Structural changes in V2O5-P2O5 glasses: non-constant force field molecular dynamics and IR spectroscopy

Quasi-binary phosphate-vanadate glasses have been studied by both IR spectroscopy and a novel method of molecular dynamics with a non-constant force field. This method is used for the self-assembly of structural models of glasses. The obtained models and the glass network structure are analyzed quantitatively using element distribution by the number of R–O–R bonds (R is phosphorous or vanadium) and 4-, 6-, and 8-membered cycles. The bends on the concentration dependences of atoms distribution in the second coordination sphere agree well with changing the shape of IR spectra. Based on the cycle analysis, the formation of cycles is shown to be more characteristic for vanadate fragments that can form 4-membered cycles, which, according to Zachariasen’s rule, negatively affects glass-forming ability.


Introduction
Oxide semiconducting glasses possess electron conductivity due to the electron transfer between ions of transition metals (V 4+ /V 5+ , Fe 2+ /Fe 3+ , Pb 2+ /Pb 4+ , etc.) [1][2][3]. Such glasses attract scientific attention because of the combination of electron conductivity and features of the vitreous state: glasses can be given any shape, and their composition can be varied in a wide range of concentrations, achieving the required properties. Glasses containing transition metal oxides can be used as electrode materials of batteries, gas sensors, for the disposal of high-level waste, etc. [4][5][6]. Vanadate glasses have the highest conductivity among semiconducting oxide glasses [7]; vanadate phosphate glasses are the most studied. At the same time, despite numerous works devoted to the study of vanadate-phosphate glasses in terms of their conductivity, insufficient attention has been paid to the explanation of some concentration dependence of non-electrical properties. For example, we have carried out a detailed study of the thermal and transport properties of xV 2 O 5 -(1-x)P 2 O 5 (x = 35-95 mol %) glasses, and it appeared that the properties change non-linearly with vanadium content and their concentration dependences demonstrate several clearly distinguished ranges [8]. It has been suggested that the observed phenomena are connected with the structural changes of the glass network. Vanadium glasses have a complex structure because vanadium ions can exist in four-, five-and six-coordinated states (tetrahedron, square pyramid, trigonal bipyramid, and octahedron [9]). Moreover, vanadium oxide is a transition metal which possesses at least four-and five-valent states in glasses, and ions with different valence can form the same structural groups [10]. According to [11], amorphous vanadium oxide consists of VO 4 and VO 5 structural units sharing edges and corners, while VO 4 units dominate in the molted V 2 O 5 and VO 5 units are formed due to the transformation of VO 4 ones at quenching. In binary vanadium phosphate glasses, VO 4 structural units can substitute similar PO 4 units [12] forming a continuous glass network. The structure of binary V 2 O 5 -P 2 O 5 glasses was studied in a wide composition range by NMR spectroscopy [13,14], ESR [15], and IR spectroscopy [16]. It was established that there are two types of VO 5 units in glasses involving VO 5 units sharing oxygen with only VO 5 units and VO 5 units sharing oxygen atoms with PO 4 units [14]. The structure of glasses containing 50 mol % P 2 O 5 was proven to be similar to the structure of VPO 5 crystalline compounds [17]. It was shown in [18] by studying the crystallization products of vanadium phosphate glasses that a Chimica Techno Acta 2021, vol. 8(2), № 20218211 ARTICLE 2 of 11 significant difference should be observed between the structures of glasses with compositions lying below and above 78 mol % V 2 O 5 . When V 2 O 5 content is more than 78 mol %, the crystallization of V 2 O 5 is observed; at V 2 O 5 concentration below 78 mol %, the crystallization of VPO 5 solid solution is possible, which is confirmed by IR spectroscopy. According to the recent research, main vanadium structural units in binary vanadate-phosphate glasses are presented by symmetric-V 5+ O 4 (s-VO 4 ), distorted-V 5+ O 4 (d-VO 4 ), V 5+ O 6 (VO 6 ), and V 5+ O 5 units (VO 5 ) [10].
In the present work, structural models of xV 2 O 5 -(1-x)P 2 O 5 (x = 35-95 mol %) glasses were obtained by means of self-assembly using non-constant field molecular dynamics. This method simulates the formation of glasses from a completely dissociated state by creating covalence bonds, and its applicability was demonstrated on boratevanadate glasses [19]. The obtained models were analyzed by different structural criteria, and some quantitative factors connected with the network structure were obtained. It was shown that the bends on concentration dependences of these parameters correlate with the bends on such dependences of physicochemical properties. Similar bends were observed in the IR spectra.
To study the structural changes occurring at a change in V 2 O 5 /P 2 O 5 ratio, infrared (IR) spectroscopy was used. The spectra were collected in the absorption mode at room temperature in a wavelength range of 400-4000 cm -1 using a Tensor 27 Fourier IR spectrometer (Bruker, Germany). Before measurements, glass powders were mixed with KBr in a weight ratio of 1:500, and the pellets were compacted in an evacuated mold. Then, pre-treatment (baseline subtraction) and deconvolution of the spectra into Gaussian components were performed using the Fityk software [20]. ions with the corresponding correction of the O 2number. This corrective was made according to some experimental evidence. The box sizes were chosen to match the earlier obtained density data [8]. A summary of the simulated system's size is given in Table 1. The following selfassembly procedure was performed in two stages. In the first stage, V 5+ and P 5+ ions can form double bonds with oxygen ions O 2when the distance between couples of vanadium(phosphorus) and oxygen ions is short enough. This process leads to species conversion: (1) Note that the formal oxidation degree of ions remains the same, but the partial charges are changed. V 3+2δ in Eq. (1) means five-valent vanadium with two electron pairs shared with the double-bonded oxygen (O d -2δ ). The value of δ was chosen as 0.6e, as in [22]. Hereinafter, we will omit the partial part of the charges (multiple δ) for brevity. This stage was stopped after all pentavalent cations were bonded to oxygen. In the next stage, the obtained cations as well as the remained V 4+ cations are sequentially linked to oxygen in a similar way up to the formation of four bonds for each cation (V 5+ and P 5+ had one double bond and three single, V 4+ had four singles). The oxygen ions converted sequentially into the terminal (O t , one bond) and bridging (O c , two bonds): (3) In Eq. (3), V 3+ means V 3+δ , not V 3+2δ as in Eqs. (1,2). At these stages, covalent bonds were taken in the form of harmonic potentials with the spring constant of 10- 30 eVÅ -2 and the corresponding equilibrium distance. The creation of bridging oxygen (O c ) was accompanied by adding a valent angle potential. Finally, after the selfassembly procedure, the main part of the simulation was provided during 3'000'000 steps with a timestep of 1 fs. The force field for this stage was replaced with the one from our previous work [8]. The electrostatics was calculated in a way suggested by Fennel and Gezelter [23]. In all cases, the temperature was kept around 298 K by the Nose-Hoover thermostat. , the considered wavelength range was reduced to 1800 cm -1 . The following peaks corresponding to the vibrations of the glass network can be distinguished in the spectra: ~480, 530, 665-690, 760, 800-820, 960-980, 1040-1060, 1230, 1370, and 1750 cm -1 . However, these peaks are broadened, and, in most cases, they are a superposition of several peaks, as typical for amorphous materials. A slight bend is seen near ~1010 cm -1 , after which a sharp growth of the intensity of the spectra is observed; such behavior might be connected with the presence of bound water in samples. Therefore, the spectra were considered in two wavelengths of 400-1010 and 1010-1800 cm -1 to increase the deconvolution accuracy. For clarity, the peaks observed in the spectra given in Fig. 1 are listed in Table 2. 4.1. Wavelength range from 400 to 1010 cm -1 As was mentioned above, several concentration ranges with the same spectrum shape exist. As V 2 O 5 content grows, a redistribution of intensity is observed between the wavelength ranges of 400-650 and 850-1010 cm -1 .  For the middle of the spectra (650-850 cm -1 ), no strong correlation between the glass composition and peak intensity is observed; however, its shape depends on the glass composition: when V 2 O 5 concentration is reduced from 95 mol %, the peak becomes smoother, and it almost disappears at V  [29][30][31]. Considering that the intensity of this peak increases as P 2 O 5 content grows, the main contribution might be assigned to the vibrations of PO 2 structural units [24,31].

Results and Discussion
In the wavelength range of 650-850 cm -1 , a gradual degeneration of the most intense peak is observed, and it shifts towards lower wavelengths (from 757 to 742 cm -1 for V 2 O 5 content of 95 and 75 mol %, respectively). The following vibrations can be excited in this spectrum area: stretching vibrations of P-O-P [24,31] and V-O-P [32] bonds and the vibrations of the vanadium-oxygen network such as asymmetric stretching vibrations of VO 2 groups in VO 4 tetrahedra [30,33]. Since the vibrations of phosphate and vanadate networks in this area are overlapped, it is complicated to unambiguously relate the peaks obtained by spectra deconvolution. It might be assumed that the main contribution to the intensity of the peak at ~750 cm -1 is made by the vibrations of the vanadium-oxygen network, as well as V-O-P vibrations, in the V 2 O 5 concentration range from 85 to 95 mol %. As P 2 O 5 content increases, this peak shifts towards the low-frequency area, which is followed by its vanishing; that may be connected with both a low intensity of vanadium-oxygen network vibration and restructuring of the glass network (transition from predominantly vanadate to mixed phosphatevanadate glass network).
Within the wavelength range of 850-1010 cm -1 , the most noticeable changes occur at V 2 O 5 content of 80 mol %. In the 85 ≤ x ≤ 95 (mol %) composition range, three peaks are observed in the spectra deconvolution: ~985, 965, and 933 cm -1 . The intensity of the peak at 985 cm -1 remains the same within this V 2 O 5 range, while the intensity is redistributed between the peaks at 933 and 965 cm -1 . The peak at 965 cm -1 can be attributed to the stretching vibrations of V-O bonds in VO 4 structural units [29]. Hence, decreasing its intensity is connected with the reduction of vanadium oxide concentration and, consequently, the number of bonds formed with vanadium ions. The peak at ~935 cm -1 is connected with the stretching vibrations of P-O-P bonds in Q 0 structural units [29,[34][35][36]; therefore, the growth of its intensity is also natural. It is interesting to mention the peak at 985 cm -1, which vanishes with decreasing the V 2 O 5 content from 95 to 80 mol %. According to the literature data, it can be caused by the vibrations of vanadium-oxygen tetrahedra VO 4 [37], constituting the glass network at high vanadium oxide content [38], or vanadium clusters [37]. Probably, the glass network changes drastically with the growth of P 2 O 5 content, which results in the decay of vanadium clusters.
As was mentioned above, a change in the shape of the spectra begins at V 2 O 5 content of 70 mol %, which is accompanied by the disappearance of a number of peaks and redistribution of vibration intensities; this change finishes at x = 65 mol % (Fig. 2). Apparently, the transition from predominantly vanadate to mixed phosphate-vanadate glass network occurs. Beginning from x = 85 mol %, there is a peak at ~530-540 cm -1 , whose intensity and position are kept with a further increase in P 2 O 5 concentration. This indicates that this peak is attributed to the vibrations of the phosphate structural units.
More noticeable changes in the shape of the spectra occur in the wavelength range of 650-900 cm -1 . The intensity of peaks in the area of 650-850 cm -1 increases sharply; the position of the peak at ~700 cm -1 observed for the glass with x = 70 mol % is almost unchanged, while the peak with the maximum at ~780 cm -1 shifts towards larger wavenumbers. The vibrations in this spectrum area are attributed to the symmetric stretching vibrations of P-O-P bonds in Q 2 structural groups [39][40][41][42]. The growth of the intensity of these peaks clearly indicates increasing the connectivity of the phosphate network with an increase in P 2 O 5 content. The peak at ~700 cm -1 appears in the spectra of glass with x = 70 mol % and maintains with the P 2 O 5 content growth up to x = 55 mol %; in the spectrum of x = 50 mol % composition, this peak completely disappears. According to the literature data [26], this peak is typical for P-O-P vibrations and symmetric stretching vibrations of phosphate rings [25]. Further distortion of the spectrum shape begins at V 2 O 5 content of 55 mol % and ends at x = 50 mol %. A slight narrowing of the top of the spectra accompanied by a redistribution of intensity of vibrations in all studied wavelength range is observed. The appearance of the peak at 488 cm -1 can be mentioned in the spectra of glasses with x = 55 mol %; this peak shifts to the low-frequency area as P 2 O 5 content grows. This peak may be attributed to the bending vibrations of PO 4 tetrahedra [43]. Its appearance indicates the formation of the metaphosphate chain with decreasing V 2 O 5 content, which indirectly indicates the modifying role of V 2 O 5 in this concentration range. The peak near 620-640 cm -1 becomes more pronounced, and it shifts towards the high-frequency area with the growth of P 2 O 5 concentration (660-690 cm -1 ). In this area, bending vibrations of O-P-O bonds [43] and stretching vibrations of P-O-P bonds [44] are observed. Increasing the intensity of this peak indicates the growth of connectivity of the phosphate network and the possible formation of this network with inclusions of vanadiumoxygen clusters. This suggestion is indirectly confirmed by the constancy of the peak near 980-990 cm -1 , whose appearance points up the existence of vanadium-oxygen clusters in the glass network [31]. -1 In the wavelength range of 1010-1800 cm -1 , changes in the shape and intensity of the spectra are similar to those observed in the 400-1010 cm -1 range. The broadening of the spectra is observed in this frequency range as the P 2 O 5 content increases (Fig. 1). This might be connected with the growth of hygroscopicity of glasses with an increase in P 2 O 5 and, consequently, a large amount of bound water in the glass structure. It can cause a broadening of the spectrum associated with the absorption of water molecules and the appearance of P-OH bonds. For these reasons, unambiguous interpretation of the results of spectrum deconvolution is difficult; therefore, only a brief discussion of the peaks which can be interpreted based on the literature data is given below. The peaks at ~1230-1270 and 1350-1370 cm -1 are present in the spectra of all studied compositions (Fig. 3), and they can be attributed to the vibrations of P=O bonds [45]. In addition, there is a peak at ~1040-1060 cm -1 , which indicates the vibrations of V=O bonds [46] or asymmetric stretching vibrations of pyrophosphate groups [47]; its intensity decreases with the growth of P 2 O 5 content. A small peak at ~1745 cm -1 is likely connected with the vibrations of the OH groups [48]. The rest of the peaks in the range of 1400-1800 cm -1 are assigned to the vibrations of P-OH bonds and adsorbed water molecules [48].

Simulation results
Examples of snapshots of the simulated systems are given in Fig. 4; for clarity, we presented only thin slices of the systems cut from the middle of the box. One can see phosphate-vanadate chains and valent angles R-O-R. The interatomic distances correspond to experimental data on single and double bond lengths, as was shown in detail in our previous work [8]. Fig. 5 shows distributions of vanadium ions by coordination number (at a cutoff of 2.21 Å) and mean coordination number as a function of vanadium content. The dominated coordination is five (about 60%), and, in general, the proportion of four-coordinated vanadium decreases, and that of six-coordinated increases with an increase in vanadium content.
To estimate the structure of the glass network, we analyzed the number of atoms with different quantities of triatomic bond sequences; for this purpose, we introduced R n OR notation, where n is the number of R-O-R bonds starting from a given atom R, see Fig. 6 for the explanation. Fig. 7 shows the dependences of P n OP as a percentage of the total number of P atoms; for example, P 2 OP = 30% means that 30% of phosphorous atoms have exactly two bonds P-O-P starting from them. All  It is interesting that P 2 OV fluctuates around 30% for all studied compositions but two last, i.e., every third P atom has two P-O-V bonds, Fig. 9. The fraction of P atoms without P-O-V bonds (P 0 OV ) drops dramatically down to the value of 2.4% at x = 65 mol %, then decreases smoothly and disappears around x = 85 mol %. In addition, we analyzed the obtained configurations for 4-, 6-and 8-membered cycles using the written script. The preliminary results, averaged over five configurations, are summarized in Table 3. As one can see from the table, the total number of cycles, as well as the number of participated in cycles atoms, grows with increasing of V 2 O 5 content. It is interesting that 4-membered cycles were detected. In terms of coordination polyhedra, such cycles mean that two coordination polyhedra have a shared edge, not a corner which is against Zachariasen's rule [49]. to the number of corresponding atoms; these fractions directly correlate with the amount of phosphorous/vanadium in the system. However, the formation of 4-membered cycles is less typical for phosphorous than for vanadium. Even at high P 2 O 5 content, less than 5% of phosphorous participates in such cycles. One can conclude that phosphorous oxide obeys Zachariasen's rule strictly than vanadium oxide. This leads to the fact that P 2 O 5 is the main glass former and V 2 O 5 is an intermediate. In general, vanadium is more prone to form cycles due to higher coordination numbers and longer V-O bonds. Fig. 11(b) shows the dependences of the number of 6membered cycles normalized to the total number of P and V atoms. Here is also evident that vanadium tends to form cycles better than phosphorous. Even at the equimolar ratio of the oxides in the glass (x = 50 mol %), the dominated cycle's species is VVP. Starting from x = 55 mol %, the numbers of the VVV and VVP cycles are comparable, and from x = 70 mol %, the VVV cycles become dominated among 6-membered cycles. Examples of the found cycles are given in Fig. 12.  glasses found earlier [8]. A bend was observed in the concentration dependence of glass transition temperature in the area of x = 65-70 mol % (Fig. 13a). It was suggested that this effect could be explained by changing the glass structure, for example, by the transition from predominant vanadium-oxygen units to a mixed vanadiumphosphate network with a predominance of phosphate structural units. According to the IR spectroscopy results, a noticeable restructuring is observed in this range of V 2 O 5 concentration which agrees with our hypothesis. It was confirmed by the simulation results (Fig. 13a): above x = 65 mol %, phosphorous atoms without vanadium ones in the second coordination sphere are almost absent. When x < 65 mol %, the number of such phosphorous atoms sharply increases, i.e., isolated phosphorous clusters are formed (Fig. 9).
Moreover, a change in conductivity behavior was observed in the same range of V 2 O 5 concentrations which was expressed as a change in the impedance spectra shape and a sudden decrease in conductivity (Fig. 13b). It was assumed that such behavior is connected with a change in the charge transfer mechanism in the studied glasses. Based on the data on the structure of phosphate-vanadate glasses, one can assume that structural changes strongly affected the conductivity. Similar changes were observed at x = 45-50 mol % that can also be explained by the change of the glass network from mixed to predominantly phosphate with vanadium-oxygen clusters. When x < 50 mol %, the number of phosphorous atoms with two bonds P-O-P (which could be considered as phosphate chains with vanadate branches) is about a third of all phosphorus atoms, and it decreases with the x growth (Fig. 7). Moreover, it is seen that at x = 50 mol %, a sharp increase in the fraction of phosphorus atoms isolated by the vanadate matrix begins. Thus, it can be concluded that the observed deviations in the concentration dependences of some physicochemical properties of xV 2 O 5 -(100-x)P 2 O 5 glasses can be explained by the detailed investigation of their structure by IR spectroscopy combined with the analysis of atomic configurations obtained by molecular simulation. a b c Fig. 13 (a) concentration dependences of glass transition temperature (Tg) and P 3 OP cycles and (b) concentration dependences of lgσ50 and V 5 OV cycles. Data on glass transition temperature and electrical conductivity are taken from [8] 5. Conclusions xV 2 O 5 -(100-x)P 2 O 5 (x = 35-95 mol %) glasses were studied by IR spectroscopy. By means of deconvolution of the IR spectra, the data were obtained on changes in the glass structure, and the correlation of these changes with a number of physicochemical properties was shown. To obtain the structural models of the glasses, self-assembly was performed using non-constant field molecular dynamics. The models were analyzed to find the atom distribution by the number of defined triatomic sequences. Bends and kinks are observed in their concentration dependences, which indicate the transitions between the main glassforming elements. These points coincide with the changes observed in the IR spectra.
It was established from the analysis of obtained molecular configurations of glasses in the presence of cycles that the vanadate fragments are much more prone to the ring formation, including 4-membered cycles. According to Zachariasen's rule, systems with 4-membered cycles (edge connected coordination polyhedra) cannot have a strong tendency to vitrification. So, our results can explain why V 2 O 5 has much lower glass-forming ability than P 2 O 5 (melts enriched with vanadium require higher cooling rate to vitrify).