Hysteresis effects in the semiconductor-metal transition of Cr-doped VO2

Solid State Communications,

Vol. 13, pp. 1953—1957, 1973.

Pergamon Press.

Printed in Great Britain

HYSTERESIS EFFECTS IN THE SEMICONDUCTOR—METAL TRANSITION OF Cr-DOPED V02 * J.M. Reyes, J.R. Marko and M. Sayert Department of Physics, Queen’s University, Kingston, Ontario, Canada (Received 25 July 1973 by M.F. Collins)

The thermal hysteresis of the semiconductor to metal transitionin V1_~Cr~O2with 0 x ~ 0.023 was found to increase with x because of a corresponding rise in the heating transition temperature. No significant change was observed in the temperature associated with the cooling transition. A qualitative explanation is offered for these results on the basis of an assumed relationship between the free energy and a lattice distortion parameter. ‘~

1’2 on IN SPITE of properties the large amount of datadioxide, available the physical of vanadium there is still no universally accepted explanation for the semi. conductor to metal (SM) transition in this material. One possibility, suggested by Goodenough,8 is that, in general, two distinct transitions occur. One of these corresponds with increasing temperature to a change at T = T~from homopolar to metallic bonding between the vanadium ions. The second transition occurs between the low temperature antiferroelectric and high temperature paraelectric phases. The latter process takes place at a temperature T~>T, and corresponds to the observed SM transition. Goodenough has argued that although the equality T~= T is accidentally satisfied in pure VO substitutional impurities such as Cr, Al, Fe, etc.,2,have different effects on the two processes so that two distinct transitions may be experimentally observed, A good deal of attention4’6 has recently been devoted to these matters in Cr.doped VO 2(V1_~Cr~O2), particularly with regard to the crystal structures obtained at temperatures T < T~.Although differences *

Work supported by the Defence Research Board of Canada, Grant No. 951099. ~

still exist among the various sets of reported data (see Fig. I below), there now appears to be some general agreement7 that for x < 0.05, three different monodinic phases can occur below the SM transition temperature. The experimental results to be described in this letter were obtained by differential thermal analysis (DTA) and measurements of the d.c. electrical conductivity and 6328 A optical reflectance. The SM transition was also studied by monitoring, as a function of T, the 20 GHz resonant frequency of a sample-loaded TM~ microwave cavity. In order to avoid difficulties due to crystal shattering during the 6 our measurements were carried out SM transition single crystal samples. These crystals on powdered with 0 ~ x ~ 0.023 were grown at 1200°Cby isothermal flux evaporation under an argon atmosphere.8’9 With the exception of the DTA results. no anomalies were detected which could be associated with the previously reported structural changes (SC) in the semiconducting state. Minor transitions, usually double-peaked, with integrated intensities less than 10% of that associated with SM transition were observed by DTA. These were assumed to indicate the boundaries between the three monoclimc phases 6 The (M1 M2heating andM3) reportedtemperatures by Marezio etassociated al. average transition ,

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HYSTERESIS EFFECTS IN Cr.DOPED V02

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account for the details of the impurity-induced changes in this phenomenon. In this letter, particular attention is given to the differences observed in the transitions associated with the alternative heating

R 0

and cooling portions of the experimental cycle; although there has beeneffects no previous evidence that the involved hysteresis have fundamental

70

60

significance in pure VO2. However, since it would appear that impurities and/or nonstoichiometry

-

—50.

__________________________________

(-)

material, a detailed study of these effects in doped 3”°in the ‘pure’ determine samples seems the extent appropriate. of hysteresis A large shift (-‘ 1%) in the resonant frequency of

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~4Q

.

MAREZIO ET AL

30

a microwave cavity was found to occur when an enclosed VO 2 or V1_~Cr~O2 sample was heated or cooled through an SM transition (on the other hand, the cavity resonant frequency was found to be unaffected by the observed SC transitions).

VLLEPUVE ET AL

.

THIS WORK

M3’~

20

-

10

-

Unfortunately because of the large ‘metallic’ electrical conductivity, skin-depth effects and the accompanying breakdown of the cavity perturbation technique

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0— 0

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I

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3

AT % Cr FIG. 1. Temperature-composition phase diagram for V1_~Cr~O2. Following Marexio eta!., (See reference 6) the phases below the SM transition are assigned the monocinic M1 M~and M3 structures. Above the SM transition, the structureis tetragonal rutile (R). Dashed lines indicate the phase boundaries proposed by Marezio eta!., reference 6. ,

with the various measured parameters are plotted against Cr-concentrated in Fig.1 which includes temperatures previously identified4’6 with SM and SC transitions. While the agreement with the Villeneuve et aL4 results is not good in general, our observations differ from those of Marezio eta!.6 only in the quantitative details of the lower temperature transitions. In particular, the concentration dependence of the M 2 M3 phase boundary observed in our work differs from that obtained by the latter workers. This suggests that the lower temperature transitions are sensitive to the details of sample preparation. ,

—

Our interests, at present, are primarily directed toward the relationship between the Cr-concentration and the characteristics of the SM transition in Vi~Cr~O2. A reasonable test of the alternative theories of the pure VO2 transition would be to

prevented the extraction of corresponding dielectric data from the observed shifts. The magnitudes of these shifts were found to decrease with decreasing sample particle size. This result is consistent with the explanation that these shifts are due to the large increase in conductivity associated with the SM transition which lets the sample act as an extension of the wail of the brass cavity, effectively transforming it into a ‘re-entrant’ cavity ofeffect lowerisresonant 1 We believe this responsible for frequency.’ the anomalous and extremely pronounced rises in E(24 GHz) noted by Kawakubo er aL~2in the tem. perature region immediately below T~. In Fig. 2, we have plotted the normalized sampleinduced shifts in cavity frequency as functions of T. In accord with previous results,13 the magnitude of the thermal hysteresis was found to increase with impurity concentration. However, the most striking feature of the curves is the fact that the temperatures (Tfl and widths (in temperature) associated with the cooling transitions are essentially independent of the Cr concentration. In other words, it would appear that the additional hysteresis introduced by the Cr impurities can be attributed solely to a rise in the heating transition temperature (T~).Similar conclusions were drawn from the measured values of the other experimental parameters of both powder and single crystal samples.

HYSTERESIS EFFECTS IN Cr-DOPED V02

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AT % Cr

I I

1955

5~ionsstate which have ionic radiiinsmaller crystal in the trivalent they may result the production V Hence, than that ofof V’. regardless of the valence state of the Cr impurity, the net effect is to introduce cations with smaller ionic radii into the VO 2 lattice. It appears likely that the nature of the impurity. induced shift in the transition temperature may be understood on this basis. A phenomenological model capable of reproducing the important features of the observed hysteresis may be based upon a free energy expansion method which 4”5 In terms of a parameter 6 which ismaterials.’ apreviously measure of the distortion of the lattice has been electric applied with successto ferrofrom the tetragonal high temperature structure, the free energy may be written as:

t~T~O-7

F

2+F 4+F 6. (1) 0+F26 46 66 In the case of a first order transition, the inequalities F 4 <0 and F6> 0 must be satisfied. To a first approximation, only F2 is assumed to change with temperature and is taken to be of the form: =

F

F2

I

•

60

T

I

I

‘70

80

(°C)

FIG. 2. Normalized shift ~ Efi~ fHT]/FILT fHT], wherefRT and fLT are the high temperature and low temperature cavity resonant frequencies, respectively), in the cavity resonant frequency (fR) in the region of the SM transition. It was previously pointed out by MacChesney and 3 that a correspondence exists between Guggenheim’ the radii of impurity ions in the tetravalent state and the sign of the resulting shift in the transition temperature. In general, impurities which raise and lower the transition temperature have ionic radii which are respectively smaller and larger than that of the V~ion. The lack of detailed information on the valence states of the Cr impurities in the crystal makes the interpretation of the results on the basis of this correlation difficult. However, if the Cr impurities enter the

=

a(T—T~).

(2)

The resultant expression has local minimum at 6free = 0 energy for all Atlocal highaminimum enough temperatures, say T> T’,T> no T~. other occurs. On the other hand, when T< T~,,F always has a single minimum at some value 6 * 0. To illustrate our model within this picture, we have drawn a set of free energy curves in Fig. 3 for the temperature range T~
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HYSTERESIS EFFECTS IN Cr-DOPED V02

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changes of the type suggested may be realized by increasing the magnitude ofF4 and/or decreasing that ofF6 in equation I. T~ Io

~-_-

Ui

T

S ‘•5

Ui

- - -‘

Ui

Tc

Ui U-

_______________________________________ 6 FIG. 3. Curves of free energy (F) as a function of a distortion parameter (6) at several temperatures. The heating (cooling) transition occurs at T~(Ti) when the energy barrier in the direction of decreasing (increasing) 6 is decreased to a value small enough for nucleation to occur. Ti’, is on the order of 1K and may in fact be determined by the practical limitations of sample purity, —

The role of substitutionalimpurities in affecting the transitions may be related to the established correlation between the spatial dimensions of the impurity ions and the sign of the resulting change in the heating transition temperature. Thus, in the case 4~or alternatively V5~ions, their smaller radii of Cr introduces a tendency toward an anion coordination which is smaller than that associated with the displaced V~ions. As a result, such ionic impurities produce a lowering of the intrinsic free energy which increases with 6 leading to the broken curves sketched in Fig. 3. The greater importance of the impurity-induced stabilization at larger values of 6 has the effect of increasing the free energy barrier seen by the 6 * 0 low temperature structure while leaving that seen by the tetragonal & = 0 state relatively unchanged. As a result, this mechanism would reproduce the experimentally observed results whereby TI is essentially independentwith ofx(for x ~ 0.023) while T~risesIt significantly increasing Cr concentration. should be note that the impurity-induced free energy

Within our suggested model, the introduction of impurities with radii larger than that of the V~ion would stabilize the tetragonal structure leading to 3’9the “s. observed reduction in the transition temperature However, the thermal hysteresis may not be expected9 to have a simple form analogous to that discussed above since the obvious requirement that T~>T~ constrains Ti” to decrease with x at a rate at least equivalent to that governing r~. The above model is a very simple one and within its context we have made no attempt to describe such relevant experimental features as the noted differences between the pure and doped sample monoclinic structural forms. Instead this model was directed toward achieving some qualitative understanding of the rather striking asymmetry in the observed hysteresis. This asymmetry, from our preliminary studies of other V0 2 -impurity systems, would appear to be characteristic of dopings which raise the SM transition temperature. The consistency of this kind of model with these effects would appear to offer yet another argument for the proposition that a lattice distortion provides the driving force for the SM transition. The Goodenough antiferroelectric instability proposal is a specific example of such a mechanism in which the electronic properties on each side of the transition are determined by the respective crystal structures. Further picture studies are before a of more quantitative can needed be constructed the effects of impurities on the SM transition in VO 2. Measurements on samples doped with other impurities such as Al, Fe and Ti are essential to further explorations of this kind. A complete model of these impurity-induced effects must explain not only the observed values of T~and TI but must also account for the instances in which the high temperature transition is of a semiconductor to semiconductor (SS) type. Note A recent study of Vi_~Cr~Oi(O x <0.20) 6 ~was called toby J.B. Goodenough Hong’ our attention afterand thisH.Y-P. paper was submitted. They report similar asymmetrical behaviour of the heating and cooling SM transition temperatures in the range 0~x<0.05.

Vol. 13, No. 12

HYSTERESIS EFFECTS IN Cr-DOPED V02 REFERENCES

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L’hystéresis thermale de la transition semiconducteur—métal dans Vi_~Cr~O2 ayant 0
1957