Synthesis and structural investigation of the Eu, _,Bi,VO4 scheelite phase: X-ray diffraction, Raman scattering and Eu3 luminescence+ Jean Luc Blin," Annick Lorriaux-Rubbens,"" Francis Wallart" and Jean Pierre Wignacourt' "Laboratoire de Spectrochimie Infrarouge et Raman, UPR CNRS A2631 L, Universitk des Sciences et Technologies de Lille, Bdtiment C5, 59655 Villeneuve d'Ascq Cedex, France 'Laboratoire de Cristallochimie et de Physicochimie du Solide, URA CNRS 452, Universitk des Sciences et Technologies de Lille, Ecole Supkrieure de Chimie de Lille, Boite Postale 108, 59652 Villeneuve d 'Ascq Cedex, France A previous investigation of EuV0,-BiVO, binary system has indicated the existence of a solid-solution of Eu, -,Bi,V04 up to x =0.60, with a zircon-type structure.In this work we have reinvestigated the synthesis conditions, thus extending the upper limit to x =0.74. X-Ray diffraction techniques associated with Raman and luminescence spectroscopies have led to the unambiguous identification of the structural type as a scheelite model. Rare-earth-metal ions are introduced as dopants in many systems because of their potential applications in communi- cations or lasers technologies.lp2 Partial rare-earth-metal sub- stitutions have already been investigated in vanadate systems such as Ln,-,Bi,VO, (Ln=Eu, Gd), mainly for their ferro- electric proper tie^,^,^ and also for Eu3 l~minescence.~+ In studies aimed at developing new materials, we have studied the binary diagram EuV04-BiV04 in a limited com- position domain corresponding to a 35-90% bismuth for europium substitution. This particular limitation results from a previous in~estigation:',~ a maximum substitution ratio of 0.65 was then proposed; and near this limit, an interesting enhancement of the Eu3+ luminescence was noted, but the synthesis problems were not completely clarified.On the basis of the structural results of the starting compounds BiV04 and EuVO,,'** we have studied the reproducibility of the synthesis conditions, found the upper limit of the solid-solution domain, and have investigated the structure of the corresponding composition. Experimental Synthesis conditions The studied compounds were obtained from the solid-state reaction of the appropriate proportions of decarbonated and dehydrated bismuth, europium and vanadium oxides, according to reaction (1).( 1-u)Eu,03 +xBi,O, +V205+2Eu, -,Bi,V04 ( 1) The starting materials were ground and mixed in an agate mortar; the resulting mixture was then calcined at 850°C in an aluminium crucible for several hours and then air quenched. After several intermediate regrindings, the completeness of the reaction was checked by powder X-ray diffraction (XRD). Powder diffraction Room-temperature diffraction patterns were obtained from a Guinier de Wolff camera, using Cu-Ka radiation (0.154178 nm). NH4Br was introduced as an internal standard, and allowed the indexation of the pattern, the refinement of the lattice parameters and the identification of the solid-solution limit.The Rietveld refinement of the structure was made from a powder sample; the data was taken on a Siemens D5000 diffractometer equipped with a Cu anode, a back monochroma- tor and a rotating sample holder (2mm depth) in the range 8 <28 <120°, with a scanning step of 0.02". Molecular spectroscopy Raman scattering and luminescence spectra were recorded on a computerized RT30 Dilor spectrometer, consisting of three dispersive stages (800 mm focal length) in an additive optical scheme, each stage having a plane holographic grating with 1800 lines permm. This apparatus is equipped with two different laser sources, i.e. ionised argon and krypton (2025 Spectra Physics), thus giving access to a set of exciting lines.The detector is a photomultiplier (Hamatsu R943 02) with an AsGa photocathode cooled by the Peltier effect.' In order to investigate polycrystalline compounds, we chose the 647.1 nm exciting line of the Kr' laser with 25 mW power at the sample. The spectrum obtained at 0.7 cm-I resolution is recorded in the spectral range 6-1500 cm-l. Results X-Ray diffraction analysis We have investigated samples within a composition range 0.35<x<0.90, using 0.05 steps for x, and identified two different behaviours: first, a tetragonal solid-solution is unam- biguously observed for 0.35 <x <0.70 (domain 1);then, in the range 0.75 <x <0.90, a biphasic mixture of the previous tetra- gonal phase and a monoclinic one, is noted (domain 2).X-Ray patterns of domain 2 compositions were the key to this identification:" we used mixtures of constant NH,Br mass (q),as an internal standard, and a constant mass m2 of a sample of a given composition x;then a non-saturated NH4Br diffraction line is used as a reference, and the evolution of a selected reflection of the monoclinic phase is plotted versus x. The desired limit point, x =0.74, is obtained when I(mono- clinic)/l(standard)=0, i.e. when the tetragonal phase is pure (Fig. 1).This was confirmed by a specific sample preparation, and the corresponding compound EUO$%O 74vo4 has tke following lattice parameters: a =7.282(2) A and c =6.430( 2) A. This new limit, which is quite different from the literat~re,~ results explicitly from the longer synthesis time: 1% weeks at 850 "C, thus leading to complete reaction.Several attempts at preparing single crystals of Euo 26Bi0 74v0, have failed; thus we decided to investigate the structure from a powder sample, using the Rietveld method. In order to avoid any composition problems, we selected a sample (x=0.72) just below the limit of the solid-solution. According to the literature, EuVO, presents a tetragonal structure of the ~ircon-type,~ space group 14,/arnd(D,,19), with J. Muter. Chem., 1996, 6(3), 385-389 385 cl 0 70 075 0 80 0 85 090 subshtution ratio, x Fig. 1 Determination of the solid-solution limit from the X-ray patterns (a) I(sample/I, (NH,Br), (b) I(sample)/I,( NH,Br), (c), I(sample/I,(NH,Br) [I,(NH,Br) =most intense line of NH,Br, 12(NH4Br) =less intense line of NH,Br, Z,(NH,Br) = Il(NH4Br)+ 12(NH,Br)] 4 1 I I(a) 28/degrees Fig.2 Computed Rietveld diffraction profiles compared with exper- imental data for the Euo2,Bio7zV04 composition (a) in the 14,la space group, (b) in the 14, famd space group lattice parameters a=7 2373(2) A,c=6 3661(3) A and 2=4 As for BiV04, at room temperature, the structure is monq- ~linic,~space 6roup 12/b (C2,6),ofergusonite type, a =5 196(1)A, b= 11 704(2) A, c=5 093(2) A, ,!?=90 38(2)", Z=4 Both phases present a high-temperature variety, the scheelite-type structure,'' l2 tetragonal, in the space group 14,/a (C4h6) The data refinement was performed for the two tetragonal possibil- ities, Fig 2(a) and (b),and a good fit between the experimental and simulated data is noded in both cases, the ~$11parameters are then a =7 28296(9) A and c =6 43407( 10)A The corres- ponding structural features are given in Table 1 The only difference results from the oxygen position, in a special (025) or a general position, but even in the latter case, the deviation compensates for the difference The reliability factors (Table 2) are very similar in both models, and no clear conclusion about the space group can be made at this stage Structural information from molecular spectroscopy In the composition range O<x<O74, Raman spectra of Eul-,Bi,V04 samples (Fig 3) show in the frequency range 6-1000 cm-l some spectral modifications in the low-frequency region (lattice modes) as well as in the typical internal modes such as the V-0 stretching motions in the frequency range 700-900 cm-' Such a change may be due to a space group modification directly linked to the composition When x is close to the maximum value of 074, the Raman spectra can be interpreted in the C4h6factor group, this is confirmed by the luminescence spectra of the corresponding samples irradiated with the 568 2 nm excitation line (Fig 4) where the transition 5DO+7F0 is noted at 398 cm-' As a matter of fact, this band is absent for EuVO,, in good agreement with the selection rules, when Eu is located in the site 4b (D2d),space group 14,lamd But it is observed when the bismuth content is increased, which implies a symmetry change for the Eu/Bi sites, related to a space group modification The location of the mixed cation in the site 4b (S,) of the 14,/a space group, verifies the selection rules and fits the other electronic transitions, such as 5DO-+7F4 which is noted between 1100 and 1300 cm-' under the 647 1 nm exciting line These preliminary results obtained from Raman and lumi- nescence spectroscopies, allow the selection of a space group compatible with the scheelite form, and corresponding to a stabilisation of the high-temperature forms of BiVO, and EuVO,, which both have this structural type 'I l2 Table 2 Final reliability factors for the refinement using the Rietveld method I4,lamd space group 14,/U space group 14 6 14 6 18 5 18 5 13 46 13 45 188 188 3 82 3 84 2 52 2 61 Table 1 Atomic fractional coordinates and thermal coefficients for the two possible tetragonal cells I4,lamd 14, la Xla Ylb z/c B/A2 xla Ylb ZIC B/A2 72% Bi 00 0 75 0 625 0 39(2) 72% Bi 00 0 25 0 625 0 39( 2) 28% ELI 28% Eu 4b 4b v 00 0 75 0 125 032(6) V 00 0 25 0 125 0 31(7) 4a 4a 0 0 1779(7) 025 0 0428 (8) 0 6(1) 0 0 1778(7) 0743(6) 00429(8) 05(2) 16h 16f 386 J Mater Chem , 1996,6(3), 385-389 X = 0.40 I X= 0.60 r X= 0.65 I 200 400 600 800 lo00 1200 1400 200 400 600 800 1000 1200 1400 relative wavenumberkm-I Fig.3 Raman and luminescence spectra of the Eu,-,Bi,VO, compounds in the 6-1500 cm-' frequency range (A,,,=647.1 nm) Eu,.,Bi,,,VO, structure Thus, the crystal structure of Euo.2sBio.72V04 can be described as having tetragonal symmetry, space group 14,/a, with four formula units (Fig.5). The atomic positions in the unit cell are given in Table 1, and the cation position has been refined with 72% and 28% imposed occupancy factors for Bi3+ and Eu3+, Table 3 Interatomic distances and angles in Bi, 72E~0 28V04 interaction number distance/A angleldegrees 4 2 4 2 1.687( 5) 2.59( 5) 2.83(5) 4 4 4 2 4 4 4 4 4 4 2 2 2.406( 5) 2.499( 5) 2.59(5) 2.87(2) 3.07( 5) 3.18(5) 3.48(6) 4.5 1(4) 4.65(3) 4.69( 5) 4 4 4 4 4 4 2 respectively; the thermal coefficients are given in Table 1. The crystal structure is formed from tetrahedral vo43-anions, + +and mixed Bi3 /Eu3 cations; the orthovanadate ions are described by four identical bond lengths of 1.687( 5) A, and six bond angles, four at 114(3)" and two at 100( 3)".In comparison, the geometry of the orthovanadate ion in the highly symmetri- cal form of EuV0413 is clper to an ideal tetrahedron, with V-0 distances of 1.66(7) A and angles of 109(8)Oand llO(9)O; in the BiVO, scheelite form, obtained either by heat treatment', or by high pres?ure" synthesis, the V-0 bond lengths are similar, 1.72(1)A, with four angles at 105.9(4)' and two others at 116.9(8)". Thus our results show an angular deformation of the orthovanadate ion, but they are still compatible with the literature data. A closer analysis of the data of Sleight et a1.,12 David and Glazer14 or Mariathasan et ~1.~'~links the ferro- elastic-paraelastic transition (fergusonite-scheelite) in BiV0, to a deformation of the Bi3+ coordination polyhedron, with a minor influence of the VOd3-anions.In all cases, Bi3+ is surrounded by 8 oxyge; atoms, with bond lengths14 of 2.343, 2.379, 2.50? and 2.640A in the monoclinic phase, and 2.448 and 2.497 A in the tetragonal phase. In this investigation, the mixed trivalent cation coordination is described in Fig. 6: six different vo43-tetrahedra are involved; four of the oxygen atoms are closer than the other four, which are provided by two Vo43- edges. The M"'0, polyhedrons are linked either by a tetrahedral anion, or by two oxygen atoms delimiting a parallelogram with the two corresponding trivalent cations.These results are featured in Table 3. Molecular spectrum assignment Raman scattering. The Raman spectra of these compounds are located below 900cm-' for the Stokes domain, which is J. Muter. Chem., 1996,6(3), 385-389 387 200 400 600 800 11 10 relative wavenumber/cm-1 Fig. 4 Raman and luminescence spectra of the Eul-,Bi,V04 com-pounds in the 6-1000 cm-' frequency range (Ibe,,=568 2 nm) 8 cation M (M = 61,Eu) Fig. 5 Tetragonal cell scheme of the Eu, 28B10 72vo4 scheelite type structure the most intense side (excitation of the electrons of the funda- mental vibrational level) The anti-Stokes side is symmetrical with the Stokes side, with the excitation line as a reference, but the band intensities quickly decrease because only the electrons of the excited vibrational level contribute Thus, it is easy to distinguish the Raman effect from other phenomena such as luminescence in our case For our compounds, we obtained the internal vibrational modes of the V043-anion between 700 and 900 cm-' for the symmetrical and antisymmetrical stretching motions, the most intense band being assigned to the v,(A,) frequency, and the others to v3(F,)vibration, and between 350 and 480 cm-' for 388 J Muter Chem, 1996, 6(3), 385-389 vanadium 8 catDon M (M = Bi.Eu)111 o oxygen Fig. 6 Surroundings of the (a) M3+ cations by the oxygen atoms and their arrangement, (b)vanadium atoms I 5D0-7F0 1 -500 -1000 -1500 wavenumber/cm 1 Fig.7 (a) Raman' Stokes and luminescence spectrum in the 6-1500 cm frequency range, and (b) Raman antistokes and lumi- nescence spectrum in the -2000--200 cm ' frequency range of the Eu, 26Bi0 74vo4 sample (Ae,, =647 1nm) the symmetrical and antisymmetrical bending motions, corre- sponding to the vz( E) and v4( F,) frequencies In the very low- frequency domain, the obtained lines are due to lattice modes On the basis of our previous work,' and using the correlation tables between molecular, site and factor groups, we can assign the different lines observed by Raman scattering as indicated in Table 4 Luminescence spectroscopy.Using 647 1 nm radiation as the exciting line, two bands are clearly obtained in the 1080-1300cm-1 frequency range, and some others in the -1800 to -400 cm-' anti-Stokes domain, which are not due to vibrational motions [Fig 7(u) and (b)] We have already explained the Eu3+ luminescence observed under experimental conditions by a phonon-assistance mechanism l6 These elec- tronic transitions involve the first emitting level, 'DO,located in our compound at 17212cm-l, and the 7FJlevel of this cation Those transitions ascribed to 5Do+7F3transitions are expected in the exciting line range Because of the strong intensity of this exciting line, the presence of the lattice vibrations and the weakness of these electronic transitions.it Table 4 Internal vibrational mode assignment and lattice mode localisation of the Bio,74Euo,,,V04 Raman spectrum molecular group site group factor group Td s4 C4h frequency/cm~ A 8 59 A 368 B B 738 E 767 B 448 E lattice modes 3T(M3-): B+E 250 3T(V043-): 208 B+E 119 76 59 3L(V043-): A+E Table 5 Observed emission wavelength of the Eu3+ luminescence in the Bi0,74E~0,26V04 scheelite structure and its expected numbers, using the 647.1 nm excitation line of the Kr' laser" number of electronic expected observed relative emission transition transition types transition types frequency/cm-' wavelength/nm 5D, j7F0 1 md 5~o+7~, 1 md 1 (ed, md) 'Do +7F2 1 md 2 ed 1 (md, ed) 'Do j7F3 1 md 2 ed 2 (md, ed) 5D0+7F4 3 md 2 ed 2 (md, ed) "md =magnetic dipole; ed =electric dipole.would be difficult to detect them under such experimental conditions. The expected 5D,+7Fj transitions with the Eu3+ cation in an S4 symmetry site of the scheelite cell are presented in Table 5, where the observed relative wavenumbers, taking into account the exciting line reference, and their corresponding emission wavelengths are summarized. Conclusion An optimization of the synthesis conditions has led to a new definition of the tetragonal solid-solution limits. The refinement -1758 581.0 -1385 593.9 -1397 593.4 -998 607.8 -906 61 1.3 -805 615.1 -707 6 18.8 162 653.9 1118 697.6 1233 703.2 3 J. W. Hur, H. C. Lee, M. S. Jang, D. H. Yo0 and H. K. Kim, Ferroelectrics, 1990, 109, 197. 4 M. S. Jang, M.S. Lee, H. C. Lee, J. W. Hur and H. K. Kim, Ferroelectrics, 1990, 109, 185. 5 J. Ghamri, PhD Thesis, Universite de Lille I, 1990. 6 A. Lorriaux-Rubbens, J. Corset, J. Ghamri and H. Baussart, Adu. Mater. Res., 1994, 1-2,433. 7 A. T. Aldred, Acta Crystallogr., Sect. B, 1984,40,569. 8 J. D. Bierlein and A. W. Sleight, Solid State Commun., 1975, 16, 69. 9 G. Walker, Cryocoolers, Plenum Press, New York, 1983. 10 P. Conflant, These de 3" Cycle, Lille I, 1975. 11 I. H. Ismailzade, R. N. Iskenderov, A. I. Alekberov, R. M. Ismailov, A. M. Habibov and F. M. Salayev, Ferroelectrics, 1981,31,45. of this composition are developed. References 1 F. Auzel, Opto. 65, Echo des Recherches, 1992,143,24. 2 C. Hsu and R. C. Powell, J. Luminescence, 1995,10,273. 12 A. W. Sleight, H. Y. Chen and A. Ferretti, Nut. Res. Bull., 1979,of the structure of a composition Eu~.~~B~~.~~V~~,using the 14, 1571. Rietveld method on a polycrystalline sample near the solid- 13 W. 0.Milligan and L. W. Vernon, J. Phys. Chem., 1952,56, 145. solution limit, associated with Raman scattering and lumi- 14 W. I. F. David and A. N. Glazer, Phase Transitions, 1979, 1, 155. nescence spectroscopies, describes the crystal structure in a 15 J. W. E. Nariathasan, R. N. Hazen and Z. W. Finger, Phase scheelite model. The structural and molecular characterisations Transitions, 1986, 6, 165. 16 A. Lorriaux-Rubbens, J. L. Blin, L. Rghioui, F. Wallart, J. P. Wignacourt, A. Mizrahi, M. Drache, P. Conflant, Proceedings of the 14th International Conference on Raman Spectroscopy, ed. N. T. Yu and X. Y. Li, Wiley, New York, 1994, pp. 566-567. Paper 51048736; Received 24th July, 1995 J. Mater. Chem., 1996, 6(3), 385-389 389