T I Melnikova, G M Kuzmicheva, V B Rybakov - Structure and morphology peculiarities of Bi24(SiM)2O40 (M Mn, V) with sillenite structure - страница 1
Chemistry of Metals and Alloys
Chem. Met. Alloys 3 (2010) 96-100 Ivan Franko National University of Lviv www. chemetal-j ournal. org
Structure and morphology peculiarities of Bi24(SiM)2O40 (M = Mn, V) with sillenite structure
Tatyana I. MEL'NIKOVA1*, Galina M. KUZ'MICHEVA1, Victor B. RYBAKOV2, Nadezhda B. BOLOTINA3, Alain COUSSON4, Alexander B. DUBOVSKY5
Lomonosov State Academy of Fine Chemical Technology, Vernadsky Ave 86, 119571 Moscow, Russia
2 Lomonosov State University, Vorob 'evy Gory, 119992 Moscow, Russia
3 A. V. Shubnikov Institute of Crystallography, Leninsky Ave 59, 119333 Moscow, Russia
4 Laboratoire Leon Brillouin, Cea/Saclay, 91191 Gif-sur-Yvette Cedex, France
5 Russian Research Institute for the Synthesis of Minerals, 601600 Aleksandrov, Russia * Corresponding author. E-mail: firstname.lastname@example.org
Received May 30, 2010; accepted October 29, 2010; available on-line March 2, 2011
Single crystals in the form of cubes, tetrahedra or combinations of cubes and tetrahedra with sillenite structure in the Bi2O3-SiO2-MnO2 system, as well as bulk crystals (Bi24Ge2O40 used as a seed crystal) in the Bi2O3-SiO2-V2O5 system, have been grown by the hydrothermal method. The results of the crystal chemical analysis by X-ray diffraction, together with the formal charges of the cations calculated by the bond-valence method, allowed establishing the formation of phases of the general compositions Bi24(Si,Mn)2O40 and Bi24(Si,Bi,Mn)2O40 with Bi34, Si44, and Mn44 ions, and Bi24(Si,Bi,V)2O40 with Bi34, Si44, V44, and V54 ions. It was found that the phases of refined compositions
Bi24(Si 0.9(1)Mn 1.1)O404 Bi24(Si 0.04Bi 0.60Mn 1.36)(O39.70□0.30), Bi24(Si 0.17(1)Bi 0.01Mn 1.82)(O39.93(5)□0.07), Bi24(Si 1.160(6)Bi 0.430(V ,V )0.410)(O39.970(20)□0.030), and Bi24(Si 1.150(6)Bi 0.452V 0.398)(O39.930(20)D0.070) crystallize
in space group Р23, in contrast to Bi24(Si4411Bi3401Mn4408)(O3995n005) and Bi24(Si44058Bi34002Mn4414)(O3999n001), which adopt space group /23. The lower symmetry can be explained by a kinetic phase transition of order-disorder type associated with peculiarities of the structure (different atoms in the same crystallographic site) and growth conditions. The crystals with a combination of cubes and tetrahedra (initial charge Na2SiO3-9H20 : Mn(NO3)2-6H2O = 1:1) have space group /23, whereas the crystals with cube (Na2SiO39H20 : Mn(NO3)26H2O > 1) or tetrahedron (Na2SiO39H20 : Mn(NO3)26H2O < 1) habit crystallize in space groups /23 and P23.
Sillenites / Composition / Crystal structure Introduction
The members of the sillenite family, nowadays known as the phases Bi24M2O40±g and solid solutions Bi24(M/xM//1-x)2O40±5 (where M,M,M' are cations with different formal charges in tetrahedral coordination), crystallize in space group I23 (Z = 1) [1,2] (Fig. 1a). These materials are of interest due to photoconductivity, piezo- and electrooptical effects and photorefractive properties. The physical properties depend on the cations in the tetrahedral site and their formal charge (FC).
The presence in tetrahedral sites of cations with different crystal chemical properties (dimension, electronegativity, cation FC) can lead to a kinetic phase transition of order-disorder type . The ordering depends on structure peculiarities, properties of the components, and the method and conditions of preparation of the samples. There exists according to our knowledge no information about similar phenomena for sillenites.
The aim of this paper was to determine the composition and structure of sillenites in the Bi2O3-SiO2-MnO2 and Bi2O3-SiO2-V2O5 systems. Structure refinements have been reported for some compositions in these systems: Bi24Si199Mn001O40 (а = 10.109(1) A) , Bi24Si2O40 (а = 10.10433(5) A) , Bi24Mn2O40 (а = 10.206(1) A) , Bi24(V5+1.7sBi3+0.06n0.16)O40.54 (а = 10.247(8) A) , and Bi3+24[(V5+O4)(Bi3+O4)]O32 (а = 10.222(4) A) .
All the crystals in the Bi2O3-SiO2-MnO2 and Bi2O3-SiO2-V2O5 systems were grown by hydrothermal synthesis with initial charge compositions NaBiO3 + Na2SiO3-9H2Q +
a b Fig. 1 Arrangement of polyhedra in sillenites: space group /23 (a), space group P23 (b). Table 1 Characterization of investigated crystals in the Bi2O3-SiO2-MxOy (M = Mn, V) systems.
Refined composition, reliability factors Ra, D (%); FCcalc/FCexp
Cell parameter а (A)
R1 = 11.04, wR2 = 30.64; D = 3.9; FCcalc./FCexp = 3.3(6)/3.95
tetrahedron/ dark green
R1 = 3.74, wR2 = 3.95; D = 3.5; FCcalc./FCexp. = 3.5(3)/3.95
cube+ tetrahedron/ dark green
R1 = 4.13, wR2 = 4.52; D = 3.6; FCcalc./FCexp .= 4.2(1)/4
cube/ dark green
Bi24(Si 0.04Bi 0.60Mn 1.36)(O39.70°0.30)
R1 = 4.00, wR2 = 4.55; D = 4.5; FCcalc./FCexp = 3.7(2)/3.7
tetrahedron/ dark green
Bi24(Si 0.58Bi 0.02Mn 1.4)(O39.99D0.01)
R1 = 5.75, wR2 = 5.27; D = 4.7; FC3+c./FCexp. = 4.0(4)/3.99
cube + tetrahedron/ dark green
Bi24(Si 0.17(1)Bi 0.01Mn 1.82)(O39.93(5)a0.07)
R1 = 3.84, wR2 = 4.13; D = 3.6; FCc+lc./FCexp. = 4.2(2)/3.995
cube/ dark green
Bi24(Si 1.150(6)Bi 0.452V 0.398)(O39.930(20)Q0.070)
R1 = 7.73, wR2 = 23.35; D = 5.2; FCcalc/FCexi, = 5.18/4.55
Bi24(Si 1.160(6)Bi 0.430(V ,V )0.410)(O39.970(20)Q0.030)
R1 = 7.65, wR2 = 21.98; D = 5.1; FCcalc./FCexp. = 5.12/4.57
The structure refinements were performed in space group /23. Space group not analyzed, space group /23 was used.
Mn(NO3)2-6H2O, and NaBiO3 + Na2SiO3-9H2Q + V2O5, respectively (alkaline solvent NaOH, T = 310°C, AT = 40°C, p = 500 kg/cm2).
The crystals in the Bi2O3-SiO2-MnO2 system were grown as spontaneous single crystals (dark green color, cubes, tetrahedra and combinations of cube and tetrahedron crystal habits, dimensions < 1 mm3) (samples 1-6 in Table 1). The crystals in the Bi2O3-SiO2-V2O5 system were grown as bulk crystals (green and orange color, Bi24Ge2O40 seed crystal, dimensions -3*3*3 mm) (samples 7, 8 in Table 1).
The morphology of the spontaneous single crystals in the Bi2O3-SiO2-MnO2 system depends on the initial charge composition: cubes for Na2SiO3*9H2Q : Mn(NO3)2-6H2O > 1, tetrahedra for Na2SiO3-9H2Q : Mn(NO3)2-6H2O < 1 and a combination of cube and tetrahedron habit for Na2SiO3-9H2Q : Mn(NO3)2-6H2O = 1:1.
Structural analyses of the spontaneous single crystals (samples 1-6, Table 2) were carried out on Xcalibur and CAD-4 diffractometers at room temperature (Mo Ka radiation, graphite monochromator, ю scan mode). For preliminary data processing, we used the CrysAlis RED and WinGX packages, respectively. A neutron diffraction study of the samples prepared in the Bi2O3-SiO2-V2O5 system (samples 7, 8, Table 2) was carried out on the Orphee 5С2 reactor (LLB, France; 1 = 0.828 A). Full-matrix least squares refinements, using the atomic coordinates of Bi24Si2O40  (space group /23) as starting parameters, were carried out with the SHELXL97  and JANA2000  program packages and included anisotropic displacement parameters for all the atoms. The individual occupancies of three different elements (Si, Mn, Bi and Si, V, Bi) on the tetrahedral site were assessed by the SHELXL97 program. The conditions for the data collection and refinement procedure were the same for all of the samples.
X-ray spectral microanalysis was carried out using an Oxford INCA Penta Fetx 3 instrument.
The formal charge (FC) of the tetrahedral site was controlled by the bond-valence method . The electrostatic bond strength
Sy = exp[(Ry - dj) / 0.37] (1) where Rij is the bond-valence parameter for a particular ion pair (tabulated data), and dij experimental values of the interatomic M-O or (M'M'O-O distances; FCcalc = 4*Sj (Table 1).
The value of the parameter D  was employed to test the validity of the crystal structures:
where Vy = kj / rn is the electrostatic bond strength from cations j to anions i, and
Fig. 2 Unit cell parameter a vs. x in the systems (1-x)Bi3+24Si4+2O40-x"Bi3+24Mn4+2O40" (I) and (1-x)Bi3+24Bi3+2(O39n1)-x"Bi3+24Mn4+2O40" (//).
is the constant of the cation polyhedra, vc is the cation valency, ri is the cation-anion distance, va is the anion valency. According to , the value of D must be
less than 5 % (Table 1).
Results and discussion
On the basis of the cell parameters of Bi3+24Si4+2O40 (a = 10.104(1) A) and Bi3+24Ge4+2O40 (a = 10.153(4) A ) and the corresponding ionic radii r(Si4+) = 0.26 A and r(Ge4+) = 0.39 A  we derived an expression for the cell parameters of all Bi3+24M4+2O40 phases (a = 0.3769r(M4+) + 10.006 A) assuming the fulfillment of Vegard's law. Fig. 2 shows the dependence of the cell parameters of the sillenites in the (1-x)Bi3+24Si4+2O40 - x"Bi3+24Mn4+2O40" (line I) and (1-x)Bi3+24Bi3+2(O39^1) - x"Bi3+24Mn4+2O40" (line II) systems on the composition (x value) using literature data for the Bi3+24Si4+2O40  and Bi3+24Bi3+2(O39n1)  phases and the calculated value a ~ 10.15 A for the "Bi3+24Mn4+2O40" phase (r(Mn4+) = r(Ge4+) = 0.39 A). These dependencies have been outlined by straight lines obeying Vegard's law.
The experimental points corresponding to (sample 3) and
Bi24(Si4+0.04Bi3+0.60Mn4+1.36)(O39.70D0.30) (sample 4) (□ -vacancy) fit well on the straight lines I and II, respectively. There is good agreement between the values of FC of the tetrahedral sites calculated by the bond-valence method and the experimental ones for samples 3 and 4 (Table 1). The positive deviation from the linear dependence I is ascribed to Bi3+ ions. The composition of sample 2 obtained using quantitative X-ray spectral microanalysis (Bi242(1)(Si07(1)Mn11)O40) does not contradict the composition calculated from X-ray data (Table 1). This fact supports the presence of additional Bi3+ ions in samples 1, 5 and 6.
The crystal structures were refined in the /-centered space group /23; atomic coordinates, equivalent displacement parameters and occupancies of the tetrahedral cationic positions are listed in Table 2. The relatively low R discrepancy factors and the positive displacement parameters for all the atoms confirm the main features of the structure. However, the presence of a certain number of hkl reflections with F > 3o(F) (F is the structure factor) that are systematically absent in space group /23 (hkl with h+k+l Ф 2n, 0kl with k+l Ф 2n, hhl with l * 2n, h00 with h Ф 2n) for samples 3, 4, 6, 7 and 8 was a clear indication for lower symmetry. Data for sample 1 were collected for space group /23. The opportunity of changing to P23 symmetry was noticed in a later experiment for sample 3 and for samples 2-8 the space group was analyzed. All the crystallographic sites in the structure are split by the transition from space group /23 to space group P23 (Fig. 1). For example, the structure of sample 6
(Bi24(Si4+0.17(1)Bi3+0.01Mn4+1.82)(O39.93(5)^0.07)) refined in
Table 2 Atomic coordinates, equivalent isotropic thermal parameters Ueq*102 (A2), site occupancies ft, and selected interatomic distances d (A) determined on single crystals. Space group /23, Bi2,SiM in Wyckoff position 2a (0 0 0), O2 and O3 in 8c (x x x).