Investigation of structural transition in molybdates CuMo1-xWxO4 prepared by polymeric precursor method

Solid solutions (CuMo1-xWxO4 where 0 ≤ x ≤ 0.12) were synthesized by using a polymeric precursor method. The crystallization process of the polymeric precursors was investigated using TG-DTA, FTIR and XRD techniques. Crystallization of the CuMo1-xWxO4 powders was detected at 300 °C, and entirely completed at 420 °C. XRD patterns revealed that the polymorphic variety was dependent upon the tungsten content. For x ≤ 0.075, the solid solutions were isostructural to α-CuMoO4 whereas, for 0.075 < x ≤ 0.12, they were isostructural to γ-CuMoO4. The obtained homogeneous powders are constituted of relatively spherical grains with sizes in the range 0.5–2.0μm. The phase transition of the as-synthesized CuMo1-xWxO4 powders was investigated by X-ray diffraction at elevated temperature and dilatometry. The temperature of the structural transition γ → α during heating was dependent upon the tungsten content.


I. Introduction
Because of their potential applications as temperature indicator, an intensive research on thermo-chromic materials was carried out [1][2][3].Most of them are of organic type and their use is limited to applications at low temperature because of their low thermal stability.The thermo-chromic oxides are of special interest because of their high stability at high temperatures.Several mechanisms explain the change of their colour with temperature: dilatation and reduction of the ligand field on a chromophoric cation (e.g. in chromium doped alumina) [4], reduction of the band gap in semiconductors [5], or phase transition (e.g. in molybdates AMoO 4 , where A is a 3d-transitional metal) [3,6,7].The molybdate CuMoO 4 particularly exhibits a reversible phase transition α (green) → γ (brown-red) [1,3].In the polymorph α-CuMoO 4 , stable at high temperature and low pressure, the Cu 2+ ions are located both in octahedral (CuO 6 ) and in pyramidal (CuO 5 ) sites.The Mo 6+ fax: +212 522 230 674, e-mail: Benchikhi_mohamed@yahoo.fr cations are in tetrahedral environment (MoO 4 ) [3,8].In the polymorph γ-CuMoO 4 , stable at low temperature and high pressure, both metallic cations are in octahedral environment [3,8].From standard conditions of temperature and pressure, the α → γ transition occurs either by increase of pressure above 2.5 kbar or by cooling below 200 K [3].Recently it has been shown that, in tungsten doped copper molybdate CuMo 1-x W x O 4 (x ≤ 0.12), the transition temperature and pressure are dependent upon the tungsten content [1].Numerous authors have studied the optical, electrical, magnetic, and catalytic properties of CuMoO 4 [2,[8][9][10][11].
The most general method to prepare molybdate powders involves solid state reaction of mixed parent oxides [8], which needs high temperature to overcome the energy barrier [12].The powders obtained by this method are composed of large particles with an inhomogeneous composition.The inhomogeneous structure is formed because MoO 3 has tendency to volatilize at high temperatures [13].For these reasons, various chemical methods have been investigated to prepare molybdates powders, such as co-precipitation method [1], polyol process [9], pyrolysis technique [14], electrochemical route [15] and recently the citrate complex method [16,17].This latter, allowing the control of the composition, the grain size and the homogeneity of powders [16][17][18], was chosen for the preparation of tungsten doped copper molybdate.
Solid solutions CuMo 1-x W x O 4 were synthesized within the domain of solubility of tungsten in CuMoO 4 (0 ≤ x ≤ 0.12) and the phase transformation γ → α was studied by X-ray diffraction and dilatometry.

II. Experimental
The solid solutions CuMo 1-x W x O 4 were synthesized by the polymeric precursor method using the appropriate amounts of copper nitrate Cu(NO 3 ) 2 ×3 H 2 O (Acros organics, 99.9%), ammonium molybdate (NH 4 ) 6 Mo 7 O 24 ×3 H 2 O (Sigma Aldrich, 99.8%), ammonium tungstate (NH 4 ) 10 W 12 O 41 ×5 H 2 O (Acros organics, 99.0%) and citric acid (Acros organics, 99.9%).The synthesis procedure was similar to that used for the preparation of undoped α-CuMoO 4 [16].The stoichiometric amounts of metal salts were dissolved separately in deionized water under magnetic stirring at room temperature.The obtained solutions were mixed together in order to obtain final molar ratio Cu : Mo : W = 1 : 1 − x : x (with x = 0.025, 0.05, 0.075, 0.100, 0,120).A citric acid (CA) solution was added to the mixture in a proportion such that CA/cation = 3.The solution was heated under magnetic stirring at 80-100 °C until the formation of a viscous liquid.The obtained gels were dried at 120 °C for 24 h and then pre-calcined at 300 °C for 12 h.
The crystallization process of the polymeric precursors was evaluated by thermogravimetry-differential thermal analysis (SETERAM TG-DTA 92) and infrared spectroscopy (FTIR, Perkin Elmer 1760).The crystalline structure was investigated by X-ray diffraction (Brucker AXSD4, λ CuKα = 0.154056 nm, operating voltage 40 kV and current 40 mA) with a 0.02°step scan and 3.6 s/step.The microstructure and chemical composition of samples were analysed using scanning electron microscopy (SEM, JEOL JSM 6400 provided with energy dispersion X-EDS).The phase transition was investigated by X-ray diffraction (Brucker ASXD8 fitted with a high temperature chamber of MRI Radiation type) and dilatometry under air flux at a temperature rate of 2.5 °C/min (Setsys Evolution TMA Setaram).

Structural characterization
Figure 1 shows the TG-DTA curves for the CuMo 0.9 W 0.1 O 4 gel dried at 120 °C.Considerable weight loss can be seen in the TG curve with temperature increase up to 420 °C.For temperature higher than 420 °C, there is no weight loss in the TG curve, confirming that combustion of all organic matter occurs below this temperature.The DTA curve indicates that there are  the carboxylate group, respectively [19].The absorption bands at 1080 and 900 cm -1 are possibly due to δ(CO) and υ(C−C), respectively [20].The bands at 1230 and 856 cm -1 are attributed to asymmetric NO 3 -stretching and bending vibrations, respectively [21].The absorption intensity of these bands decreased with increasing temperature up to 420 °C.This indicates that the organic species have been decomposed completely at 420 °C, agreeing basically with TG/DTA analysis.The new absorption bands at 966, 945, 907, 874, 846, 823, 799, 744, 728 and 496 cm -1 are attributed to the vibrational mode of copper molybdate crystalline phase [22].
The microstructure of the solid solution CuMo 0.9 W 0.1 O 4 prepared by the citrate complex method was compared with that of a solid solution obtained by a conventional way according to the process described by Ehrenberg [8] (Fig. 4).The former powder was constituted of relatively spherical grains with sizes in the range 0.5-2.0µm whereas the latter was constituted of rectangular grains with sizes larger than 10 µm.The chemical composition of the samples was checked by elemental analyses.The results are summarized in Table 1, indicating that the W/(Mo+W) ratio is in agreement with the intended stoichiometry.

Investigation of the phase transition γ → α
As shown on the X-ray thermo-patterns of the powder γ-CuMo 0.9 W 0.1 O 4 (Fig. 5), the γ-CuMoO 4 phase was sole up to 50 °C.Above this temperature, the peak 201 characteristic of the phase α-CuMoO 4 (2θ = 23.8°) was detected.In the temperature range 50-200 °C, the intensities of the peaks of the γ-CuMoO 4 phase decreased at the benefit of the ones of the α-CuMoO 4 type  powders versus temperature (Fig. 6).The α-phase content (C) was calculated according to the formula: where I α and I γ are the relative intensities of the strongest XRD peaks (201 for α and 202 for γ), respectively.It can be seen that T 1/2 increases with the increase of tungsten content (Table 2).Our values are of the same order of magnitude as those measured by Yanase [1] using the differential scanning calorimetry.
The γ → α transition occurs with a lattice volume increase of about 13% [3] which allows characterization of the transition by dilatometry.In order to specify the transition temperature, dilatometric measurements were carried out on the green pellets without any binder.For the solid solution γ-CuMo 0.9 W 0.1 O 4 (Fig. 7) a fast increase of the pellet volume, attributed to the transformation γ → α, was observed in the temperature range 95-183 °C.Then no volume change was noticed until the temperature reaches about 400 °C and above 400 °C the volume decrease was imputed to the sintering.The phase transition temperature was that for which the standardized ∆L/L 0 = 0.5 [24].The obtained values (Table 2) were in agreement with those determined by XRD in temperature.The partial isovalent substitution of molybdenum by tungsten in copper molybdate CuMoO 4 increased the γ → α phase transition temperature from −23 °C for γ-CuMoO 4 up to 127 °C for γ-CuMo 0.88 W 0.12 O 4 .

IV. Conclusions
Solid solutions (CuMo 1-x W x O 4 , where 0 ≤ x ≤ 0.12) were obtained by pyrolysis at 700 °C for 2 hours of a precursors prepared by the citrate complex method.The XRD results revealed that the polymorphic form was dependent upon the tungsten content.For x < 0.075 they were isostructural with the polymorph α-CuMoO 4 and for 0.075 ≤ x ≤ 0.120 isostructural with the polymorph γ-CuMoO 4 .The grain size was in the range 0.5-2.0µm.The in situ high temperature X-ray diffraction (HTXRD) and dilatometry studies revealed that the γ → α phase transformation temperature increased with the rise of tungsten content.

Figure 1 .
Figure 1.Simultaneous TG/DTA data measured at heating rate of 2.5 °C/min under air for CuMo 0.9 W 0.1 O 4 precursor powder

Figure 4 .
Figure 4. SEM micrographs of CuMo 0.9 W 0.1 O 4 obtained by: a) conventional method and b) citrate complex method phase.Above 200 °C, only the α-CuMoO 4 phase was detectable.So, the phase transition γ → α occurred in the temperature range 50-200 °C.The spreading of the transition temperature was attributed to the dispersion of the grain sizes of the powder, in agreement with Ponceblanc[23].The transition temperature T 1/2 , corresponding to the transformation of the half of the γ-type phase into α-type phase, was determined from the evolution of the content of the α-type phase in the CuMo 1-x W x O 4 powders versus temperature (Fig.6).The α-phase content (C) was calculated according to the formula:

Table 1 .
Chemical compositions determined by energy dispersive spectroscopy (EDS) and crystalline phases detected in CuMo 1-x W x O 4 prepared by the citrate method at 420 and 700 °C for 2 h