Materials Research Bulletin 44 (2009) 1771–1777

(Ca2+, V5+) co-doped Y2Ti2O7 yellow pigment

N. Pailhe, M. Gaudon *, A. Demourgues

Bordeaux Institute of Condensed Matter Chemistry, ICMCB,-CNRS, 87 avenue du Dr. Albert Schweitzer, 33608 Pessac cedex, France

A R T I C L E I N F O

Article history:

Received 15 January 2009

Received in revised form 20 February 2009

Accepted 12 March 2009

Available online 26 March 2009

Keywords:

A. Oxides

C. X-ray diffraction

D. Optical properties

D. Crystal structure

A B S T R A C T

(Ca2+, V5+) co-doped Y2Ti2O7 yellow pigments were prepared by solid route. The influence of the

annealing temperature, the vanadium rate (0.02 � x � 0.3) and a post-mechanical grinding on the phase

purity and the colour were studied using X-ray diffraction and UV–vis spectroscopy. The correlation

between pigment colouration, synthesis parameters and structural feature is then discussed. Pigments

with various colourations from yellow to deep orange can be obtained depending on the synthesis

parameters linked to a slight modification of the vanadium coordination environment. For a same

composition, the pigment colouration depends on the creation of Frenkel pairs inside the structure

corresponding to a displacement of an oxygen atom toward one interstitial empty site.

� 2009 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Materials Research Bulletin

journa l homepage: www.e lsev ier .com/ locate /matresbu

1. Introduction

Thanks to their higher thermal and chemical stability comparedto organic pigments, inorganic pigments are widely used in variousapplications such as paints, inks, ceramics, enamels and glasses [1].However, most of them contain toxic elements (Cd, Pb, Hg, Cr, Co,Sb) which are now forbidden and expelled from the worldwidemarket [2]. Several mixed metal oxides containing a transitionmetal or/and a rare earth as a chromophore ion have beenproposed in literature as yellow/orange pigments [3–8] but few ofthem are developed [6–8]. Among transition metals, vanadium,and more particularly the pentavalent state, is well known for itscolouring properties. As example, vanadium pentoxide V2O5,whose colour is due to the charge transfer band from O2p

2� toV3d

5+, is commonly used as classic yellow pigment in glazes field[9]. Numerous AxVyOz ternary oxides are described in literature.The networks of most of these compounds are built from tetra-coordinated V5+, indicating a natural preference of the pentavalentelement for the tetrahedral coordination. A brief screening on thecolouration of various ternary vanadates is presented hereafter.The colour of these compounds depends on the coordination stateof V5+ cations and on the O2�–V5+ bond lengths. XVO4 with X = La(P21/n), Y (I41/amd) [10,11] and A2V2O7 with A = Ca, Sr (P�1)[12,13] compounds, where V5+ is tetra-coordinated and O–V bondlength is closed to 1.7 A, exhibit a pale yellow colour. In the case ofMg2V2O7 (P-1), where the distorted [VO5] pentahedra present O–Vbond lengths ranging from 1.6 to 2.9 A, the colour is ocre [14,15].

* Corresponding author. Tel.: +33 5 40 00 66 85; fax: +33 5 40 00 27 61.

E-mail address: gaudon@icmcb-bordeaux.cnrs.fr (M. Gaudon).

0025-5408/$ – see front matter � 2009 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2009.03.009

Then, oxides consisted of V5+ six-fold coordinated to O2� anionsusually exhibit a more pronounced yellowish/orange colouration.For instance, MgV2O6 [16] and ZnV2O6 [17] (C2/m) exhibit a brightyellow colour. In such compounds, the distorted [VO6] octahedraget O–V bond lengths ranging from 1.7 to 2.7 A. Compared tocompounds with V5+ in tetrahedral sites, the electronic transitionoccurring between O2p

2� and V3d5+ in the former oxides is less

energetic because of the longer O–V bonds.Consequently, it can be interesting to incorporate V5+ in an oxide

network where only octahedral sites are available. Such compoundscould exhibit a tuneable hue from yellow to orange and so offer thepossibility to get colour-scale pigments. In ABO3 perovskite (Pm-3m) and A2B2O7 pyrochlore (Fd-3m) materials, the twelve/eight-fold coordinated A-site and the octahedral B-site can be occupied bya wide-range of cations; steric effects allow controlling the structurestability. Particularly, pyrochlore-type phases can offer a large rangeof bond lengths, these ones being stable for ionic radius ratio RA/RB

ranging from 1.46 up to 1.80. V5+/Ca2+ co-doped Y2Ti2O7 pyrochlore(Fd-3m) have been already synthesized and characterized as a newyellow ceramic pigment [18]. The authors mentioned that the x

solubility limit corresponding to the composition close to x = 0.13and explain the yellow colouration is well due to V5+ substitution forTi4+ ions in octahedral sites.

In this paper, Y2�xCaxTi2�xVxO7 compositions with x varyingbetween 0.02 and 0.3 were prepared by solid route. Moreover,various synthesis temperatures were performed and the influenceof a post-mechanical grinding of the sample was studied. Sampleshave been investigated by X-ray diffraction and UV–vis spectro-scopy in order to show the correlation between the material colour,the structural feature, notably the O–V bond lengths, and thesynthesis parameters.

Fig. 1. (a) Arrangement of oxygen tetrahedral sites in pyrochlore structure. The

double arrow illustrates the cationic disorder (swapping of cations), the full arrow

the O1 migration (Frenkel pair). (b) Zoom underlining the impact of the Frenkel pair

formation.

N. Pailhe et al. / Materials Research Bulletin 44 (2009) 1771–17771772

2. Experimental details

2.1. Synthesis process

The co-doped pyrochlore compounds with target formulaY2�xCaxTi2�xVxO7 (0.02 � x � 0.3) were synthesized by solid-stateprocess. Stoichiometric powder mixtures of Y2O3, V2O5, TiO2 andCaCO3 anhydrous raw powders were firstly calcinated 5 h at 600 8Cand finally heated 10 h at the final temperature (typically 1350 8C).

2.2. X-ray powder diffraction

A Philips PW 1820 apparatus equipped with a Cu(Ka1/Ka2)source (laverage = 1.5424 A) was used to identify structure and tocontrol the purity of compounds. Diffraction patterns werecollected with a 2u step of 0.028 with a counting time of 10 sper step. For more advanced structural investigations, X-raydiffraction (XRD) measurements were carried out on a PANalyticalX’PERT PRO diffractometer, using a monochromatic CuKa radia-tion (l = 1.54056 nm). Diffractograms have been refined withRietveld refinement method [19,20] using FULLPROF1 package[21].

2.3. Grinding experiments

The post-mechanical grindings were performed using a highenergy SPEX-8000 3D vibrational mill. 1 g of powder was added to10 agate balls (size = 10 mm) into a 45 mL agate bowl and wasground during different times (2 h, 4 h, and 6 h) with 1 h grindingcycle.

2.4. Diffuse reflectance measurements

The UV–vis spectroscopies were carried out on a VARIAN CARY5000 spectrophotometer equipped with an integrating spherecoated with polytetrafluoroethylene (PTFE). Measurements wereperformed at room temperature for wavelengths varying from200 nm up to 800 nm. HALON was used as white reference. L*a*b*colouring space parameters were calculated from diffuse reflec-tance curves R(l) and from the three relative sensibility curves:xðlÞ; yðlÞ and zðlÞ defined by the CIE-1964. In this system, L* is thelightness axis [black (0) to white (100)], a* is the green (<0) to red(>0) axis, and b* is the blue (<0) to yellow (>0) axis.

3. Results

3.1. Structural description

The Y2Ti2O7 phase adopts a cubic symmetry with the Fd-3mspace group related to the pyrochlore-type structure. Such anoxide with A2B2O7 formula has got two different kinds of oxygenanions O1 and O2. Representation of oxygen tetrahedral sites andthe arrangement in chains of these oxygen tetrahedral sites in thepyrochlore network is schematized in Fig. 1. The trivalent A3+

cations are surrounded by eight oxygen anions (6O1 + 2O2)forming a distorted cubic site (16d). The tetravalent B4+ cationshave six nearest neighbours (O1) which form a distorted octahedralsite (16c). The 48f tetrahedral sites consist of O1 anions as ‘‘central’’atom, two A cations and two B cations as apexes, while the 8btetrahedral sites are constituted by O2 anions surrounded by four Acations. At last, pyrochlore structure contains vacant 8a tetrahedralsites with four B cations as apexes.

A priori, in the case of the studied V5+/Ca2+ co-doped Y2Ti2O7

compounds, V5+ ions (rVI = 0.54 A) are in substitution for Ti4+ ions(rVI = 0.605 A) while Ca2+ ions (rVIII = 1.12 A) are in substitution forY3+ ones (rVIII = 1.015 A) leading to the general formula

Y2�xCaxTi2�xVxO7. The RA/RB ratio in this case is higher than theone in un-doped Y2Ti2O7 (1.68) whatever the x value (e.g. 1.71 forx = 0.16). Thanks to the wide stability domain of pyrochlorecompounds (with RA/RB ranging from 1.46 up to 1.80) thepyrochlore structure remains stable despite the increase of theaverage RA/RB due to the Ca2+/V5+ co-substitution. The maximumvanadium rate theoretically allowed is about 0.67 (RA/RB = 1.80).Then a limitation of vanadium incorporation into pyrochlore phasecannot be due to steric effects.

Moreover, anionic disorder can be created into the pyrochlorenetwork: 48f oxygen vacancies and interstitial oxygen on 8a sitescan be associated such as O1 oxygen atoms migrate from 48f siteto 8a site in order to form a Frenkel pair. Energies of these oxygenpoint defects were calculated on Y2Ti2O7 by Wilde and Catlow[22]. Results of calculation showed that vacancies are morestable in 48f than in 8b and that the oxygen migration from 48fsite to 8a site seems to be quite easy: the cost energy of theFrenkel pairs creation is calculated by the authors equal to 4.6 eVin a pure Y2Ti2O7 oxide. The authors also explained that Frenkelpairs are stabilized by cationic disorder inherent to thepyrochlore structure (Fig. 1b). Indeed, the authors consider thatcrystallographic sites exchange between two 16c–16d adjacentcations (Ti4+–Y3+) leads to the displacement of 48f oxygenposition near to the 8b one, enhancing hence the possibility ofFrenkel disorder.

3.2. Colour and phase purity of compounds

As previously mentioned in literature [1,23] whatever thetarget composition and the thermal treatment, such co-dopedtitanate pyrochlores prepared by ceramic route exhibit brightyellow to orange colour. One diffuse reflectance curve correspond-ing to the Y2�xCaxTi2�xVxO7 composition with x = 0.16 andobtained after annealing at 1350 8C is presented on Fig. 2a. Aspreviously discussed in the literature, the electronic transferbetween the 2p(O2�) ligand and the 3d(V5+) metallic centre in 16coctahedral site should be at the origin of the observed colour, onlyvanadium being chromophore element in this system. Thesecompounds exhibit colours varying from pale yellow (Eg � 2.7 eV)to deep orange (Eg = 2.2 eV) depending on the vanadium rate andthe synthesis temperature. X-ray diffraction analyses show thatthese pigments contain an YVO4-type impurity, besides the mainpyrochlore phase, whatever V5+ dopant rate and sample thermaltreatment. One can easily explain the systematic formation of thisimpurity by the natural preference of V5+ for the tetrahedral

Fig. 2. Diffuse reflectance visible spectra of Y2�xCaxTi2�xVxO7 (x = 0.16, 1350 8C-fired) pyrochlore compound (a) and YVO4 (b). lg represents the charge transfer band

wavelength.

Fig. 3. (a) Influence of the initial vanadium rate on the (1350 8C-fired) pyrochlore

compounds purity. (b) Evolution of YVO4 cell parameters versus the initial

vanadium concentration used.

N. Pailhe et al. / Materials Research Bulletin 44 (2009) 1771–1777 1773

coordination. It has been checked here again that the tetra-coordination of the vanadium element in an YVO4 structure cannotbe at the origin of a charge transfer in visible range. The diffusereflectance spectrum of YVO4 is reported in Fig. 2b. YVO4

compounds being a white to very pale yellow compound, onecan highlight on the reported spectrum that the O2�–V5+ chargetransfer band is located at the UV–vis border around 370 nm. Onecan conclude that the observed yellow colour of these compoundsis really due to the vanadium-doped pyrochlore phase and not tothe impurities. Nevertheless, it is obvious that the presence of thisimpurity will be problematic for any interpretation linked to thereal composition of the main phase, particularly for the vanadiumcontent.

Hereafter, the studies on the influence of the vanadium rate, theannealing temperature and the grinding on the purity and thecolour of the V5+/Ca2+ co-doped Y2Ti2O7 compounds, are reportedseparately. In order to clarify the colour mechanisms, diffusereflectance spectra and X-ray diffraction analyses are finallycorrelated and discussed.

3.3. Initial vanadium rate effect

In order to study the effect of the vanadium substitution rateinto the pyrochlore network, Y2�xCaxTi2�xVxO7 target composi-tions, with x varying from 0.02 to 0.3, were synthesized. For allattempted compositions, X-ray diffraction patterns reveal Y2Ti2O7

(pdf number: 42-0413 [24]) as major phase and YVO4-type phase(pdf number: 17-0341 [25]) as minor impurity: few low intensityextra peaks are always observed. Furthermore, in the case of a largeCa2+/V5+ rate (x = 0.3), CaTiO3 perovskite-type structure is formed.Higher the initial vanadium rate, more ‘‘YVO4’’ impurity is formed,i.e. more intense is the ratio between the major diffraction peaksareas of each phase: A200, YVO4/A222, Y2Ti2O7 as shown in Fig. 3.However, the observed impurity does not correspond exactly to‘‘standard’’ YVO4 phase. Indeed, while the vanadium ratedecreases, the peaks (see the illustration on the hkl = 200 one)of YVO4-type impurity phase are shifted towards lower and lowervalues of 2u. As it can be seen in Fig. 3b, the a and c tetragonalparameters both asymptotically increase with the initial vanadiumrate used until the standard literature values: 7.12 and 6.29 A for a

and c, respectively (reference data 42-0413). The decrease of thecell parameters of the ‘‘YVO4’’ impurity versus the decrease ofinitial doping rate is surprising. Indeed, Ca2+/Ti4+ substitutions for

Y3+/V5+ should normally lead to a shift towards low values of 2uwith regard to the reference, i.e. the opposite of our observation. Inzircon-type compounds creation of cationic and/or anionicvacancies can be excluded. It was indeed check that YVO4-type

Fig. 4. Diffuse reflectance curves of various pyrochlore compounds (1350 8C-fired).

( ), ( ), and ( ): the three charge transfer gaps.

Fig. 5. Influence of the synthesis temperature on (x = 0.16) pyrochlore compound

purity.

Table 2Sintering temperature effect on charge transfer energy and trichromatic

parameters.

T (8C) lg (nm) Eg (eV) L* a* b*

1350 536 2.32 82.0 15.3 77.0

1450 554 2.24 76.4 24.1 75.1

1475 560 2.22 71.7 24.8 64.8

L*a*b* of x = 0.16 compounds.

N. Pailhe et al. / Materials Research Bulletin 44 (2009) 1771–17771774

impurity phase cannot exhibit yttrium deficiency from the failureof sub-stoichiometric phases obtaining. An intergrowth phenom-enon would not act a priori in the same manner on both cellparameters. None explanation of the zircon-type impurity exoticcell dimensions can be proposed yet.

A refinement of the global profile using LeBail method [26](pattern matching) was performed on each X-ray diffractionpattern in order to determine cell parameters of the pyrochlorephase. These last ones slightly increase from a = 10.0957(4) A up toa = 10.0969(4) A for initial vanadium rate (x) ranging from 0.02 to0.16. Thus, a higher cell parameter seems to be linked to apyrochlore structure richer in V5+/Ca2+rates, considering that evenif the impurity quantity grows with x, the real doping rate increasesversus target one. While the initial x doping rate reaches 0.3, a thirdphase (CaTiO3 perovskite-type phase) formation, containing thelargest cations, lead to a very slight decrease of the cell parameter(a = 10.0963(4) A) of the pyrochlore phase, i.e. globally the aparameter remains quasi-constant versus the x co-substitutionrate.

It appears interesting to mention that the notion of a ‘‘vanadiummaximal solubility in pyrochlore network’’ reported in theprevious paper [18], is not adapted because YVO4 related phasealways forms whatever the initial vanadium rate, i.e. even for verylow rate; then, its quantity increases versus the initial doping ratein a quasi proportional way (Fig. 3). Concerning the diffusereflectance properties of the pyrochlore pigments, higher theinitial vanadium rate, more reddish the pigment: as reported onTable 1, the a* parameter value increases with the initial vanadiumrate x. Furthermore, the vanadium concentration increase leads tomore saturated yellow colouration, the b* parameter increasesfrom about 50–70. A more detailed study based on the reflectance

Table 1Initial vanadium rate effect on charge transfer energy and trichromatic parameters.

x lg (nm) Eg (eV) L* a* b*

0.02 507 2.45 87.5 1.90 51.7

0.04 515 2.41 86.7 4.70 60.1

0.08 527 2.36 84.3 11.0 73.6

0.10 533 2.33 81.9 13.8 75.7

0.16 536 2.32 82.0 15.3 77.0

L*a*b* of 1350 8C-fired compounds.

spectra (Fig. 4) clearly shows that several convoluted phenomenaoccur: (i) three charge transfer gaps can be detected for low ratewhile only one gap (the intermediate/main one) seems to remainwhen doping rate increases, (ii) the main gap shifts to lowerenergies while the initial doping rate increases (what explains botha* and b* parameters variations), and (iii) the main gap is less andless spread out in energy while the initial doping rate increases.

3.4. Synthesis temperature effect

In order to obtain pure pyrochlore phase and to avoid the YVO4

impurity phase, high temperature thermal treatments up to

Fig. 6. Influence of the synthesis temperature effect on (x = 0.16) pyrochlore

compound reflectance curves.

Fig. 7. Influence of the grinding time effect on the (x = 0.16/1350 8C-fired)

pyrochlore compound purity and crystallization.

Fig. 8. Influence of the grinding time on the diffuse reflectance curves of the

(x = 0.16/1350 8C-fired) pyrochlore compound. ( ), ( ), and ( ): the three charge

transfer gaps.

N. Pailhe et al. / Materials Research Bulletin 44 (2009) 1771–1777 1775

1450 8C have been attempted. Results obtained on theY1.84Ca0.16Ti0.84V0.16O7 target composition from diffraction pat-terns, clearly show that higher the synthesis temperature, lower isthe YVO4 impurity content (less intense is the ratio between majorpeak area of each phase: A200, YVO4/A222, Y2Ti2O7 as shown inFig. 5). However, it can be noticed that the composition of theimpurity seems to remain the same whatever the temperaturesince no shift of the diffraction peaks is observed.

Concerning the diffuse reflectance properties, higher thesynthesis temperature, more reddish the pigment as illustratedby Fig. 6 and Table 2. Hence, comparing the powder colourdependence versus the target composition and the final synthesistemperature influence it can be concluded that the increase of theannealing temperature allow the incorporation of larger amount ofV5+ in the main phase since it was shown while the pyrochlorephase contains a more and more important vanadium rate, itscolour is more and more reddish (a* increasing) and more andmore saturated (b* also increasing).

3.5. Grinding effect

This last study was performed on the Y1.84Ca0.16Ti1.84V0.16O7

target compound with a final calcination temperature equal to1350 8C. Different times of grinding were tested: 2 h, 4 h and 6 h.Actually, grinding experiments result on a partial amorphization ofthe Y2Ti2O7 main phase as well as the YVO4 impurity phase (Fig. 7).In order to check if the disappearance of the diffraction peaks

Table 3Grinding time effect (tG) on charge transfer energy and trichromatic parameters.

tG (h) lg (nm) Eg (eV) L* a* b*

0 536 2.32 82.0 15.3 77.0

2 495 2.51 87.0 (�)0.5 40.0

4 485 2.56 87.7 (�)1.4 36.5

6 475 2.61 89.5 (�)2.4 31.8

L*a*b* of x = 0.16, 1350 8C-fired compounds.

relative to the YVO4 impurity are related to the amorphization ofthis latter and not caused by its incorporation of V5+ ions inside thepyrochlore phase, the 2 h ground compound was annealed at800 8C to induce a re-crystallization of the solid phases. The areaunder peak analysis has shown that the ratio of major peak areas ofeach phase (A200, YVO4/A222, Y2Ti2O7) remains constant before andafter grinding whatever the grinding time. This proves that thegrinding does not modify the chemical composition of thepyrochlore and the YVO4 phases. Nevertheless after 6 h grinding,the sample is clearly polluted by grinding agate balls or mortar.

Whereas the chemical composition of the main phase is notaffected by the mechanical grinding, reflective spectra are stronglyshifted as it is shown in Fig. 8. (i) Firstly, longer the grinding time,lighter is the colour (more energetic is the charge transfer andlower the a* and the b* parameter values as reported in Table 3);(ii) furthermore, as previously described into the paragraphdealing with target composition influence, the blue shift isassociated here again with the gap splitting into three ‘‘sub-gaps’’: a central/main one and two satellites.

4. Discussion

The pyrochlore colour evolution versus the incorporatedvanadium rate, the effect of the synthesis temperature and thepost-mechanical grinding time is complex because many para-meters have to be taken into account. By the way, considering thatthe colouration is due to the main phase (the pyrochlore one) andonly the O2� ! V5+ transfers, the powder colour only depends on theV5+ environment inside this main phase (insertion site, distortiondegree, bond lengths distribution) [27]. Hence, pyrochlore colourmodification has to be interpreted as the result of an increase of theO2�–V5+ bond length involving a decrease of the bond ionicity(charge localization) and so, a charge transfer gap shifting at lowerenergies. It was chosen to accurately analyse from X-ray diffractionpatterns the grinding effect and its consequences on the pyrochlorephase colour. Indeed, in this case, the pyrochlore composition is notmodified and then Rietveld refinements allow following with someaccuracy the evolution of the structural parameters. Rietveldanalyses were performed on a x = 0.16 composition sample sinteredat 1350 8C before any grinding, after 2 h and 4 h grinding andregenerated after 4 h grinding by a last calcination step at 800 8C to

Fig. 9. Fourier maps for layer z = 0.125 of green (a) and 4h-ground (b) (x = 0.16/

1350 8C-fired) pyrochlore compounds.

N. Pailhe et al. / Materials Research Bulletin 44 (2009) 1771–17771776

10 h. For these refinements it was considered that the smallquantities of YVO4 and CaTiO3 impurities can be neglected and thatthe main phase composition stills near the target composition:Y1.84Ca0.16Ti1.84V0.16O7. Furthermore, Ca2+, Ti4+ and V5+ get quitethe same structural factor because of an equivalent number ofelectrons, then a refinement of the diffractogram considering the16d(Y1.84�zX0.16+z)

16c(X2�zYz)48fO6�y

8aO18bOy oxide formula (X = Ca,

Ti or V) can be made. Thus, only two parameters were refined: (i) z

Table 4Rietveld refinements results of x = 0.16/1350 8C-fired ground compounds (post-

annealed at 800 8C) with the corresponding formula (Y1.84�zX0.16+z)(X2�zYz)48f

O6�y8aO1

8bOy.

Compound z y

tG = 0 h 0.16(1) 0.16(1)

tG = 2 h 0.10(1) 0.38(1)

tG = 4 h 0.03(1) 0.75(1)

tG = 4 h; 800 8C 0.12(1) 0.18(1)

parameter, representing the disorder between A and B cationic sites(considering the calcium ions are not concerned by the atomicdisorder because of its too large ionic size) and (ii) y parameter,characteristic to the atomic disorder between the 48f and 8b anionicsites (Frenkel pairs quantification). In order to be comparablebetween the various studied samples for these refinements, theisotropic displacement factors Biso were fixed to 1 and 1.5 A2 forcations and anions, respectively. Results are collected in Table 4. Therefinement results clearly show a significant increase of the Frenkelpairs with the grinding, i.e. 8a occupation rate (y), largely increasesversus grinding time. Two Fourier maps reported in Fig. 9 illustratethe oxygen migration related to the grinding, by the appearance ofelectronic density on 8a sites and its decrease on 48f sites. Thecreation of anionic Frenkel pairs is not here correlated to cationicdisorder on the contrary to the mechanism proposed in literature[23]. In an opposite way, the Y3+ disorder associated to the z

parameter is higher before than after grinding. However, it isdifficult to know if the associated B cation to the Y3+ disorder is theV5+ or the Ti4+ cation. Moreover, one can notice that the initialstructural organization is regenerated by performing 800 8C post-annealing step after the grinding.

The colour change versus grinding is the consequence of a blueshift of the main charge transfer band (a splitting into three ‘‘sub-gaps’’ being also observed) linked to an internal atomic reorga-nization. Thus, it seems that the creation of Frenkel pairs induces adecrease of the V5+–O2� average bond length. For each Frenkel paircreated, the coordination of the B-site cations is strongly modified:two B sites get a coordinence increasing from 6 to ‘‘6 + 1’’ (six 8banions + one 8a anion), whereas two other B sites get acoordination shifting from 6 to 5 + 1 (five 8b anions + one 8aanion), as illustrated on Fig. 1. On a global point of view, the8aO2�–16dV5+ average bond lengths are significantly longer thanthe 8bO2�–16dV5+ ones (about 2.2 A instead of 1.8–1.9 A). Hence, itis difficult to know if one has to consider 8a oxygen ion in the firstcoordination sphere of the concerned B sites and furthermore localatomic migrations acclimating the local constrains created by theatomic disorder are also present. In the case of the participation ofthe 8a anion to the coordination sphere of the neighbouring B sites(16d), Frenkel pairs would lead to an increase of the coordinationnumber of these metallic cations associated to an increase of thecation–ligand average bond lengths and so a red-shift. Here theopposite phenomenon is observed (blue shift) and can be onlylinked to an average V5+–O2� bond contraction; so the 8ainterstitial sites are not participating to the first coordinationsphere of the neighbouring V5+ cations.

Nevertheless, it is really difficult here to conclude without anydoubt about this direct correlation between diffuse reflectancespectra (and so oxides colour) and Frenkel pairs apparition insidepyrochlore network. However, herein is clearly evidenced thatvery slight variation of composition-structure of vanadium-doped Y2Ti2O7 oxides allows the formation of a wide colour-scale from yellow to deep orange with a* colour parameter varyingfrom 0 to 25 and b* from 35 to 80. The studied pyrochlore oxidesare very promising pigments; inside these systems, the incorpo-rated V5+ rate as well as the V5+ environment (cationic disorder,Frenkel defects neighbouring) are the key parameters to control inorder to produce a new generation of non toxic mineral pigmentswith tuneable colouration from more or less saturated/brightyellows to deep red (via the a* and b* and L colorimetricparameters control).

References

[1] H.M. Smith, High Performances Pigments, Wiley/VCH, Weinheim, 2002.[2] G. Buxhaum, Industrial Inorganic Pigments, VCH, Germany, 1993.[3] M. Jansen, H.P. Letschert, Nature 404 (2000) 980.[4] P. Sulcova, M. Trojan, Dyes Pigments 40 (1998) 87.

N. Pailhe et al. / Materials Research Bulletin 44 (2009) 1771–1777 1777

[5] P.P. Rao, M.L.P. Reddy, Dyes Pigments 63 (2004) 169.[6] D.H. Piltingsrud, US Patent no. 4,115,141 (1978).[7] D. Huguenin, et al., US Patent no. 5,560,772 (1996).[8] D.R. Swiler, et al., US Patent no. 6,582,814 (2003).[9] http://www.claymaker.com/ceramic_central/info/glazes.htm.

[10] A. Watabe, National Inst. for Inorganic Materials, Private Communication, Tsu-kuba, Japan, 1999.

[11] U.S. Natl. Bur. Stand. Monogr. 25(5) 1967, p. 59.[12] F. Larson, G. McCarthy, ICDD Grant in Aid, North Dakota State, Fargo, 1987.[13] J. Huang, A. Sleight, Mater. Res. Bull. 27 (1992) 581.[14] R. Gopal, C. Calvo, Acta Crystallogr. B 30 (1974) 2491.[15] A.G. Nord, G. Aberg, T. Stefanidis, P. Kierkegaard, V. Grigoriadis, Chem. Script. 25

(3) (1985) 212.[16] A.G. Nord, P. Kierkegaard, T. Stefanidis, G. Aberg, E.J. Baran, Chem. Script. 28 (2)

(1988) 133.

[17] J. Angenault, A.C.R. Rimsky, Seances de l’Academie des Sciences—Serie C 267(1968) 227.

[18] S. Ishida, et al. J. Am. Soc. 76 (1993) 2644.[19] H.M. Rietveld, Acta Crystallogr. 22 (1967) 151.[20] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65.[21] J. Rodriguez-Carvajal, FULLPROF: a program for Rietveld refinement and pattern

matching analysis, in: Abstracts of the Satellite Meeting on Powder Diffraction ofthe XV Congress of the IUCr, Toulouse and Grenoble, France, (1990), p. 127.

[22] P.J. Wilde, C.R.A. Catlow, Solid State Ionics 112 (1998) 173.[23] K. Fujiyoshi, et al. J. Am. Ceram. Soc. 76 (1993) 981.[24] W.J. Becker, G. Will, Zeitschrift fur Naturforschung, Teil B. Anorganische Chemie

254 (1969) 259.[25] J.A. Baglio, G. Gashurov, Acta Crystallogr. B 24 (1968) 292.[26] A. LeBail, D. Louer, J. Appl. Crystallogr. 11 (1978) 50.[27] I.D. Brown, D. Altermatt, Acta Crystallogr. B 41 (4) (1985) 244.