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22Ученые записки Таврического национального университета им. В. И. Вернадского Серия «Биология, химия». Том 24 (63). 2011. № 3. С. 48-56.
UDC 548.32
STUDY THE CRYSTAL STRUCTURE OF THE COMPOSITION
Pb8-xEuxNa2(PO4)6D2-x/2Ox/2
Getman E.I., Ignatov A. V., Mohammed A.B. Abdul jabar, Loboda S.N.
Donetsk National University, Donetsk, Ukraine
E-mail: [email protected]
The substitution of europium by lead in the compound Pb8Na2(PO4)6n2 , in accordance with the scheme Pb2+ + ViU ^ Eu3+ + AO2- has been investigated by infrared spectroscopy, scanning electron microscopy and powder X-ray diffraction methods, that corresponds to the compound of solid solutions Pb(8 -x)EuxNa2(PO4)6 □ (2-x/2)O(x/2) (0 £ x < 2,0). It was established that single-phase solid solutions, synthesized by solid state reaction at 800°C, are formed in the range from x=0.0 up to x=1.0. Refinement of the crystal structure of some samples was performed using the Rietveld method. Established that europium ions are located in positions Pb(2), resulting in the distance in a polyhedron Pb(2) the structure of apatite decreased. Keywords: apatite structure, solid solution, lead, europium.
INTRODUCTION
Apatites are generally described in the hexagonal symmetry group P63/m and have the general composition M10(EO4)6(Z)2, where M- one, two and trivalent cations (Na+, K+, Ca2+, Sr2+, Ba2+, Pb2+, Cd2+, Eu3+, Y3+, La3+, ions lanthanide etc.), E - four-, five- and hexavalent cations (Si4+, Ge4+, P5+, V5+, As5+, S6+, Cr6+, etc.), Z - anions OH-, F-, Cl-, Br-, I-, O2- and vacancy (□).
The structure of apatite is characterized by the presence of two structurally nonequivalent positions in the cation sublattice conventionally designated M(1) and M(2). Position M(1) has a circle of nine oxygen atoms (each of which is part of the PO4 tetrahedral), forming a coordination polyhedron - nine vertex polyhedron. Coordination environment positions M(2) consists of six oxygen atoms belonging to the PO4 tetrahedral, and F-(Cl-, OH-, O2-, etc.) ions, which form the coordination polyhedron -seven vertex polyhedron. Equilateral triangles M(2) in the apatite structure form a channel, which are F-(Cl-, OH-, O2-, etc.) ions.
In recent years, the interest of researchers toward compounds with this structure does not weaken, at least for two reasons. First, they possess a practically important property and can be used, for example, as solid stable forms for disposal of radioactive waste, sorbents [1, 2, p. 10], as solid electrolytes [3, p. 10], catalysts [4, p. 10], phosphors, laser materials [5, p. 10], and in many other cases. Secondly, they characterized by a wide range of isomorphism substitutions, allowing isomorphic components by introducing adjust their properties. In particular, by partial substitution in the apatite structure of divalent ions of rare-earth elements on the other elements are luminescent and laser materials [6, 7, p. 10].
Therefore, relevant research heterovalent substitution scheme M2+ + Z- ® Ln3+ + O2- in the systems M(i0-x)Lnx(EO4)6Z(2-x)Ox, where M2+ - ions of divalent elements, Ln3+ - ions of rare-earth elements. So far, substitution of alkaline studied in most rare earth elements (for example, [8-10, p. 10]). However, despite the fact that the ionic radius of lead is similar in size to the radii of ions of alkaline earth elements, no published information about the substitution of lead by rare earth elements in the systems Pb(10-x)Lnx(PO4)6OH(2-x)Ox. Advantages systems of the lead apatites are significantly lower temperature synthesis (800°C [11, p. 10]) compared with systems of the apatite of alkaline earth elements (1200-1450°C [8, p. 10]), which simplifies the method of synthesis and helps to ensure a fine grain.
Thereby, it is interesting to study the substitution in following scheme: 2Pb2++D^2Eu3++O2-, as described for the systems Pb(8-x)LnxNa2(PO4)6D(2-x/2)O(x/2) (Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er). However, these systems have been studied only for compositions with x= 0.25 [12, p. 10]. In this paper we study the substitution of lead by europium in the compound Pb8Na2(PO4)6n2-x/2 in a wide range of compositions.
MATERIALS AND METHODS
1.1. Synthesis: As starting reagents for apatite synthesis, we used PbO (chemically pure), Eu2O3 (99.99%), Na2CO3 (chemically pure) and (NH4)2HPO4 (analytical grade). Samples corresponding to Pb(8-x)EuxNa2(PO4)6D(2-x/2)O(x/2) (0 < x < 2) were synthesized by solid state reaction. One sample was prepared for each value of x in the preceding formula from 0 to 2.0 with an x increment of 0.2. All samples were synthesized according to the following reaction:
(8-x)PbO+6(NH4)2HPO4+Na2CO3+(x/2)Eu2O3® Pb(8-x)EuxNa2(PO 4)6 0 (2-x/2)O(x/2) +
9H2O+ CO2+ 12NH3
A mixture of initial components with a total weight of 1 g was ground in a mortar, followed by successive annealing steps in alumina crucibles at 300 C for 3 h, and finally at 800°C for 72 h with intermittent grindings at the last temperature step.
The duration of annealing for the last temperature step was determined by attaining stable phase composition of synthesized samples. Reaching equilibrium and stable phase composition in Pb(8-x)EuxNa2(PO4)6D(2-x/2)O(x/2) apatites becomes more difficult as europium atomic number increase. The system takes several intermittent grindings to reach equilibrium at 800°C.
1.2. Characterizations: The materials synthesized were characterized by the following conventional techniques. X-ray powder diffraction (XRD) was carried out with a DRON-3 diffractometer, using monochromatic Ni-filtered Cu Ka radiation (^=1.54178). The pattern was scanned in steps of 29=0.05°, in the range 15-140° at a counting time 3 s for each step. The data were analyzed using the Rietveld method with the program FullProf.2k (version 3.40) [13, p. 10] and with graphic interface WinPLOTR [14, p. 10].
Transmission infrared spectra were taken by the KBr method using a Fourier transform infrared spectroremeter (FT-IR TENSOR 27 (Bruker Optics)) in the range (4000-400) cm-1. The infrared absorption spectrum was carried out on a pellet sample, which prepared by crushing 1 mg samples with 600 mg KBr (samples + KBr). Estimate grain size and semi-quantitative elemental analysis was performed on a scanning electron microscopy with X-ray microanalysis in a scanning electron microscope JSM-6490L V
(JEOL, Japan) with energy dispersive spectrometer INCA Penta FETx3 (OXFORD Instruments, England). Differences in the experimental and theoretical content of the elements do not exceed 2%, which is acceptable for this method of analysis in such systems.
RESULTS AND DISCUSSION
The X-ray diffraction patterns of the samples with different value x obtained by solid state reaction are shown in Fig. 1. As seen from Fig. 1, in the composition range x = 0.0 up to 0.8 diffractogram shows only reflections of phase with the structure of apatite (reflections of pure apatite) [15, p. 10]. In samples x = 1.0 and x =1.2 in the diffractogram shown, beside reflections the structure of apatite, found a reflection whose intensity is about 3 % compared with the maximum intensity reflection structure of apatite. Since its intensity is almost independent of the value of x, we can assume that either the superstructure reflections or reflection component that is not part isomorphic to the structure. In the composition range x = 1.4 - 2.0 in X-ray diffractogram, there are also reflections of the structure of europium phosphate EuPO4, whose intensity increases with the value of x increases regularly. This suggests that the limit of the isomorphism substitution corresponds with the value x <1.0.
■t5 ' 20 ' № ' 30 ' 35 ' 40 45 ' 50 55 29 • r (CuKi)
NaPbttP04)t OD-2B-1230 {ICDD)
..J..III., I,
15 2Q ZS d'J V -i; 45 SB
EuPO, 00-2&-10B5 (ICDD)
20 ZO 3 i- 0 . | :. , 1)1, U ill 30 40 4S SO S9
Fig. 1. The X-ray diffraction patterns of Pb(8-x)EuxNa2(PO4)6D(2-x/2)O(x/2), and diffractograms the phase identification of Pb8Na2(PO4)3 and EuPO4 based on data base pdf-2 (ICDD).
A plot of parameters a and c of the Pb(8-x)EuxNa2(PO4)6D(2-x/2)O(x/2) hexagonal unit cell vs degree of substitution by europium (x) is shown in (Fig. 2). The unit cell parameters a and c do not linearly with the compositions (fig. 2). Reason that, the crystal ionic radius of ion Eu3+ (1.087 A) smaller than ion Pb2+ (1,33 A) at 0,21 A (hereafter
sizes taken for coordination number 6 [16, p. 10]) causes a decrease of unit cell parameters a and c with the increase of value x from 0.0 to 2.0 (the error in determining the cell parameters is within ± 0,003 A).
Fig. 2. Plot of hexagonal unit cell parameters a and c vs degree of Eu substitution Pb in the system Pb(8-x)EuxNa2(PO4)6D(2-x/2)O(x/2).
Due to a decrease of cell parameters refinement limit of substitution was also carried out by the "disappearing phase" method. We estimated the europium substitution limit in the apatite by extrapolating the linear relationship of intensity of the largest phosphate europium EuPO4 peak (120) vs the degree of substitution to the intersection with the abscissa axis. Accordingly, the horizontal axis (Fig. 3) gives the limit of substitution of europium in Pb(8-x)EuxNa2(PO4)6D(2-x/2)O(x/2) at x =1,206, in good agreement with the value which shown in Figure 2.
DagritufsifcibtuliwX'
Figure 3. Plot of the intensity of (120) reflection for EuPO4 vs degree of substitution, x.
The samples of the composition Pb(8-x)EuxNa2(PO4)6D(2-x/2)O(x/2) were examination by SEM revealed particles 2.0 and 5.0 ^m successively. Figs. 4(a), 4(b), 4(c) and 4(d) and present SEM images the grain sizes ranged from several dozen to several hundred nanometers. Due to the fact that lead oxide in the synthesis conditions can sublimate was conducted semi-quantitative elemental analysis for Pb, P, Eu, O and Na for samples with the values x=0.0 and 0.4, whose result are presented in Table 1. For the sample with x=0 determine the content of elements was carried out in 24 points on five sections (in brackets are calculated values). Table 1 show results of many samples, which composition by SEM (scanning electronic microscopy) analyze.
c d
Figs. 4. SEM photographs of the samples synthesized by solid state reaction with various revealed particles Pb7.6Eu0.4Na2(PO4)6D0.8O0.2 (a and b), Pb8Na2(PO4)6D(2-x/2) (c and d).
Table 1
Results of the SEM of Pb(8-x)EuxNa2(PO4>6^(2-x/2) for x=0.0 and x=0.4, which synthesized by solid state reaction
x P Pb Eu Na O
Calcd found Calcd found Calcd found Calcd found Calcd found
0 8.18 8.86 72.80 74.84 - - 2.02 1.51 16.9 15.21
0.4 8.24 8.72 69.84 69.59 2.69 2.54 2.03 1.42 17.17 16.74
As initial data for the refinement of the crystal structure using the coordinates of the atoms in the structure of calcium hydroxyapatite, which are presented in [17, p. 10], as well the results of the work [18, p. 10], which showed that the sodium ions in two
predominantly the structure Pb8Na2(PO4)6 localized in position Pb(1). Refinement was carried out for compositions x=0 and 0.8 for 851 and 907 reflections and 34 parameters and 33, respectively. Reliability factors for x=0.0 and x=0.8: 8.13 and 5.68 (Rp); 10.5 and 7.15 (Rwp); 6.48 and 5.79 (Rf); 6.51and 6.03 (Rb); 1.56 and 1.69 (ft2), respectively.
Previously it was shown that the replacement of the structure of calcium hydroxyapatite the predominantly cationic positions determined by the difference of the effective charges of ions replace each other. In that case if the effective charge of the substitute ion is smaller than the ion Ca2+, it take the place of Ca(1), if more - Ca(2) [19, p. 10]. A similar pattern is observed upon substitution of the lead by europium in the solid solutions structure Pb(8-X)EuxNa2(PO4)6D(2-x/2)O(X/2). Since the effective charge of Pb2+ ions is less than the effective charge of ions Eu3+, then the last substitution in the apatite structure of Pb2+ ions are located in positions Pb(2) structure is presented in Table 2 and 3.
Table 2
Crystallographic data and atomic coordinates of Pb7.2Eu08Na2(PO4)6D0.6O04 sample obtained by solid state reaction from Rietveld analysis
Space group: P63/m (VOL. A, 176) Unit cell parameters: a =9. 73074 (Â); c = 7. 17399 (Â); v =588.279 (Â3)
Atom Site Site Occupancy x y z Beq (Â2)
Pb1 Na1 Eu1 Pb2 Eu2 Na2 P 01 02 03 04 4f 4f 4f 6h 6h 6h 6h 6h 6h 12i 4e 1.491( 50) 2.000 0.509( 50) 5.709( 50) 0.291( 50) 6.000 6.000 6.000 12.000 0.400 2/3 2/3 2/3 0.25456( 54) 0.25456( 54) 0.25456( 54) 0.40550( 237) 0.44449( 500) 0.66653( 502) 0.35079( 308) 0 1/3 1/3 1/3 0.99628( 85) 0.99628( 85) 0.99628( 85) 0.37854( 219) 0.56545( 628) 0.49304( 552) 0.25664( 366) 0 -0.00181( 335) -0.00181( 335) -0.00181( 335) 1/4 1/4 1/4 1/4 1/4 1/4 0.08083( 364) 1/4 1.239(162) 1.239(162) 1.239(162) 1.128( 84) 1.128( 84) 1.128( 84) 0.451(448) 5.083(793) 5.083(793) 5.083(793) 5.083(793)
Table 2 showed the atomic parameters, Site Occupancy and Equivalent Isotropic Thermal Displacement Parameters for Pb7.6Eu0.4Na2(PO4)6D0.6O0.4 , moreover Table 3 showed the occupancy of Pb(1) and Pb(2) in the structures Pb8Na2(PO4)6 and
Pb7.2Eu0.8Na2(PO4)6D0.6O0.4.
As a result, refinement of the crystal structure was calculated interatomic distances; some of them are showed in Table 3. As can be seen from Table 3 the localization of europium ion in the position of Pb(2) structure causes a decrease in the average distances Pb(2)-O(1,2,3), that due to the lower crystal ionic radius of Eu3+ in comparison with ionic radius of Pb2+ [16, p. 10], but ion Eu3+ has a large charge in comparison with the charge
of ion Pb2+. Decrease in the interatomic distances in the polyhedron Pb(2) in turn causes an increase in the interatomic distances in the polyhedron Pb(1).
Table 3
Selected Interatomic Distances Pb(8-x)Na2Eux(PO4)6D(2-x/2)O(x/2) for Varying Values of x
x = 0 x = 0.8
Pb(1)-O(1) x 3 2.35(3) 2.55(4)
Pb(1)-O(2) x 3 2.59(3) 2.40(4)
Pb(1)-O(3) x 3 2.81(3) 2.85(4)
< Pb(1)-O > 2.58 2.60
Pb(2)-O(1) 2.63(4) 2.52(7)
Pb(2)-O(2) 2.29(4) 2.19(4)
Pb(2)-O(3) x 2 2.43(2) 2.53(4)
Pb(2)-O(3) x 2 2.77(3) 2.54(3)
< Pb(2) - O(1-3) > 2.55 2.475
Pb(2) - O(4) - 2.496(7)
< Pb(2)-O > - 2.486
P-O(1) 1.65(5) 1.42 (6)
P-O(2) 2.21(5) 1.81 (5)
P-O(3) x 2 1.59 (3) 1.68(3)
< P-O > 1.76 1.65
Pb(2) - Pb(2) 4.349(8) 4.323(9)
The infrared absorption spectra of Pty^E^Na^PO^Dp^O^) with x=0.0, 0.2, 0.4 and 0.6 show characteristic absorption bands of the phosphate groups (PO4)3- and H2O of the apatitic structure. All bands are shown in infrared spectrum Fig. 5.
The two strong lines (x=0.0), located at 987 and 1050 cm-1, and assigned to the triply-degenerate asymmetric stretching (V3) mode of phosphate, appearing at high wavenumbers than Pbi0(PO4№ (1041 and 980 cm-1) and Pb^(PO4)6(OH)2 (1041 and 985 cm-1) [21, p. 10], but approximate to Pb8Na2(PO4^(1056 and 990 cm-1) [22, p. 10,11].
wavenurmber (cm"1)
Fig. 5. Infrared spectra of Pb(8-X)Na2EuX(PO4)6D(2-X/2)O(X/2) solid solutions with x=0.0, 0.2, 0.4, 0.6.
CONCLUSIONS
1. Experimental syntheses, FTIR, XRD, and SEM investigations of PbAP were reported, focusing upon a composition Pb(8-x)EuxNa2(PO4)6D(2-x/2)O(x/2). The site of the atoms in the solid solutions was also analyzed by the Rietveld method. The grain sizes ranged from several dozen to several hundred nanometers. It was established, that the limit of the isomorphism substitution in the composition Pb(8-X)EuXNa2(PO4)6D(2-X/2)O(X/2) corresponds with the value x <1.2. The results of lattice parameters showed, that the unit cells a and c and were decreased, when the value x is increasing..
2. The infrared spectrum showed, a shifting of the internal vibration modes of the (PO4)3-toward higher wavenumber as the lead contents decreases. Refinement of the crystal structure showed that the europium ions are mainly occupied in the sites of Pb(2) the structure of apatite, resulting in mean interatomic distances Pb(1)-O(1,2,3) increases, while the P-O decreases. This unexpected fact demonstrates that the changes in interatomic distances are controlled not only by the geometrical factor, the difference in sizes of the ions involved in the substitution, but also by their charges as well. This finding has practical importance for choosing new substituents and enhancing adsorptive, catalytic, and other important properties of apatite.
References
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Гетьман Е.И. Исследование кристаллической структуры композиции Pbs-xEuxNa2(PO4)6□2-x/2Ox/2 / Е.И. Гетьман, А.В. Игнатов, Мухаммед А.Б. Абдуль Джабар, С.Н. Лобода // Ученые записки Таврического национального университета им. В.И. Вернадского. Серия «Биология, химия». - 2011. -Т. 24 (63), № 3. - С.48-56
Методом рентгенофазового анализа изучено замещение ионов свинца ионам европия в соединении РЪ8Ка2(Р04)6П2 в соответствии со схемой РЬ2+ + !/□ ^ Еи3+ + !/02- ", что соответствует образованию твердых растворов состава РЪ8-хЕих№2(Р04)6П2-х/20х/2 (0 < х < 2,0). Найдено, что замещение при температуре 800 °С, происходит в области составов до х < 1,0. Уточнение кристаллической структуры некоторых образцов проведено с помощью метода Ритвельда. Установлено, что ионы празеодима локализуются преимущественно в позиции РЪ(2). Показано, что параметры ячеек практически не изменяются, в то время как средние межатомные расстояния РЪ(1) - 0(1,2,3) заметно возрастают, а Р -О - уменьшаются.
Ключевые слова структура апатита, твердые растворы, свинец, европий.
Гетьман €.1. Дослвдження кристалiчноl структури композищ1 Pbs-д:EuдNa2(PO4)6□2-д:/2Oд:/2 / €.1. Гетьман, О.В. 1гнатов, Мухамед А.Б. Абдуль Джабар, С.М. Лобода // Вчет записки Тавршського национального университету ш.В.1. Вернадського. Серш „Бюлопя, хiмiя". - 2011. - Т. 24 (63), № 3. - С. 48-56. Методом рентгенофазового анатзу дослвджено замщення юшв плюмбуму юнами евротю в сполущ РЪ8Ка2(Р04)602 у вдаовдаост зi схемою РЪ2+ + !/□ ^ Еи3+ + !02-, що вдаовщае утворенню твердих розчишв складу РЪ8-1Еи1Ка2(Р04)6^2-1/201/2 (0 < х < 2,0). Знайдено, що однофазш твердi розчини, за температури 800°С, утворюються в област до х < 1,0. Уточнення кристатчно! структури деяких зразкгв проведено з використанням алгоритму Ргтвельда. Встановлено, що юни Празеодиму переважно локатзуються в позицп РЪ(2). Показано, що параметри юмрок практично не змшюються, у той час як середт мiжатомнi ввдстат РЪ(1) - 0(1,2,3) помггно зменшуються, а Р - О - збшьшуються. Ключот слова структура апатиту, твердi розчини, плюмбум, европгй.
Поступила в редакцию 10.09.2011 г.
