Synthesis and homogeneity range of Yb_8−xY_xV₂O₁₇ in the Yb₈V₂O₁₇–Y₈V₂O₁₇ system

Abstract

New substitutional limited solid solution of the formula Yb_8−xY_xV₂O₁₇ and 0 < x < 4.0 was synthesised by the solid-state reaction from the mixtures of the compounds Yb₈V₂O₁₇ and Y₈V₂O₁₇. The monophasic samples containing Yb_8−xY_xV₂O₁₇ were characterised by powder XRD, DTA, IR, UV–Vis–DRS and SEM methods. The influence of the degree of Y³⁺ ion incorporation in the crystal lattice of Yb₈V₂O₁₇ replacing Yb³⁺ ions, on the thermal stability, unit cell volume, band gap energies as well as on the position of the IR absorption bands in the spectrum of Yb_8−xY_xV₂O₁₇ was determined. The morphology of Yb₈V₂O₁₇ and solid solution was analysed.

Introduction

Yttrium vanadates(V) doped with rare earth metals have been for a few decades the subject of interest because of their unique and attractive physicochemical properties and have been applied in many branches of industry [1–13]. Yttrium orthovanadates(V) doped with Nd³⁺, Er³⁺ or Yb³⁺ ions make interesting laser materials employed in lasers used in medicine or electronic industry [1–7]. YVO₄ doped with Nd³⁺, Yb³⁺, Eu³⁺, Bi³⁺ or Er³⁺ ions has been applied as a luminophore or for production of plasma displays, electroluminescent diodes or fluorescence lamps [8–13].

Many scientific reports have been devoted to yttrium or ytterbium orthovanadates(V) [1–18], but much less attention has been paid to other vanadates(V) forming in the binary systems V₂O₅–Yb₂O₃ and V₂O₅–Y₂O₃, that is to the compounds Yb₈V₂O₁₇ and Y₈V₂O₁₇ [19–22]. The first of them Yb₈V₂O₁₇ has been for the first time obtained by Brusset et al. [19] as a result of heating of a mixture of oxides Yb₂O₃/V₂O₅ at the molar ratio 4:1 to 1550 °C. This compound crystallises in the monoclinic system, and the parameters of its elementary cell are: a = 1.0397 nm, b = 0.8242 nm, c = 1.5891 nm, β = 98.47° [19]. The same authors [20] constructed the phase diagram of V₂O₅–Yb₂O₃, taking into account the formation of YbVO₄ and Yb₈V₂O₁₇.

According to literature data on the system V₂O₅–Y₂O₃ [21, 22], the compound Y₈V₂O₁₇ has been obtained as a result of heating a mixture of oxides Y₂O₃/V₂O₅ at the molar ratio 4:1 at 1549 °C [21] and by the alkoxy method from VO(OC₂H₅)₃ and YCl₃ [22]. The authors of [22] have washed the sediment obtained many times with hot water and then after drying, subjected the sediment to calcination at 1500 °C. However, the X-ray structural data for the compound Y₈V₂O₁₇ obtained by the above-mentioned two methods are significantly different [21, 22]. The authors of [21, 22] agree that Y₈V₂O₁₇ occurs in two polymorphic varieties and that the low-temperature variety of Y₈V₂O₁₇ undergoes a monotropic phase transition to the high-temperature variety at 1550 °C. Only Yamaguchi et al. [22] have reported that the low-temperature variety crystallises in tetragonal system, while the high-temperature one in monoclinic system and determined the parameters of their elementary cells.

The state of knowledge on Yb₈V₂O₁₇ and Y₈V₂O₁₇, and in particular the lack of information on their thermal stability and incomplete information on their structures, has prompted us to supplement the gap.

Taking into account the fact that the title pseudo-binary system Yb₈V₂O₁₇–Y₈V₂O₁₇ has not been studied yet, the general aim of our study was to which phases–if any–form as a result of reaction between the components of this system, determination of their thermal stability and basic physicochemical properties. These new phases can potentially find similar or wider application than the already known ones formed with yttrium orthovanadate(V) or orthovanadates of rare earth elements.

Experimental

The following reagents were used in our experiments: Yb₂O₃, a.p. (Alfa Aesar, Germany), Y₂O₃, a.p. (Alfa Aesar, Germany), V₂O₅, a.p. (POCh, Poland). The components of investigated system, i.e. Yb₈V₂O₁₇ and Y₈V₂O₁₇, were obtained by heating in air the mixtures of V₂O₅/Yb₂O₃ and V₂O₅/Y₂O₃ at the molar ratio of 1:4 in the temperature range 600–1300 °C [19, 21].

Nine samples were prepared for the experiments. The samples were contained from 5.00 to 62.50 mol% of Y₈V₂O₁₇ in mixtures with Yb₈V₂O₁₇. The solid solution, Yb_8−xY_xV₂O₁₇, where 0 < x < 4, was prepared by the high-temperature solid-state reaction from the compounds: Yb₈V₂O₁₇ and Y₈V₂O₁₇ by the method described inter alia in [23–26]. Reagents weighed in suitable proportions were homogenised and calcined in air atmosphere in the temperature range 1400–1600 °C. The samples were heated in a tube furnace PRC 50/170/M (Czylok, Poland) equipped with a stationary optical pyrometer MARATHON MM (Raytek, Germany). The optical pyrometer was calibrated to the melting points of Ca₂P₂O₇ (1353 °C), CaF₂ (1420 °C) and Na₃PO₄ (1583 °C). After each heating stage, the samples were gradually cooled in the furnace to room temperature and analysed by the powder XRD or DTA methods too. The powder diffraction patterns of the samples obtained were recorded with the aid of the diffractometer EMPYREAN II (PANalytical, Netherlands) using CuKα with graphite monochromator. The phases were identified on the basis of XRD characteristics presented in the PDF cards [27]. The powder diffraction pattern of Yb_8−xY_xV₂O₁₇ was indexed in analogy to other phases [28], i.e. by means of the POWDER program [29]. The parameters of selected unit cell were refined using the REFINEMENT program of DHN/PDS package.

The DTA study was performed using an SDT 2960 thermoanalyser (TA Instruments, USA) under the air flow (110 mL min⁻¹), at the heating rate of 10° min⁻¹ in temperatures from the range 20–1450 °C. The DTA method for testing the thermal stability of the various phases forming in the multicomponent oxide systems was described, inter alia, in [30–33].

Selected samples were investigated also by means of an electron scanning microscope–SEM (JSM-6100, Jeol, Japan). The densities of Yb_8−xY_xV₂O₁₇ were determined in argon (5 N purity) with the help of an Ultrapyc 1200e ultrapycnometer (Quantachrome Instruments, USA). Initial mixtures and monophasic samples were examined by IR spectroscopy. The measurements were made within the wavenumber range of 1200–250 cm⁻¹, using a spectrophotometer SPECORD M-80 (Carl Zeiss, Jena, Germany). A technique of pressing pellets with KBr at the mass ratio of 1:300 was applied. The UV–Vis–DR spectra were measured using a UV–Vis spectrometer V-670 (JASCO, Japan) equipped with a reflecting attachment for the solid-state investigation (integrating sphere attachment with horizontal sample platform PIV-756/(PIN-757). The spectra were recorded in the wavelength region of 200–750 nm at room temperature.

Results and discussion

The first stage of the study was to verify the literature crystallographic data [19–22] on the compounds Yb₈V₂O₁₇ and Y₈V₂O₁₇ and to evaluate their thermal stability. Analysis of XRD diffractograms of Yb₈V₂O₁₇ and the low-temperature polymorph of Y₈V₂O₁₇ synthesised by us revealed that only the diffractogram of Yb₈V₂O₁₇ differed slightly from that reported by Brusset et al. [19]. Because of these differences including the presence of additional low-intensity XRD lines (d = 0.4675; 0.4492; 0.3911 nm), the Yb₈V₂O₁₇ diffractogram was subjected to indexation using the program POWDER [29].

Indexation was performed for selected 20 reflections from the range of 2θ angles from 6 to 40° (CuKα). The best agreement with experimental data was obtained for the triclinic elementary cell model. The results of indexation of the powder diffractogram of Yb₈V₂O₁₇ are presented in Table 1.

Table 1 Results of indexation of the powder diffractogram of Yb₈V₂O₁₇

The elementary cell parameters of Yb₈V₂O₁₇ were refined with the use of the program REFINEMENT (package DHN/PDS) to be:

• a = 0.8932 nm, b = 0.9259 nm, c = 0.9794 nm,

• α = 77.684°, β = 106.367°, γ = 116.348°

• elementary cell volume V = 0.6928 nm³,

• number of molecules in the elementary cell Z = 2.

The quality coefficient for this refined solution is F(20) = 12.37 (0.0192.84). The calculated density of Yb₈V₂O₁₇ for the selected solution is equal to 8.42 g cm⁻³ and experimentally determined with a gas ultrapycnometer was 8.26 ± 0.05 g cm⁻³. The difference in densities is probably connected with porosity of obtained compound.

As the solution proposed by us is significantly different from that given in [19] from 1973, we decided to refine the Yb₈V₂O₁₇ diffractogram known from literature with the use of program REFINEMENT. For the solution proposed by the authors of [19] which was the monoclinic system, the quality coefficient FM was very low, of only 2.24, which indicated a very small probability of crystallisation of this compound in the monoclinic system.

As thermal stability of the compounds Yb₈V₂O₁₇ and Y₈V₂O₁₇ has not been known, they were subjected to DTA–TG measurements in air atmosphere and in the temperature range 20–1450 °C. In this range no thermal effects were observed on both DTA and TG curves. This result means that these compounds undergo decomposition or melting at temperatures higher than 1450 °C. In view of this result, the study of reactivity between Yb₈V₂O₁₇ and Y₈V₂O₁₇ was started from a lower temperature of 1400 °C. The mixtures of Yb₈V₂O₁₇ and Y₈V₂O₁₇ of compositions specified in Table 2, were heated at the following stages: I:1400 °C (24 h) → II:1450 °C (24 h) → III:1500 °C (24 h) → IV:1550 °C (24 h).

Table 2 Composition of initial mixtures of the system Yb₈V₂O₁₇–Y₈V₂O₁₇ and results of phase analysis the samples after the last stage of heating

As evidenced by XRD analysis, already after the first stage of heating the phase composition of all samples changed. The XRD diffractograms recorded for samples 1–7 did not reveal the reflections characteristic of Y₈V₂O₁₇, and besides the lines characteristic of Yb₈V₂O₁₇ slightly shifted towards smaller angles 2θ, it showed the lines undoubtedly evidencing the presence of YVO₄ and Yb₂O₃. The subsequent two stages of the sample heating, at 1450 and 1500 °C, did not cause changes in the phase composition but changed the proportion of phases, i.e. the phases YVO₄ and Yb₂O₃ were present in small amounts. The diffractograms recorded after the last stage of heating showed only the lines characteristic of Yb₈V₂O₁₇. With increasing content of Y₈V₂O₁₇ in the initial mixtures of reagents, the lines were increasingly shifted towards smaller 2θ angles that evidenced increasing interplanar distances d_hkl relative to those in undoped Yb₈V₂O₁₇ [19].

According to these results, in the system Yb₈V₂O₁₇–Y₈V₂O₁₇ in the concentration range studied, the reactions leading to formation of substitutive solid solution in which Y³⁺ replaced Yb³⁺ in the crystalline lattice of Yb₈V₂O₁₇ took place. It is highly probable because of similar values of ionic rings of Yb³⁺ and Y³⁺ that can occur in the polyhedrons of NC = 6 (Yb³⁺-86.8 pm and Y³⁺-90.0 pm) as well as of NC = 8 (Yb³⁺-98.5 pm and Y³⁺-101.9 pm) [34]. The method for synthesis of this solid solution has been applied for patent protection in Poland [35].

According to phase composition determination in samples 8 and 9, representing the concentration ranges of the system components above 40.00 mol% Y₈V₂O₁₇, after the last stage of heating (1550 °C) the samples were biphasic and besides the Yb_8−xY_xV₂O₁₇ contained the high-temperature polymorph of Y₈V₂O₁₇ (Table 2).

The phase composition of all samples after the last stage of heating, presented in Table 2, shows that the solid solution formed in the system is substitutive, of limited solubility of components and general formula Yb_8–xY_xV₂O₁₇. The results collected in Table 2 additionally imply that the maximum degree of Y³⁺ ions incorporation into Yb₈V₂O₁₇ reaches at least 40.00% mol (x = 3.2) and does not exceed 50.00% mol. The above-discussed results lead to the conclusion that in the reaction mixtures studied the following reaction took place:

$$\left( {1{-}0.125x} \right){\text{Yb}}_{8} {\text{V}}_{2} {\text{O}}_{{17({\text{s}})}} + 0.125x{\text{ Y}}_{8} {\text{V}}_{2} {\text{O}}_{{17({\text{s}})}} = {\text{Yb}}_{8 - x} {\text{Y}}_{x} {\text{V}}_{2} {\text{O}}_{{17 \, ({\text{s}}.{\text{s}}.)}}$$

(1)

Figure 1 presents a fragment of XRD diffractogram of Yb₈V₂O₁₇ (Fig. 1a) to be compared with the analogous fragments of diffractograms of the new solid solution Yb_8−xY_xV₂O₁₇ for x = 0.8 (Fig. 1b), x = 2.8 (Fig. 1c) and x = 3.2 (Fig. 1d).

Fig. 1

Fragments of diffractograms of: a Yb₈V₂O₁₇, b Yb_7.2Y_0.8V₂O₁₇, c Yb_5.2Y_2.8V₂O₁₇, d Yb_4.8Y_3.2V₂O₁₇

At the next stage of the study our aim was to confirm that the new solid solution Yb_8−xY_xV₂O₁₇ has the structure of the matrix, that is it crystallises in the triclinic system. Thus, the powder diffractograms of the new solution Yb_8−xY_xV₂O₁₇ for x = 0.80; 2.00; 2.80 and 3.20 were subjected to indexation. The elementary cell parameters were refined with the program REFINEMENT. The results confirmed that the new solid solution crystallises in the triclinic system and permitted calculation of the parameters of its elementary cell as a function of the degree of Yb³⁺ substitution with Y³⁺ in the crystal lattice of Yb₈V₂O₁₇. Table 3 presents the elementary cell parameters, their volumes and densities for Yb_8−xY_xV₂O₁₇ with x = 0.00; 0.80; 2.00; 2.80 and 3.20.

Table 3 Unit cell parameters, volumes and densities for Yb₈V₂O₁₇ and solid solutions Yb_8−xY_xV₂O₁₇

According to the data presented in Table 3, with increasing x in Yb_8−xY_xV₂O₁₇, that is with increasing degree of Yb³⁺ substitution with Y³⁺, the crystal lattice expansion takes place; that is the elementary cell volume increases with respect to that of the matrix Yb₈V₂O₁₇. Moreover, the density of the solid solution determined with the use of a gas ultrapycnometer is lower than the value calculated on the basis of parameters selected elementary cells.

With increasing x in Yb_8−xY_xV₂O₁₇ a change in its colour from orange to yellow was observed.

At the subsequent stage the compound Yb₈V₂O₁₇ and the solid solution Yb_8−xY_xV₂O₁₇ with x = 3.2 were studied by scanning electron microscopy (SEM). The SEM images of the two polycrystalline samples are presented in Figs. 2 and 3.

Fig. 2

SEM image of Yb₈V₂O₁₇

Fig. 3

SEM image of Yb_8−xY_xV₂O₁₇ (x = 3.2)

The morphology of the solid solution Yb_8−xY_xV₂O₁₇ crystallites is very similar to that of the matrix Yb₈V₂O₁₇. They look like polygons of irregular shapes and different sizes, varying from 1 to 8 µm (Fig. 3). It should be pointed out that some crystallites of the matrix have ball-like shape of diameters close to ~6 µm (Fig. 2).

As the structures of Yb₈V₂O₁₇ and the solid solution Yb_8−xY_xV₂O₁₇ were not fully resolved, their IR spectra were measured to get some preliminary information on the type of metal oxide polyhedrons in structures these phases. Figure 4 presents the IR spectra of Yb₈V₂O₁₇ (Fig. 4a) and the newly obtained solid solution Yb_8–xY_xV₂O₁₇ with x = 0.80; 2.80 and 3.20 (Fig. 4b–d).

Fig. 4

Fragments of the IR spectra of: a Yb₈V₂O₁₇, b Yb_7.2Y_0.8V₂O₁₇, c Yb_5.2Y_2.8V₂O₁₇, d Yb_4.8Y_3.2V₂O₁₇

The IR spectrum of Yb₈V₂O₁₇ (Fig. 4a) shows a few absorption bands of maxima appearing at 940, 820, 450 cm⁻¹. The bands in the wavenumber range 970–620 cm⁻¹ can be assigned to stretching vibrations of the symmetric and asymmetric V–O bonds in VO₄ tetrahedrons and to the vibrations of Yb–O bonds in the YbO₆ octahedrons [36–38]. The presence of YbO₈ polyhedrons cannot be excluded [38]. On the basis of the IR spectrum of ytterbium vanadate(V) of the known structure [36–38] the absorption band in the range 650–300 cm⁻¹ can be assigned to bending vibrations of V–O bonds in VO₄ tetrahedrons and the band at ~630 cm⁻¹ can be attributed to V–O–V bond. On the other hand a broad band with a maximum at 450 cm⁻¹ can likely be attributed to the stretching vibrations of M–O bonds in the YbO₆ octahedrons [36–38].

As follows from analysis of the IR spectrum of the new solid solution (Fig. 4b–d) the positions of absorption bands in this spectrum are very similar to those in the IR spectrum of Yb₈V₂O₁₇. With increasing degree of Y³⁺ incorporation to replace Yb³⁺ in the crystal lattice of Yb₈V₂O₁₇, the positions of absorption bands shift towards smaller wavenumbers or remain unchanged with respect to their positions in the IR spectrum of the matrix. The results of this part of study confirmed that the solid solution Yb_8–xY_xV₂O₁₇ shows the structure of Yb₈V₂O₁₇, so contains VO₄ tetrahedrons and MO₆ polyhedrons (M=Y and Yb) or/and YbO₈ [36–39]. It is not possible to conclude from IR spectra on the way the polyhedron is joined and therefore studies are only qualitative.

In order to assess the thermal stability of Yb₈V₂O₁₇ and the solid solution Yb_4.8Y_3.2V₂O₁₇ obtained at 1550 °C, the samples containing these two phases were heated from 1450 to 1700 ± 20 °C in air atmosphere in a horizontal tube furnace equipped with an optical pyrometer.

The samples heated to 1600 °C did not show signs of melting, and their phase composition was unchanged. The diffractograms of the samples heated up to ~1650 °C show the set of XRD lines which, according to the PDF chart no. 00-035-0153, were assigned to the high-temperature polymorph of Yb₈V₂O₁₇.

The lines in the diffractogram of Yb_4.8Y_3.2V₂O₁₇ corresponded to slightly greater interplanar distances, which suggest the presence of the solid solution Yb_8–xY_xV₂O₁₇, as the high-temperature polymorph of Yb₈V₂O₁₇ is its matrix. The results obtained at this stage imply that at 1650 ± 20 °C the compound Yb₈V₂O₁₇ undergoes monotropic polymorphous transformation.

Heating of the samples up to ~1700 °C did not result in their melting and did not cause changes in their phase composition. As we have no possibility to heat the samples above 1700 °C, we could only conclude that the high-temperature polymorph of Yb₈V₂O₁₇ and the solid solution of the same structure are thermally stable in air at least to ~1700 °C. Figure 5 presents a fragment of XRD diffractogram of Yb₈V₂O₁₇ (Fig. 5a) with the analogous fragment of diffractogram of Yb_4.8Y_3.2V₂O₁₇ (of the new solid solution Yb_8−xY_xV₂O₁₇ for x = 3.2), i.e. of samples which were heated at ~1700 °C (Fig. 5b).

Fig. 5

Fragments of diffractograms of samples were heated at ~1700 °C: a Yb₈V₂O₁₇ b Yb_4.8Y_3.2V₂O₁₇

Literature data [40] on the high-temperature polymorph of Yb₈V₂O₁₇ are fragmentary so the studies should be continued to provide its physicochemical characterisation and detail structure.

In this study the physicochemical characterisation of Yb₈V₂O₁₇ and the solid solution Yb_8−xY_xV₂O₁₇ was extended by the UV–Vis–DR measurements that helped evaluate the energy gap for these phases.

The reflectance spectra of Yb₈V₂O₁₇ and the solid solution Yb_8–xY_xV₂O₁₇ for x = 0.80; 2.80; 3.20 after Kubelka–Munk transformation are shown in Fig. 6.

Fig. 6

Kubelka–Munk transformation of reflectance spectra of Yb₈V₂O₁₇ and the solid solution Yb_8–xY_xV₂O₁₇ for x = 0.80; 2.80; 3.20

The Kubelka–Munk transformation of measured reflectance performed using package Spectra Manager Version 2 (Spectra Analysis program) according to equation:

$$K = \frac{{(1 - R)^{2} }}{2R}$$

where K is reflectance transformed according to Kubelka–Munk and R is reflectance (%).

Then plotted the relationship (K·hν)² = f(hν)—see Fig. 6, where on the ordinate variable (K/M) means (K·hν)². The intersection between the linear fit and the photon energy axis gives the value to E _g.

The energy gap (E _g) obtained for the compound Yb₈V₂O₁₇ is ~2.60 eV, while the energy gap for the solid solution Yb_8–xY_xV₂O₁₇ evaluated by UV–Vis–DR method decreases with increasing degree of substitution x from E _g = ~ 2.52 eV for Yb_7.2Y_0.8V₂O₁₇ to ~2.44 eV for Yb_4.8Y_3.2V₂O₁₇. The above energy gap values mean that both the compound Yb₈V₂O₁₇ and the solid solution Yb_8−xY_xV₂O₁₇ are semiconductors.

Conclusions

• The substitutional, limited solid solution of the formula Yb_8−xY_xV₂O₁₇ and 0 < x < 4.0 is formed in the Yb₈V₂O₁₇–Y₈V₂O₁₇ system.

• This phase has been obtained in air by high-temperature reaction from the specially synthesised compounds: Yb₈V₂O₁₇ and Y₈V₂O₁₇.

• Yb_8−xY_xV₂O₁₇ crystalises in the triclinic system.

• With increasing x in Yb_8−xY_xV₂O₁₇, the crystal lattice of solid solution expands.

• The solid solution Yb_8−xY_xV₂O₁₇ is stable in air atmosphere up to ~1700 °C.

• Yb₈V₂O₁₇ and Yb_8−xY_xV₂O₁₇ belong to the group of semiconductors with the band gab energies from 2.60 to 2.44 eV.