Metal-insulator Phase Transition in Vo2

-The effects of doping and uniaxial stress on the structural and magnetic properties of V02 are reviewed. Important electron-electron correlation effects are deduced : (i) in the metallic phase from the results obtained in V I ~ N ~ ~ O ~ alloys, (ii) in the insulating phases of pure V02 at ambiant pressure (MI), under uniaxial stress or in the presence of Cr impurities (T and M2). Discovered in 1959 by Morin [l], VO, undergoes a first order metal-insulator phase transition at 340 K from a high temperature rutile phase (R) to a low temperature monoclinic phase (M,). The first experiments showed the absence of any magnetic ordering [2] and the presence of V-V pairing, suggesting a one electron description of the insulating phase ; the band gap being induced by the crystallographic distorsion forming the pairs (Adler et al. [3] Goodenough [4]). Describing the metallic phase in term of a simple band theory, Berglund and Guggenheim [5] proposed that most of the large entropy change (AS = 1.5 k/VO, unit) coming from lattice vibrations, stabilizes the metallic phase. A phonon softening mechanism due to strong d electron-phonon interactions was proposed by Paul [6] and Hearn [7]. On the other hand, following an idea put forward some years ago by Goodenough [S], Rice et al. [9] suggested that each cationic electron gets trapped in bonded singlet V-V pairs forming the insulating phase. (They also calculated that, even in the atomic limit, the energy of such a phase can compare favorably with that of a Mott insulator in its antiferromagnetic state.) Considering also the strong similarities (electrical conductivity, magnetic susceptibility) between properties of the metallic phases of VO, and VzO,, they proposed a unified description of the two phases in terms of a strongly correlated or exchange enhanced metal. In such a description, a sizeable contribution to the entropy change at the metalinsulator transition comes from the electrons. This controversy needed a much more detailed study of VO,. A fruitful way was opened by the study of the effects of various dopants on the metal-insulator transition. In this paper we present in the first part the phase diagrams of the two classes of impurities (Nb and Cr classes) and their relation to that of pure VO,. In the second part we discuss the metallic phase, using mainly the V,-,Nb,O, system. The properties of the insulating phases, of VO, under uniaxial stress and of the V, -,Cr,O, system, 'are presented in the third part. The last section discusses the problem of the metalinsulator phase transition in VO,. l . The phase diagram of V 0 2 and its alloys. In its high temperature phase VO,, has a rutile (R) structure (Fig. l) composed of two equivalent vanadium atoms A (center) and B (corner) per cell ; each V atom being surrounded by an oxygen octahedron whose axis point in the (110), and (lie), directions for the A and B vanadium atoms, respectively. The FIG. 1. Rutile cell of the high-temperature phase of V02. (*) Associ6 au Centre National de la Recherche Scientifique. Distances, in angstrom, are taken from reference [10]. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976408 04-50 3. P. POUGET AND H. LAUNOIS monoclinic insulating phase (M,) can be presented as the result of a two components distortion : a pairing of V atoms along the (OOl), axis and a zig zag (or antiferroelectric) distortion along the octahedron axis [4]. Due to this distortion all the vanadium atoms belong to equivalent V-V pairs slightly tilted from the (OOl), axis, as schematically shown in figure 2a. FIG. 2. a ) The phase diagram of V I ~ C T ~ O ~ alloys (after ref. [13]). In each insulating phase, vanadium chains of the A and B sublattices, contained in the ( l i 0 ) R and ( 1 1 0 ) ~ planes respectively, are represented. The arrows in the T phase indicate the displacement of the V atoms when the temperature increases. b) The phase diagram of pure V02 vs temperature and stress. After reference [H]. T( K) b Pseudo R Figure 2a presents the phase diagram of the V1-xCrs02 system. For only few thousandths of chromium atoms two new insulating phases M, and T are stabilized between the R and M, phases. The simplest way to understand these phases is to view the rutile phase as two interpenetrating sublattices A and B of V chains parallel to the (001), axis ; each type of sublattice consists either of A vanadium atoms or of B vanadium atoms as previously mentioned. In the monoclinic M, phase, Marezio et al. [l11 have shown that the V atoms of the A sublattice are strongly paired along the (001), axis while the V atoms of the B sublattice form zig zag chains along the same direction, as shown schematically in figure 2a. Because of the interaction between the two vanadium sublattices via 3401 oxygen atoms, the pairing in A sublattice causes the zig zag (or antiferroelectric) distortion in B sublattice, giving rise to two inequivalent sublattices, each exhibiting half of the distortion of M,. This consequence of the rutile structure shows that, contrary to Goodenough suggestion [4], the two components of the crystallographic distortion of VO,, cannot be separated by atomic substitutions. Electric field gradient (E. F. G.) measurements show that the T phase corresponds to a progressive dimerization of the zig' zag chains in the B sublattice and a progressive tilting of the V pairs in the A sublattice leading to the two equivalent V sublattices in the M, phase (Fig. 2a) [12]. X ray structural determination [12, 131 gives a triclinic symmetry for this transitional phase T. This type of phase diagram is also found with others impurities like A1 [14], Fe [15, 161 oxidizing the V4+ state to the VSC state in the M, phase. The exact mechanism by which a particular group of impurities can stabilize T and M, and break the symmetry between sublattices A and B is not yet clear (migration of holes (V5+ sites), to the B sublattice, which compensate the trivalent ions (Cr3 + for example) of the A sublattice has been suggested [12, 171). The break in the symmetry has been done more directly by applying a uniaxial stress in the (110), direction in pure VO, [18]. The phase diagram thus obtained is presented in iigure 2b. This experiment proves that the new insulating phases T and M, are alternative phases of pure VO,. Figure 2b shows also unambiguously that the critical stress, Sc 100-300 bars, for which the M, phase appears is so small that the free energy of M , and M, are extremely close at temperatures just below the metallic rutile phase in pure VO,. ----:---+-l.+____ M2 FIG. 3. The phase diagram of Vl-z Nbx 0 2 alloys deduced from X ray measurements. After reference [201. r h-+-----.T "" 8