Фазовая диаграмма системы Os-V
К оглавлению: Другие диаграммы (Others phase diargams)
Os-V (Osmium-Vanadium)
J.F. Smith
Two rather different phase diagrams have been proposed for the Os-V system by [
66Rau] and [79Sus]. Many of the differences between the two proposed phase
diagrams can be rationalized. The diagram of [79Sus] is based on more
extensive data and is preferred as the assessed diagram.
The two diagrams both show a cph Os-rich phase representing the terminal solid
solution, a Cr3Si-type intermediate phase, a CsCl-type structure, and an
elemental bcc structure corresponding to the V-rich terminal solution at high
V compositions. Thus, basic details of the two diagrams are in rough accord,
but phase boundaries agree only with regard to the Os-rich solvus, which in
the investigation of [66Rau] was determined from the lattice parameters of
quenched samples. This boundary increases from ~28 at.% V at 900 C to ~38 at.%
V at 1570 C. There is also qualitative agreement between the two diagrams
with regard to the liquidus-solidus contours, with both showing a eutectic
reaction near the central portion of the system and a melting maximum in the V-
rich portion of the system.
There are significant differences between the two diagrams with regard to the
temperatures and compositions associated with the invariant reactions and with
the remaining phase boundaries. [66Rau] determined liquidus temperatures by
thermal analysis, whereas [79Sus] determined solidus temperatures by direct
observation of melting. Because data from thermal analysis are more likely to
suffer error from supercooling, the results of [79Sus] are preferred.
The two investigations interpreted the melting maximum in quite different ways,
with [66Rau] taking it to be the congruent melting point of a CsCl-type phase
and with [79Sus] taking it to be an azeotropic melting of the V-rich solid
solution. The difference in interpretations hinges on whether the formation of
the CsCl-type structure is first order. [66Rau] assumed the CsCl-type
structure to represent a separate and distinct phase with a median
stoichiometry of V3Os2 and with the requirement of a two-phase field to
separate it from the V-rich terminal solution. The diagram of [66Rau] was
drawn accordingly and used dashed lines to show a two-phase field whose
existence was unsupported by experimental evidence. [79Sus] made a careful
search for such a two-phase field and was unable to detect it. Their
interpretation of the CsCl-type structure was that it was associated with an
order-disorder reaction whose formation was thermodynamically second rather
than first order. In this sense, the Os-V system is analogous to the Ru-V
system in the V-rich region with the ordering becoming detectable by the
appearance of superlattice lines below some threshold V composition, which in
Os-V is near 78 at.% V.
Though [79Sus] admitted the possibility of a two-phase field between the bcc
and CsCl-type phase above 1500 C, the probability seems small. The
preponderant weight of their data supports the view that the terminal solution
grades into the CsCl-type structure by an ordering reaction without an
intervening two-phase field. In the assessed diagram, the temperature-
composition contour separating their observation of the presence or absence of
superlattice lines is shown by a dashed line. In the investigations of [66Rau]
and [79Sus], the CsCl-type structure was reported not to extend to equiatomic
stoichiometry; rather, the Os-rich limit is indicated to be near 55 at.% V.
[66Rau] and [79Sus] agree that a Cr3Si-type phase occurs nonstoichoimetrically
near equiatomic composition; however, there is a significant difference
between the two diagrams with regard to the temperature and composition ranges
of stability of this phase. The reason may be associated with the fact that
the Cr3Si-type phase tends to metastability and can easily be retained at
lower temperatures with only moderate rates of quenching [79Sus].
Within the limits of 50 to 55 at.% V, the superconducting transition
temperature was reported to decrease from 5.7 to 3 K [79Sus]. Earlier
measurements of the superconducting transition temperature of this phase [
69Bla, 70Spi] fall within this range. In the V-rich region, additions of Os to
V cause the superconducting transition temperature of elemental V to fall
below 1 K at 90 at.% V and to reappear below 60 at.% V in the ordered CsCl-
type region to reach 1.7 K at the Os-rich boundary of 55 at.% V. Low to
vanishing superconductivity between 90 and 60 at.% V is corroborated by the
observation of [63Mat] that a 71 at.% V alloy was a normal conductor to the
lowest temperature of measurement of 0.37 K. The decrease in the
superconducting transition temperature of V with Os additions followed by a
subsequent rise with further Os enrichment is qualitatively similar to the
behavior of Ru additions to V in the Ru-V system [79Sus].
58Kna: A.G. Knapton, J. Inst. Met., 87, 28-32 (1958-59).
63Mat: B.T. Matthias, T.H. Geballe, and V.B. Compton, Rev. Mod. Phys., 35, 1-
22 (1963).
66Rau: E. Raub and E. R”schel, Z. Metallkd., 57, 470-474 (1966).
69Bla: R.D. Blaugher, R.A. Hein, J.E. Cox, and R.M. Waterstrat, J. Low Temp.
Phys., 1, 539-547 (1969).
70Spi: P. Spitzli, R. FlЃkiger, F. Heiniger, A. Junod, J. Muller, and J.L.
Staudenmann, J. Phys. Chem. Solids, 31, 1531-1537 (1970) in French.
79Sus: C. Susz, R. FlЃkiger, J.L. Jorda, and J. MЃller, J. Less-Common Met.,
63, 45-52 (1979).
Published in Phase Diagrams of Binary Vanadium Alloys, 1989. Complete
evaluation contains 1 figure, 2 tables, and 9 references.
Special Points of the Os-V System