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111Physics of Complex Systems, 2020, vol. 1, no. 3 _www.physcomsys.ru
Physics of Semiconductors. Semiconductor Physics
UDC 536.42+539.19 DOI: 10.33910/2687-153X-2020-1-3-113-122
Comparative analysis of semiconductor-metal phase transition mechanisms in vanadium oxides (V2O3 and VO2)
A. V. Ilinskiy01, E. I. Nikulin1, E. B. Shadrin1
1 Ioffe Institute, 26 Politekhnicheskaya Str., Saint Petersburg 194021, Russia
Authors
Aleksandr V. Ilinskiy, e-mail: Ilinskiy@mail.ioffe.ru Evgeniy I. Nikulin
Evgeniy B. Shadrin, ORCID: 0000-0002-1423-2852
For citation: Ilinskiy, A. V., Nikulin, E. I., Shadrin, E. B. (2020) Comparative analysis of semiconductor-metal phase transition mechanisms in vanadium oxides (V2O3 and VO2). Physics of Complex Systems, 1 (3), 113-122. DOI: 10.33910/2687-153X-2020-1-3-113-122
Received 31 May 2020; reviewed 6 July 2020; accepted 6 July 2020.
Copyright: © The Authors (2020). Published by Herzen State Pedagogical University of Russia. Open access under CC BY-NC License 4.0.
Abstract. In this study, phase transformation mechanisms in thin V2O3 and VO2 films are analysed based on experimental data and on the qualitative model for vanadium oxides proposed by the authors of the study. New features of semiconductor-metal phase transformation mechanism revealed in V2O3 are discussed. Characteristics of phase transition process in V2O3 is compared with the features of a similar phase transformation in thin VO2 in detail.
Keywords: vanadium oxides, Magneli phases, semiconductor-metal phase transition, phase transformations, correlation energy.
Introduction
Recently, there has been a revival of interest in studying Semiconductor-Metal Phase Transition (SMPT) in vanadium oxides in the Magneli series: VO2, V2O3, V6O13, V3O5, V4O7, V6O11, etc. This interest is due to unusual results for vanadium dioxide obtained using modern research methods: optical, femtosecond and dielectric spectroscopy (Wegkamp, Stahler 2015; Ilinskiy et al. 2020), spectroscopy of vanadium-oxide photonic crystals (Fan et al. 2015; Golubev et al. 2001; Lu, Zhao 2012; Ye et al. 2015), atomic force microscopy (Tselev et al. 2013), micro-Raman light scattering (Schilbe 2002) and others. Such studies have resulted in a comprehensive understanding of the processes that occur during phase transition in vanadium oxides.
However, such comprehensive understanding only exists for vanadium dioxide. Analysis of the mechanisms of phase transformations in other oxides of the Magneli series has to a large extent been neglected by researchers. Therefore, in this work, the main focus is on vanadium sesquioxide (V2O3), a compound possessing a number of unusual physical properties. It seems reasonable to consider these unusual properties by comparing them with the properties of the well-studied VO2 (temperature SMPT Tc = 340 K).
SMPT also occurs in V2O3 with increasing temperature, but it occurs at lower temperatures than in VO2 (at Tc = 140 K < Tc = 340 K). Moreover, the increase in the electrical conductivity of V2O3 after phase transition is much larger (for a single V2O3 crystal, the change is 7 orders of magnitude, and in VO2 it is 4 orders of magnitude). There is no doubt that these differences are associated with differences in vanadium oxide crystal structure. It is crucial to note that vanadium atom has an unfinished
¿-shell, and also that strong interactions between electrons play an important role in the phase transition when the energy structure of the crystal is rearranged.
Namely, a strong temperature dependence of the semiconductor energy gap width is associated with correlation effects in V2O3 and VO2 compounds (Gatti et al. 2007). In such materials, the position of energy bands depends on their occupancy by electrons. Since V2O3 and VO2 are oxides of V, a transition metal (No. 23 in the periodic table), they exhibit a fundamental property of vanadium: the dependence of the energy position of atomic levels on their electron occupancy. This conclusion is confirmed by the analysis of electronic configurations of Ti, V, and Cr, i.e. elements located in the 4th period of the table.
Namely, during the transition from element 22 (Ti with 3cP4s2 configuration) to element 23 (V with 3d34s2 configuration), the 3d shell is filled with electrons. But when going to element 24 (Cr: 3d54s1), the filling sequence is violated. This is due to the fact that the additional electron acquired by Cr greatly reduces the energy of the 3d level and initiates a transition of an electron from the 4s level to the 3d level. The reasons for the lower 3d level energy in V and Cr atoms are the strong interactions between the electrons. These interactions take place simultaneously with the interaction of electrons with the V nucleus. The ability of V levels to lower energy when they are occupied by electrons is transferred to vanadium oxides, in which the positions of the bands on the energy scale are also dependent on their electron occupancy.
Despite the persuasiveness of these arguments, the specific details of V2O3 and VO2 electronic spectrum and crystal structure transformation during the phase transition are of great interest, and for V2O3 they still remain unclear.
The objective of this work, therefore, was to study the features of the phase transition in nanocrystal-lites of thin V2O3 films in detail, as well as to compare these features with well-studied features of SMPT in thin VO films.
2
General information on the physical properties of V2O3 and VO2 single crystals
Vanadium sesquioxide V2O3
The V2O3 crystal lattice is a corundum (Al2O3) type lattice (Tan et al. 2012). The unit cell of the rhom-bohedral (trigonal) phase contains 4 vanadium atoms and 6 oxygen atoms (V4O6). In the V2O3 lattice, for every two oxygen octahedra containing V3 +, one octahedron does not contain the V3+ ion. Such octahedra with empty bases are located on both sides of a pair of octahedra with filled bases. The coordination number of the V3+ ion is 6, the coordination number of the O2- ion is 4.
In V2O3 single crystals, SMPT occurs at an increased temperature of Tc = 140 K. In this case, single crystal symmetry rises from monoclinic to rhombohedral (trigonal) (Nagaosa et al. 2010). The band gap in the semiconductor low-temperature phase is estimated at Eg = 0.3 eV (Keller et al. 2004).
Vanadium Dioxide VO2
The crystal lattice of the VO2 metal phase has tetragonal symmetry, while the semiconductor lattice has monoclinic symmetry (Shimazu et al. 2015). The unit cell contains 2 vanadium atoms and 4 oxygen atoms, (V2O4). In the VO2 lattice, all oxygen octahedra contain vanadium atoms in the centres of their bases. The coordination number of the V4+ ion is 6, the coordination number of the O2- ion is 3.
In vanadium dioxide single crystals, SMPT occurs with the temperature increase to T = 340 K.
The band gap in the semiconductor low-temperature phase is estimated at Eg = 0.7 eV.
SMPT in V2O3 and VO2 films
This work is focused, as indicated above, on the study of phase transition in thin films of vanadium oxides consisting of crystallites with sizes of tens of nanometers. Therefore, when describing the phase transition process, it should be taken into account that the grains of V2O3 and VO2 films have different sizes, which follow a normal distribution. As such, grains differ from each other by the contributions of surface energy to the phase transition process, and, consequently, differ in phase transition temperatures, the width and shape of the elementary hysteresis loops and the degree of grain adhesion to the substrate. It follows that all of the above factors have to be taken into account to describe the SMPT mechanism in nanocrystalline films. At the initial stage of phase transition modelling, this work will only
discuss the features of SMPT in individual nanocrystallites, while at the final stage it will be possible to make phase transition in the entire film as a whole.
Samples
The samples were thin (about 70 nm thickness) V2O3 and VO2 films synthesized by laser ablation method on a SiAl ceramic substrate. For optical experiments, films synthesized on a mirror aluminum layer preliminarily deposited on a SiAl substrate were used. A thin-film optical interferometer arose. The use of such a thin-film Fabry-Perot interferometer allows one to enhance the effect of a change in the reflection coefficient of the film structure during a phase transition.
Fig. 1. The atomic force image of V2O3-a and VO2-b films
Figure 1 shows the AFM images of V2O3 and VO2 films. A comparison of the images shows that the films have a uniform granular structure with a typically occurring grain size of 150-200 nm. At the same time, histograms show that the grain size of the films is distributed. The shape and width of the temperature hysteresis loops of the films as a whole depends on the size distribution.
The results of the experiments
Temperature hysteresis loops of the electrical conductivity of films
Fig. 2. The temperature hysteresis loops a of the film V2O3-a, and VO2-b
Figure 2a shows temperature dependence of electrical conductivity (lna(T)) of the V2O3 film obtained by us. The graphs show that electrical conductivity of the film increases by more than an order of magnitude with an increase in temperature from 100 to 300 K. At high temperatures, the conductivity is metallic with a weak decrease of conductivity with increasing temperature deep into the metal phase. This is typical of metals. In the heating-cooling cycle, a hysteresis loop extended in temperature is observed (the length of the loop branches is ~120 K). The width of the loop is 7 K, the centre of gravity of the branch of the loop corresponding to heating falls onto the temperature Tc = 140 K. This temperature is to a first approximation taken as the temperature of SMPT.
A detailed analysis shows that the value of Tc depends on the size of the crystallites of the film and the degree of imperfection of their crystal structure. Specifically, due to the size distribution of grains, they differ in the widths of the elementary hysteresis loops out of which the main loop is composed. The difference in loop widths indicates the martensitic nature of SMPT in V2O3. In addition, a difference in the degree of defectiveness of the grains, for example, a difference in the concentration of oxygen vacancies in film grains, creates a difference in the equilibrium temperatures of the Tc phases in these grains. This is due to the correlation nature of that part of SMPT, which is the Mott electronic phase transition (Gatti et al. 2007). The fact is that oxygen vacancies are electron donors that lower Tc. The shape of hysteresis loop branch, which corresponds to heating and whose centres of gravity are taken as the temperature of the phase transition of the film, is determined as the temperature of an infinite percolation conduction cluster formation. It follows that the numerical value of Tc only on average characterizes the temperature of the phase transition in the V2O3 film.
It is also important to note here that during the hydrogenation of V2O3 film, the entire temperature hysteresis loop shifts toward low temperatures by 4-6 K by the atomic % of embedded hydrogen (Andreev et al. 2017). This circumstance will be used in discussing SMPT mechanisms in the studied film structures.
Figure 2b shows the temperature dependence of the electrical conductivity of VO2 film. Here, during the phase transition, the electrical conductivity increases by an order of magnitude, and at large Tc temperatures, it is of a metallic nature. But the branches of the hysteresis temperature loop are less extended (30 K), and the loop width is 12 K. Here, when the film is hydrogenated, the entire temperature hysteresis loop shifts toward lower temperatures, but by a larger value than V2O3: 10-15 K (Ilinskiy et
al. 2011). 2 3
Temperature hysteresis loops of film optical reflectivity
Fig. 3. Temperature hysteresis loops of the intensity of light reflected from films: a — V2O3 and b — VO2 (1 — the initial loop, 2 — after annealing in vacuum for 15 min. at 250°C, 3 — after annealing in vacuum for 15 min. at 350°C)
Figure 3a demonstrates temperature dependence of optical reflectance (I(T)) of the V2O3 film obtained by us. The graphs show that the intensity of light reflected from the film increases by more than an order of magnitude with temperature increase from 100 to 300 K. Therefore, we can assume that at high temperatures the film grains are in the metal phase. In the heating-cooling cycle, an intensity hysteresis loop of the light reflected from the film is observed. It is high and extended along temperature (~120 K). The width of the loop is 7 K, the centre of gravity of the branch of the loop corresponding to the heating of the sample also falls on the temperature Tc = 140 K.
Figure 3b shows temperature dependence of optical reflectivity of VO2 film for an ordinary film (curve 1). Here, during a phase transition, the intensity of the light reflected from the film increases by an order of magnitude, and at high temperatures the conductivity is metallic. However, the thermal hysteresis loop is less extended in temperature (30 K), and its width is 12 K. In Fig. 3b, loops 2 and 3 were obtained for a film treated in a special way: the film was vacuum annealed. The treating process is described in more detail in the signature to Fig. 3. The figure shows that after this treatment, the hysteresis loops shift to the low-temperature region.
The optical reflectivity of V2O3 and VO2 films is also characterized by the fact that during the hydrogenation of films, all temperature hysteresis loops are shifted toward low temperatures by 10-15 K (Andreev et al. 2017; Ilinskiy et al. 2011).
Acoustic emission
During SMPT, V2O3 and VO2 films exhibit effective acoustic emission in the immediate vicinity of Tc (Andreev et al. 2000; McBride et al. 1974). This fact is interesting for detailing the processes of phase transformations in these oxides.
Hall constant
For V2O3, we measured the Hall constant Rh at low (T = 95 K) and high (T = 300 K) temperatures: Rh = 1200 cm3/K and Rh = 1.3 cm3/K, respectively. For VO2, the Hall constant obtained by us at low (T = 100 K) and high (T = 380 K) temperatures turned out to be Rh = 6000 cm3/K and Rh = 0.6 cm3/K, respectively. The numerical values of the Hall constant indicate a large increase of the concentration of free electrons in both oxides with increasing temperature.
Discussion of the results
Peierls semiconductor-metal structural phase transition
Peierls phase transition is a change in the symmetry and parameters of the crystal lattice during the formation (destruction) of V-V dimers. Dimer is a system of two V ions coupled in a pair located in neighbouring octahedra. This takes place both in V2O3 and in VO2. Each V ion gives one electron, which is free from the formation of a skeleton, to create a strong a-bond with the V ion of the neighbouring octahedron in both oxides. Another free electron of the V3+ ion, which exists only in V2O3, creates, in addition, a strong n-bond with the neighbouring V3+ ion. As the temperature decreases (increases), new bonds that are not capable of (destroying) the temperature factor kT arise (disappear) between the atoms of the lattice, i.e., new dimers arise (or are destroyed). When the critical concentration of arising (destroyed) dimers is reached, the symmetry of the entire lattice of the single crystal changes abruptly. The Peierls thermal phase transition in the films of vanadium oxides discussed here has hysteresis with a loop width of the order of several degrees. The large loop width is due to the fact that the small sizes (100-200 nm) of the nanocrystallite films make, according to the Laplace theorem, a significant contribution of surface energy to the energy of SMPT. Namely, the contribution of surface energy to the energy of SMPT is so large that a deviation of several phase equilibrium temperatures Tc is required to complete the phase transition. This effect occurs both when the temperature changes in the direction of the metal phase, and in the direction of the semiconductor phase (Aliev et al. 2006). Let us consider in more detail the process of phase transition in V2O3 and VO2 nanocrystallites.
Vanadium sesquioxide V2O3
Figure 4 shows the bases of ten octahedra of the V2O3 crystal lattice, in the centres of which V3+ ions are located adjacent to each other. These ions form n-bonds between each other due to overlapping 3d -3d and 3d -3d branches of vanadium ion orbitals.
xz x z yz yz
The vanadium atom, located in the centre of the base of the oxygen octahedron, forms 3dxy-bonds with six oxygen atoms due to its 3d1xy(1)3d0z2(1)4s2(1)4p0(3) hybridization (the superscript is the number of electrons in the orbitals). During the formation of the lattice, a vanadium atom gives on average % of its electron density to the oxygen atoms of the environment and turns into a V3+ ion. At the same time, each oxygen atom forms four 2s2(1)2px1(1)2py1(1)2pz2(1)-hybrid orbitals and gives 3/2 of its electron density in connection with vanadium atoms. It forms four complete a-bonds with vanadium atoms in the centres of the bases of the surrounding octahedra (2 electrons per bond). And the V3+ ion creates 6 full-fledged a-bonds with oxygen ions of its own octahedron. Thus, a strong octahedral V2O3 framework is formed.
Since, to ensure the strength of the crystal lattice, each V3+ ion gives only 3 electrons to the formation of a-bonds (one from 3d1xy and two from the 4s2 orbitals), each V3+ ion has two electrons not used to form bonds with O2- ions - octahedron. Due to the participation of one unoccupied electron, a strong a-bond is formed between 3dx2 y2-3dx2 y2 orbitals of V3+ ions, which are located at the bases of neighbouring octahedra. Durable 3dx2 y2-3dx2 y2 a-dimers appear in V2O3, which are not destroyed either in the semiconductor or in the metal phases (see the lower part of Fig. 4).
Fig. 4. Fragment of V2O3 crystal lattice illustrating the formation of dimers. The upper part of the figure shows the formation at T < T of stable n-dimers formed from the dynamic 3d -3d, ,s bonds existing at T> T .
c / xy xy " c
The lower part of the figure shows a-3d -3d -dimers stable in V2O3 at temperatures both larger and lower than Tc
Electron density corresponding to the second free electron of the V3+ ion in the metal phase is uniformly distributed between the 3d and 3d orbitals of the vanadium ion in the octahedron. Due to
J xz yz
the overlapping branches of the orbitals of the V3+ ion of neighbouring octahedra (Fig. 4), V-V pairs arise, connected by n-bonds. A set of such dynamic n-bonds perpendicular to each other creates a one-dimensional zigzag structure along the hexagonal axis CR, the elements of which are interconnected. Since each V3+ ion gives away only one free electron (another is used to create a non-destructive a-bond), in V2O3 there is a system of one-dimensional zigzag filaments created with metallic type conductivity. The value of the Hall constant measured by us unambiguously indicates electronic type of conductivity.
The lattice of the semiconductor V2O3 phase has monoclinic symmetry, which contains fewer symmetry elements than the metal phase of rhombohedral symmetry. This is due to the fact that at temperatures lower than Tc, n-dimers are formed. In such dimers, V3+ ions are located in the metal phase at the base centres of neighbouring octahedra, which are inclined to each other at an angle of 150°. With SMPT, V3+ ions exit base centres and approach each other.
The reason for vanadium ions leaving base centres is that due to the tilt, the overlap of the branches of 3d and 3d,,- orbitals in 3d, ,-3d -n- bonds is 60% larger than that of 3d -3d -n-bonds. From this,
xz xz xz xz t? yz yz
according to the theory of molecular orbitals, it follows that the binding energy of 3dx,z,-3dxz-n dimers is almost twice as high as the binding energy of 3dyz-3dyz-n dimers, and the distance between the centres of V3+ ions is 37% less. The consequence of this is the formation of precisely 3d , ,-3d -n-dimers
^ r J x z xz
and a doubling of the repetition period of the peaks of the thermodynamic potential of the one-dimensional system of n-dimers, where CR. 3dxz-3dx,z,-n-dimers with a higher binding energy than 3d z-3d z-n-dimers also capture a single free electron. This excludes the participation of 3dyz-3dyz-n bonds in conductivity. The low-temperature monoclinic phase V2O3 acquires sharply reduced conductivity.
In the crystal lattice, the energy levels corresponding to the binding (n) and loosening (n*) of orbitals expand on the energy scale in the n and n* zones, which differ sharply in their energy positions in the conducting and semiconductor phases (Fig. 5). It is crucial that in the semiconductor phase, a gap appears on the energy scale between binding 3dxz-3dx,z,n-band and binding 3dyz-3dyz-n band (experimentally estimated by Eg = 0.3 eV (Keller et al. 2004)). Thus, the V2O3 crystal is in the low-temperature phase a semiconductor with a band gap of 0.3 eV.
The aforesaid is illustrated in Fig. 5a, which shows a diagram of energy bands of low-temperature V2O3 semiconductor phase.
Fig. 5. Diagram of energy zones of V2O3 crystal. a — semiconductor phase, b — metal phase
Vanadium Dioxide VO2
Vanadium V4+ ion in each oxygen octahedron, resulting from 3d1xy(1)3d1z2(1)4s2(1)4p0(3) hybridization of vanadium atom, forms 6 a-bonds with O2- ions, which arise during 2s2(1)2px1(1)2py1(1) hybridization of an oxygen atom. Each oxygen atom is connected by 3—but not 4, as in V2O3—a-bonds with V4+ ions, which are located in neighbouring octahedra. The V4+ ion gives an average of 4/3 of its electron density to the a-bond with O2-, which gives an average of 2/3 to the a-bond. A complete V-O a-bond arises, having two electrons with oppositely directed spins.
In VO2 metal phase, each V-octahedron has one free electron per V4+ ion, which is not involved in the stabilization of the lattice framework. This electron is located on 3d orbital, which is located
x2-y2
in the plane of the base. The cruciform branches of this orbital are parallel to the ribs of the base of the frame. V4+ ions form a system of one-dimensional filaments along the tetragonal axis CR, which creates a one-dimensional metal-type conductivity in a half-filled energy zone. This zone arises in VO2 when the energy level corresponding to the 3dx2-y2 orbitals expands in crystal to zone. The number of energy levels in such zone is equal to the number of V4+ ions and, at the same time, is equal to the number of electrons in the zone. Due to Pauli exclusion principle, electrons occupy only half of the levels of the lower part of the zone. The upper half of the zone remains free. This corresponds to the metallic conductivity of the material in high-temperature phase.
Fig. 6. Diagram of the energy zones of the VO2 crystal. a — semiconductor phase, b — metal phase
In low-temperature VO2 phase of monoclinic symmetry a 3dx2- 2 orbitals of neighbouring octahedra form 3dx2-y2-3dx2-y2 a-dimers due to the fact that the temperature; factor is not enough to destroy them, as it was in the metal phase. Therefore, V4+ ions are pairwise shifted towards each other and the period of repetition of the thermodynamic potential along CR doubles. The crystal conductivity sharply
decreases, and the material acquires semiconductor properties (EG = 0.7 eV), since free electrons are now fixed in 3dx2-y2-3dx2-y2 a bonds. A structural phase transition to low-symmetry phase occurs.
Fig. 6a is a diagram of the energy zones of low-temperature semiconductor VO2 phase.
Mott electronic semiconductor-metal phase transition
The Mott phase transition represents a continuous increase in the metallization of a single crystal with an increase of crystals temperature, which was in a semiconductor state with Eg = 0,7 eV. In this case, a change in the symmetry of the crystal lattice does not occur. With increasing temperature, the concentration of free electrons in the conduction band of the semiconductor continuously increases. The main feature of the Mott phase transition is that experimental data indicates a—very marked— decrease of the band gap of the semiconductor when the transition is completed. When the temperature reaches a critically high value, the energy gap of the semiconductor becomes equal to zero. The semiconductor goes into a metallic state with a lattice that maintains a low symmetry of the semiconductor phase. This second-order phase transition does not have thermal hysteresis. However, in the vanadium oxides studied by us, the Peierls and Mott phase transitions do not occur in the "pure state". The actual situation is more complicated: when the temperature changes, both phase transitions occur. Moreover, they mutually influence each other. Common schemes of the process of making SMPT in V2O3 and VO2 in the first approximation coincide.
The key finding is that in both oxides, electrons obeying the Fermi distribution cannot receive energy greater than 30 meV at temperatures below Tc, which does not allow efficient transfer through the energy gap (0.3 eV for V2O3 and 0.7 eV for VO2) of such a number of electrons that is necessary for the transition of the material from semiconductor into metallic phase. Such transition is possible when a critical number of dimers that stabilize the semiconductor phase are destroyed (3d -3d, ,-n dimers for
r ' v xz xz
VO and 3d0 „-3d0 -a-dimers for VO).From the bonds of these dimers, there is a transfer of electrons
2 3 x2-y2 x2-y2 2'
to the allowed band. It follows that V2O3 and VO2 must remain semiconductors even at room temperature. This, however, conflicts with the results of the experiment. The contradiction is eliminated by Motts fundamental idea, where phase transition energy has to include high correlation energy of interaction between the electrons moving in the periodic potential of the crystal lattice. According to Mott (Mott 1990), the transfer of electrons into the conduction band causes it to lower in energy, which then increases the transfer of new electrons. This causes additional zone lowering, etc. The zone from which the electrons leave, on the contrary, rises in energy (Gatti et al. 2007). It follows that the standard thermal transfer of electrons into the conduction band of a semiconductor from its valence band with increasing temperature causes a decrease in the band gap if correlation interaction between the electrons exists. At the same time, the concentration of electrons in the conduction band increases. This process is a temperature-extended Mott semiconductor-metal phase transition.
Figures 5 and 6 illustrate the aforesaid. They show the energy diagrams of the bands in the semiconductor and metal phases for both vanadium oxides.
This process in both oxides leads to effective destruction of dimers stabilizing the semiconductor phase and which, according to the theory of valence bonds, serve as electron donors. The semiconductor phase in both oxides rapidly passes into the metallic phase (Peierls transition) upon reaching a critical number of destroyed dimers. Thus, in each grain of the vanadium oxide film, Peierls structural transition is initiated by Mott electronic transition. That is, dimers that remain undisturbed cannot keep the crystal lattice in a state of low symmetry. In all grains, the Peierls phase transition occurs at different temperatures—Tc in a defect-free crystal—depending on their size.
The presence of correlation effects is additionally confirmed by the fact that during VO2 film annealing in vacuum, the hysteresis loops shift to lower temperatures (Fig. 3b, curves 2, 3). Specifically, during vacuum annealing, oxygen vacancies are formed, which are electron donors. Donor electrons pass into the conduction band. Due to correlation effects, the conduction band decreases in energy, while the band gap and Tc narrow. Hydrogenation of the films also leads to the shift in the temperature hysteresis loops toward lower temperatures in both V2O3 and VO2. Indeed, introduced hydrogen, it being an electron donor, transfers free charge carriers to the conduction band, causing additional correlation lowering in energy and a decrease in Tc.
The destruction of n-dimers (V2O3) and a-dimers (VO2) with increasing temperature leads to a decrease in the number of dimers to a critical level, so that they cannot keep the lattice in a state of low symmetry. After that, an abrupt Peierls structural transition occurs. Vanadium ions move in an avalanche-
like direction toward the centres of oxygen octahedra under the influence of the valence forces of the oxygen skeleton. All of crystallite dramatically changes its symmetry and size. In both oxides, a strong acoustic emission arises (Andreev et al. 2000; McBride et al. 1974).
Crucially, it should be noted that acoustic emission in single crystals of both oxides is observed after Tc is reached both when the temperature moves deep into the metal phase and after Tc is reached when the temperature moves deep into the semiconductor phase. This directly indicates the displacement of V ions in both phases in a rather wide temperature range (4-5 K in V2O3 and 20 K in VO2). The width of the temperature range depends on the width of the grain size distribution of the film, since SMPT occurs in grains of different sizes at different temperatures.
Conclusion
This work provides a detailed comparison of the mechanisms of SMPT in sesquioxide (V2O3) and vanadium dioxide (VO2). The comparison is based on the analysis of both experimental data obtained by the authors and the data described in scientific literature. In addition to this, a number of points regarding the SMPT mechanism are described for the first time, such as the two types of n-bonds that can arise in V2O3, as well as the new types of n-dimers and a-dimers that can appear. For the first time, the integrated Mott-Peierls character of SMPT in V2O3 has been demonstrated and the causes of near SMPT acoustic emission in both oxides were revealed. The reasons for the difference in the numerical values of the conductivity jumps during SMPT in both oxides have been outlined.
A comparative analysis of the SMPT mechanisms in vanadium oxides expands the possibilities of the practical use of vanadium oxide film structures, as it allows the construction of combined systems for memorization, processing, and storing digital information. Such devices will be of a higher quality than older devices due to the reduction of thermal noise at low SMPT temperatures. In addition, detailed information on the SMPT mechanisms in nanocrystallites of vanadium oxide films will improve the synthesis of thin-film vanadium oxide structures that use the martensitic nature of SMPT in practical devices.
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