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ORNL-4191 - the Molten Salt Energy Technologies Web Site

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The MoF, so obtained has been identified by its<br />

x-ray diffraction pattern established by LaValIe<br />

et aI on <strong>the</strong> material syn<strong>the</strong>sized by o<strong>the</strong>r methods.<br />

This convenient method of preparation, which seems<br />

not to have been used before, has served to prepare<br />

several pure batches of MoFCI.<br />

When MoF, is pumped, in this same way at 15OoC,<br />

fhe reaction has been reportedsv7 to be<br />

We have established that <strong>the</strong> solid product is not<br />

MoF, (as produced in <strong>the</strong> 200°C: reaction), but we<br />

have not yet completed our identification of <strong>the</strong><br />

material.<br />

The compound MoF, has been shown to react<br />

with EiF to form at least two binary compounds.<br />

These materials, whose stoichiometry has not yet<br />

been established, are both birefringent. The mean<br />

index of refraction for compound I is at 1.520,<br />

while its major x-ray diffraction peaks (copper<br />

x-ray target, 20 values in degrees) are at 21.4,<br />

19.4, and 27 0; for compound 11, <strong>the</strong> mean refrac-<br />

tive index is 1.480 and <strong>the</strong> major diffraction peaks<br />

are at 22.3 and 20.3. The stoichiometry and <strong>the</strong><br />

optical and x-ray data will be established as soon<br />

as well-crystallj zed samples are available.<br />

1 A .2 REACTION OF MOLYBDENUM FLUORIDES<br />

WITH MOLTEN LiF-BeF, MIXTURES<br />

C. F. Weaver 11. A. Friedman<br />

The appearance of ''Mo in <strong>the</strong> exit gas from <strong>the</strong><br />

MSRE has suggested <strong>the</strong> possibility that a volatile<br />

fluoride of molybdenum exists in <strong>the</strong> MSKE. We<br />

have accordingly begun an examination of reactions<br />

ol molybdenum fluorides with MSRE fuel and fuel<br />

sol vent mixtures.<br />

Molybdenum hexafluoride is a very volatile<br />

material (boiling point, 3473 and is a very strong<br />

oxidant. The nickel container was rapidly attacked<br />

when MoF, diluted to -50% with helium was<br />

passed through <strong>the</strong> MSRE fuel mixture (in which all<br />

of <strong>the</strong> uranium was as UF,) at 69°C; after 1 hr <strong>the</strong><br />

fuel melt contained 9500 ppm of nickel. A small<br />

quantity of uranium was transported (presumably<br />

as UF,) by <strong>the</strong> gas, but, as expected, no appre-<br />

'G. W. Cody and G. B. Hargreaves, J. Chem. SOC.<br />

1568-74 (1 961).<br />

143<br />

ciable reaction of <strong>the</strong> IvIQF, with <strong>the</strong> melt was ob-<br />

served. When MoF6 was passed through a fuel<br />

mixture to which 0.08 mole 7L UF, bad been added,<br />

<strong>the</strong> UF, was rapidly oxidized to UF,,. A small<br />

quantity of uranium was again observed in <strong>the</strong> exit<br />

gas lines, and violent cornsion of tlie nickel was<br />

observed. The valence state to which <strong>the</strong> MoF,<br />

was reduced was not determined in <strong>the</strong>se experi-<br />

ments. It is probable, however, that <strong>the</strong> molybde-<br />

num was present as ei<strong>the</strong>r molybdenum metal or<br />

MoF,, since MoF and MoF, have not been reported<br />

to exist and MoF,, Mo,F,, and MoF, seem to be<br />

<strong>the</strong>rmally unstable abovc 200°C.<br />

The compound MoF, disproportionates readily6<br />

under a vacuum at temperatures greater than about<br />

6OOOC to form molybdenum and MoF,. However,<br />

this material has been heated under its own pres-<br />

sure in closed nickel and copper capsules at 500<br />

and 710"C, respectively, for periods in excess ot<br />

ten days without disproportionation. In addibon,<br />

it has been shown that MoF, at 500 mm will react<br />

at 560T with molybdenum to form MoF,. Conse-<br />

quently, an equilibnum<br />

2NIoF, e N I o t MoF,<br />

must exist in this temperature range with MoF<br />

pressures of a fraction of an atmosphere. When<br />

MoF, was heated in a nickel capsule to 500°C in<br />

<strong>the</strong> presence of 2LiF.ReF' (SO-SO mole %), <strong>the</strong><br />

reaction<br />

21CloF3 + 3Ni 4 3NiF, c 2M0<br />

occurred. Apparently <strong>the</strong> 21,iF.BeF liquid dis-<br />

solved <strong>the</strong> protective coating on tlie nickel wall.<br />

Repeating <strong>the</strong> experiment with copper indicated<br />

that much less corrosion occurred. In this case<br />

<strong>the</strong> molybdenum was found both as MoF, and as<br />

<strong>the</strong> 1.480 refractive index LiF-MoF compound of<br />

unknown stoichiometry. The reaction<br />

MoF, (1 mole %) + 3UF, (4 aole %)----:* Mo t 3UF,<br />

was demonstrated to proceed essentially to comple-<br />

tion at 500°C in a copper container. The reverse<br />

reaction was not observed, although it is possible<br />

that equilibrium was achieved wilh very small con-<br />

centrations of MoF and IIF ,.<br />

Present information suggests that <strong>the</strong> following<br />

events (equations unbalanced) may be significant<br />

in <strong>the</strong> kinetics of <strong>the</strong> reduction of MoF, to molyb-<br />

denum metal by UF,:

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