Synthesis, structure, and thermal stability of new scheelite-type Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x ceramic materials

Abstract

Monophasic polycrystalline samples of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution with limited homogeneity (0 < x ≤ 0.2222) and cationic vacancies (□) have been prepared by high-temperature annealing of PbMoO₄/Pr₂(WO₄)₃ mixtures composed of 40.00 mol% and less of praseodymium tungstate. Initial reactants and obtained ceramic materials were characterized by XRD, simultaneous DTA–TG, IR and UV–Vis–NIR techniques. The X-ray diffraction analysis showed that the monophasic samples crystallize in a tetragonal symmetry, with space group I4 ₁ /a (a scheelite-type structure), and PbMoO₄ is a matrix of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution. Thermal stability of samples under study strongly depends on concentration of Pr³⁺ ions. The Pb_0.9286□_0.0238Pr_0.0476(MoO₄)_0.9286(WO₄)_0.0714 solid solution (x = 0.0238) shows the highest melting point (1055 °C), and this value is slightly higher than the melting point of PbMoO₄ (1040 °C). Lead molybdate and samples of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution are insulators having indirect band gap E _g > 3 eV. The observed band gap of monophasic samples shows a nonlinear variation with a change of Pr³⁺ ions concentration in the scheelite framework.

Introduction

Divalent metal molybdates and tungstates (AMO₄, where M = Ca, Sr, Ba, Ca, Cd, and Pb; M = Mo or W) have wide industrial applications in many fields. They have been used as laser host materials, Raman lasers, optical fibers, humidity sensors, magnetic and photoluminescence materials, and catalysts [1–13]. Among them, lead molybdate and lead tungstate are very attractive materials because of their applications as acousto-optical modulators, deflectors, ion conductors, and solid-state scintillators for a nuclear instrumental application [14–22]. These materials doped with RE³⁺ ions exhibit good photoluminescence, and they are promising as laser hosts [14, 22]. Lead molybdate crystallizes with a scheelite-type tetragonal structure and has a space group (I4 ₁ /a), as well as four molecular formulas per unit cell (Z = 4) [1, 11, 15, 19, 20, 23, 24]. Its structure is made up of PbO₈ dodecahedra and MoO₄ tetrahedra connected via common vertices (Fig. 1). PbO₈ polyhedra are connected via common edges and form a 3D framework (Fig. 1) [11, 19].

Fig. 1

Structure of PbMoO₄ (cyan—PbO₈ dodecahedra, violet—MoO₄ tetrahedra)

Our earlier studies on a reactivity in a solid state of divalent metal molybdates or tungstates (CdMoO₄, CdWO₄ and others) with rare-earth compounds (molybdates or tungstates) have been showed an existence of new interesting materials for applications in optoelectronics [25–34]. The scheelite-type ceramic materials, i.e., Cd_0.25RE_0.50□_0.25WO₄ (RE = Pr, Nd, Sm, Eu, and Gd, vacancy is denoted by □), were synthesized via a solid-state reaction of CdWO₄ with rare-earth ditungstates RE₂W₂O₉ [25–28]. In the scheelite Cd_0.25Gd_0.50□_0.25WO₄ host, where trivalent europium ions occupy only one low symmetry site, a very strong red emission was observed [25]. This Eu³⁺-doped cadmium and gadolinium tungstate proved to be a new promising polycrystalline red phosphor for WLEDs [25]. As compared to optical parameters of YAG:Nd, new cadmium and gadolinium tungstate doped with Nd³⁺ ions (Cd_0.25Gd_0.50□_0.25WO₄:Nd³⁺, the concentration of active ions 5 mol%) showed larger absorption cross section (2.5·10⁻¹⁹ cm² at λ = 805 nm) and much higher emission at 1064 nm [26]. The broad spectral emission band observed in Cd_0.25Gd_0.50□_0.25WO₄:Nd³⁺ allows tuning of laser action over the 1030- to 1080-nm range for generation of very short pulses, and therefore, it has applications in pico- or femtosecond lasers [26]. Important group of very good and new candidates for applications in optoelectronics are scheelite-type cadmium and rare-earth metal molybdates Cd_1−3x□_xRE_2xMoO₄, where RE = Pr–Yb [29–34]. Value of x parameter strongly depends on a radius of trivalent rare-earth ion, and it can reach a maximum value of 0.25 [29–34]. The substitution of divalent cadmium ions by RE³⁺ ones leads to formation of cationic vacancies (□) in CdMoO₄ framework [29–34]. The appearance of vacancies results in some disorder of crystal lattice around RE³⁺ ions. This fact is manifested by an emission of broad bands associated with f–f transitions in RE³⁺ laser ions (e.g., Nd³⁺ or Yb³⁺), and this phenomenon could be used for ultrashort laser pulses [30, 31]. Single crystals of some Cd_1−3x □ _x RE_2x MoO₄ (RE = Gd, Dy) solid solutions with different concentrations of RE³⁺ ion have been successfully grown by the Czochralski method [35, 36]. Their structure, dielectric and magnetic properties, and FTIR and Raman spectra were recorded and discussed in detail [35, 36].

The aim of this work is to make a detailed study of new Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution that could be used as scintillator material in polycrystalline or single-crystal form. Thus, we present the results on a homogeneity range of the solid solution under study, its structure, and thermal and spectroscopic properties.

Experimental

Sample preparation

Lead molybdate (PbMoO₄) and praseodymium tungstate (Pr₂(WO₄)₃) were used as the starting materials. Lead molybdate was prepared by heating PbO with MoO₃ (both with the purity 99.95 %, Alfa Aesar) mixed in an equimolar ratio, and in the following thermal conditions: 550 °C (6 h), 600 °C (6 h), 700 °C (6 h), 800 °C (12 h), 900 °C (12 h), and 950 °C (12 h). Praseodymium tungstate was obtained using a solid-state reaction method [37–45] and Pr₆O₁₁ with WO₃ (both with the purity 99.95 %, Alfa Aesar) mixed in the 1:9 molar ratio according to the procedure used by us in a synthesis of other rare-earth tungstates, and under thermal conditions reported earlier [25–28, 37–39]. Mixtures of starting reactants with Pr₂(WO₄)₃ content changing from 0.50 to 50.00 mol% were homogenized in an agate mortar. Their initial content is presented in Table 1. Next, they were heated in air, in ceramics crucibles, with 12-h annealing stages, and in the temperature range from 900 to 975 °C. After each heating stage, all samples were cooled slowly down to ambient temperature, and for better reactivity, ground in an agate mortar. After a final sintering stage, monophasic samples were examined by XRD, DTA-TG, IR, and UV–Vis–NIR techniques. Additionally, their density was measured.

Table 1 Pr₂(WO₄)₃ content in initial PbMoO₄/Pr₂(WO₄)₃ mixtures, results of XRD analysis of samples obtained after the last annealing stage (the formula of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x was calculated on phase compositions of initial mixtures), calculated lattice constants for an adequate solid solution, calculated as well as experimental values of density, and determined band gap energy

Methods

Powder X-ray diffraction patterns were collected in the 10–80° 2Θ range with the step 0.013° on a diffractometer Empyrean II (PANalytical) using Cu Kα radiation (λ = 0.15418 nm). XRD patterns were analyzed by a HighScore Plus 4.0 software package, and lattice parameters were calculated using the least squares refinement procedure and a POWDER software [47]. Simultaneous DTA and TG measurements were carried out on a TA Instruments thermal analyzer (model SDT 2960) at the heating and cooling rate of 10 °C min⁻¹, in air (the gas flow 110 mL h⁻¹), and using alumina crucibles. The mass of each sample for DTA-TG measurements was ~30 mg. DTA and TG curves were recorded in the temperature range of 20–1100 °C. UV–Vis–NIR diffuse reflectance spectra were recorded at room temperature and in the wavelength range of 190–1000 nm using a JASCO V-670 spectrophotometer equipped with an integrating sphere. The reflectance measurements were carried out using scan step 0.5 nm and scan speed 400 nm/min. IR spectra of samples were collected in the 1500–200 cm⁻¹ spectral range on a Specord M-80 spectrometer (Carl Zeiss Jena) using pellets with KBr. The density of samples was measured on a Quantachrome Instruments Ultrapycnometer (model Ultrapyc 1200 e) using nitrogen (99.99 %) as a pycnometric gas. The measurements were carried out in five repetitions using ~4 g of each sample for the test. The error determine a density was estimated as ±0.01 g cm⁻³.

Results and discussion

XRD and IR studies

The X-ray powder diffraction patterns of initial reactants and all samples under study were recorded and analyzed. Figure 2 shows experimental XRD patterns of PbMoO₄ and products obtained after the final sintering stage of some PbMoO₄/Pr₂(WO₄)₃ mixtures. It was observed that when the initial content of praseodymium tungstate was 40.00 mol% and less, the XRD patterns consisted of diffraction lines that could be attributed to a scheelite-type lattice. No additional peaks of other phases, e.g., initial reactants, have been observed. Additionally, all observed diffraction peaks shift toward higher 2θ angle with an increase in Pr₂(WO₄)₃ content in initial mixtures. Very sharp and strong peaks clearly confirm good crystallinity of the as-prepared samples. All diffractions lines observed in the XRD patterns of single-phase materials were successfully indexed with a tetragonal symmetry and space group I4 ₁ /a. This fact has indicated a formation of a new solid solution and its synthesis can be described by the following equation:

$$\left( {1 - 3x} \right){\text{ PbMoO}}_{{4({\text{s}})}} + \, x{ \Pr }_{2} \left( {{\text{WO}}_{4} } \right)_{{3({\text{s}})}} = {\text{ Pb}}_{1 - 3{\rm x}} \square_{\rm x} {\text{Pr}}_{2{\rm x}} \left( {{\text{MoO}}_{4} } \right)_{1 - 3{\rm x}} \left( {{\text{WO}}_{4} } \right)_{{3{\rm x}({\text{s}})}}$$

(1)

A location of this solution is marked in the tetrahedron of PbO–WO₃–Pr₂O₃–MoO₃ system (Fig. 3). The lattice constants calculated by cell refinement fitting on the basis of XRD data for an adequate Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution identified in monophasic samples (Nos. 2–10), as well as experimental and calculated values of density are presented in Table 1. The cell parameters gradually decrease with an increase in value of x parameter only up to 0.2222 suggesting the solid solution limit has been achieved. Both, unit cell parameters and volume calculated for single-phase samples fulfill the Vegard law, i.e., are nearly linear function of x (Fig. 4). The diffraction pattern of a sample comprising initially 50.00 mol% of Pr₂(WO₄)₃ consisted of diffraction lines due to Pb_0.3334□_0.2222Pr_0.4444(MoO₄)_0.3334(WO₄)_0.6666 (the saturated solid solution), and additionally, the peaks corresponding to praseodymium tungstate have appeared. The above observations indicate that the solubility limit of Pr₂(WO₄)₃ in a scheelite-type framework of PbMoO₄ is not higher than 40.00 mol% and Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution exists for 0 < x ≤ 0.2222. Moreover, the results have indicated that Pr³⁺ (CN (coordination number) = 8, ionic radius −112.6 pm [46]) and W⁶⁺ (CN = 4, 42 pm [46]) ions were introduced to PbMoO₄ matrix instead of Pb²⁺ (CN = 8, 129 pm [46]) and Mo⁶⁺ (CN = 4, 41 pm [46]) ions, respectively. Compensation of excess positive charge resulting from substitution of divalent ions by trivalent ones is accomplished by an appearance of cationic vacancies denoted as □.

Fig. 2

Powder XRD patterns of PbMoO₄ (a), Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x for: x = 0.0050 (b); x = 0.0098 (c); x = 0.0238 (d); x = 0.0455 (e); x = 0.0839 (f); x = 0.1430 (g); x = 0.1667 (h); x = 0.2000 (i); x = 0.2222 (j)

Fig. 3

Tetrahedron of the PbO–WO₃–Pr₂O₃–MoO₃ system with a location of Pb_1−3x□_xPr_2x(MoO_4)1−3x(WO₄)_3x solid solution

Fig. 4

Linear fit of cell volume of Pb_1−3x□_xPr_2x(MoO_4)1−3x(WO₄)_3x solid solution versus x parameter

Figure 5 shows IR spectra recorded for polycrystalline samples of PbMoO₄ and Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x with different concentration of Pr³⁺ ions. For solid molybdates with a scheelite-type structure, the frequencies active in IR are observed in the wave number ranges 900–700 cm⁻¹ (the stretching multiples ν ₁ and ν ₃) and 450–250 cm⁻¹ (the bending modes ν ₂ and ν ₄) [33, 34, 48, 49]. Very narrow absorption bands can be observed in the IR spectrum of PbMoO₄ (pure matrix). The first group of bands sequentially recorded at 853, 816, and 768 cm⁻¹ can be assigned to the stretching modes of Mo–O bonds in MoO₄ tetrahedra [33, 34, 48, 49]. On the other hand, the absorption bands with their maxima at 380 and 296 cm⁻¹ can be related to the bending modes of Mo–O bonds in MoO₄ tetrahedra [33, 34, 48, 49]. The experimental results for pure lead molybdate confirm the presence of only regular MoO₄ tetrahedra occupying sites with the S₄ symmetry in scheelite-type framework. In the case of IR spectra of the samples of solid solution, an additional absorption band, apart to those ones recorded for PbMoO₄, was observed (Fig. 5). This band is clearly visible in the IR spectra of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x for x ≥ 0.0238 (at 925 cm⁻¹ for x = 0.0238), and it moves toward higher wavenumbers up to 937 cm⁻¹ (the saturated solid solution). Additionally, the intensity of this band significantly increases with increasing of praseodymium ions content in samples under study. Moreover, the absorption bands due to the stretching as well as bending modes of Mo–O bonds in MoO₄ tetrahedra observed also in the IR spectra of PbMoO₄ clearly move toward higher wave numbers when a concentration of praseodymium ions increased in doped samples. This fact suggests the presence of WO₄ tetrahedra in crystal lattice of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x [48, 49]. The appearance of additional absorption band and a change in its position suggests the presence of other types of molybdenum and tungsten polyhedra, i.e., distorted MoO₄ as well as WO₄ tetrahedra with cationic vacancy near one or two corners of MoO₄ and WO₄. The same phenomenon has been observed in the case of a similar solid solution with gadolinium ions (Cd_1−3x□_xGd_2xMoO₄) [33, 34]. Gradual increase in the intensity of the additional IR band (at ~930 cm⁻¹) and simultaneous reduction in the intensity of the bands observed at 853, 816, 768, 380, and 296 cm⁻¹ indicate the increasing number of deformed MoO₄ and WO₄ tetrahedra in comparison with a number of regular ones. It seems to be evident because with increase in concentration of Pr³⁺ ions the number of vacancies generated in the crystal structure of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution is increased.

Fig. 5

IR spectra of PbMoO₄ (a), Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x for: x = 0.0050 (b); x = 0.0098 (c); x = 0.0238 (d); x = 0.0455 (e); x = 0.0839 (f); x = 0.1430 (g); x = 0.1667 (h); x = 0.2000 (i); x = 0.2222 (j)

DTA-TG studies

Appropriate DTA-TG studies of obtained ceramic materials were preceded by similar studies for PbMoO₄ and Pr₂(WO₄)₃. Figure 6 shows DTA curves of PbMoO₄ (TG curve is not presented here) recorded during heating and cooling runs. The endothermic effect started at 1040 °C is due to congruent melting of lead molybdate. The crystallization process from a melt starts at a little higher temperature, i.e., 1053 °C. These results are similar to those ones obtained by Senguttuvan et al. and by Zeng [21, 50]. Lead molybdate does not show polymorphism. Two endothermic effects were observed on the DTA curve recorded during controlled heating of Pr₂(WO₄)₃ (Fig. 7, TG curve is not presented). The first one started at 1037 °C is connected with a phase transformation of a monoclinic α-Pr₂(WO₄)₃ (space group C2/c) to a high-temperature polymorph (β) with a orthorhombic symmetry (space group Pnca, a structure of Sc₂(MoO₄)₃-type) [51, 52]. A similar polymorphic change is observed in other rare-earth metal tungstates, e.g., for Gd₂(WO₄)₃ [38]. Praseodymium tungstate melts congruently in air at 1112 °C (the second endothermic effect on DTA curve of this compound recorded during controlled heating). The crystallization process from a melt and the phase transition of β- to α-polymorph start at 1107 and 903 °C, respectively (Fig. 7). Figure 8 shows DTA curves of some samples of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution (Nos. 2–10). On each DTA curve only one endothermic effect (symmetrical or asymmetrical) was recorded. This effect was caused by melting of each sample under study. When a concentration of Pr³⁺ ions in PbMoO₄ matrix increased up to x = 0.0238 (up to 2.50 mol% of praseodymium tungstate in initial PbMoO₄/Pr₂(WO₄)₃ mixtures), the onset of endothermic peak increased up to 1055 °C. The endothermic effects recorded on DTA curves of monophasic samples for x ≥ 0.0455 are clearly asymmetric with a visible inflection observed at a temperature slightly lower than a minimum temperature of each effect. This inflection can suggest a simultaneous presence of two endothermic peaks with slightly different onsets. In a case of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x samples for x ≥ 0.0455, the onset of observed peaks decreased from 1048 up to 1025 °C (for the saturated solid solution). In the whole homogeneity range melting point of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x samples shows nonlinear dependence on x parameter.

Fig. 6

DTA curves of PbMoO₄ recorded during controlled heating and cooling runs

Fig. 7

DTA curves of Pr₂(WO₄)₃ recorded during controlled heating and cooling runs

Fig. 8

DTA curves of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x for: x = 0.0050 (a); x = 0.0098 (b); x = 0.0238 (c); x = 0.0455 (d); x = 0.0839 (e); x = 0.1430 (f); x = 0.1667 (g); x = 0.2000 (h); x = 0.2222 (i)

UV–Vis–NIR spectra and band gap determination

The band gap energy, E _g, is a very important attribute of semiconductors because it determines their applications, e.g., in optoelectronics. Optical absorption studies are used to characterize the electronic properties of materials, through the determination of such parameters as band gap and excited state lifetime. The band gap for bulk PbMoO₄ and samples of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution was determined from their diffuse reflectance spectra showed in Fig. 9. The reflectance data were converted into absorption ones using the Kubelka–Munk function according to the following Eq. [53]:

$$F\left( R \right) \, = \, \left( {1 - R} \right)^{2} /2R$$

(2)

where R is reflectance and F(R) is the Kubelka–Munk transformation. It is known that the band gap E _g and absorption coefficient α are related as in the following Eq. [53]:

$$F\left( R \right) \cdot h\nu = \alpha \cdot h\nu = \, A(h\nu - E_{\text{g}} )^{\rm n}$$

(3)

where A is a constant characteristic for material under study, h is the Plank constant, ν is light frequency, and n is a constant associated with the different types of electronic transitions (n = 1/2, 2, 3/2, or 3) depending upon an nature of electronic transitions responsible for an absorption [54]. The Tauc optical gap for PbMoO₄ and monophasic samples of solid solution was determined through an extrapolation of the linear trend observed in the spectral dependence of (α·hν)^1/n, i.e., [F(R)·hν]^1/n over a limited range of photon energies hν. The values of band gap energy for n = 2, i.e., for indirect allowed transition of an electron from a valance to a conduction band are showed in Table 1. The E _g value for PbMoO₄ was found to be 3.17 eV, and this value is very close to those ones reported earlier, i.e., 3.14–3.19 eV [22], 3.20 eV [23], and 3.30 eV [1]. The observed band gap of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x samples increases from 3.14 eV (x = 0.0050) up to 3.22 eV (x = 0.2000). The E _g value of saturated solid solution, i.e., Pb_0.3334□_0.2222Pr_0.4444(MoO₄)_0.3334(WO₄)_0.6666 (x = 0.2222) is a slightly lower and equals 3.21 eV. It was also shown a nonlinear (parabolic) variation in the band gap with x parameter (Fig. 10). Similar dependence of E _g (nonlinear) on x parameter was observed for scheelite Ca_1-x Sr _x MoO₄ solid solution [55].

Fig. 9

UV-vis–NIR diffuse reflectance spectra of PbMoO₄ and Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x for different values of x parameter

Fig. 10

Dependence of band gap energy (E _g) on x parameter

Conclusions

Polycrystalline samples of new Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution, where □ are cationic vacancies, have been successfully synthesized by a solid-state reaction at the temperatures ranging from 900 to 975 °C. As the starting materials for synthesis, PbMoO₄ and Pr₂(WO₄)₃ were used. The homogeneity range of solid solution under study is 0 < x ≤ 0.2222, i.e., the maximum content of praseodymium tungstate in initial PbMoO₄/Pr₂(WO₄)₃ equals 40.00 mol%. All monophasic samples crystallize in a tetragonal body-centered scheelite-type structure. Samples of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution do not show polymorphism, and their melting point strongly depends on a value of x parameter. The highest melting point, i.e., 1055 °C is for a solid solution with x = 0.0238, and this temperature is slightly higher than a melting point of pure matrix (PbMoO₄, 1040 °C). IR spectra showed a presence of MoO₄ and WO₄ tetrahedra in the scheelite structure of new phases. Additionally, in comparison to regular MoO₄/WO₄ tetrahedra occupying sites with the S₄ symmetry in a scheelite-type framework, a some deformation of MoO₄ and WO₄ tetrahedra in structure of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x is observed. Both, pure matrix (PbMoO₄) and samples of Pb_1−3x□_xPr_2x(MoO₄)_1−3x(WO₄)_3x solid solution are insulators with indirect band gap E _g > 3 eV. The nonlinear dependence of E _g on a value of x parameter is also observed.