Synthesis and structural characterization of a new yttrium phosphate: Na2.5Y0.5Mg7(PO4)6 with fillowite-type structure

JOURNAL OF RARE EARTHS, Vol. 28, No. 4, Aug. 2010, p. 481

Synthesis and structural characterization of a new yttrium phosphate: Na2.5Y0.5Mg7(PO4)6 with fillowite-type structure Hasna Jerbi, Mourad Hidouri, Mongi Ben Amara (R.U. Inorganic materials, Faculty of Sciences, 5019, Monastir, Tunisia) Received 24 April 2009; revised 20 April 2010

Abstract: A new phosphate, Na2.5Y0.5Mg7(PO4)6 was synthesized as single crystals by the flux method and as a powdered sample by the Pechini technique, and investigated by single crystal X-ray diffraction, charge distribution (CD) and 31P NMR spectroscopy. This compound crystallized in the rhombohedral space group R 3 and its equivalent hexagonal cell had the following parameters: a=1.4976(6) nm, c= 4.2599(7) nm, Z=18. The structure consisted of MgO5, MgO6, YO6, (Na,Y)O8 and NaOx (x=6, 7 and 9) polyhedra which were linked either directly through common corners, edges and faces and by means of the PO4 tetrahedra via common corners and edges, giving rise to a complex three-dimensional framework, similar to that of the fillowite-like structure. The cation distribution was confirmed by charge distribution (CD) calculations and 31P NMR spectroscopy study. Keywords: phosphate; fillowite; X-ray diffraction; 31P NMR spectroscopy; rare earths

The fillowite-like compounds form a small group of phosphates with chemical compositions approximating Na2CaM7(PO4)6 where M is a divalent cation. Most of them occur as accessory minerals in granitic pegmatites[1–9] or are extracted from some meteoritic rocks[10–14]. A few number of synthetic terms has been reported up to date, being mostly limited to rich-Mn phases[15–20]. The fillowite, itself Na2Ca(Mn,Fe)7(PO4)6 has been described by Moore when studying a natural sample from Branchville Connecticut[5]. The hexagonal cell contains 18 formula units and the asymmetric unit encloses 45 non equivalent atoms. Its complex three-dimensional framework was described in terms of a hexagonal packing based on three kinds of rods, called I, II and III, all running along the c crystallographic direction. Rods I and II pass through the (a, b) plane at (0, 0) and (1/3, 1/3), respectively and are both consisted by large cations X such as Ca, Na and Mn. Rod III consists of the X cations, T (T=P, Si) tetrahedrally coordinated ones and two types of ordered vacancies, called ͚(1) and ͚(2), respectively. It passes through the (a, b) plane at (2/9, 1/9) and has a sequence of -T-X-T-X-T-͙(1)-T-X-T-X-T-͙(2)-. Rare earth phosphates have attracted much attention in recent years because of their interesting optical applications as luminiphors in fluorescent lamps and in laser source devices[21–24]. Moreover, these materials have shown to exhibit a rich structural chemistry due to the tendency of the rare earth element to adopt various coordination polyhedra which can be linked to the phosphate groups in several ways, leading to a large variety of frameworks. Our interest in these materials concerns the preparation of new compounds and their structural characterization by the X-ray diffraction Corresponding author: Mongi Ben Amara (E-mail: [email protected]) DOI: 10.1016/S1002-0721(09)60137-X

techniques and the 31P MAS NMR spectroscopy. In this context, we have recently undertaken a systematic investigation of the system Na3PO4-M3(PO4)2-LnPO4 (M=Mg, Ca, Zn; Ln=La, Y, Nd, Er, Yb) phosphates using several synthetic routes. As a part of this investigation, we reported here the synthesis and structural characterization of a new yttrium phosphate Na2.5Y0.5Mg7(PO4)6 with a fillowite-type structure.

1 Experimental 1.1 Synthesis Single crystals of Na2.5Y0.5Mg7(PO4)6 were grown in a flux of sodium molybdate Na2MoO4 starting from a mixture of 0.622 g of Na2CO3, 0.282 g of Y2O3, 3.399 g of MgCO3, 3.450 g of (NH4)H2PO4 and 3.620 g of Na2MoO4·2H2O. This mixture was firstly homogenized and heated in stages up to 473, 673 and 873 K for 24 h at each stage with intermediate grinding. Then, it was melted for 1 h at 1273 K and subsequently cooled down to 673 K at a 10 K/h rate and then at a 50 K/h rate to room temperature. The final product was washed with warm water to dissolve the flux. From this mixture, quasi-spherical colorless crystals were extracted. Their analysis by ICP performed by "Spectroflame Modula ICP" indicated the presence of Na, Y, Mg and P in an atomic ratio Na:Y:Mg:P|2.7:0.51:7.00:5.89 in accordance with the Na2.5Y0.5Mg7(PO4)6 formula. The powder form was successfully synthesized by the Pechini method in five steps. (1) 0.225 g of Y2O3 and 2.733 g of MgCO3 were dissolved in diluted nitric acid. (2) 0.535 g of Na2CO3 and 2.770 g of (NH4)H2PO4 were added to the

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obtained solution. (3) 0.22 g of citric acid and 0.1 g of ethylene glycol were added. (4) The limpid solution was kept at 333 K for 8 h under continuous stirring. (5) The transparent gel was heated at 423 K leading to a brown resin. A white powder was obtained by firing the precursor at 673, 873 and 1173 K. Its purity was confirmed by Rietveld analysis[25] of its X-ray powder diagram recorded by a PANatycal diffractometer using Co KD radiation (O=1.7910×10-1 nm) (Fig. 1). Attempts to prepare the powder by the conventional solid state reaction were unsuccessful. 1.2 X-ray study The structure was determined by single crystal X-ray diffraction using a quasi-spherical crystal of dimensions 0.23 mmu0.23 mmu0.23 mm. Data were collected by an Enraf-Nonius CAD4 diffractometer using a graphite monochromated MoKD radiation (O=0.71073u10–1 nm). The unit cell parameters and the orientation matrix were determined on the basis of 25 intense reflections in the range 9.92°dTd 10.03°. A total of 5361 unique reflections were collected with a maximum 2T of 60° for the (r h, k, l) hemisphere (Rint=0.028), using the Ȧ-2T scan mode. Only 4295 reflections were considered observed according to the statistic criterion [Fo2>4V(Fo2)]. The intensity data were corrected for the Lorentz and polarization effects. The absorption was corrected empirically using < scan data (Tmin,max=0.49, 0.56)[26]. On the basis of systematic absences, statistics of intensity distribution and subsequent successful solution and refinement of the structure, the space group was determined to be R 3 but the refinement was done in the equivalent hexagonal cell. The structure was solved by direct methods[27] which revealed the positions of the yttrium atoms. The remaining atomic positions were located by a full matrix least squares refinement[28]. A last refinement including all atomic coordinates and anisotropic thermal parameters converged at R1=0.039 and ZR2=0.095 for the observed reflections. X-ray scattering factors together with anomalous dispersion coefficients were taken from International Tables for Crystallography[29]. Crystal data, experimental conditions for intensity measurement, and refinement parameters are given in Table 1. Final atomic coordinates and equivalent isotropic temperature factors are reported in Table 2.

1.3 Characterization The differential thermal analysis (DTA) curve was registered on ground crystals by a SETARM TGDTA-92 apparatus with a heating rate of 10 K/min. The mass of the sample was about 50 mg and alumina was used as reference material. The curve shows a unique endothermic peak at 1391 K corresponding to the melting of the compound. The 31P NMR measurements were performed on a powdered sample at 121.2 MHz by a Bruker spectrometer using a 5 mm prob head allowing spinning rate up to 10 kHz. Chemical shifts are quoted relatively to 85% H3PO4. The spectrum was deconvoluted using the DMfit program[30].

2 Results and discussion The crystal data for the Na2.5Mg7Y0.5(PO4)6 structure as well as its projection along c-axis (Fig. 2) show an isotypism with those of the fillowite-like compounds. The asymmetric unit contains 21 distinct coordination polyhedra: six PO4, four MgO5, five MgO6, two YO6, one NaO6, one NaO7, one NaO9 and one (Na,Y)O8. The latter site is the only disorTable 1 Crystal data, experimental conditions for data collection and structure refinement Crystal data Chemical formula

Na2.5Y0.5Mg7(PO4)6

Formula weight/(g/mol)

841.92

Crystal system

trigonal

Space group

R3

a/nm

1.4976(6)

c/nm

4.2599(7)

V/nm3

8.274(6)

Z

18

Ucal /(kg/m3)

3.04×103

Intensity measurements Crystal dimensions/mm

0.23×0.23×0.23

Apparatus

CAD4( Enraf -Nonius)

O MoKD/10–1 nm

0.71073

Monochromator

Graphite

—/mm–1

2.57

Scan mode

Z/2T

2Tmax

60°

Unique reflections; Rint

5361; 0.028

Observed reflections (F0>4V(F0))

4295

Indices

–21
F(000)

7434

Structure solution and refinement Intensity correction

Lorentz and polarization

Absorption correction

\-scan

Tmin, Tmax

0.49, 0.56

Resolution method

Direct method

Agreement factors

R1=0.039; ZR2=0.095; S=1.070 (F0>4V(F0))

Fig. 1 Powder XRD pattern of Na2.5Mg7Y0.5(PO4)6

Number of refined parameters

365

Extinction coefficient

0.00038(3)

Weighting scheme

w=1/[ı2(F02)+0.0412P)2+106.855P] where P=(F02+2Fc2)/3

('U)max,min/(10 e/nm ) 3

3

1.14, –1.16

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483

dered one with occupancy factors of Na:Y=5:1. These polyhedra are connected to each other to form what have been called by Moore as an hexagonal rod packing built up from three kinds of rods. Rod I (Fig. 3(a)) consists of units of face-sharing Mg(i)O6 (i=1, 2, 3), Na(3)O6 and Na(2)O9 polyhedra. Such units are separated by Y(1)O6 and Y(2)O6 octahedra leading to a stacking sequence of : Y(1)Mg(1)Na(3)Na(2)Mg(2)Mg(3)Y(2)Mg(3) Mg(2)Na(2)Na(3)Mg(1). Table 2 Atomic positions and equivalent thermal parameters Ueq(10–2 nm2) for Na2.5Y0.5Mg7(PO4)6 Atom

x (V)

y (V)

z (V)

Ueq (V)

Y(1)

0

0

0

0.0077(2)

Y(2)

0.2963(2)

0

0.5

0.0058(2)

(Y1/6,Na5/6)

0.3179(2)

0.2701(1)

0.0001(1)

0.0296(3)

Na(1)

0

–0.0784(2)

0.1697(1)

0.0288(4)

Na(2)

0

0

0.2497(1)

0.0220(6)

Na(3)

0

0

0.1769(1)

0.0149(5)

Mg(1)

0

0

0.1039(1)

0.0082(4)

Mg(2)

0

0

0.3200(1)

0.0072(4)

Mg(3)

0.1301(1)

0

0.3954(1)

0.0070(4)

Mg(4)

0.9884(1)

–0.0921(1)

0.3717(1)

0.0143(3)

Mg(5)

0.8872(1)

–0.3430(1)

0.4139(1)

0.0045(2)

Mg(6)

–0.1769(1)

–0.8927(1)

0.1330(1)

0.0130(2)

Mg(7)

–0.3284(1)

0.2499(1)

0.0515(1)

0.0037(2)

Mg(8)

–0.0813(1)

–0.2587(1)

0.0859(1)

0.0104(2)

Mg(9)

0.2006(1)

–0.5743(1)

0.1226(1)

0.0152(3)

P(1)

0.2219(2)

–0.2489(1)

0.3717(1)

0.0069(2)

O(11)

0.9944(2)

–0.1449(2)

0.3849(1)

0.0133(5)

O(12)

0.1597(2)

–0.7965(2)

0.2742(1)

0.0105(5)

O(13)

0.3068(2)

–0.2674(2)

0.3383(1)

0.0141(5)

O(14)

0.9058(1)

–0.2433(2)

0.3724(1)

0.0102(5)

P(2)

0.9042(2)

–0.8740(1)

0.2098(1)

0.0083(2)

O(21)

0.8409(2)

–0.7740(2)

0.2177(1)

0.0088(4)

O(22)

0.0144(2)

–0.9581(2)

0.2344(1)

0.0129(5)

O(23)

0.8524(2)

–0.8585(2)

0.2090(1)

0.0136(5)

O(24)

0.7491(1)

–0.9151(2)

0.1786(1)

0.0232(6)

P(3)

0.6877(2)

–0.7835(1)

0.2769(1)

0.0081(2)

O(31)

0.8644(2)

–0.7783(2)

0.3042(1)

0.0225(6)

O(32)

0.7219(2)

–0.7196(2)

0.2825(1)

0.0165(5)

O(33)

0.7207(2)

–0.7518(2)

0.2459(1)

0.0181(6)

O(34)

0.7836(1)

–0.8985(1)

–0.2727(6)

0.0116(5)

P(4)

0.6822(2)

–0.4258(1)

0.4681(1)

0.0077(2)

O(41)

0.8401(2)

–0.4232(2)

0.4649(1)

0.0107(5)

O(42)

0.8461(2)

–0.3681(2)

0.4978(1)

0.0114(5)

O(43)

0.7569(2)

–0.3711(2)

0.4383(1)

0.0125(5)

O(44)

0.9119(1)

–0.5395(2)

0.4695(1)

0.0115(5)

P(5)

0.9886(2)

–0.2315(6)

0.3614(1)

0.0063(2)

O(51)

0.9665(2)

–0.2534(2)

0.3784(1)

0.0148(5)

O(52)

0.8114(2)

–0.1122(1)

0.3590(1)

0.0140(5)

O(53)

0.8882(2)

–0.2662(2)

0.3796(1)

0.0152(5)

O(54)

0.1106(1)

–0.2821(1)

0.3291(1)

0.0106(5)

P(6)

0.0641(2)

–0.0983(1)

0.4430(1)

0.0059(2)

O(61)

0.0801(2)

–0.0789(2)

0.4721(1)

0.0143(5)

O(62)

0.2301(2)

–0.0630(2)

0.4131(1)

0.0096(4)

O(63)

0.0817(1)

–0.0398(2)

0.4456(1)

0.0094(4)

–0.2126(2)

0.4423(1)

0.0093(4)

O(64)

Fig. 2 Projection along the [001] direction of the Na2.5Mg7Y0.5 (PO4)6 structure showing the hexagonal rod packing (The polyhedra of rod II and III were omitted for clarity)

Rod II (Fig. 3(b)) consists of corner- and edge-sharing Mg(8)O5, Mg(5)O6, Na(1)O7, and (Y,Na)O8 polyedra which form a sequence of : (Y,Na)Mg(5)Na(1)Mg(8). Rod III (Fig. 3(c)) consists of units of corner-sharing PO4, Mg(i)O5 (i=6,7,9) and Mg(4)O6 polyhedra. These units are separated by ordered vacancies ͙1 and ͙2 to form a sequence of: P(5)Mg(7)P(2)Mg(6)P(3)͙1P(4)Mg(9)P(1) Mg(4)P(6)͙2. These rods are connected to each other through common corners or edges in such a way that each of the I and II rods alternates with two rods of type III forming two sorts of planes, labelled (I III III I) and (II III III II), stacked parallel to the [210] direction. This arrangement shown in Fig. 4 is analogous to that observed in the glaserite type-structure[31]. Table 3 gives main interatomic distances in Na2.5Y0.5Mg7(PO4)6. There are nine symmetry non equivalent Mg atoms. Mg(1) to Mg(5) are six coordinated with Mg-O distances ranging from 1.938(3)×10–1 to 2.377(3)× 10–1 nm. Corresponding average values included between 2.069(3)×10–1 to 2.165(3)×10–1 nm are in a good agreement with that 2.14×10–1 nm observed for hexacoordinated Mg2+ ions in Mg3(PO4)2[32]. Mg(6) to Mg(9) are five-coordinated with Mg–O distances varying from 1.968(3)×10–1 to 2.245(3)× 10–1 nm. Their average values (from 2.054×10–1 to 2.089×

Fig. 3 Rods I (a), II (b) and III (c) in Na2.5Y0.5Mg7 (PO4)6

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Fig. 4 The (I III III I) and (II III III II) planes stacked parallel to the [210] direction in Na2.5Y0.5Mg7(PO4)6 (a) and the (BAAB) planes in the glaserite-type structure (b) Table 3 Selected interatomic distances (10–1 nm) for Na2.5Y0.5Mg7(PO4)6 Bond

Interatomic distances/10–1 nm

Bond

Interatomic distances/10–1 nm

Bond

Interatomic distances/10–1 nm

Mg(1)-O(33) Mg(1)-O(44)

1.964(3) (×3) 2.184(3) (×3) 2.074(3)

Mg(2)-O(14) Mg(2)-O(52)

2.040(3) (×3) 2.099(3) (×3) 2.069(3)

Mg(9)-O(23) Mg(9)-O(22) Mg(9)-O(42) Mg(9)-O(63) Mg(9)-O(64)

1.968(3) 2.049(3) 2.063(3) 2.136(3) 2.230(3) 2.089(3)

Mg(3)-O(62) Mg(3)-O(52)

2.010(2) (×3) 2.153(3) (×3) 2.081(3)

Y(1)-O(31)

2.226(3) (×6) 2.226(3)

Na(3)-O(44) Na(3)-O(44) Na(3)-O(44) Na(3)-O(23) Na(3)-O(23) Na(3)-O(23)

2.427(3) 2.427(3) 2.427(3) 2.438(3) 2.438(3) 2.438(3) 2.433(3)

Mg(4)-O(13) Mg(4)-O(11) Mg(4)-O(62) Mg(4)-O(51) Mg(4)-O(53) Mg(4)-O(52)

1.938(3) 1.979(3) 2.045(3) 2.299(3) 2.356(3) 2.377(3) 2.165(3)

Y(2)-O(61)

2.206(2) (×6) 2.206(2)

2.021(3) 2.074(3) 2.109(3) 2.121(3) 2.206(3) 2.213(3) 2.124(3)

2.361(3) 2.477(3) 2.544(3) 2.591(3) 2.601(3) 2.759(3) 2.781(4) 2.809(3) 2.615(4)

1.533(3) 1.536(2) 1.520(3) 1.550(2) 1.534(3)

Mg(5)-O(51) Mg(5)-O(32) Mg(5)-O(21) Mg(5)-O(64) Mg(5)-O(12) Mg(5)-O(43)

(Na,Y)-O(32) (Na,Y)-O(54) (Na,Y)-O(54) (Na,Y)-O(53) (Na,Y)-O(13) (Na,Y)-O(51) (Na,Y)-O(31) (Na,Y)-O(34) <(Na,Y)-O>

P(1)-O(11) P(1)-O(12) P(1)-O(13) P(1)-O(14) P(2)-O(21) P(2)-O(22) P(2)-O(23) P(2)-O(24)

1.547(2) 1.549(3) 1.525(3) 1.514(3) 1.533(3)

1.984(3) 1.996(3) 2.012(3) 2.063(3) 2.219(3) 2.054(3)

2.305(3) 2.484(3) 2.513(3) 2.555(3) 2.571(3) 2.583(3) 2.985(4) 2.571(4)

1.510(3) 1.517(3) 1.527(3) 1.563(3) 1.529(3)

Mg(6)-O(41) Mg(6)-O(24) Mg(6)-O(21) Mg(6)-O(44) Mg(6)-O(33)

Na(1)-O(42) Na(1)-O(64) Na(1)-O(42) Na(1)-O(63) Na(1)-O(41) Na(1)-O(61) Na(1)-O(61)

P(3)-O(31) P(3)-O(32) P(3)-O(33) P(3)-O(34) P(4)-O(41) P(4)-O(42) P(4)-O(43) P(4)-O(44) < P(4)-O>

1.544(2) 1.528(3) 1.547(3) 1.543(3) 1.540(3)

Mg(7)-O(34) Mg(7)-O(14) Mg(7)-O(12) Mg(7)-O(54) Mg(7)-O(22)

2.027(3) 2.081(3) 2.091(3) 2.094(3) 2.108(3) 2.08(3)

1.526(3) 1.552(3) 1.536(3) 1.523(3) 1.534(3)

2.002(3) 2.012(3) 2.015(3) 2.103(3) 2.245(3) 2.075(3)

2.470(3) 2.470(3) 2.470(3) 2.659(3) 2.659(3) 2.659(3) 2.826(3) 2.826(3) 2.826(3) 2.651(3)

P(5)-O(51) P(5)-O(52) P(5)-O(53) P(5)-O(54)

Mg(8)-O(34) Mg(8)-O(63) Mg(8)-O(53) Mg(8)-O(43) Mg(8)-O(41)

Na(2)-O(14) Na(2)-O(14) Na(2)-O(14) Na(2)-O(23) Na(2)-O(23) Na(2)-O(23) Na(2)-O(22) Na(2)-O(22) Na(2)-O(22)

P(6)-O(61) P(6)-O(62) P(6)-O(63) P(6)-O(64)

1.518(3) 1.533(3) 1.554(2) 1.542(2) 1.536(3)

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10–1 nm) are close to that 2.08×10–1 nm reported for the five coordinated Mg2+ ion in NaMg4(PO4)3[33]. The two distinct yttrium atoms are octahedrally coordinated with a single Y–O distance of 2.226(3)×10–1 nm for Y(1) and 2.206(2)×10–1 nm for Y(2), close to that 2.250×10–1 nm observed for the six coordinated Y3+ in KYP2O7[34]. The only disordered site in the structure (Na,Y) has occupancy factors of 5/6 for Na and 1/6 for Y. These values were imposed by the electrical neutrality condition. Similar distribution between Na+ and Y3+ has already been observed in molybdates and oxyapatite compounds[35,36]. This site is eight coordinated with (Na,Y)-O distances in the range 2.361(3)×10–1–2.809(3)×10–1 nm, showing that the (Na,Y)O8 polyhedron is highly distorted. The average distance <(Na,Y)–O>=2.615(4)×10–1 nm is between 2.347×10–1 nm observed for eight-coordinated Y3+ in Na2Y(MoO4)(PO4)[37] and 2.698×10–1 nm reported for Na+ with the same coordination in Na4CaFe4(PO4)6[38]. The six crystallographically distinct PO4 tetrahedra have P–O distances in the range 1.510(3)×10–1–1.563(3)×10–1 nm and their overall value of 1.534×10–1 nm is in a good agreement with 1.537×10–1 nm, predicted by Baur for the monophosphate groups[39]. The environments of the three crystallographic distinct pure Na sites were determined assuming a maximum cation-oxygen distance Lmax=3.13×10–1 nm, suggested by Donnay and Allmann[40]. The Na(1) environment is consisted by seven oxygen atoms with Na-O distances varying from 2.305(3)×10–1 to 2.985(4)×10–1 nm. That of Na(2) is composed by nine oxygen atoms at distances ranging from 2.470(3)×10–1 to 2.826(3)×10–1 nm. Na(3) is bounded to only six oxygen atoms with two Na–O distances of 2.427(3)×10–1 nm and 2.438(3)×10–1 nm.

The charge distribution (CD) was calculated using the CHARDI-IT program[41] leading to the results given in Table 4. The calculated valences of all the sites are consistent with their formal charges. In particular, the valence of 1.304 calculated for the (Na,Y) site is in a good agreement with 1.331 predicted by the structural refinement. Na2.5Y0.5Mg7(PO4)6 displays strong similarities with the Mg-rich fillowite of related composition, Na4/3Ca4/3 Mg7(PO4)6 reported by Domanskii et al.[15] but it features two differences: the presence of one supplementary site of sodium occupation Na(3) and the distribution of the yttrium over three sites among which one, (Na,Y), is shared with Na. These differences in the cationic distribution in the Na(3) and (Na,Y) sites seem to be correlated with the difference in the geometry of their neighbouring polyhedra (Na(2)O9 and Mg(6)O5 for Na(3) and Mg(4)O6 for (Na,Y)) as shown by a calculation of the bond length deviation (BLD) (Table 5). A 31P NMR spectroscopy study was undertaken to confirm the disorder in the (Na,Y) site. The spectrum, shown in Fig. 5, consists of a broad and dissymmetric signal arising from the overlap of several peaks. Its fit revealed nine resonances at 8.32, 7.57, 4.77, 3.61, 1.84, 0.28, –0.86, –2.39 and –3.52 ppm, which are in a good agreement with the values previously reported for other monophosphates[42,43]. The large number of observed peaks, and therefore phosphorus environments, compared to six crystallographic P sites can be assigned to the disorder in the (Na,Y) site. In fact, as illustrated by Fig. 6, among the six P sites, three (P(1), P(3) and P(5)) have the (Na,Y) site as a first-neighbour cation. As

Table 4 Charge distribution (CD) sum calculation of Na2.5Y0.5 Mg7(PO4)6*

Site

n

Ca(1)

8

0.2663

4.32

(Na,Y) 8

0.2615

4.81

Cation (Na,Y) Y(1) Y(2) Na(1) Na(2) Na(3) Mg(1) Mg(2) Mg(3) Mg(4) Mg(5) Mg(6) Mg(7) Mg(8) Mg(9) P(1) P(2) P(3) P(4) P(5) P(6)

Ca(2)

6

0.2324

0

Y(1)

6

0.2226

0

Ca(3)

6

0.2208

0

Y(2)

6

0.2206

0

Na(1)

7

0.2536

3.75

Na(1)

7

0.2571

4.74

Na(2)

9

0.2652

1.34

Na(2)

9

0.2651

4.57

Na(3)

–

–

–

Na(3)

6

0.2432

0.22

Mg(1) 6

0.2087

5.15

Mg(1)

6

0.2074

5.3

Mg(2) 6

0.2056

1.77

Mg(2)

6

0.2069

1.42

Mg(3) 6

0.2114

5.29

Mg(3)

6

0.2081

3.43

Mg(4) 6

0.2141

8.45

Mg(4)

6

0.2165

4.12

Mg(5) 6

0.2133

2.11

Mg(5)

6

0.2124

2.68

Mg(6) 5

0.2088

7.03

Mg(6)

5

0.2054

4.13

Mg(7) 5

0.2067

1.56

Mg(7)

5

0.208

1.02

Mg(8) 5

0.2075

3.03

Mg(8)

5

0.2075

3.79

Mg(9) 5

0.211

3.84

Mg(9)

5

0.2089

3.59

P(1)

4

0.1534

0.49

P(1)

4

0.1534

0.53

P(2)

4

0.1533

1.12

P(2)

4

0.1533

0.94

P(3)

4

0.1532

1.33

P(3)

4

0.1529

1.09

P(4)

4

0.1537

0.47

P(4)

4

0.154

0.32

P(5)

4

0.1536

0.42

P(5)

4

0.1534

0.28

P(6)

4

0.1531

1.42

P(6)

4

0.1536

0.73

q 1.331 3.000 3.000 1.000 1.000 1.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 5.000 5.000 5.000 5.000 5.000 5.000

Anion O(11) O(12) O(13) O(14) O(21) O(22) O(23) O(24) O(31) O(32) O(33) O(34) O(41) O(42) O(43) O(44) O(51) O(52) O(53) O(54) O(61) O(62) O(63) O(64) V=0.05 V=0.10 * Q: computed charge; q: formal oxidation number; Charge dispersion:

Q 1.304 3.085 3.006 0.965 0.954 0.969 2.054 2.033 1.997 2.007 2.061 2.050 1.953 1.973 1.912 5.087 5.045 5.052 4.988 5.035 4.882

q –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000 –2.000

Q –1.826 –1.882 –2.093 –2.093 –1.945 –2.065 –2.124 –1.829 –1.945 –1.969 –1.919 –2.117 –2.058 –2.141 –1.828 –2.008 –1.999 –1.837 –2.007 –2.105 –1.996 –2.128 –2.120 –1.966

Table 5 Geometry parameters of the cationic sites in Na2.5Y0.5 Mg7(PO4)6 and Na4/3Ca4/3Mg7(PO4)6* Na4/3Ca4/3Mg7(PO4)6

100 *BLD = n

Na2.5Y0.5Mg7(PO4)6

/nm

¦

n i 1

BLD/%

M  O   M  O

Site

n

/nm BLD/%

i

M O

%, where n is the number of bonds and

(M–O) is the central cation-oxygene length

486

JOURNAL OF RARE EARTHS, Vol. 28, No. 4, Aug. 2010

apatite NaPb9(PO4)6F(H2O)0.33[44]. It is worth pointing out that the great complexity of the structure did not permit any attribution of the observed peaks to corresponding phosphorus sites.

3 Conclusions

Fig. 5 The 31P NMR spectrum for Na2.5Y0.5Mg7 (PO4)6 and its deconvolution

a consequence, each of them, leads to two distinct local environments whether the (Na,Y) site is occupied by Na+ or Y3+. Similar phenomena have already been observed in the

Na2.5Y0.5Mg7(PO4)6 was synthesized by both the flux and the Pechini techniques and shown to belong to the fillowite type phosphates. Compared to the Mg containing fillowite with related composition Na4/3Ca4/3Mg7(PO4)6 this structure is distinguished by the presence of one supplementary Na site and the disorder in an other site (Na,Y). This disorder revealed by the X-ray study was confirmed by a charge distribution calculation and a 31P NMR spectroscopy study.

Fig. 6 Cationic environments of the phosphorus sites Acknowledgments: The authors are grateful to Pr. Fourati of the Sciences University of Sfax, Tunisia, for the 31P NMR measurements.

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