Structural characterization and colour of Mg_xCu_3-xV₂O₈ (0 ≤ x ≤ 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) compositions

Abstract

In this study, Mg_xCu_3-xV₂O₈ (0 ≤ x ≤ 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) compositions were synthesized by the chemical coprecipitation method and characterized by X-ray diffraction, UV–vis-NIR spectroscopy and CIE L* a* b* parameters measurements. Melting points of compounds Cu₃V₂O₈ and Cu₂V₂O₇ are 780°C and 790°C, respectively. The addition of small amounts of Mg (II), Mg_xCu_3-xV₂O₈ (x < 1.0) and Mg_yCu_2-yV₂O₇ (y < 0.5) fused compositions, was not sufficient to stabilize structures at 800°C. For the Mg₂CuV₂O₈ (x = 2.0) composition fired at 800°C, Mg (II) incorporated into the monoclinic Cu₃V₂O₈ structure stabilizes this crystalline phase. At 1000°C, orthorhombic Mg₃V₂O₈ structure from this composition was obtained. Solid solutions with orthorhombic symmetry were detected from the prepared compositions fired at 1000°C when 1.0 ≤ x ≤ 3.0. The difference of coloration of Cu, Mg vanadates might be explained by the presence of a strong charge transfer band in visible spectra.

Background

The preparation of materials with applications on ceramic pigments industry by the conventional ceramic method presents some drawbacks. Thus, the necessary high temperatures and long soaking times give rise to loss of volatile reagents and consequently to deviation of the stoichiometric conditions of the initial systems. Some improvements were obtained with alternative synthesis methods (García et al. 2001, Tena et al. 2003). For example, no desirable brown materials were obtained from oxide mixtures in V_xTi_1-xO₂ (x < 0.10) rutile solid solutions with potential usefulness as gray ceramic pigments but only bluish gray colorations were obtained from gels (Tena et al. 2003). However, the ceramic pigments industry tends towards cheap and simple processing. The synthesis from mixtures of solid starting materials with addition of halides as flux agents is habitual (Sorlí et al. 2004). Alternative synthetic methods are used exceptionally. The choice of precursors materials is a possibility to obtain desired final industrial product. The bright colours of vanadates could be considered from ceramic industry to development ceramic pigments but their melting points are low. These melting points can be modified by the formation of solid solutions (West 1984). Solid solutions are very common in crystalline materials. A solid solution is basically a crystalline phase that can have variable composition.

Cu₃V₂O₈ and Mg₃V₂O₈ compounds melt incongruently at 780°C and 1212°C respectively (Fleury 1966, Clark and Morley 1976). Mg₂V₂O₇ melts incongruently at 1132°C (Clark and Morley 1976) and Cu₂V₂O₇ melts at 790°C (Fleury 1966). Considering these melting points, Mg (II) orthovanadate and Mg (II) divanadate structures can be more suitable for ceramic industry than Cu (II) orthovanadate and Cu (II) divanadate structures.

Colour is often, but not always, associated with transition metal ions. In molecular chemistry, colour can arise from two possible common causes. The d-d electronic transitions within transition metal ions give rise to many of the familiar colours of transition metal compounds, e. g. the various shades of blue and green associated with different copper (II) complexes. Charge transfer effects in which an electron is transferred between an anion and a cation are often responsible for intense colours as in, for example, permanganate (purple) and chromates (yellow). In solids, there is an additional source of colour; it involves the transition of electrons between energy bands. Colour may be measured in a quantitative way by spectroscopic techniques. At higher frequencies than the infrared, electronic transitions associated with d-level splitting, impurity ions, crystal defects, etc., are possible. Many of these occur in the visible region and are responsible for colour.

Most of divalent metal orthovanadates of small ionic radius have orthorhombic symmetry. Copper orthovanadate presents triclinic symmetry at atmospheric pressure (Coing-Boyat 1982). A monoclinic form of this compound prepared under pressure at high temperature (4 GPa, 1173K) was described (Shannon and Calvo 1972). The structure of Cu₃V₂O₈ compound with monoclinic symmetry and space group P12₁/c1 is similar to the orthorhombic structure of Mg₃V₂O₈ compound with space group Cmca (Krishnamachari and Calvo 1971). Most of divalent metal divanadates, M₂V₂O₇, are polymorphic. Copper divanadate, Cu₂V₂O₇, presents orthorhombic (Calvo and Faggiani 1975), monoclinic (Mercurio-Lavaud and Bernard Frit 1973) and triclinic crystalline forms and Mg₂V₂O₇ presents monoclinic and triclinic symmetries (Gopal and Calvo 1974). Information about these crystal structures is included in an Additional file 1.

In this study, structural characterization of Mg_xCu_3-xV₂O₈ (0 ≤ x ≤ 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) compositions was made to investigate the possible formation of solid solutions with orthovanadate or divanadate structure. Coloration of these materials was also measured and related with structure in prepared compositions. A partial substitution of Cu (II) by Mg (II) ions might increase the melting point of materials with orthovanadates and divanadates structures. These potential solid solutions could be applied in ceramic industry.

Methods

Mg_xCu_3-xV₂O₈ (0 ≤ x ≤ 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) compositions were synthesized by the chemical coprecipitation method. The starting materials were Cu(NO₃)₂ · 2.5H₂O (Sigma-Aldrich, 98%), MgCl₂.6H₂O (Panreac) of reagent grade chemical quality, and NH₄VO₃ (Sigma-Aldrich, 99%). The stoichiometric amount of NH₄VO₃, Cu(NO₃)₂ · 2.5H₂O and MgCl₂ · 6H₂O was added on 200 mL of water with vigorous stirring at room temperature. These starting materials were added in solid state and the concentration of the various cations is different in each prepared composition. After that, a solution of ammonium hydroxide was added dropwise until pH = 8. The obtained precipitates were dried by an infrared lamp and dry samples were fired at 300°C for 12 hours, 600°C for 12 hours, 800°C for 1 hour and 1000°C for 1 hour.

The resulting materials were examined by X-ray diffraction with CuKα radiation to study the development of the crystalline phases at different temperatures. A structure profile refinement was carried out by the Rietveld method (Fullprof.2 k computer program) (Rietveld 1969, Rodriguez-Carvajal 2012, Chapon and Rodriguez-Carvajal 2008). Unit cell parameters and interatomic distances (M-O and V-O) in divanadates and orthovanadates structures were obtained from Mg_xCu_3-xV₂O₈ (0 ≤ x ≤ 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) fired compositions to investigate the possibility of formation of solid solution in these synthetic conditions. The diffraction patterns were collected running from 5 to 110 ^o2θ, using monochromatic CuK_α radiation, a step size of 0.02 ^o2θ and a sampling time of 10 s. The initial structural information was obtained of the Inorganic Crystal Structure Database (Inorganic Crystal Structure Database ICSD 2013). Table 1 includes the reference ICSD to every structure. This initial structural information also appears in the references (Coing-Boyat 1982, Shannon and Calvo 1972, Krishnamachari and Calvo 1971, Calvo and Faggiani 1975, Mercurio-Lavaud and Bernard Frit 1973, Gopal and Calvo 1974) for the main structures of this study. Dicvol program (Boultif and Loüer 2007) was used to obtain initial cell parameters in some compositions.

Table 1 Structural information of Cu and Mg orthovanadates and divanadates

UV–vis-NIR spectroscopy (diffuse reflectance) allows the Cu (II) site and the V (V)-O and Cu (II)-O charge bands in samples to be studied. A Jasco V-670 spectrophotometer was used to obtain the UV–vis-NIR (ultraviolet visible near infrared) spectra in the 200 to 2500 nm range. X-Rite spectrophotometer (SP60, an illuminant D65, an observer 10°, and a reference sample of MgO) was used to obtain CIEL*a*b* colour parameters on fired samples: L* is the lightness axis (black (0) → white (100)), a* the green (−) → red (+) axis, and b* is the blue (−) → yellow (+) axis (CIE 1971).

Results and discussion

Tables 2 and 3 show crystalline phase evolution with composition and temperature in Mg_xCu_3-xV₂O₈ (0 ≤ x ≤ 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) compositions. XRD patterns from Mg_xCu_3-xV₂O₈ and MgyCu_2-yV₂O₇ compositions are shown in Additional files 2 and 3.

Table 2 Evolution of crystalline phases with temperature in Mg _x Cu _3-x V ₂ O ₈ (0 ≤ x ≤ 3) samples

Table 3 Evolution of crystalline phases with temperature in Mg _y Cu _2-y V ₂ O ₇ (0 ≤ y ≤ 2) samples

In Mg_xCu_3-xV₂O₈ (0 ≤ x < 1.0) compositions the major crystalline phase is the triclinic Cu₃V₂O₈ polymorph at 600°C. Cu₃V₂O₈ compound melts incongruently at 780°C. This fact can explain the melting of samples with a presence higher than 50% in Cu₃V₂O₈ triclinic crystalline phase (small amount of Mg in compositions). This crystalline phase with triclinic structure is not detected in samples fired at 800 and 1000°C.

A crystalline phase with Cu₃V₂O₈ structure and monoclinic symmetry was obtained at 300 and 600°C in compositions with 1.0 ≤ x ≤ 2.0. This crystalline phase with monoclinic Cu₃V₂O₈ structure is also present when 1.5 ≤ x ≤ 2.0 at 800°C. The crystalline phase with the structure of MgCu₂V₂O₈ compound (β = 90.44 (2) and Z = 4) is detected when 1.0 ≤ x ≤ 2.0 at 800°C but the crystalline phase with the ordered metal distributions in Mg₂CuV₂O₈ compound (β = 116.22 (3), Z = 2, Cu1 in 2a sites and Mg1 in 4e sites, ICSD-404851) is not detected in conditions of this study. In the prepared Mg₂CuV₂O₈ composition (Mg_xCu_3-xV₂O₈ with x =2.0), crystalline phase with Cu₃V₂O₈ structure and monoclinic symmetry (Z = 2 and M1 (M1 = Mg, Cu) in 2b sites and M2 (M2 = Mg, Cu) in 4e sites) is the only crystalline phase detected at 800°C. Figure 1 shows the major crystalline phase detected in MgCu₂V₂O₈ (x = 1.0), Mg₂CuV₂O₈ (x = 2.0) and Mg_2.5Cu_0.5V₂O₈ (x = 2.5) compositions fired at 800°C.

Figure 1

Diffractograms from Mg _x Cu _3-x V ₂ O ₈ compositions at 800°C. Diffraction maxima from the major crystalline phase in : (a) x = 1.0, MgCu₂V₂O₈; (b) x = 2.0, Mg₂CuV₂O₈ and (c) x = 2.5, Mg_2.5Cu_0.5V₂O₈ compositions. Crystalline phases: MC (MgCu₂V₂O₈, monoclinic), CM (Cu₃V₂O₈, monoclinic) and MO (Mg₃V₂O₈, orthorhombic).

Crystalline phase with orthorhombic Mg₃V₂O₈ structure is obtained when 2.0 ≤ x ≤ 3.0 at 600°C and when 2.5 ≤ x ≤ 3.0 at 800°C. At 1000°C, this phase is the major crystalline phase in the unfused samples (x ≥ 1.0). Figure 2 shows the positions of diffraction lines and their intensities from Mg_xCu_3-xV₂O₈ compositions with x = 1.0, x = 2.0 and x = 2.5 fired at 1000°C. The orthorhombic Mg₃V₂O₈ crystalline phase is the major phase in them.

Figure 2

Diffractograms from Mg _x Cu _3-x V ₂ O ₈ compositions at 1000°C. Diffraction maxima from the major phase in: (a) x = 1.0, MgCu₂V₂O₈; (b) x = 2.0, Mg₂Cu₂V₂O₈ and (c) x = 2.5, Mg_2.5Cu_0.5V₂O₈ compositions. MO: orthorhombic Mg₃V₂O₈ crystalline phase.

Cu divanadates and Mg divanadates are present together with Cu and Mg orthovanadates in some samples.

Crystalline phase with α-Cu₂V₂O₇ structure (orthorhombic symmetry) is only present in Mg_yCu_2-yV₂O₇ compositions when y < 0.5 at 600°C. These compositions melt at 800°C. Diffraction peak intensity associated with this crystalline phase is weak or very weak at 300°C.

Presence of crystalline phase with β-Cu₂V₂O₇ structure (monoclinic symmetry) is more extended than the crystalline phase with α-Cu₂V₂O₇ structure in the prepared compositions. At 300°C, crystalline phase with β-Cu₂V₂O₇ structure is present when y < 0.5 with diffraction peaks of medium intensity. This crystalline phase is present when 0.25 ≤ y < 1.50 at 600°C and when 0.50 ≤ y ≤ 0.75 with diffraction peak of strong or medium intensity. Figure 3 shows graphical result of the diffraction profile refinement by Rietveld’s method from the Mg_0.25Cu_1.75V₂O₇ composition fired at 600°C.

Figure 3

An example of the diffraction profile refinement. The diffraction profile refinement by Rietveld’s method from Mg_0.25Cu_1.75V₂O₇ composition fired at 600°C. Crystalline phases: α-Cu₂V₂O₇ (C1) and β- Cu₂V₂O₇ (C2).

Crystalline phase with γ-Cu₂V₂O₇ structure (triclinic symmetry) is present in compositions when 0.25 ≤ y ≤ 0.50 at 300°C but its intensity of diffraction peak is weak at 600 and 800°C (Table 3).

In Mg_yCu_2-yV₂O₇ compositions fired at 300°C, the assignation to α- Mn₂V₂O₇ structure of a crystalline phase detected when y ≥ 1.25 might be explained by the existence of a polymorph of Mg₂V₂O₇ compound with this structure. It was not detected any match with the existing single-crystal related in the bibliography. Synthesis of single-crystal is the most used method in synthesis of vanadates.

Crystalline phase with α-Mg₂V₂O₇ structure (monoclinic symmetry) is detected in a compositional range more extended than the crystalline phase with triclinic Mg₂V₂O₇ structure from Mg_yCu_2-yV₂O₇ prepared compositions. Crystalline phase with monoclinic α-Mg₂V₂O₇ structure is identified when 0.75 ≤ y ≤ 2.00 at 600°C, when 0.50 ≤ y ≤ 2.00 at 800°C, and when 1.50 ≤ y ≤ 2.00 at 1000°C.

Crystalline phase with triclinic Mg₂V₂O₇ structure is observed when 1.5 ≤ y ≤ 2.00 in compositions fired at 800°C and only when y = 2.0 at 300°C.

Tables 4, 5, 6, 7, 8 and 9 show the unit cell parameters and weight fractions in crystalline phases with weight fractions higher than 8% from samples fired at 600, 800 and 1000°C. The variation of these parameters with composition confirms the formation of solid solutions.

Table 4 Unit cell parameters in crystalline phases with weight fractions higher than 8% from Mg _x Cu _3-x V ₂ O ₈ (0 ≤ x ≤ 3) compositions fired at 600°C/12 h

Table 5 Unit cell parameters in crystalline phases with weight fractions higher than 8% from Mg _x Cu _3-x V ₂ O ₈ (0 ≤ x ≤ 3) compositions fired at 800°C/1 h

Table 6 Unit cell parameters in crystalline phases with weight fractions higher than 8% from Mg _x Cu _3-x V ₂ O ₈ (0 ≤ x ≤ 3) compositions fired at 1000°C/1 h

Table 7 Unit cell parameters in crystalline phases with weight fractions higher than 8% from Mg _y Cu _2-y V ₂ O ₇ (0 ≤ y ≤ 2) compositions fired at 600°C/12 h

Table 8 Unit cell parameters in crystalline phases with weight fractions higher than 8% from Mg _y Cu _2-y V ₂ O ₇ (0 ≤ y ≤ 2) compositions fired at 800°C/1 h

Table 9 Unit cell parameters in crystalline phases with weight fractions higher than 8% from Mg _y Cu _2-y V ₂ O ₇ (0 ≤ y ≤ 2) compositions fired at 1000°C/1 h

From Mg_xCu_3-xV₂O₈ compositions solid solutions with triclinic Cu₃V₂O₈ structure are obtained when 0.0 ≤ x ≤ 1.0 at 600°C. At this temperature, the c unit cell parameter increases slightly with the replacement of Cu (II) ion by a slightly smaller one (Mg (II)) from Mg_xCu_3-xV₂O₈ compositions when 0.0 ≤ x ≤ 1.0. Ionic radii values do not explain this fact. A slight contraction of unit cell is expected. Structural distortion might explain the c increase with incorporation of Mg (II) ions in a structure (Tena 2012).

Variation of unit cell parameters obtained from Mg_xCu_3-xV₂O₈ (x ≥ 1.0) compositions fired at 800°C is noticeable in monoclinic Cu₃V₂O₈ structure (detected when 1.5 ≤ x ≤ 2.0) and it indicates the formation of solid solutions with monoclinic Cu₃V₂O₈ structure in 1.5 ≤ x ≤ 2 compositional range at 600°C and 800°C.

Solid solutions with orthorhombic Mg₃V₂O₈ structure are also obtained when 2.5 ≤ x ≤ 3.0 at 600, 800°C and when 1.0 ≤ x ≤ 3.0 at 1000°C. Figure 4 shows unit cell parameters and interatomic distances in Mg orthovanadate structure (orthorhombic symmetry) obtained from Mg_xCu_3-xV₂O₈ samples fired at 1000°C/1 h. At 1000°C, the a and b unit cell parameters decrease with the replacement of Cu (II) ion by a slightly smaller one (Mg (II)). Average changes of interatomic distances are very slight. Changes in intensities with composition (Figure 2) are due to changes in both the atomic coordinates and the Mg/Cu ratio in these sites in orthorhombic Mg₃V₂O₈ structure when solid solutions are formed. These solid solutions are the most stable solid solutions obtained in this study from Mg_xCu_3-xV₂O₈ compositions.

Figure 4

Cell parameters in the Mg ₃ V ₂ O ₈ structure. Variation of unit cell parameters (a) and interatomic distances (b) in Mg orthovanadate structure with composition obtained from Mg_xCu_3-xV₂O₈ samples fired at 1000°C/1 h.

From unit cell parameters values obtained in Mg_yCu_2-yV₂O₇ compositions, the formation of two kinds of solid solutions with monoclinic β-Cu₂V₂O₇ and monoclinic α-Mg₂V₂O₇ structures can be confirmed. Two crystalline phases with the same structure (β-Cu₂V₂O₇ or α-Mg₂V₂O₇) are detected in some samples (Tables 7 and 8). This fact indicates that the composition of these two crystalline phase (C2: β < 110.6 ^o and C2’: β > 110.6° with β-Cu₂V₂O₇ structure or two of M1: β = 100.0-100.6°, M2: β = 98.0-99.6°, M3: β = 94.5-97.0°, M4: β = 88.0-92.0° phases with α-Mg₂V₂O₇ structure) is slightly different at 600 and 800°C in the conditions of this study. This slight difference between the two crystalline phases with the same structure is evidenced with differences in unit cell parameters values obtained. In Mg_yCu_2-yV₂O₇ compositions fired at 600°C, the values of a and β parameters decrease when y increases in β-Cu₂V₂O₇ structure (crystalline phase is obtained with a weight fraction > 35% when 0.25 ≤ y ≤ 1.5). At 800°C this crystalline phase is obtained with a weight fraction > 35% only when 0.5 ≤ y ≤ 0.75. In crystalline phase with α-Mg₂V₂O₇ structure, β angle value is close to 100° at 300°C and it is close to 90° at 800°C. When two crystalline phases with this structure are detected in the same sample, unit cell parameters are 0.2 Å (a, b, c parameters) or 8 degrees (β parameter) longer and wider in one of them. This fact is in accordance with the formation of solid solutions in this monoclinic α-Mg₂V₂O₇ structure with the replacement of Mg (II) ion by Cu (II) and with an important distortion structural in these formed solid solutions. Solid solutions with monoclinic α-Mg₂V₂O₇ structure are obtained when 0.5 ≤ y ≤ 2.0 at 600 and 800°C. At 1000°C, these last solid solutions are obtained when y ≥ 1.5, including all unmelted samples (Table 9).

Coordination number of V (V) ion in Cu and Mg orthovanadate and divanadate structures is four. Coordination number of Cu (II) ion is four and five and coordination number of Mg (II) ion is six in these structures. Values obtained from samples are in accordance with literature about these structures. In distorted monoclinic structure of Cu₃V₂O₈ obtained from Mg₂CuV₂O₈ composition at 800°C, the coordination number of Cu (II) and Mg (II) ions are also six.

Average interatomic V-O distances is smaller than average interatomic Cu-O or Mg-O distances in all of detected crystalline phases. Figure 5 shows the average interatomic V-O and Cu-O or Mg-O distances obtained from Mg_yCu_2-yV₂O₇ compositions considering all the crystalline phases in each composition and its weight fraction in samples fired at 600, 800 and 1000°C. These M-O (M = Cu, Mg) interatomic distances increase with magnesium amount (y) when 0 ≤ y ≤ 1.25. This increased average M-O distance is coincident with destabilization of structures of Cu divanadate and a change is observed when y > 1.25. Structures of Cu divanadate are unstable with temperature and structures of Mg divanadates are stable at 1000°C when 1.5 ≤ y ≤ 2.0 (Table 3).

Figure 5

Interatomic distances in Mg _y Cu _2-y V ₂ O ₇ compositions. Average interatomic V-O (a) and M-O (b) distances in Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 1) compositions fired at 600°C/12 h, 800°C/1 h and 1000°C/1 h considering weight fraction of crystalline phases.

Figures 6 and 7 show UV–vis-NIR spectra of Mg_xCu_3-xV₂O₈ (0 ≤ x ≤ 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) compositions fired at 600, 800 and 1000°C. Visible spectra obtained from Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) compositions fired at 800°C/1 h and Mg_xCu_2-xP₂O₇ (0 ≤ x ≤ 1.5) compositions fired at 800°C/5d (Tena 2012) are shown in Figure 8. Most of complex with Cu (II) ion are formed with four ligands (IC = 4). The reason for the unusual behaviour is connected with the Jahn-Teller effect. Because of that, the Cu (II) ions do not bind the fifth and sixth ligands strongly. Structural distortion due to this Jahn-Teller effect can explain the asymmetry in observed bands. From Cu (II) ion (d⁹) a transition d-d is allowed. Experimentally electronic spectra of Cu (II) ion are often characterized by a single highly asymmetric band. In this study spectra show a strong absorbance in 700–1400 nm wavelength range with the absorption maximum at 800–900 nm. It can be associated with Cu (II) d-d transition. Bands due to d-d transitions are not expected from V (V) ion. Strong absorbance in 430–600 nm wavelength range (in visible wavelength range) with the absorption maximum depending of copper amount in the sample (x or y values) is detected in Mg_xCu_3-xV₂O₈ (0 ≤ x ≤ 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) compositions at 300°C/12 h, 600°C/12 h, 800°C/1 h and 1000°C/1 h. The strong absorbance in 350–600 nm wavelength range is not observed in Mg_xCu_2-xP₂O₇ compositions (Figure 8) and in Mg_xCu_3-xV₂O₈ (x = 3) and Mg_yCu_2-yV₂O₇ (y = 2) compositions. It is associated with charge transfer between Cu-O in orthovanadates and divanadates when 0 ≤ x < 3 or when 0 ≤ y < 2. When x = 3 or y = 2, the charge transfer is associated with V-O charge transfer (λ < 430 nm in studied temperature range). Maximum of this band is observed at higher wavelength when copper amount is hight than when the copper amount is low. This strong absorbance in visible wavelength range explains the colour of these materials. At 600 and 800°C, coloration from samples with vanadate structures is red-brown, orange and yellow and is very different to the weak blue coloration obtained from samples with phosphate structures (Tena 2012). At 1000°C, brown-dark and black colorations are obtained when x < 3.0 or y < 2.0 and a strong absorbance in 400–1800 nm range is obtained in spectra. In the most of samples band due to d-d transition and band due to charge transfer can not be differentiated at 1000°C. Vanadate structure does not seem to be the only important factor for the presence of this charge transfer band because it is present in Mg_xCu_3-xV₂O₈ (x ≠ 3) compositions and in Mg_xCu_2-yV₂O₇ (y ≠ 2) compositions with different crystalline phases detected by XRD (Tables 2 and 3). The difference of coloration of Cu phosphates and Cu vanadates might be explained from differences in interatomic Cu-O distances due to the presence of bonds P-O or V-O. Interatomic Cu-O distances are smaller in vanadate structures than in phosphate structures (Cu-O bond is more strong in vanadates than in phosphates) because the V-O bond is weaker than the P-O bond (interatomic V-O distances are slightly greater than interatomic P-O distances). Table 10 shows interatomic distances in some of these compositions.

Figure 6

Variation of UV–vis-NIR spectra from Mg _x Cu _3-x V ₂ O ₈ compositions. UV–vis-NIR spectra of Mg_xCu_3-xV₂O₈ (0.0 ≤ x ≤ 3.0) samples fired at: (a) 600, (b) 800 and (c) 1000 ºC; x = 0.0 (1), x = 0.5 (2), x = 1.0 (3), x = 1.5 (4), x = 2.0 (5), x = 2.5 (6) and x = 3.0 (7).

Figure 7

Variation of UV–vis-NIR spectra from Mg _y Cu _2-y V ₂ O ₇ compositions. UV–vis-NIR spectra of Mg_yCu_2-yV₂O₇ (0.0 ≤ y ≤ 2.0) samples fired at : (a) 600, (b) 800 and (c) 1000 ºC; y = 0.0 (1), y = 0.50 (2), y = 0.75 (3), y = 1.0 (4), y = 1.25 (5), y = 1.50 (6), y = 1.75 (7) and y = 2.0 (8).

Figure 8

Visible spectra of compositions at 800°C. Visible spectra of: (a) Mg_yCu_2-yV₂O₇ (0.0 ≤ y ≤ 2.0) and (b) Mg_xCu_2-xP₂O₇ (0.0 ≤ x ≤ 1.5) samples fired at 800°C; y = 0.50 (2), y = 0.75 (3), y = 1.00 (4), y = 1.25 (5), y = 1.50 (6), y = 1.75 (7), y =2.00 (8).

Table 10 Interatomic distances in orthovanadates, divanadates and diphosphates of Cu and Mg

CIE L*, a* and b* parameters of Mg_xCu_3-xV₂O₈ (0 ≤ x ≤ 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y ≤ 2) samples fired at 600, 800 and 1000°C are shown in Table 11.

Table 11 CIE L* a* b* colour parameters in Mg _x Cu _3-x V ₂ O ₈ (0 ≤ x ≤ 3) and Mg _y Cu _2-y V ₂ O ₇ (0 ≤ y ≤ 2) samples

Interesting yellow colorations are obtained when x = 1.5 and 2.0 at 600°C/12 and when x = 2.0 at 800°C/1 h. In these samples that development yellow colorations, crystalline phase with monoclinic Cu₃V₂O₈ structure is detected. The best yellow colour is obtained from Mg₂CuV₂O₈ (x = 2.0) solid solution with monoclinic Cu₃V₂O₈ structure. This is the only crystalline phase detected at 800°C in conditions of this study. The Mg (II) incorporated into this structure stabilizes this crystalline phase at temperatures higher than 780°C (melting point of Cu₃V₂O₈). In Mg₂CuV₂O₈ composition fired at 800°C, average distances of M1-O (M1 = Cu, Mg) = 1.9587 Å and M2-O (M2 = Cu, Mg) = 2.1129 Å are obtained. Yellow colorations are not obtained from Cu, Mg divanadates.

The colour red-brown obtained from Mg_0.5Cu_1.5V₂O₇ (y = 0.5) fired at 600°C /12 h is the most noticeable colour but it is unstable at 800°C /1 h. Dark brown colour is obtained from this composition at 800°C. The major crystalline phase detected by XRD is the crystalline phase with β-Cu₂V₂O₇ structure in this sample fired at 600°C.

Orange colour is obtained when y = 0.75, 1.00 and 1.25 at 600°C/12 h and when y = 1.25 and 1.50 at 800°C/1 h. In these orange materials, a mixture of monoclinic crystalline phases with β-Cu₂V₂O₇ and Cu₃V₂O₈ or β-Cu₂V₂O₇ and α-Mg₂V₂O₇ structures is detected by XRD.

Figure 9 shows the wavelength of charge transfer in Mg_yCu_2-yV₂O₇ compositions fired at 600°C/12 h (bands in Figure 7 (a)) and the variation of average V-O and M-O distances with wavelength of charge transfer and with composition (y). Inflection point between maximum and minimum absorbance in band is considered in assignation of wavelength values. Distances are calculated considering all the crystalline phases detected by XRD in each composition and its weight fraction in this sample. When Cu (II) amount (y) decreases in samples, the wavelength of charge transfer decreases in all compositions at this temperature and colour of samples changes from red-brown to yellow. From variation of average interatomic distances at 600°C/12 h, the longest average M-O distances is detected when 0.75 ≤ y ≤ 1.25 and orange coloration is obtained in these materials (charge transfer about 550 nm). At this temperature red-brown coloration is obtained in samples with the shortest average M-O distances (charge transfer about 570 nm). The obtained colorations are different with similar average distances when Mg amount is high (y > 1.25) and when Cu amount is high (y < 1.25) because Mg (II) and Cu (II) radius are similar. Structural changes must be also considered to explain the complete evolution of colour with composition.

Figure 9

Variation of interatomic distances with wavelength and composition. Variation of wavelength of Cu-O charge transfer with composition (a), variation of average interatomic V-O and M-O (M = Cu, Mg) distances with wavelength (b) and variation of these average interatomic distances with composition (c) from Mg_yCu_2-yV₂O₇ samples fired at 600°C. Solid marks: y < 1.5; Empty marks: y ≥ 1.5.

Conclusions

From Mg_xCu_3-xV₂O₈ compositions solid solutions with triclinic Cu₃V₂O₈ structure are obtained in 0.0 ≤ x ≤ 1.0 at 600°C. When x = 1.0, MgCu₂V₂O₈ compound is detected at 800°C. When x = 2.0, Mg₂CuV₂O₈ compound with the ordered metal distributions is not detected in the conditions of this study. From this composition, solid solutions with monoclinic Cu₃V₂O₈ structure (Cu1 in 2b sites) are obtained. These solid solutions with monoclinic Cu₃V₂O₈ structure are obtained when 1.5 ≤ x ≤ 2.0 at 600°C and 800°C. Solid solutions with orthorhombic Mg₃V₂O₈ structure are obtained when 2.5 ≤ x ≤ 3.0 at 600, 800°C and when 1.0 ≤ x ≤ 3.0 at 1000°C. In this study, the most stable solid solutions are obtained with Mg orthovanadate structure (orthorhombic).

From Mg_yCu_2-yV₂O₇ compositions, the formation of two kind of solid solutions with β-Cu₂V₂O₇ and α-Mg₂V₂O₇ structures is detected. Solid solutions with monoclinic β-Cu₂V₂O₇ structure are obtained when 0.25 ≤ y ≤ 1.50 at 600°C and when 0.50 ≤ y ≤ 0.75 at 800°C. Solid solutions with monoclinic α-Mg₂V₂O₇ structure are obtained when 0.5 ≤ y ≤ 2.0 at 600 and 800°C and showing an important structural distortion. At 1000°C, solid solutions with α-Mg₂V₂O₇ structure are obtained in 1.5 ≤ y ≤ 2.0 compositional range. It is proposed the existence of a new polymorph of Mg₂V₂O₇ compound with α-Mn₂V₂O₇ structure detected when y ≥ 1.25 at 300°C.

Strong absorbance in visible spectra is detected in Mg_xCu_3-xV₂O₈ (0 ≤ x < 3) and Mg_yCu_2-yV₂O₇ (0 ≤ y < 2) compositions which is associated with charge transfer between Cu-O in orthovanadates and divanadates. Wavelength of the abrupt change in absorbance is in accordance with Cu-O interatomic distances in these structures. In this study, the best yellow colour is obtained from Mg₂CuV₂O₈ (x = 2.0) solid solution with monoclinic Cu₃V₂O₈ structure. The colour red-brown is obtained from Mg_0.5Cu_1.5V₂O₇ (y = 0.5) fired at 600°C /12 h and it is unstable at 800°C /1 h. This red-brown colour is obtained when the average M-O distances are the shortest from divanadates. Orange colour is also obtained from some divanadates when average M-O distance is long.

Structural changes must be also considered to explain the colour of these materials. Thus, yellow colorations are obtained from orthovanadates and red-brown and orange colorations are obtained from divanadates.