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The title compound, lead bismuth calcium sodium phosphate, Pb4.6Bi0.4Ca2.6Na2.4(PO4)6 crystallizes in the apatite structure type, with vacancies in sites 2a or 2b that are normally occupied by anions. The fact that the Bi and Pb ions are mainly localized in the 6h sites confirms the electron lone-pair influence on the apatite structure.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270103024971/sk1662sup1.cif
Contains datablocks biapat, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270103024971/sk1662Isup2.hkl
Contains datablock I

Comment top

Because of their biologic occurrences and their chemical properties, apatite-type compounds have been widely investigated. The general formula of these compounds is M10(YO4)6X2 (M is an alkaline earth metal, lead, alkali, or a rare earth metal etc., Y = P, As, Si, Ge, S, V and Mn, and X = OH, a halogen, 0.5O, 0.5S or 0.5CO3. Many studies have reported the synthesis, structure, thermal stability, and infrared and Raman spectroscopy ?of these compounds (El Feki et al., 2000). Apatite crystallizes in the hexagonal system (space group P63/m), with cations occupying 4(f) and 6(h) sites in the crystal lattice. Lead apatites, with the general formula Pb8M2(PO4)6, have been prepared, M being a monovalent ion, usually Na, K or Rb (Mayer et al., 1981). Apatites with mixed anions, such as Pb10(PO4)4(SiO4)2, which exhibit anionic lacunae, have also been investigated (Merker & Wondratschek, 1960). Moreover, chemical and physical studies have been performed on lacunary lead apatite partially substituted by calcium, Pb6Ca2Na2(PO4)6, which has vacancies in the Y anion sites (Naddari et al., 2002). We present here a structural determination of a lacuna lead apatite with calcium and bismuth partially substituted for lead, viz. Pb4.6Bi0.4Ca2.6Na2.4(PO4)6. This formula diverges from that expected and reveals that during the heating process a loss of PbO and Bi2O3 occurs. This compound was found to be isostructural with Pb6Ca2Na2(PO4)6 (Naddari et al., 2003). Including restraints, the best refinement gives the formula Pb0.12Ca2.59Na1.29]4f[Pb4.49Bi0.40Na1.09]6h(PO4)6. The Bi/Pb allotment is arbitrary because, with the quality of the measurement, X-ray analysis cannot discriminate between Bi and Pb atoms; there is a difference of only one electron between them. However, constraint of the electroneutrality helps to obtain the Bi3+-ion quantity. Effectively, when M3+ ions are substituted for M2+ ions, a charge compensation mechanism occurrs and two processes may be invoked, viz.

3M2+ 2M3+ + 1Vacancy

or 2M2+ 1M3+ + 1M1+

The first mechanism is unrealistic, because Na atoms were introduced in the synthesis since we assumed that the second mechanism would occur? Moreover, the formula given by the structure solution confirms this assumption. The 4(f) site is preferentially occupied by Na or Ca atoms. Nevertheless, a small amount of a heavy atom is found in this site, although the corresponding z coordinate is found at z =1/4. For structures with lead in this site [e.g. Pb8M2(PO4)6], the z coordinate is moved to 0.29, which behavior has been attributed to lone-pair interaction. We may thus assume that, if there is a Pb atom in this site, it is so diluted that interactions are unobservable. The 6(h) site is filled by Pb, Bi and Na atoms. Although this amount of sodium appears important (18%)?, the Na distribution in the 6(h) and 4(f) sites (45.8 and 54.2%, respectively) does not match the statistical distribution (60 and 40%, respectively), thus revealing that sodium prefers the 4(f) site. Previous studies explained that lone-pair interactions stabilize the triangular shape of atoms with an electron lone pair. Thus, in the title compound, triangles of Pb32+ to Bi33+, with substitutions in between, are expected. Sodium, which has? the lowest electric charge, can form triangles of lower repulsive energy than calcium can, which can explain why calcium is not observed in this site. Assuming such a triangular arrangement, we propose a super cell made of 15 cells in the [001] direction, which allows a composition of 50 (Pb, Bi) triangles and 11 Na triangles stacked in a statistical way. The PO43− tetrahedra, which occupy 6(h) positions similar to cations M(6 h), form an expanded triangular shape. Adjacent M(4f) and M(6 h) polyhedra are linked through O atoms of the phosphate group. Comparison of the PO43− tetrahedron in our compound and in Pb8Na2(PO4)6 (Ternane et al., 2000) shows a slight shortening of the average P—O distance [1.533 (14) and 1.545 (8) Å, respectively]. The P—O2 distance appears to be the shortest P—O bond, which can be attributed to the fact that atom O2 is mainly connected to two M(4f) sites, which contain the M+ cation, while atoms O1 and O3 principally interact with the M(6 h) sites, which are preferentialy occupied by M3+ and M2+ cations. As expected, the average O—P—O angle is 109.41°, but the standard error (3.0°) confirms that distortion of the tetrahedron occurs. Angles including the O2—P bond are greater than the other angles (112.65 and 106.0°, respectively), thus confirming the P—O2 axial deformation and the influence of substitution in the cationic sites on the PO4 distortion.

Experimental top

Crystals of the title compound were obtained from a mixture of Na2CO3, (BiO)2CO3, PbO and (NH4)2HPO4 with stoichiometry according to the formula Pb4BiCa2Na3(PO4)6. A series of stages of heating followed by grinding were performed until a pure apatite was obtained. Between each stage, the temperature was increased by 50 K, up to a maximum of 1373 K. Crystals were obtained after heating for 1 h at 1373 K and slow cooling at 20 K h−1 to 1173 K. Infrared spectroscopic analysis did not reveal any carbonate absorption in the compound.

Refinement top

The main difficulty arises from the occupation of both 4(f) and 6(h) cationic sites by Pb, Bi, Ca or Na atoms. Nevertheless, Fourier summation revealed that the electron density was much more intense in the 6(h) site than in the 4(f) site, thus inferring that the 6(h) site is preferentially occupied by heavy atoms and the 4(f) site is occupied by light atoms. In order to obtain the distribution of the various atoms in each site, constraints on occupancy (assuming full occupation) and electroneutrality were applied. Any attempt to remove the constraints causes the refinement to diverge, which behavior can be attributed to the number of different kinds of atom in one site. Because of the very large absorption coefficient, weak reflections were poorly measured, thus producing a large residual near the heavy atoms (Pb or Bi) in the 6(h) site.

Computing details top

Data collection: Collect (Nonius, 1998); cell refinement: DIRAX/LSQ (Duisenberg, 1992); data reduction: EVALCCD (Duisenberg et al., 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: CrystalMaker (CrystalMaker Software, 1999); software used to prepare material for publication: PLATON (Spek, 2003).

Figures top
[Figure 1] Fig. 1. A view of the structure of the title compound, showing the atom-labelling scheme.
Pb4.6Bi0.4Ca2.6Na2.4(PO4)6. top
Crystal data top
Pb4.6Bi0.4Ca2.6Na2.4(PO4)6Dx = 5.231 Mg m3
Mr = 1768.64Melting point: 1373K K
Hexagonal, P63/mMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 6cCell parameters from KappaCCD reflections
a = 9.5953 (6) Åθ = 5.0–48.0°
c = 7.0418 (4) ŵ = 38.79 mm1
V = 561.48 (6) Å3T = 293 K
Z = 1Hexagonal needle, colourless
F(000) = 771.80.20 × 0.04 × 0.03 mm
Data collection top
Nonis KappaCCD
diffractometer
1863 independent reflections
Radiation source: fine-focus sealed tube1199 reflections with I > 2σ(I)
Vertically mounted graphite crystal monochromatorRint = 0.090
Detector resolution: 9 pixels mm-1θmax = 48.0°, θmin = 3.8°
CCD scansh = 1620
Absorption correction: gaussian
(Coppens et al., 1965)
k = 2019
Tmin = 0.066, Tmax = 0.372l = 1413
17792 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.041 w = 1/[σ2(Fo2) + (0.0119P)2 + 3.2549P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.075(Δ/σ)max = 0.030
S = 1.04Δρmax = 3.05 e Å3
1863 reflectionsΔρmin = 2.45 e Å3
44 parametersExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
3 restraintsExtinction coefficient: 0.0019 (4)
Crystal data top
Pb4.6Bi0.4Ca2.6Na2.4(PO4)6Z = 1
Mr = 1768.64Mo Kα radiation
Hexagonal, P63/mµ = 38.79 mm1
a = 9.5953 (6) ÅT = 293 K
c = 7.0418 (4) Å0.20 × 0.04 × 0.03 mm
V = 561.48 (6) Å3
Data collection top
Nonis KappaCCD
diffractometer
1863 independent reflections
Absorption correction: gaussian
(Coppens et al., 1965)
1199 reflections with I > 2σ(I)
Tmin = 0.066, Tmax = 0.372Rint = 0.090
17792 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.04144 parameters
wR(F2) = 0.0753 restraints
S = 1.04Δρmax = 3.05 e Å3
1863 reflectionsΔρmin = 2.45 e Å3
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ (F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Ca10.66670.33330.0052 (2)0.0143 (4)0.648 (5)
Na10.66670.33330.0052 (2)0.0143 (4)0.324 (12)
Pb10.66670.33330.0052 (2)0.0143 (4)0.030 (2)
Bi20.74931 (3)0.00171 (3)0.25000.01689 (7)0.067 (12)
Pb20.74931 (3)0.00171 (3)0.25000.01689 (7)0.749 (15)
Na20.74931 (3)0.00171 (3)0.25000.01689 (7)0.182 (13)
P0.40658 (14)0.02788 (13)0.25000.0103 (3)
O10.5932 (5)0.1229 (5)0.25000.0238 (9)
O20.3432 (5)0.1518 (4)0.25000.0181 (7)
O30.3512 (4)0.0836 (4)0.4240 (5)0.0262 (7)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Ca10.0159 (4)0.0158 (4)0.0112 (6)0.00791 (19)0.0000.000
Na10.0159 (4)0.0158 (4)0.0112 (6)0.00791 (19)0.0000.000
Pb10.0159 (4)0.0158 (4)0.0112 (6)0.00791 (19)0.0000.000
Bi20.02006 (11)0.01364 (9)0.01789 (11)0.00912 (7)0.0000.000
Pb20.02006 (11)0.01364 (9)0.01789 (11)0.00912 (7)0.0000.000
Na20.02006 (11)0.01364 (9)0.01789 (11)0.00912 (7)0.0000.000
P0.0121 (4)0.0089 (4)0.0095 (5)0.0050 (3)0.0000.000
O10.0137 (14)0.0190 (16)0.035 (3)0.0056 (13)0.0000.000
O20.0233 (17)0.0100 (12)0.0204 (19)0.0078 (12)0.0000.000
O30.0451 (18)0.0239 (13)0.0161 (14)0.0221 (13)0.0131 (13)0.0050 (11)
Geometric parameters (Å, º) top
Pb1—O2i2.418 (3)Pb2—Piv3.1500 (12)
Pb1—O2ii2.418 (3)Pb2—Pb1ii4.0505 (7)
Pb1—O2iii2.418 (3)Pb2—Pb1xii4.0505 (7)
Pb1—O12.526 (3)Pb2—Pb1viii4.0630 (8)
Pb1—O1iv2.526 (3)P—O21.515 (4)
Pb1—O1v2.526 (3)P—O3viii1.534 (3)
Pb1—O3vi2.824 (4)P—O31.534 (3)
Pb1—O3vii2.824 (4)P—O11.551 (4)
Pb1—O3viii2.824 (4)P—Pb2v3.1500 (12)
Pb1—P3.2762 (12)P—Pb1viii3.2762 (12)
Pb1—Piv3.2762 (12)O1—Pb1viii2.526 (3)
Pb1—Pv3.2762 (12)O2—Pb1ii2.418 (3)
Pb2—O12.310 (4)O2—Pb1xii2.418 (3)
Pb2—O3ix2.466 (3)O2—Pb2xiii2.948 (4)
Pb2—O3x2.466 (3)O3—Pb2x2.466 (3)
Pb2—O3vi2.548 (3)O3—Pb2v2.548 (3)
Pb2—O3iv2.548 (3)O3—Pb1viii2.824 (4)
Pb2—O2xi2.948 (4)
O2i—Pb1—O2ii74.81 (10)O1—Pb2—Piv70.15 (10)
O2i—Pb1—O2iii74.81 (10)O3ix—Pb2—Piv108.03 (8)
O2ii—Pb1—O2iii74.81 (10)O3x—Pb2—Piv108.03 (8)
O2i—Pb1—O1150.41 (12)O3vi—Pb2—Piv28.84 (7)
O2ii—Pb1—O192.10 (9)O3iv—Pb2—Piv28.84 (7)
O2iii—Pb1—O1127.97 (12)O1—Pb2—Pb1ii76.71 (9)
O2i—Pb1—O1iv92.10 (9)O3ix—Pb2—Pb1ii43.42 (9)
O2ii—Pb1—O1iv127.97 (12)O3x—Pb2—Pb1ii93.78 (9)
O2iii—Pb1—O1iv150.41 (12)O3vi—Pb2—Pb1ii117.84 (8)
O1—Pb1—O1iv74.96 (11)O3iv—Bi2—Pb1ii151.22 (8)
O2i—Pb1—O1v127.97 (12)Piv—Pb2—Pb1ii138.169 (19)
O2ii—Pb1—O1v150.41 (12)O1—Pb2—Pb1xii76.71 (9)
O2iii—Pb1—O1v92.10 (9)O3ix—Pb2—Pb1xii93.78 (9)
O1—Pb1—O1v74.96 (11)O3x—Pb2—Pb1xii43.42 (9)
O1iv—Pb1—O1v74.96 (11)O3vi—Pb2—Pb1xii151.22 (8)
O2i—Pb1—O3vi83.90 (11)O3iv—Pb2—Pb1xii117.84 (8)
O2ii—Pb1—O3vi73.48 (10)Piv—Pb2—Pb1xii138.168 (19)
O2iii—Pb1—O3vi145.43 (10)Pb1ii—Pb2—Pb1xii50.37 (4)
O1—Pb1—O3vi66.82 (11)O1—Pb2—Pb134.53 (6)
O1iv—Pb1—O3vi54.94 (11)O3ix—Pb2—Pb169.58 (7)
O1v—Pb1—O3vi122.42 (11)O3x—Pb2—Pb1118.33 (7)
O2i—Pb1—O3vii73.48 (10)O3vi—Pb2—Pb143.48 (8)
O2ii—Pb1—O3vii145.43 (10)O3iv—Pb2—Pb172.54 (7)
O2iii—Pb1—O3vii83.90 (11)Piv—Pb2—Pb152.177 (19)
O1—Pb1—O3vii122.42 (11)Pb1ii—Pb2—Pb186.136 (7)
O1iv—Pb1—O3vii66.82 (11)Pb1xii—Pb2—Pb1108.004 (6)
O1v—Pb1—O3vii54.94 (11)O1—Pb2—Pb1viii34.53 (6)
O3vi—Pb1—O3vii116.01 (5)O3ix—Pb2—Pb1viii118.33 (7)
O2i—Pb1—O3viii145.43 (10)O3x—Pb2—Pb1viii69.58 (7)
O2ii—Pb1—O3viii83.90 (11)O3vi—Pb2—Pb1viii72.54 (7)
O2iii—Pb1—O3viii73.48 (10)O3iv—Pb2—Pb1viii43.48 (8)
O1—Pb1—O3viii54.94 (11)Piv—Pb2—Pb1viii52.177 (19)
O1iv—Pb1—O3viii122.42 (11)Pb1ii—Pb2—Pb1viii108.003 (6)
O1v—Pb1—O3viii66.81 (11)Pb1xii—Pb2—Pb1viii86.135 (7)
O3vi—Pb1—O3viii116.01 (5)Pb1—Pb2—Pb1viii52.50 (4)
O3vii—Pb1—O3viii116.01 (5)O2—P—O3viii112.64 (14)
O2i—Pb1—P165.30 (9)O2—P—O3112.64 (14)
O2ii—Pb1—P90.50 (7)O3viii—P—O3106.0 (3)
O2iii—Pb1—P101.36 (9)O2—P—O1111.0 (2)
O1—Pb1—P27.30 (9)O3viii—P—O1107.11 (17)
O1iv—Pb1—P97.37 (8)O3—P—O1107.11 (17)
O1v—Pb1—P65.76 (9)O2—P—Pb2v135.66 (16)
O3vi—Pb1—P92.35 (7)O3viii—P—Pb2v53.24 (13)
O3vii—Pb1—P120.64 (7)O3—P—Pb2v53.24 (13)
O3viii—Pb1—P27.89 (6)O1—P—Pb2v113.39 (16)
O2i—Pb1—Piv90.50 (7)O2—P—Pb1135.32 (10)
O2ii—Pb1—Piv101.36 (9)O3viii—P—Pb159.45 (14)
O2iii—Pb1—Piv165.30 (9)O3—P—Pb1111.59 (13)
O1—Pb1—Piv65.76 (9)O1—P—Pb148.31 (11)
O1iv—Pb1—Piv27.30 (9)Pb2v—P—Pb178.41 (2)
O1v—Pb1—Piv97.37 (8)Pb2v—P—Pb1viii78.40 (2)
O3vi—Pb1—Piv27.89 (6)Pb1—P—Pb1viii66.53 (5)
O3vii—Pb1—Piv92.35 (7)O2—P—Pb1viii135.32 (10)
O3viii—Pb1—Piv120.65 (7)O3viii—P—Pb1viii111.59 (13)
P—Pb1—Piv92.79 (3)O3—P—Pb1viii59.45 (14)
O2i—Pb1—Pv101.36 (9)O1—P—Pb1viii48.31 (11)
O2ii—Pb1—Pv165.30 (9)Pb2v—P—Pb1viii78.40 (2)
O2iii—Pb1—Pv90.50 (7)Pb1—P—Pb1viii66.53 (5)
O1—Pb1—Pv97.37 (8)P—O1—Pb2123.5 (2)
O1iv—Pb1—Pv65.76 (9)P—O1—Pb1104.40 (15)
O1v—Pb1—Pv27.30 (9)Pb2—O1—Pb1114.23 (12)
O3vi—Pb1—Pv120.65 (7)P—O1—Pb1viii104.40 (15)
O3vii—Pb1—Pv27.89 (6)Pb2—O1—Pb1viii114.23 (12)
O3viii—Pb1—Pv92.35 (7)Pb1—O1—Pb1viii90.72 (14)
P—Pb1—Pv92.79 (3)P—O2—Pb1ii131.97 (9)
Piv—Pb1—Pv92.79 (3)P—O2—Pb1xii131.97 (9)
O1—Pb2—O3ix85.56 (8)Pb1ii—O2—Pb1xii90.92 (13)
O1—Pb2—O3x85.56 (8)P—O2—Pb2xiii96.81 (18)
O3ix—Pb2—O3x137.15 (17)Pb1ii—O2—Pb2xiii97.52 (10)
O1—Pb2—O3vi74.78 (12)Pb1xii—O2—Pb2xiii97.52 (10)
O3ix—Pb2—O3vi80.24 (6)P—O3—Pb2x140.72 (17)
O3x—Pb2—O3vi136.55 (12)P—O3—Pb2v97.92 (16)
O1—Pb2—O3iv74.78 (12)Pb2x—O3—Pb2v116.72 (12)
O3ix—Pb2—O3iv136.56 (12)P—O3—Pb1viii92.66 (16)
O3x—Pb2—O3iv80.24 (6)Pb2x—O3—Pb1viii99.71 (13)
O3vi—Pb2—O3iv57.50 (14)Pb2v—O3—Pb1viii98.14 (10)
Symmetry codes: (i) y+1, x+y+1, z; (ii) x+1, y, z; (iii) xy, x, z; (iv) y+1, xy, z; (v) x+y+1, x+1, z; (vi) y+1, xy, z+1/2; (vii) x+y+1, x+1, z+1/2; (viii) x, y, z+1/2; (ix) x+1, y, z1/2; (x) x+1, y, z+1; (xi) x+y+1, x, z; (xii) x+1, y, z+1/2; (xiii) y, xy1, z.

Experimental details

Crystal data
Chemical formulaPb4.6Bi0.4Ca2.6Na2.4(PO4)6
Mr1768.64
Crystal system, space groupHexagonal, P63/m
Temperature (K)293
a, c (Å)9.5953 (6), 7.0418 (4)
V3)561.48 (6)
Z1
Radiation typeMo Kα
µ (mm1)38.79
Crystal size (mm)0.20 × 0.04 × 0.03
Data collection
DiffractometerNonis KappaCCD
diffractometer
Absorption correctionGaussian
(Coppens et al., 1965)
Tmin, Tmax0.066, 0.372
No. of measured, independent and
observed [I > 2σ(I)] reflections
17792, 1863, 1199
Rint0.090
(sin θ/λ)max1)1.045
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.041, 0.075, 1.04
No. of reflections1863
No. of parameters44
No. of restraints3
Δρmax, Δρmin (e Å3)3.05, 2.45

Computer programs: Collect (Nonius, 1998), DIRAX/LSQ (Duisenberg, 1992), EVALCCD (Duisenberg et al., 2003), SHELXS97 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), CrystalMaker (CrystalMaker Software, 1999), PLATON (Spek, 2003).

Selected geometric parameters (Å, º) top
Pb1—O2i2.418 (3)Pb2—O3v2.548 (3)
Pb1—O12.526 (3)Pb2—O2vi2.948 (4)
Pb1—O3ii2.824 (4)P—O21.515 (4)
Pb2—O12.310 (4)P—O3vii1.534 (3)
Pb2—O3iii2.466 (3)P—O31.534 (3)
Pb2—O3iv2.466 (3)P—O11.551 (4)
Pb2—O3ii2.548 (3)
O2—P—O3vii112.64 (14)O2—P—O1111.0 (2)
O2—P—O3112.64 (14)O3vii—P—O1107.11 (17)
O3vii—P—O3106.0 (3)O3—P—O1107.11 (17)
Symmetry codes: (i) y+1, x+y+1, z; (ii) y+1, xy, z+1/2; (iii) x+1, y, z1/2; (iv) x+1, y, z+1; (v) y+1, xy, z; (vi) x+y+1, x, z; (vii) x, y, z+1/2.
 

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