Na(,4x~Eu,Ti,(P0,), phosphates Ann. Chim. Sci. Mat, 1998,23, pp. 77-80
PREPARATION AND STRUCTURAL STUDY OF Na,,.,E&Ti,(PO,), PHOSPHATES
H. FAKRANE, M. LAMIRE, A. EL JAZOULI*, G. LE FLEM**, R. OLAZCUAGA**
Laboratoire de Chimie du Solide, Facult6 des Sciences &n Chock, Casablanca, Maroc.
* Laboratoire de Chimie des Mat&iaux Solides, Faculto des Sciences Ben M’Sik, Casablanca, Mnroc.
** Institut de Chimie de la Mat&e Condensoe de Bordeaux, Universito de Bordeaux I, France.
Summary : The phosphates Na(t-sXjEuxTis(P04)3 (0 <x 5 0.25) have been obtained by solid state reaction at 950°C. They crystallize in the Nasicon-type structure. The progressive substitution of europium for sodium results in an increase of the at, parameter and a decrease of the ch parameter. This variation is mainly due to the creation of vacancies in the M(1) sites. The structure of Nao.ssEuo.s5Tis(P0& has been solved by the Rietveld protilation method in the R 5 space group. The europium and sodium occupy statistically half of the M(1) sites with a 1-l vacancy-Na/Eu ordering along the c-axis. In this compound, the enropium (III) emission spectrum evidences the actual location of the rare- earth in a unique non-centrosymmetric site. The strong absorption at high energy can be ascribed to the oxygen europium (III) and oxygen titanium (IV) charge transfers.
RCsumC : Preoaration et etude struct
-. ~,ohosohates3x&&gQ &. Les phosphates Nqr.3xrEuxTi2(P04)3 (0 5 x 2
0.25) ont W obtenus par tiaction ir l’etat solide Q 950°C. Ces matbriaux cristallisent dans le type structural Nasicon. La substitution progressive du sodium par l’europium provoque une augmentation du parametre ch et une diminution du parametre a. Cette variation resulte smtout de la creation de lacunes darts les sites M(1). La stmcture de Naa,s5EUo,zsTir(PO& a CtC resolue par la methode de protilation de raies de Rietveld darts le groupe d’espace R 3. L’europium et le sodium occupent statistiquement la moitie des sites M(1) avec un ordre l-l entre les lacunes et les ions Naf/Eu3+ le long de I’axe c. Le spectre d’emission de ce compost monk que l’europium occupe en fait nn site unique non-centrosymettique. La forte bande d’absorption a haute etmrgie est attribuee aux transfer& de charge oxyg6ne europium (III) et oxygene titane (IV).
l- INTRODUCTION
The introduction of rare earth elements in Nasicon-type compounds was first reported for the phosphates Na4.sLnr,s(P0& (Ln = Yb, Tm, Lu) and for the arseniates Na4.sLnr,s (AsO& (Ln = Er, Tm, Yb, Lu) (1). The detailed
resolution of the structure of the ytterbium phosphate allowed the structural formula of this compounds to be written as INa~]M?cNa]M,[NaasLn,.s](x04), (X = P, As). In other words the rare earth are located in the octahedral sites of the covalent framework typical of this structure.
More recently the phases LnusZrz(PO& (Ln = rare earth) were prepared and the rare earth ions are located in the antiprism usually labelled M(1) (2). The investigation of the luminescent properties of the europium phosphate concludes to the non-centrosymmetric character of the europium site due to a possible offcenttred position. In the context of rare earth phosphate of the Nasicon-type, the present paper reports a description of the new phosphate Na(l- 3x)EuxTi2(P04)3. (0 I x 5 0.25)
Remin@ : M. LAMIRE, Laboratoire de Chimie du Solide, Faculte des Sciences Ain Chock, B.P.5366, Maarif, Casablanca MAROC.
78 H. Fakrane et al.
Nao&u~Ti#O& solid solution compositions have been prepared by solid state reaction m stoichiometrtc proportions of N&CO,, Ti(X, EuzOx and WH2P0., according to the reaction :
( 1-3x)NaZC01 + xEu*@ + 4TiOr + 6NH.&P04 -+ 2Na+rKjEu;ri,(PO& + (I-3x)Cq + 6NH, + 9H,O Six thermal processings with intermittent regrinding at 2OO’C (24h). 400°C (24h). 600°C (96h), 800°C (48h).
900°C (48h) and 950°C (5 days) are necessary to obtain these compounds without impurities.
In the 0 5 x < 0.25 domain, the X-ray powder diffraction p&terns of these samples reveal the existence of just one phase of Nasicon-type structure. For x > 0.25, a mixture of Tip&. .EuPO, and EuzO, is detected in addition to the
“Nasicon” phase. The effect of the progressive replacement of sodium by europium is a decrease of the ah parameter and an increase of the cb parameter (fiti, although the ionic radius of Et?* is smaller than that of Na’. Actually this evolution is controlled by the strong electrostatic repulsion existing between the neighbouring plans of oxygen limiting the empty site. The limit phase Nao.z~E~.a~Tiz(PO& (r(Na’, Eu? = 0.985 A) possesses cell parameters practically identical to those of Ca,,,Ti@O& (3) [r(Ca’+) = 1.00 A, at, = 8.36 A, ct, = 22.02 A] where the calcium occupies half of the M( 1) sites.
~T*.~~~~~~, i$y~,:,
0.00 0.05 0,lO 0.15 0,20 0.25 0.00 0.05 0.10 0.15 0.20 0.25
Figure 1 : Variation of the cell parameters ah and Q versus rare earth concentration x for Nar.&u~Ti~(P04)3
(0 < x < 0.25).
3- STRUCTURE DETERMINATION OF Nao.~l&.zsTitlPO&
The refinement of the structure of N~.z~~.2~Ti#0,,)3 has been carried out by the Rietveld profilation method in X-ray diffraction, The initial atomic coordinates were those of Cq.;Ti2(PO& (3) in the R 3 space group. Experimental conditions and the crystal structure data are given in tables 1 and 2. Fipure 2 shows the observed, calculated and difference X-my profils for Nao.a~Eu,z~Ti#O&.
Table 1 : Crystal structure data for N@.25EkZJTiZ(P04X
- dcdc.
- d.xs . wavelength (A) -Aqul.rnnge(deg.28) - Step width (deg. 20) - coullr the -Refinement program
- Law for fdl width at
- Number olrellecUons - RP
-%
- RB -RF
R? (Z = 6; hexagonal system) 8.358; 22.04; 1333
3.17 3.09 1.5418 10-120
0.02 30s DBW 3.2 S (4) Law or Ca@tI Pseudo-Voi@(Pv) PV = qL + (l-q)G
q= 0.4308 920 12.6%
14.8%
6.66%
5.40%
Figure 2 : X - Ray dihctogram Nao.&uo.~~Tiz(PO&.
Comparison between experimental and calculated data.
Nat13,@xTi2(P04)3 phosphates 79
Table 2 : Reduced coordinates and thermal vibration factors of Na&Euo.25Tiz(PO& (R 2 ).
4- DESCRIPTION OF THE STRUCTURE AND DISCUSSION
The three-dimensional network of Naa,25Euos~Ti,(P0.& is made up of lJ”iO,] octahedra and Ipo,] tetrahedra sharmg corners @Pure 3). One of the two M(1) site positions (3b : 0, 0, l/2) is totally empty, whereas the other (3a : 0.0, 0) is statistically occupied by Na+ and EL? ions.
The Na/Eu-O(l) distance (2.49A) (table) is larger than that deduced from the ionic radii (5) but it is lower than the distance (2.6OA) between the center of the 3b site and the oxygens O(2). This later high value is the consequence of the strong electrostatic repulsion between the o(2) oxygen plans, perpendicular to the c-axis and limiting the vacancy.
The cationic ordering over the M(1) site implies two titanium types : Ti( 1) neighboring the Na/Eu site and Ti(2) neighboring the empty site. In the ri(l)O,] octahedron, the Ti-O(l) distance facing M(1) is larger than the Ti-O(3) distance since the oxygens o(1) are attracted in the opposite direction by the cation in M( 1). In contrast the Ti(2h0(2) distance facing the empty M(1) site is shortened by the effect of the 0(2)-O(2) electrostatic repulsion within M( 1) along the c-axis. The Tpo,] tetrahedra are slightly distorted and the P-O distances are close to those found in NaTidPQh (6) and Caa.&PW~ (4).
Table 3 : Distances (A) and angles (“) in N~.25Euo.2sTi2(PO&.
(Ns, En)-O(l) (x6) X489(6)
(Na, lb)-Ti(1) 3.242(6) Ol-(Na, Eu)-Ol 63.87(2X
(Na, Et+Ti(2) 4.826(8) 11&12(s)
U-O(Z) (x6) 2.596(Z) 02-0-02 61.21(T),
U-n(l) 4.826(5) 118.79(3)
0-TI(2) 3.161Q
03--n(l)-03 94.61(Z) Ti(l)-O(1) (x3) 1.982(l) Ol-Tt(Q-01 83.27(7~
Ti(l)-O(J) (x3) 1.852(S) 03-Ti(l)-01 93.40(6), Ti(WX2) (13) 1.859(g) 88.27(5)
Ti(f)-O(4) (x3) 1.902(7) 04-Ti(Z)-04 88.56(s) 02-Ti(2)-02 90.62(7) 04-Tl(Z)-02 91.14@, 89.65(4) 02-P-01 111.27(6) P-0( 1) (xl) 1.517(Z) 03-P-01 108.77(Z)
P-O(Z) w l.SW7) 03-P-04 110.53(4)
P-o(J) (xl) 1.551(4) 01-P-04 107.15(t)
P-O(4) (xl) 1.58X2) 04-P-02 112.91(l)
03-P-02 106.17(6)
Figure 3 : Structure of N~.2sEue25Ti2(PO&
S- OPTICAL. PROPERTIES OF N~~,.~&,..uT%PO&
Figure 4 shows the d&se reflection spectrum of the investigated phosphate. The strong absorption observed at high energy (h < 400 mn) can be attributed to the oxygen-europium (III) and oxygen-titanium (Iv) charge transfers Figure 5 represents the emission spectrum of Eu* at 300K under a 36 I nm excitation. The observation of only one ‘Do + ‘FO band implies a unique and non-centrosymmetric site for EU 3+ ; its low intensity is typical of a small deviation from a centrosymmetric position of the luminescent center. This result can be correlated also to the strong thermal agitation of Na/Eu atoms calculated in table 2. The europium is slightly shifted from the center of M( 1) and the actual
80 H. Fakrane et al.
band implies a unique and non-centrosymmetric site for Eu ‘*; its low intensity is typical of a small deviation from a centrosymmetric position of the luminescent center. This result can be correlated also to the strong thermal agitation of Na/Eu atoms calculated in table 2. The europium is slightly shifted from the center of M(l) and the actual local symmetry is C;, or close to C3,. Finally the high energy of the ‘Do -+ ‘FO (17361cm.l ) transition implies a weak crystal field at the rare earth site and confums the ionic character of the Eu-0 bond (7-9).
I(a.u.) ni
T 1
“t
“t rE u-l 9”
2)
I I I I b I I I )
200 ’ 400 600 800 550 650 750
wd h(m)
Figure 4 : Diffuse reflection spectrum Of N% 2sE~ zsTiz(P04), at 300K.
Figure 5 : Emission spectrum of the Et?
ion luminescence in Nkz~Eua lSTi?(PO&
at 300K (hexc. = 361nm) 6- CONCLUSION
The phase Na(,.~,,Eu,Ti2(P0.& (0 5 x < 0.25) have been elaborated by solid state reaction and crystallize with the Nasicon-type structure. The substitution of europium for sodium induces the increase of the ct, parameter and the decrease of the at, parameter. The crystal structure of the compound N~~~EuO.~~T~~(PO& was solved in the context of the RS space group. Sodium and europium atoms are statistically distributed over half of the M( 1) site with a 1-l
vacancy - Na/Eu ordering along the c-axis. Actually the europium is shifted from the center of its site.
Acknowledgments
The authors would like to thank Mr F. Guillen, Institut de Chimie de la Matiere Condensee de Bordeaux, Universite de Bordeaux I, France, for his technical assistance.
7- REFERENCES
[l] R. Salmon, C. Parent, M. Vlasse and G. Le Flem, Mat. Res. Bull., 14, 85, (1979).
[2] M. Alami Taibi, R. Brochu, C. Parent, L. Rabardel and G. Le Flem, I. Solid. State Chem.. 110,330, (1994).
[3] 0. Mentre, F. Abraham, B. Deffontaines and P. Vast, Solid State lonics. 72,293, (1994).
[4] D. B. Wiles and R. Young, J. AUDI. Crvst.. 14, 149, (1981).
[5] R. D. Shannon, Acta Crvst., Sect. A, 32,751, (1976).
[6] J. L. Rodrigo, P. Carrasco and J. Alamo, Mat. Res. Bull.. 24,6 11, (1989).
[7] P. Care, 0. Beaury and E. Antic, J. de Phusiaue. 37,671, (1976).
[8] J. Dexpert-Ghys, M. Faucher and P. Caro, I. Solid State Chem.. 19, 193, (1976).
[9] M. A. Saltzberg and G. C. Farrington, J. Solid State Chem.. 83,272, (1989).
