.050
4.047
4.009
*The density calculation is based on a unit cell containing 6.77 NaF and 6 ZrF4 molecules per unit rhombohedral
cell at this composition.
4.005
71