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Phase equilibria in the vanadium-oxygen system
Journal of the Less-Cownon Metals Elsevier Sequoia S.A., Lausannc - Printed in The Netherlands
PHASE
EQ~?ILIBRIA
J. S. ANDERSON
AND
IN THE
A.
s.
~~ANADI~J~~-OXYGEN
SYSTEM
KHAN*
Iwuga?zic Chevnzstvy Laboratory, Oxfiwd UGversity, 0,rfoud (Gt. Britain) (i
Phase equilibria in the VZO~-VOZ system were established by varying the oxygen partial pressures at a number of temperatures. The phases, V~OJ, V407, V509, V&la and V7Oia, have been shown to exist as stable phases of definite composition up to 1423’K. Oxygen pressures in equilibrium with the vanadium oxides have been determined by equilibration with CO/COZor COZ/& mixtures, and the standard free energies of the various vanadium oxides were calculated. Revised free energy data are given for the reaction VzOa+ jO~=zV02. The results are also used to construct the vanadium-oxygen phase diagram between the compositions VOi.j,, and VOz at the range of temperature studied.
Grossly non-stoichiometric compounds have long been knownl, and the stability of such compounds has usually been described by the Schottky Wagner model of the defect solid state and its elaborations. However, recent X-ray phase studies have revealed successions of ordered intermediate phases, with structures closely related to each other and to the parent from which they derive, in phase diagrams where, previously, non-stoichiometric compounds of broad range had been reported. Such successions of intermediate phases arise in two different ways: by defect ordering and by crystallographic shear. Defect ordering is common in phases related to the NiAs and CaFs structures. For a proper understanding of the nature of nonstoicl~iometric conlpounds, and their relation to ordered phases of definite structure and composition, it is necessary to establish the equilibrium phase relations and the thermodynamics of the systems concerned. There is a good deal of information available about the thermodynamics of some systems in which intermediate phases arise by the ordering of vacancies or interstitials-e.g., phases related to the fluoritcl-3 and nickel arsenide structures4‘5. By contrast, there is a lack of kno~~Iedge of the thermodynamics of systems in which intermediate phases arise by the operation of “crystallographic shear” 6. In a shear structure, the cation co-ordination number usually remains unchanged but the anion co-ordination number is locally increased to accommodate a * Present address: Department of Chemistry, Iiniversity of Dacca, Dxca-2, East Pakistan.
J. S.
210
ANDERSON,
A. S. KHAN
reduced non metal-metal atom ratio. In the simplest cases all the shear planes are parallel and regularly spaced. The resulting strnctures may be regarded as built up from regular slabs of the parent structure, spliced together with a relative displacement at the shear planes. it is predictable that the free energy difference between successive members of a homologous series must be small and, as 1%increases, the lattice energy and thermodynamic properties must approach more closely to those of the parent compound (12-+ co). Examples of shear planes are found in the remarkable series of homologous oxides of molybdenum, tita.nium, and vanadium, discovered by MAGN~LI and his collaboraters@-11: Mo~O@~-~, TinO@n-l, 12=4,5 . . . .. . . . . . . .. . . 10, and V,O@,-1, $21145 ,........... 8. MoaO@,+r phases are derived from the ReOa type, and Ti,O@,-1 and VnO@n_r from the rutile-type parent structure. Not only have the thermodynamics and phase stability of shear structure oxides not been examined in detail, but it is of some importance to establish whether they persist as well-defined, ordered phases up to the highest temperatures, or whether they undergo complete or partial disordering, resulting in a detectable composition range. ROY AND KACHI~ examined the stability of the oxides V,O@,_i at 1073”-1673’K under controlled oxygen pressures and concluded that V@OS has a significant composition range (VO i.~-VOi.71), but that the shear structure phases have no detectable composition range. It was therefore decided to carry out equilibrium oxygen pressure measurements across the series &O@,-1 (M=V, Ti) derived from the rutile structures. This paper reports the results of measurements on the vanadium oxides. EXPERIMENTAL
Oxygen dissociation pressures in the vanadium III-IV oxides are neither accessible to direct measurement nor conveniently established by the binary CO-CO@ or H@-H@Obuffer mixtures. Carbon monoxide (at 500°C) effects complete reduction of VO@to V@O@,without formation of the intermediate oxides VnO@n--l,and the available the~ochemical data indicate that their equilib~um pressures fall into the range IO-@10-10 atm, corresponding to large CO@:CO ratios, not readily maintained by gasmixing methods. In this work, the requisite chemical potentials of oxygen were established by (a) use of an auxiliary metal-metal oxide buffer and (b) by using COPH2 mixtures. For (a), carbon dioxide was circulated in a closed system over an oxide buffer consisting of pellets of well-mixed AnalaR nickel metal and nickel oxide, in the uniform temperature zone of a nichrome-wound furnace. The temperature of the buffer mixture was controlled in the range 7oo”-looo°C. At equilibrium, the COz: CO ratio is determined by the standard free energy change in the reaction : Ni+$O@=NiO, at the temperature of the buffer, TB. The equilibrated gas was passed over a biphasic mixture of consecutive phases in the intermediate oxide system (V@O@+V@O5; V@OS+V~O@ etc. ; sample weight about zoo mg) contained in a platinum bucket suspended from an electromagnetic semimicrobalance. The full-scale deflection on the indicating meter for this J. Less ~ornrn~~ M&k,
22
(1970)
209-218
PHASE EgUILIBRIA
IN v-0
211
instrument was IO mg; weight changes could be read to k0.05 mg. With the weight of sample used, this corresponds to an accuracy of +0.0025 in x, in the composition of a sample VO,. The sample bucket was suspended in a mullite tube in a Crusillite furnace, which gave a 3 in. zone of uniform temperature in the range goo”-r200°C. The procedure was to hold the temperature of the sample constant at some desired value, T,, and to adjust the temperature of the buffer Tb by small amounts to that value at which the sample neither gained nor lost weight. The COZ:CO ratio established at Tt, then sets up the equilibrium oxygen pressure at T,. This method gave satisfactory data over a restricted temperature range (1173~-1z&3”K) for co-existence equilibria of biphasic VZOS/VSOSmixtures. Oxygen transport between sample and buffer was excessively slow, however. In biphasic mixtures of the shear phases, the total change in oxygen content to convert the mixture to the higher or lower oxide was small, and the slow rate of recovery from an out-of-equilibrium state made it difficult to maintain defined equilibrium conditions. Equilibrium measurements on the oxides V305--V~,Oii were therefore carried out by method (b). Carbon dioxide, dried through a tower, was flowed at a constant rate through the system. A constant gas pressure was obtained by continuously bleeding the gas off to the atmosphere through a head of a few centimetres of Apiezon oil, and the gas flow was monitored by a capillary flowmeter. Hydrogen was generated by the electrolysis of 30 7; potassium hydroxide in a cell with nickel electrodes, and the rate of generation was controlled and measured by the electrolysis current. After passage through a liquid nitrogen trap, a tube furnace containing copper strips at 5oo”--6oo’C and a magnesium perchlorate drying tower, the hydrogen was introduced by means of a fine-jet bubbler, filled with Apiezon oil, into the carbon dioxide stream. A gas-mixing bulb with a residence time of about IO min secured uniformity of the gas mixture before it was delivered to the system. The oxide sample, as in (a) was contained in a platinum bucket suspended from the semimicro balance in the Crusillite furnace. The procedure was to hold the sample at constant temperature and to vary the COZ:HZ ratio, progressively, by control of the electrolysis current. The weight, and thence the composition, of the sample in equilib~um with the gas mixture were thus found from the final constant weight attained at each ratio of COz:Hz. At the conclusion of each run the sample was removed and the phase composition checked by X-ray diffraction. MATERIALS
The starting materials have been prepared as follows: (a) varcadiumpentoxide VzOs was prepared by the thermal decomposition of AnalaR grade ammonium metavanadate. (b) Vanadium sesqwioxide VzOa was prepared by the reduction of vanadium pentoxide in a silica boat with pure, dry hydrogen in a tube furnace. The reduction was carried out in two
J. S. ANDERSON,
212
A. S. KHAN
stages; first at 600°C for about 8 h and then the temperature was raised to 900°C and the reduction was allowed to continue for about three days. (c) Va~adiz4m dioxide Powdered VsO3 and VZOS, in equimolar quantities to give the dioxide, were thoroughly mixed and pressed into pellets and annealed in evacuated quartz ampoules in two stages; first at 600°C for about two days and then at goo”C for between five and six days. The initial low-temperature annealing was necessary to prevent the melting of V~05 which attacks the quartz. (d) Preparation of oxides between compositions VOI. B7 and VOz 8 The starting materials for the preparation of oxides in this composition range were vanadium metal and vanadium dioxide. The samples of vanadium dioxide and metal were weighed out to give the appropriate oxide and were mixed thoroughly. The pressed pellets were annealed in evacuated quartz ampoules at goo”C for between seven and fourteen days and then samples were rapidly quenched in cold water. The phase analysis of various oxides of vanadium was carried out by X-ray powder photography by the use of an Enraf-Nonius Guinier camera. The samples were mounted on scotch tape. Potassium chloride was mixed with the sample as an internal standard. Nickel filtered CUKLXradiation was used. The phases present in the samples were identified by comparing positions and intensities of the lines in the diffraction patterns with those from the literatures. Atialysis The samples were analysed gravimetrically by oxidising in air in a platinum crucible. A small, known amount of ignited ma~esium oxide was added to facilitate complete oxidation, otherwise molten VzOs forms a protecting layer which prevents complete oxidation. The sample was weighed as magnesium vanadate. RESULTS
Eq~librium oxygen pressures over the phase-pair V305 + VzOs were measured in the temperature range 1173~-1248~K using CO%at I atm pressure as circulating gas and a Ni-NiO buffer. As noted above, the kinetics of the reaction were very slow. Kinetics of the reactions of the oxide with the gas phase were very rapid when COz/H2 mixtures were used at sufficiently high temperatures to permit the attainment of internal equilibrium in the solid. At 1150~--1450~K, equilibrium between the solid phase and the gas phase was attained within one hour during reduction and about two hours during oxidation. Equilibrations of the phases between V305 and VSOII were carried out in this way, but it was not practicable to extend the range of equivalent oxygen pressures to the highest known oxides in the series, V,Ora and v8015.
Reversibility of the reactions was tested at 1348°K and 1398°K by reduction runs on VO2. When the composition of the product was VOI.WI the COa:Hz ratio was gradually increased, and the change of composition followed as the oxygen activity increased. The paths followed during reduction and oxidation were coincident. At other temperatures, data were recorded during reduction only. Thus no hysteresis phenomena were observed in the isothermal interconversion of these MagnCli phases, J. Lsss-Cow#wn
Metals,
22 (1970) a~~--218
PHASE EQUILIBRIA
213
IN v-0
unlike such systems as the praseodymium to show hysteresis hysteresis
in both isothermal
occurs
readily
when
the
and terbium and isobaric
structural
oxides which have been found oxidation
relationship
and reduction. between
phases
Such is so
marked that overgrowth of one phase on the other is possible. True equilibrium data are then not obtained. That the Magneli phases did not show hysteresis indicates that the shear planes can move relatively readily through the crystals, and suggests a different mechanism
for oxidation-reduction
than that which operates in the defective
fluorite structure. -r
%;
T
I I
z
200
it :j
!
100
i
1423’
1323’
C
15
n
in VO,
Fig. I Composition of oxides VO, as function of equilibrium Cot/H? ratio at constant temperature.
In the isotherms
(Fig. I), the vertical
segments--i.e.,
equilibria
in which the
composition of the solid was independent of PO,--represent the existence of discrete compounds with an indetectable narrow homogeneity range. It follows that these are fully ordered phases. Horizontal segments represent coexistence conditions for stoichiometrically defined phase pairs. Interpolated values, judged from the onset of reactions,
involve minimal
one vertical
uncertainties
in the oxygen potentials.
Change-over
from
segment to another was sharp. Where, as for VsO9 and V~oii, coexistence
COZ: HZ ratios
have been interpolated,
additional
information
X-ray analysis: products with compositions intermediate were always biphasic mixtures of those compounds. Thermodynamw
was obtained
between
from
VsO9 and VsOii
results
For the reduction
of the data, the values assumed are:
for the reaction Ni f 402 = NiO
(I)
hG~=57,950+3.45T
log T -32.3T;
for the reaction co + $02 = CO&
(4
equilibrium constants were interpolated from the data compiled by the National Bureau of Standards20 For equilibration with COz/Hz mixtures, the equivalent oxygen pressure may be derived from eqn. (2) through the relation
(pco,) _ (pHl)i
pco, PCO
I+ Pco,/Pco qGv(q
+
Pco,/Pco
1. J. Less-Common
Metals,
22
(1970)zag--218
J. S. ANDERSON, A. S. KHAN
214 (P&I,
(PHJI are the initial
ENTROPY
AND
ENTHALPY
Reaction
FOR
of CO2
REACTIONS
Temperature AHo, range (OK) (kcal)
6VzO3+ Oz= 4V305 8VaO5 + OS= 6V407 1oV407+02=8V509 12V~09+02=IOVsO11 J. Less-Common
CHANGE
pressures
1173-1423
1273-1423 1323-1423 1348-1423
Metals, zz (1970)
-IO8.2-&7.2
- 92.2+13.4 -120.5~16.7 - 1238&26.o
209-218
OF VANADIUM
AH/atom V (kcal) -9.0 fo.6 -3.84f0.55 -3.oIlto.40 -2.06fo.43
OXIDES
WITH
OXYGEN
AS/atom V (e.u.)
ASO (e.u.7 -37.4f4.0 -28.0f
-54.5fI2.0 -5I.3fI7.5
-_3.11*0.33 8.0
-1.16&0.33 -1.36&0.50 -0.85&0.29
PHASE EQUILIBRIA
IN V-0
21.5
log PO, are reasonably accurate, the restricted range of temperature over which the controllable CO2: H2 mixtures could be used leads to rather larger uncertainties in the derived quantities ARo, and ASo,. THE STANDARD THERMODYNAMIC FUNCTIONS OF VANADIUM OXIDES
Among the vanadium oxides, V205, V305, V407, V500, V&n and VO2, only the standard free energies of formation of V202 and VO2 have been reported. The standard free energies of formation of v203 and VO2 have been compiled by COUGHLIN~~ and ELLIOT
AND GLEISER~~. They
based
their calculation
on heats of formation
of
and V02 from SIEMONSEN ANDULICH’S~~ data and the entropy data from KELLY AND KINGLY.According to COUGHLIN,the free energy equations may be written as follows : V3O3
2V(C) + 8 02(gaS) hG”T
=
2V(C)
+202
AG”T=
-
=v203
(c),
291350 + 56.49T, (600”~2000°K) (gas)
(Cal) ;
=v204(c),
-335000+73.24T,
(600-181S°K) (Cal).
KUBASCHEWSKIet ~1.16 have compiled the free energy data from SPENCER AND JUs~IcE’si7
measurements
for the reaction:
4VO3 = 2V203 + 03, based on the supposed equilibrium
and
vo2-co-co2-v2o3,
gave the expression : AGO(T) =102,800-33.5
T (1020”-1180°K).
The sample of V02 used both SIEMONSEN AND ULICHand SPENCERANDJUSTICEwas almost certainly impure, since they prepared VO2 by oxidising v203 in air at relatively low temperature. experience
It was not examined
of the present
authors
by X-ray
methods
shows that oxidation
of
for phase purity v203
and the
does not yield V03
in pure form. Further, constitute
it has been shown
an equilibrium
in the present
phase pair. Therefore,
study
apart
free energies of the oxides, V305, V407, V500 and
that V03
and V&3
from presenting
do not
the standard
V5011, an attempt has been made
to estimate the total free energy of the reaction : v&3 + $02 = 2V02
(4)
The standard free energies of the following reactions, (I)
VO1.50+0.0835 02 =VO1.67
(2) vol.6,+0.04 (3) VOl.7.s+O.O25 (4) V01.30+o.o~65
02
=vo1.75
02
=VOl.so
O2 =VO1.gg
have been calculated in the temperature range I173°-14230K. Data are reported in Table II. The standard free energies of formation per total gram atom of the oxides V305, V407, V502 and V2Oii have been obtained by a conventional plot of free energy VS. composition. The free energy of formation of V203 is known with reasonable certainty, from COUGHLIN’S compilation13 and was used as a datum point. The rest of J. Less-Common
Metals,
22 (1970) mg--218
216
J. S. ANDERSON,
TABLE
A. S. KHAN
II
STANDARD
FREE
ENERGIES
(CELS)
OF KEACTION
OF VANADIUM
OXIDES
“K
Reactions
1173”
2173”
1323’
1373”
1423”
vo1.50+o.08350z=vo1.67
- 5400 h4o
-5oao**
-4goo**
-4760**
- 4600
- 2470*
-2380
-2310
*2o
f2o
- 1320*
- 1260
V01.67+0.0402=V01.75
V01.73+o.0250~=VO~.~~
- 1450*
130 -2280 zt2o -1190
&IO
vo1.80+o.o16502==vo1.83
-
950:
-
860*
-
820*
-2200 *20 -1120 *2o
iI0
-
780 f1o
-
730 Zt1o
* Extrapolated. ** Interpolated.
the diagram was constructed from the partial molar free energy data obtained in this study. The intermediate oxides lie very close to each other along the composition axis; the difference in free energy per gram atom between successive phases is small, and differences in the relevant partial molar quantities are found to show a clear trend to zero. Irrespective of the uncertainties in partial molar free energy associated with assumptions about equilibrium over the phase pairs VaOil + V7013, V7Ois + VBOW.. VsO15 (or some higher shear phase) +VO2, the error involved in extrapolating the integral free energy of formation to VOZ is small, and is estimated at not more than + 200 Cal/g.-atom. The errors in the values of standard free energies of formation of the oxides are dependent chiefly on the accuracy of the standard free energy of formation of V203 and, to a much lesser extent, on the uncertainties in the measured values of AGO,. It has been found that the uncertainty in the AGO, values in the present data generally leads to an error not greater than a few tens of calories (per total g.-atom of the oxide) in the standard free energies of formation of the oxides. The standard free energy of the reaction, v203
+
402
=
zvoz
(5)
was calculated from the standard free energies of formation of VOZ (this study) and V203 (COUGHLIN),and is reported in Table III along with the values obtained from literature. The present values are slightly more negative than those of COUGHLIN.He based his calculation on the heats of formation of VZO3and VOZ from SIEMONSENAND ULICH and entropy from KELLY AND KING. The heat of formation of VOZ reported by SIEMONSENAND ULICH is unreliable because their measurements were carried out with VO2 of uncertain purity. The data now reported for the reaction (5) should be more accurate than those compiled by COUGHLIN. Both COUGHLIN’Sand the present data differ substantially from the free energy for reaction (5) derived by KUBASCHEWSKIet al. This is probably to be associated with the false equilibrium V203-VO2-02 on which KUBASCHEWSKI’Svalue is based. J. Less-Common
Metals,
22
(1970)
zag--218
PHASE
EQUILIBRIA
TABLE
217
IN v-0
III
STANDARD FREE ENERGIES OF FORMATION ENERGIES
OF THE REACTION
VzOa+JOz=
(PER
g.-at.)IN kcal,
OF VANADIUM
OXIDES
AND
FREE
2v02
“K
Oxide
1173”
1273”
1423’
-45.OI
-43.90 -43.10 -42.70
-42.20 -47.20 -40.80
-42.35 -42.IO -40.7~0.20 -40.30
-40.5 -40.25 -38.7o+o.20 -38.46
VZOS (CO~GHLIN) VSOS V40: VSOD V6011 *VOz (estimated) VOz (COUGHLIN,GLEISER) Freeenergy(kca1) This investigation COUGHLIN;GLEISER KuBASCHEWSRI
-44.25 -43.80 -43.60 -42.80 -42.0+0.20 -41.51
VZOS+&OP=ZVOZ
forthereaction
-z6.go+I.o -24.0
-24.7711.1 -22.33 -30.07
-31.75
-21.24*1.0 -19.82 -27.56 _
_ * errorlimitestimated.
One clear consequence V407,
VS09
1423°K
and
at least,
disorder
V~orr, without
transformations.
of the present work is to establish
persist
as stoichiometrically
undergoing This
peritectoid
is consistent
reactions
with
that the shear phases,
well-defined
ANDERSSON’S
range of composition,
Our equilibrium
measurements
as shown by measurements
are consistent
up to order-
observations
these phases showed no detectable variation in cell dimensions. However, ASBRINK et ~~1.1s reported that V205 does have detectable,
phases
or (by inference)
a narrow,
that but
of its cell dimensions.
with this and would indicate,
moreover,
that V205 has a limited field of stability. At 1423X, closely spaced CO2:H2 ratios showed no step at which the composition of the solid was independent of PC,,, corresponding to an abrupt change between two biphasic fields. When PO, over an oxide with the composition composition
crossed
when the composition a bivariant pressures.
(variable
V407
was lowered
by a small
the value VO 1.667 (V,Oj),
amount,
and the reaction
the resulting became
of the solid phase was about VO 1.62. The behaviour composition)
phase spanning
oxide
very slow was that of
a very small range of equilibrium
The slopes of the AG o, vs.T and log PO, ‘us. I/T curves for the V202 + V205
and V205+V~07 phase pairs (Figs. phase pair involves a log extrapolation.
I
and 2) accord with this, although
the former
The status of V205 and V40 7 as high-temperature phases is not finally resolved. ROY AND KACHI~ concluded that V205 had a range of composition from VOi.64 to VO1.X. The phase range probably broadens at higher temperatures, but no variability of composition was evident in equilibrium measurements up to 1273°K. They found, further,
that V407 disappeared
at 1500°K,
which is inconsistent
data, but agree that all the shear phases are oxides
of invariant
with our equilibrium composition.
In a
paper published after the completion of this experimental work, KATSURA AND HASEGAWAl” report measurements on the VO r.s-VO2.0 system at a single temperature (1600°K). They find, and substantiate, a significant range of composition for V203, but report that both V205 and V407 are stable phases of fixed composition, whereas V6Ori has an extended composition range. Their oxygen pressures are higher than the extrapolated values from the present investigation, but consideration of our flow rates, reaction tube dimensions, etc., makes it improbable that the discrepancy can be J. Less-Commolz Metals,
22
(1970) 209-218
218
J. S. ANDERSON,
A. S. KHAN
attributed to systematic errors due to thermal diffusion in our work. The discrepancy remains unexplained. The standard free energies of formation of the vanadium oxides are shown in Table III. The appearance of intermediate oxides in equilibrium tensiometric measurements depends on the difference of free energies among the successive phases. From the above Table is seen that the difference in free energies is a maximum for reaction of VOi.50 to VOr.67, and gradually diminishes for the higher phases. Although the oxides, V701a and VaOr5 have not been studied in this present work they are expected to have a very small difference in the free energies associated with them. The possible high-temperature behaviour of the shear phases remains speculative. There is evidence in the case of ordered intermediate phases based on fluorite structures that order decreases with increasing temperature; stoichiometric phases become detectably non-stoichiometric as the temperature rises and they may eventually decompose to give highly-disordered phases of broad homogeneity range. Experimental evidence reported in this investigation shows no order-disorder transformation for the shear phases nor do they show any detectable stoichiometric variability in the temperature range studied. The behaviour at temperatures close to the melting point is uncertain, however. The solid-liquid equilibria right across the V-O and Ti-0 equilibrium diagrams cover rather a narrow range of temperatures, and the available evidence does not establish whether the intermediate phases melt congruently. Recent work in our laboratory shows that the Magneli phases can be prepared by arc melting, but the phase composition of the product solidifying from a stoichiometric melt-e.g., of total composition V509--points to incongruent solidification. We therefore think it likely that the shear phases persist up to the melting point (although they may have perceptible phase ranges as a result of shear plane disorder at the highest temperature), and melt by a succession of peritectoid reactions. REFERENCES I
2
D. J. M. BEVAN AND J. KORDAS, J. Znorg. Nucl. Chem., 26 (1~64) 1509. L. EYRING, B. G. HYDE AND D. J. M. BEVAN, Phil. Trans. Roy. Sot. London, Ser. A 259 (1~65)
583. 3 L. E. J. ROBERTS AND WALTER, J. Inorg. Nucl. Chsm., zz (1~62) 53. F. GRONVOLD AND E. F. WESTRUM, Acta Chem. Stand., 1.3 (1959) 241. _ _~ : F. GRONVOLD, Acta Chem. &and., zz (1~68) 1219. 6 A. D. WADSLEY, Rev. Pure A+fiZ. Chem., 5 (1~55) 165; A. D. WADSLEY in L. MENDELCORN (ed.), Nonstoichiometric Com$ounds, Academic Press, New York, 1964, p. 98. R. ROY AND S. KACHI, U.S. Army Contract Rep. No. DA28-043 AMC-01304(E), 1964. : G. ANDERSSON, ActaChem. &and., II (1~57) 1641, 1653. 9 S. ANDERSSON AND L. JAHNBERG, Arkiv. Kemi, 21(1~63) 413. IO 0. KUBASCHEWSKI AND J. A. CATTERAL, Thermochemical Data of Alloys, Pergamon, Oxford, II 12
I3 I4 15 16
I7
18
IQ
1956, P. 176. F. D. ROSSINI, D. D. WAGMAN, J. E. KILPATRICK, W. J. TAYLOR AND K. S. PITZER, J. Res. Natl. Bur. Stand., 34 (1945) 143. J. P. COUGHLIN, U.S. BUY. Mines Bull., 542 (1~54). J. F. ELLIOTT AND M. GLEISER, Thermochemistry for Steel Making, Addison Wesley, New York, 1960. H. SIEMONSEN AND H. ULICH,~. Elektrochem., 46 (1940) 141. K. K. KELLY AND E. G. KING, Contribution to Data on Theoretical Metallurgv. X., U.S. _I BUY. Mines Bull. 476 (1949). N. P. ALLEN, 0. KUBASCHEWSKI AND 0. VON GOLDBECK, J. Electrochem. Sot., 98 (1952) 417. H. M. SPENCER AND J. L. JUSTICE, J. Am. Chem. SOL, 56 (1~34) 2306. S. ASBRINK, ActaChem. &and., 13 (1959) 603. T. KATSURA AND M. HASEGAWA, Bull. Chem. Sot. Japan, 40 (1~67) 561.
J. Less-Common
Metals, 22 (1970) 209-218
PHASE
EQ~?ILIBRIA
J. S. ANDERSON
AND
IN THE
A.
s.
~~ANADI~J~~-OXYGEN
SYSTEM
KHAN*
Iwuga?zic Chevnzstvy Laboratory, Oxfiwd UGversity, 0,rfoud (Gt. Britain) (i
Phase equilibria in the VZO~-VOZ system were established by varying the oxygen partial pressures at a number of temperatures. The phases, V~OJ, V407, V509, V&la and V7Oia, have been shown to exist as stable phases of definite composition up to 1423’K. Oxygen pressures in equilibrium with the vanadium oxides have been determined by equilibration with CO/COZor COZ/& mixtures, and the standard free energies of the various vanadium oxides were calculated. Revised free energy data are given for the reaction VzOa+ jO~=zV02. The results are also used to construct the vanadium-oxygen phase diagram between the compositions VOi.j,, and VOz at the range of temperature studied.
Grossly non-stoichiometric compounds have long been knownl, and the stability of such compounds has usually been described by the Schottky Wagner model of the defect solid state and its elaborations. However, recent X-ray phase studies have revealed successions of ordered intermediate phases, with structures closely related to each other and to the parent from which they derive, in phase diagrams where, previously, non-stoichiometric compounds of broad range had been reported. Such successions of intermediate phases arise in two different ways: by defect ordering and by crystallographic shear. Defect ordering is common in phases related to the NiAs and CaFs structures. For a proper understanding of the nature of nonstoicl~iometric conlpounds, and their relation to ordered phases of definite structure and composition, it is necessary to establish the equilibrium phase relations and the thermodynamics of the systems concerned. There is a good deal of information available about the thermodynamics of some systems in which intermediate phases arise by the ordering of vacancies or interstitials-e.g., phases related to the fluoritcl-3 and nickel arsenide structures4‘5. By contrast, there is a lack of kno~~Iedge of the thermodynamics of systems in which intermediate phases arise by the operation of “crystallographic shear” 6. In a shear structure, the cation co-ordination number usually remains unchanged but the anion co-ordination number is locally increased to accommodate a * Present address: Department of Chemistry, Iiniversity of Dacca, Dxca-2, East Pakistan.
J. S.
210
ANDERSON,
A. S. KHAN
reduced non metal-metal atom ratio. In the simplest cases all the shear planes are parallel and regularly spaced. The resulting strnctures may be regarded as built up from regular slabs of the parent structure, spliced together with a relative displacement at the shear planes. it is predictable that the free energy difference between successive members of a homologous series must be small and, as 1%increases, the lattice energy and thermodynamic properties must approach more closely to those of the parent compound (12-+ co). Examples of shear planes are found in the remarkable series of homologous oxides of molybdenum, tita.nium, and vanadium, discovered by MAGN~LI and his collaboraters@-11: Mo~O@~-~, TinO@n-l, 12=4,5 . . . .. . . . . . . .. . . 10, and V,O@,-1, $21145 ,........... 8. MoaO@,+r phases are derived from the ReOa type, and Ti,O@,-1 and VnO@n_r from the rutile-type parent structure. Not only have the thermodynamics and phase stability of shear structure oxides not been examined in detail, but it is of some importance to establish whether they persist as well-defined, ordered phases up to the highest temperatures, or whether they undergo complete or partial disordering, resulting in a detectable composition range. ROY AND KACHI~ examined the stability of the oxides V,O@,_i at 1073”-1673’K under controlled oxygen pressures and concluded that V@OS has a significant composition range (VO i.~-VOi.71), but that the shear structure phases have no detectable composition range. It was therefore decided to carry out equilibrium oxygen pressure measurements across the series &O@,-1 (M=V, Ti) derived from the rutile structures. This paper reports the results of measurements on the vanadium oxides. EXPERIMENTAL
Oxygen dissociation pressures in the vanadium III-IV oxides are neither accessible to direct measurement nor conveniently established by the binary CO-CO@ or H@-H@Obuffer mixtures. Carbon monoxide (at 500°C) effects complete reduction of VO@to V@O@,without formation of the intermediate oxides VnO@n--l,and the available the~ochemical data indicate that their equilib~um pressures fall into the range IO-@10-10 atm, corresponding to large CO@:CO ratios, not readily maintained by gasmixing methods. In this work, the requisite chemical potentials of oxygen were established by (a) use of an auxiliary metal-metal oxide buffer and (b) by using COPH2 mixtures. For (a), carbon dioxide was circulated in a closed system over an oxide buffer consisting of pellets of well-mixed AnalaR nickel metal and nickel oxide, in the uniform temperature zone of a nichrome-wound furnace. The temperature of the buffer mixture was controlled in the range 7oo”-looo°C. At equilibrium, the COz: CO ratio is determined by the standard free energy change in the reaction : Ni+$O@=NiO, at the temperature of the buffer, TB. The equilibrated gas was passed over a biphasic mixture of consecutive phases in the intermediate oxide system (V@O@+V@O5; V@OS+V~O@ etc. ; sample weight about zoo mg) contained in a platinum bucket suspended from an electromagnetic semimicrobalance. The full-scale deflection on the indicating meter for this J. Less ~ornrn~~ M&k,
22
(1970)
209-218
PHASE EgUILIBRIA
IN v-0
211
instrument was IO mg; weight changes could be read to k0.05 mg. With the weight of sample used, this corresponds to an accuracy of +0.0025 in x, in the composition of a sample VO,. The sample bucket was suspended in a mullite tube in a Crusillite furnace, which gave a 3 in. zone of uniform temperature in the range goo”-r200°C. The procedure was to hold the temperature of the sample constant at some desired value, T,, and to adjust the temperature of the buffer Tb by small amounts to that value at which the sample neither gained nor lost weight. The COZ:CO ratio established at Tt, then sets up the equilibrium oxygen pressure at T,. This method gave satisfactory data over a restricted temperature range (1173~-1z&3”K) for co-existence equilibria of biphasic VZOS/VSOSmixtures. Oxygen transport between sample and buffer was excessively slow, however. In biphasic mixtures of the shear phases, the total change in oxygen content to convert the mixture to the higher or lower oxide was small, and the slow rate of recovery from an out-of-equilibrium state made it difficult to maintain defined equilibrium conditions. Equilibrium measurements on the oxides V305--V~,Oii were therefore carried out by method (b). Carbon dioxide, dried through a tower, was flowed at a constant rate through the system. A constant gas pressure was obtained by continuously bleeding the gas off to the atmosphere through a head of a few centimetres of Apiezon oil, and the gas flow was monitored by a capillary flowmeter. Hydrogen was generated by the electrolysis of 30 7; potassium hydroxide in a cell with nickel electrodes, and the rate of generation was controlled and measured by the electrolysis current. After passage through a liquid nitrogen trap, a tube furnace containing copper strips at 5oo”--6oo’C and a magnesium perchlorate drying tower, the hydrogen was introduced by means of a fine-jet bubbler, filled with Apiezon oil, into the carbon dioxide stream. A gas-mixing bulb with a residence time of about IO min secured uniformity of the gas mixture before it was delivered to the system. The oxide sample, as in (a) was contained in a platinum bucket suspended from the semimicro balance in the Crusillite furnace. The procedure was to hold the sample at constant temperature and to vary the COZ:HZ ratio, progressively, by control of the electrolysis current. The weight, and thence the composition, of the sample in equilib~um with the gas mixture were thus found from the final constant weight attained at each ratio of COz:Hz. At the conclusion of each run the sample was removed and the phase composition checked by X-ray diffraction. MATERIALS
The starting materials have been prepared as follows: (a) varcadiumpentoxide VzOs was prepared by the thermal decomposition of AnalaR grade ammonium metavanadate. (b) Vanadium sesqwioxide VzOa was prepared by the reduction of vanadium pentoxide in a silica boat with pure, dry hydrogen in a tube furnace. The reduction was carried out in two
J. S. ANDERSON,
212
A. S. KHAN
stages; first at 600°C for about 8 h and then the temperature was raised to 900°C and the reduction was allowed to continue for about three days. (c) Va~adiz4m dioxide Powdered VsO3 and VZOS, in equimolar quantities to give the dioxide, were thoroughly mixed and pressed into pellets and annealed in evacuated quartz ampoules in two stages; first at 600°C for about two days and then at goo”C for between five and six days. The initial low-temperature annealing was necessary to prevent the melting of V~05 which attacks the quartz. (d) Preparation of oxides between compositions VOI. B7 and VOz 8 The starting materials for the preparation of oxides in this composition range were vanadium metal and vanadium dioxide. The samples of vanadium dioxide and metal were weighed out to give the appropriate oxide and were mixed thoroughly. The pressed pellets were annealed in evacuated quartz ampoules at goo”C for between seven and fourteen days and then samples were rapidly quenched in cold water. The phase analysis of various oxides of vanadium was carried out by X-ray powder photography by the use of an Enraf-Nonius Guinier camera. The samples were mounted on scotch tape. Potassium chloride was mixed with the sample as an internal standard. Nickel filtered CUKLXradiation was used. The phases present in the samples were identified by comparing positions and intensities of the lines in the diffraction patterns with those from the literatures. Atialysis The samples were analysed gravimetrically by oxidising in air in a platinum crucible. A small, known amount of ignited ma~esium oxide was added to facilitate complete oxidation, otherwise molten VzOs forms a protecting layer which prevents complete oxidation. The sample was weighed as magnesium vanadate. RESULTS
Eq~librium oxygen pressures over the phase-pair V305 + VzOs were measured in the temperature range 1173~-1248~K using CO%at I atm pressure as circulating gas and a Ni-NiO buffer. As noted above, the kinetics of the reaction were very slow. Kinetics of the reactions of the oxide with the gas phase were very rapid when COz/H2 mixtures were used at sufficiently high temperatures to permit the attainment of internal equilibrium in the solid. At 1150~--1450~K, equilibrium between the solid phase and the gas phase was attained within one hour during reduction and about two hours during oxidation. Equilibrations of the phases between V305 and VSOII were carried out in this way, but it was not practicable to extend the range of equivalent oxygen pressures to the highest known oxides in the series, V,Ora and v8015.
Reversibility of the reactions was tested at 1348°K and 1398°K by reduction runs on VO2. When the composition of the product was VOI.WI the COa:Hz ratio was gradually increased, and the change of composition followed as the oxygen activity increased. The paths followed during reduction and oxidation were coincident. At other temperatures, data were recorded during reduction only. Thus no hysteresis phenomena were observed in the isothermal interconversion of these MagnCli phases, J. Lsss-Cow#wn
Metals,
22 (1970) a~~--218
PHASE EQUILIBRIA
213
IN v-0
unlike such systems as the praseodymium to show hysteresis hysteresis
in both isothermal
occurs
readily
when
the
and terbium and isobaric
structural
oxides which have been found oxidation
relationship
and reduction. between
phases
Such is so
marked that overgrowth of one phase on the other is possible. True equilibrium data are then not obtained. That the Magneli phases did not show hysteresis indicates that the shear planes can move relatively readily through the crystals, and suggests a different mechanism
for oxidation-reduction
than that which operates in the defective
fluorite structure. -r
%;
T
I I
z
200
it :j
!
100
i
1423’
1323’
C
15
n
in VO,
Fig. I Composition of oxides VO, as function of equilibrium Cot/H? ratio at constant temperature.
In the isotherms
(Fig. I), the vertical
segments--i.e.,
equilibria
in which the
composition of the solid was independent of PO,--represent the existence of discrete compounds with an indetectable narrow homogeneity range. It follows that these are fully ordered phases. Horizontal segments represent coexistence conditions for stoichiometrically defined phase pairs. Interpolated values, judged from the onset of reactions,
involve minimal
one vertical
uncertainties
in the oxygen potentials.
Change-over
from
segment to another was sharp. Where, as for VsO9 and V~oii, coexistence
COZ: HZ ratios
have been interpolated,
additional
information
X-ray analysis: products with compositions intermediate were always biphasic mixtures of those compounds. Thermodynamw
was obtained
between
from
VsO9 and VsOii
results
For the reduction
of the data, the values assumed are:
for the reaction Ni f 402 = NiO
(I)
hG~=57,950+3.45T
log T -32.3T;
for the reaction co + $02 = CO&
(4
equilibrium constants were interpolated from the data compiled by the National Bureau of Standards20 For equilibration with COz/Hz mixtures, the equivalent oxygen pressure may be derived from eqn. (2) through the relation
(pco,) _ (pHl)i
pco, PCO
I+ Pco,/Pco qGv(q
+
Pco,/Pco
1. J. Less-Common
Metals,
22
(1970)zag--218
J. S. ANDERSON, A. S. KHAN
214 (P&I,
(PHJI are the initial
ENTROPY
AND
ENTHALPY
Reaction
FOR
of CO2
REACTIONS
Temperature AHo, range (OK) (kcal)
6VzO3+ Oz= 4V305 8VaO5 + OS= 6V407 1oV407+02=8V509 12V~09+02=IOVsO11 J. Less-Common
CHANGE
pressures
1173-1423
1273-1423 1323-1423 1348-1423
Metals, zz (1970)
-IO8.2-&7.2
- 92.2+13.4 -120.5~16.7 - 1238&26.o
209-218
OF VANADIUM
AH/atom V (kcal) -9.0 fo.6 -3.84f0.55 -3.oIlto.40 -2.06fo.43
OXIDES
WITH
OXYGEN
AS/atom V (e.u.)
ASO (e.u.7 -37.4f4.0 -28.0f
-54.5fI2.0 -5I.3fI7.5
-_3.11*0.33 8.0
-1.16&0.33 -1.36&0.50 -0.85&0.29
PHASE EQUILIBRIA
IN V-0
21.5
log PO, are reasonably accurate, the restricted range of temperature over which the controllable CO2: H2 mixtures could be used leads to rather larger uncertainties in the derived quantities ARo, and ASo,. THE STANDARD THERMODYNAMIC FUNCTIONS OF VANADIUM OXIDES
Among the vanadium oxides, V205, V305, V407, V500, V&n and VO2, only the standard free energies of formation of V202 and VO2 have been reported. The standard free energies of formation of v203 and VO2 have been compiled by COUGHLIN~~ and ELLIOT
AND GLEISER~~. They
based
their calculation
on heats of formation
of
and V02 from SIEMONSEN ANDULICH’S~~ data and the entropy data from KELLY AND KINGLY.According to COUGHLIN,the free energy equations may be written as follows : V3O3
2V(C) + 8 02(gaS) hG”T
=
2V(C)
+202
AG”T=
-
=v203
(c),
291350 + 56.49T, (600”~2000°K) (gas)
(Cal) ;
=v204(c),
-335000+73.24T,
(600-181S°K) (Cal).
KUBASCHEWSKIet ~1.16 have compiled the free energy data from SPENCER AND JUs~IcE’si7
measurements
for the reaction:
4VO3 = 2V203 + 03, based on the supposed equilibrium
and
vo2-co-co2-v2o3,
gave the expression : AGO(T) =102,800-33.5
T (1020”-1180°K).
The sample of V02 used both SIEMONSEN AND ULICHand SPENCERANDJUSTICEwas almost certainly impure, since they prepared VO2 by oxidising v203 in air at relatively low temperature. experience
It was not examined
of the present
authors
by X-ray
methods
shows that oxidation
of
for phase purity v203
and the
does not yield V03
in pure form. Further, constitute
it has been shown
an equilibrium
in the present
phase pair. Therefore,
study
apart
free energies of the oxides, V305, V407, V500 and
that V03
and V&3
from presenting
do not
the standard
V5011, an attempt has been made
to estimate the total free energy of the reaction : v&3 + $02 = 2V02
(4)
The standard free energies of the following reactions, (I)
VO1.50+0.0835 02 =VO1.67
(2) vol.6,+0.04 (3) VOl.7.s+O.O25 (4) V01.30+o.o~65
02
=vo1.75
02
=VOl.so
O2 =VO1.gg
have been calculated in the temperature range I173°-14230K. Data are reported in Table II. The standard free energies of formation per total gram atom of the oxides V305, V407, V502 and V2Oii have been obtained by a conventional plot of free energy VS. composition. The free energy of formation of V203 is known with reasonable certainty, from COUGHLIN’S compilation13 and was used as a datum point. The rest of J. Less-Common
Metals,
22 (1970) mg--218
216
J. S. ANDERSON,
TABLE
A. S. KHAN
II
STANDARD
FREE
ENERGIES
(CELS)
OF KEACTION
OF VANADIUM
OXIDES
“K
Reactions
1173”
2173”
1323’
1373”
1423”
vo1.50+o.08350z=vo1.67
- 5400 h4o
-5oao**
-4goo**
-4760**
- 4600
- 2470*
-2380
-2310
*2o
f2o
- 1320*
- 1260
V01.67+0.0402=V01.75
V01.73+o.0250~=VO~.~~
- 1450*
130 -2280 zt2o -1190
&IO
vo1.80+o.o16502==vo1.83
-
950:
-
860*
-
820*
-2200 *20 -1120 *2o
iI0
-
780 f1o
-
730 Zt1o
* Extrapolated. ** Interpolated.
the diagram was constructed from the partial molar free energy data obtained in this study. The intermediate oxides lie very close to each other along the composition axis; the difference in free energy per gram atom between successive phases is small, and differences in the relevant partial molar quantities are found to show a clear trend to zero. Irrespective of the uncertainties in partial molar free energy associated with assumptions about equilibrium over the phase pairs VaOil + V7013, V7Ois + VBOW.. VsO15 (or some higher shear phase) +VO2, the error involved in extrapolating the integral free energy of formation to VOZ is small, and is estimated at not more than + 200 Cal/g.-atom. The errors in the values of standard free energies of formation of the oxides are dependent chiefly on the accuracy of the standard free energy of formation of V203 and, to a much lesser extent, on the uncertainties in the measured values of AGO,. It has been found that the uncertainty in the AGO, values in the present data generally leads to an error not greater than a few tens of calories (per total g.-atom of the oxide) in the standard free energies of formation of the oxides. The standard free energy of the reaction, v203
+
402
=
zvoz
(5)
was calculated from the standard free energies of formation of VOZ (this study) and V203 (COUGHLIN),and is reported in Table III along with the values obtained from literature. The present values are slightly more negative than those of COUGHLIN.He based his calculation on the heats of formation of VZO3and VOZ from SIEMONSENAND ULICH and entropy from KELLY AND KING. The heat of formation of VOZ reported by SIEMONSENAND ULICH is unreliable because their measurements were carried out with VO2 of uncertain purity. The data now reported for the reaction (5) should be more accurate than those compiled by COUGHLIN. Both COUGHLIN’Sand the present data differ substantially from the free energy for reaction (5) derived by KUBASCHEWSKIet al. This is probably to be associated with the false equilibrium V203-VO2-02 on which KUBASCHEWSKI’Svalue is based. J. Less-Common
Metals,
22
(1970)
zag--218
PHASE
EQUILIBRIA
TABLE
217
IN v-0
III
STANDARD FREE ENERGIES OF FORMATION ENERGIES
OF THE REACTION
VzOa+JOz=
(PER
g.-at.)IN kcal,
OF VANADIUM
OXIDES
AND
FREE
2v02
“K
Oxide
1173”
1273”
1423’
-45.OI
-43.90 -43.10 -42.70
-42.20 -47.20 -40.80
-42.35 -42.IO -40.7~0.20 -40.30
-40.5 -40.25 -38.7o+o.20 -38.46
VZOS (CO~GHLIN) VSOS V40: VSOD V6011 *VOz (estimated) VOz (COUGHLIN,GLEISER) Freeenergy(kca1) This investigation COUGHLIN;GLEISER KuBASCHEWSRI
-44.25 -43.80 -43.60 -42.80 -42.0+0.20 -41.51
VZOS+&OP=ZVOZ
forthereaction
-z6.go+I.o -24.0
-24.7711.1 -22.33 -30.07
-31.75
-21.24*1.0 -19.82 -27.56 _
_ * errorlimitestimated.
One clear consequence V407,
VS09
1423°K
and
at least,
disorder
V~orr, without
transformations.
of the present work is to establish
persist
as stoichiometrically
undergoing This
peritectoid
is consistent
reactions
with
that the shear phases,
well-defined
ANDERSSON’S
range of composition,
Our equilibrium
measurements
as shown by measurements
are consistent
up to order-
observations
these phases showed no detectable variation in cell dimensions. However, ASBRINK et ~~1.1s reported that V205 does have detectable,
phases
or (by inference)
a narrow,
that but
of its cell dimensions.
with this and would indicate,
moreover,
that V205 has a limited field of stability. At 1423X, closely spaced CO2:H2 ratios showed no step at which the composition of the solid was independent of PC,,, corresponding to an abrupt change between two biphasic fields. When PO, over an oxide with the composition composition
crossed
when the composition a bivariant pressures.
(variable
V407
was lowered
by a small
the value VO 1.667 (V,Oj),
amount,
and the reaction
the resulting became
of the solid phase was about VO 1.62. The behaviour composition)
phase spanning
oxide
very slow was that of
a very small range of equilibrium
The slopes of the AG o, vs.T and log PO, ‘us. I/T curves for the V202 + V205
and V205+V~07 phase pairs (Figs. phase pair involves a log extrapolation.
I
and 2) accord with this, although
the former
The status of V205 and V40 7 as high-temperature phases is not finally resolved. ROY AND KACHI~ concluded that V205 had a range of composition from VOi.64 to VO1.X. The phase range probably broadens at higher temperatures, but no variability of composition was evident in equilibrium measurements up to 1273°K. They found, further,
that V407 disappeared
at 1500°K,
which is inconsistent
data, but agree that all the shear phases are oxides
of invariant
with our equilibrium composition.
In a
paper published after the completion of this experimental work, KATSURA AND HASEGAWAl” report measurements on the VO r.s-VO2.0 system at a single temperature (1600°K). They find, and substantiate, a significant range of composition for V203, but report that both V205 and V407 are stable phases of fixed composition, whereas V6Ori has an extended composition range. Their oxygen pressures are higher than the extrapolated values from the present investigation, but consideration of our flow rates, reaction tube dimensions, etc., makes it improbable that the discrepancy can be J. Less-Commolz Metals,
22
(1970) 209-218
218
J. S. ANDERSON,
A. S. KHAN
attributed to systematic errors due to thermal diffusion in our work. The discrepancy remains unexplained. The standard free energies of formation of the vanadium oxides are shown in Table III. The appearance of intermediate oxides in equilibrium tensiometric measurements depends on the difference of free energies among the successive phases. From the above Table is seen that the difference in free energies is a maximum for reaction of VOi.50 to VOr.67, and gradually diminishes for the higher phases. Although the oxides, V701a and VaOr5 have not been studied in this present work they are expected to have a very small difference in the free energies associated with them. The possible high-temperature behaviour of the shear phases remains speculative. There is evidence in the case of ordered intermediate phases based on fluorite structures that order decreases with increasing temperature; stoichiometric phases become detectably non-stoichiometric as the temperature rises and they may eventually decompose to give highly-disordered phases of broad homogeneity range. Experimental evidence reported in this investigation shows no order-disorder transformation for the shear phases nor do they show any detectable stoichiometric variability in the temperature range studied. The behaviour at temperatures close to the melting point is uncertain, however. The solid-liquid equilibria right across the V-O and Ti-0 equilibrium diagrams cover rather a narrow range of temperatures, and the available evidence does not establish whether the intermediate phases melt congruently. Recent work in our laboratory shows that the Magneli phases can be prepared by arc melting, but the phase composition of the product solidifying from a stoichiometric melt-e.g., of total composition V509--points to incongruent solidification. We therefore think it likely that the shear phases persist up to the melting point (although they may have perceptible phase ranges as a result of shear plane disorder at the highest temperature), and melt by a succession of peritectoid reactions. REFERENCES I
2
D. J. M. BEVAN AND J. KORDAS, J. Znorg. Nucl. Chem., 26 (1~64) 1509. L. EYRING, B. G. HYDE AND D. J. M. BEVAN, Phil. Trans. Roy. Sot. London, Ser. A 259 (1~65)
583. 3 L. E. J. ROBERTS AND WALTER, J. Inorg. Nucl. Chsm., zz (1~62) 53. F. GRONVOLD AND E. F. WESTRUM, Acta Chem. Stand., 1.3 (1959) 241. _ _~ : F. GRONVOLD, Acta Chem. &and., zz (1~68) 1219. 6 A. D. WADSLEY, Rev. Pure A+fiZ. Chem., 5 (1~55) 165; A. D. WADSLEY in L. MENDELCORN (ed.), Nonstoichiometric Com$ounds, Academic Press, New York, 1964, p. 98. R. ROY AND S. KACHI, U.S. Army Contract Rep. No. DA28-043 AMC-01304(E), 1964. : G. ANDERSSON, ActaChem. &and., II (1~57) 1641, 1653. 9 S. ANDERSSON AND L. JAHNBERG, Arkiv. Kemi, 21(1~63) 413. IO 0. KUBASCHEWSKI AND J. A. CATTERAL, Thermochemical Data of Alloys, Pergamon, Oxford, II 12
I3 I4 15 16
I7
18
IQ
1956, P. 176. F. D. ROSSINI, D. D. WAGMAN, J. E. KILPATRICK, W. J. TAYLOR AND K. S. PITZER, J. Res. Natl. Bur. Stand., 34 (1945) 143. J. P. COUGHLIN, U.S. BUY. Mines Bull., 542 (1~54). J. F. ELLIOTT AND M. GLEISER, Thermochemistry for Steel Making, Addison Wesley, New York, 1960. H. SIEMONSEN AND H. ULICH,~. Elektrochem., 46 (1940) 141. K. K. KELLY AND E. G. KING, Contribution to Data on Theoretical Metallurgv. X., U.S. _I BUY. Mines Bull. 476 (1949). N. P. ALLEN, 0. KUBASCHEWSKI AND 0. VON GOLDBECK, J. Electrochem. Sot., 98 (1952) 417. H. M. SPENCER AND J. L. JUSTICE, J. Am. Chem. SOL, 56 (1~34) 2306. S. ASBRINK, ActaChem. &and., 13 (1959) 603. T. KATSURA AND M. HASEGAWA, Bull. Chem. Sot. Japan, 40 (1~67) 561.
J. Less-Common
Metals, 22 (1970) 209-218