Hydrothermal Synthesis of Transition Metal Oxides

CHAPTER 4

SYNTHESIS AND CHARACTERIZATION of PbVO₃Cl

4.1 Introduction

Water is the most important solvent in the nature. The remarkable properties of water and its solutions have been used in the synthesis of a new lead chlorovanadate.

There are many studies that have been done on oxides in the Pb-V-O systems. Vanadium oxides represent a large family of compounds which have been investigated due to their wide applications, especially in the field of catalysis. For these reasons many papers about synthesis and crystal chemistry of vanadates, vanadium bronzes, and vanadium phosphates have been published in the past [58].

In contrast, very few chlorovanadates have been synthesized to date. Considering the alkaline earth chlorovanadates, only two series of compounds are actually known. The first series with the formula A(VO₄)₃Cl, obtained for A= Ca, Sr, Ba, exhibits the apatite structure. Whereas the second one corresponds to the formula A₂VO₄Cl with A= Ca, Sr and exhibits the spodiosite structure [58].

There are also many vanadium oxides containing Pb, V, O elements in Pb-V-O system such as Pb_1.32V_8.35O_16.7 [59], PbV₂O₆ [60], Pb₂V₃O_8.5 [61], and -Pb_xV₂O₅ bronzes (x = 0.3) [62].

Effectively, the V-O framework can accommodate many kind of cations including M= Li⁺, Na⁺, K⁺, Ca²⁺, Cu²⁺, Ag⁺, Cd²⁺, Pb²⁺… and resulting in a large range of structural forms depending on the M nature, and synthesis process. For instance, V₂O₅ is recognized to be particularly well suited for lithium insertions leading to the Li_xV₂O₅ system that appeared promising as positive electrode material for secondary lithium batteries [62].

In our laboratory, a novel chlorovanadate compound, PbVO₃Cl, was synthesized by hydrothermal synthesis method.

In the nature, there are several minerals such as kombatite, Pb₁₄(VO₄)₂O₉Cl₄ [63], vanadinite, Pb₅(VO₄)₃Cl [64], with the same composition of elements. The pictures of these two natural minerals are shown in Fig 4.1. Pb₁₄(VO₄)₂O₉Cl₄ has one unique V position tetrahedrally coordinated by O atoms, and there are seven Pb positions with different coordination geometries. Four Pb atoms are coordinated by three or four O atoms and three or four Cl atoms at the vertices of flattened square antiprisms, with the central Pb cations displaced strongly toward the O ligands. Three Pb atoms are coordinated by five or six O atoms but have four of the ligands to one side of the Pb cation [63].

The structural unit of kombatite has a [Pb₁₄(VO₄)₂O₉]⁴⁺ double sheet, and adjacent sheets are linked by Cl anions. Its structural unit consists of two sheets of the tetragonal PbO structure [63].

In the literature, there are three chlorovanadates, AVO₃Cl (A=Ba, Sr, Cd) [58], which exhibit same formula with the compound we synthesized. Two of them, BaVO₃Cl and SrVO₃Cl, exhibit a chain structure same as title compound, whereas the third one CdVO₃Cl, has a layered structure built up from distorted rutile slabs interconnected through double pyramidal vanadium chains. They synthesized AVO₃Cl chlorovanadates in two steps. In first step, a mixture of V₂O₅ and ACO₃ (A=Ba, Sr, Cd) according to the composition AV₂O₆ was heated up to 973 K in platinum crucible for 6 hours to liberate CO₂. In the second step ACl₂ was added to the AV₂O₆ mixture in the 1:1 ratio. After grinding, the resulting mixture was sealed in an evacuated silica ampoule and then heated up to 753 K for 1 day and cooled at 8 K per hour down to 573 K. Finally, it was quenched down to room temperature. According to this procedure they obtained yellow crystals, but their structure determination could not be done due to the poor quality of these crystals and their small size [58].

(a)

(b)
Figure 4.1 Pictures of natural minerals (a) kombatite and (b) vanadinite.

In order to grow single crystals of AVO₃Cl compounds, LiCl was added to the oxides. First, a mixture of V₂O₅ and ACO₃ (in the molar ratio 5:3) was heated up to 973 K in air in a platinum crucible for decarbonation. Then, this resulting mixture, A₃V₁₀O₂₈, was added with 2 moles of LiCl and heated in a silica ampoule under the same conditions as those described for the synthesis of polycrystalline samples. And finally yellow single crystals were extracted from the so-obtained polyphasic samples [58].

In this thesis, our goal was to synthesize new metal oxide compounds. Here a new lead chlorovanadate, PbVO₃Cl, has been synthesized in aqueous solution, structurally characterized, and some of its properties explained. In this section we report the synthesis and structural characterization of PbVO₃Cl. This compound is isostructural to BaVO₃Cl and SrVO₃Cl reported by Borel et al. [58].

4.2 Experimental Procedure

4.2.1 Synthesis of PbVO₃Cl

Single crystals of PbVO₃Cl were obtained from a reaction mixture of NaVO₃ (460.5 mg, 3.8 mmol), PbCl₂ (992 mg, 3.56 mmol). This mixture was loaded into a 23-mL Teflon-lined autoclave. 1.66M of B(OH)₃ was added to fill ~ 40% of the total volume. The mixture was heated in an oven at 170^o C for 3 days and then cooled slowly to room temperature. Cooling must be done slowly, so crystallization occurs in the cooling part of the procedure.

When opened the resulting mixture, it was seen that it had solid products. The solid products were filtered, washed several times with distilled water and acetone, and finally dried in air at the room temperature. The needle shaped yellow crystals of PbVO₃Cl and white crystals were formed in a yellow solution as reaction products. Yellow crystals were obtained with approximately 60-70% yields (Figure 4.2).


Figure 4.2 Yellow crystals of PbVO₃Cl (Nikon Eclips L150 Optic Microscope, 10 X magnitudes).

1,66M B(OH)₃

1
170^oC for 3 days
NaVO₃ (s) + 1 PbCl₂ (s) PbVO₃Cl
Similar reactions were tried with different ratios such as: 1:3:15, 5:1:15, to increase the quality of the crystals and the yield of the reaction. All these reactions gave the same products with different yields. The best yield and better crystal quality were obtained from the reaction with 1:1:7.5 ratios.

4.2.2 X-ray Crystallographic Analyses

A single crystal of PbVO₃Cl was mounted with epoxy in a capillary and was placed on a Bruker Smart 1000 CCD diffractometer under following conditions. A full reciprocal sphere corresponding to a total of 3x606 frames collected (-scan, 15 s per frame, 0.3^o oscillations for 3 different values of ). Monochromatic MoK (=0.71073 Å) was employed. Cell refinement and data reduction were carried out with the use of the program SAINT [43]. Face-indexed absorption corrections were made with the program XREP [44]. The structures were solved by direct methods with the program SHELXS and refined by full-matrix least squares techniques with the program SHELXL in the SHELXTL-97 [44] suite. Further crystallographic details were given in Table 4.1.

Atomic coordinates and equivalent isotropic displacement coefficients, and anisotropic displacement coefficients were given in Table 4.2 and Table 4.4, respectively. All bond distances and bond valence were given in Table 4.3. The bond angles were given in Table 4.5.

Table 4.1 Crystallographic Data for PbVO₃Cl.

Formula PbVO₃Cl

Fw 341.62

Crystal system Orthorhombic

Space group Pnma

Z 3

a, Å 10.022(2)

b, Å 5.2875(11)

c, Å 7.1714(14)

== deg 90

V, (Å) ³ 380.00(13)

Dcalc, g/cm³ 4.058

, mm^-1 35.376

2 range, deg 3.49 – 28.88^o

Table 4.2 Atomic Coordinates (x10⁴) and Equivalent Isotropic Thermal Parameters of

PbVO₃Cl.

Atom X Y Z Ueq

Pb1 0.32991 0.25000 0.61540 0.0093(3)

V1 0.47255 0.25000 0.06797 0.0054(4)

Cl1 0.39510 -0.25000 0.44701 0.0082(5)

O1 0.41086 0.00848 0.88669 0.0064(9)

O2 0.36055 0.25000 0.22921 0.0070(12)

Table 4.3 Bond Lengths (Å) and Bond Valence (italic) in PbVO₃Cl.

Environment of Pb(1) Environment of V(1)

Pb(1) - O(1) 2.464(4) 0.386(4) V(1) – O(1) 1.827(5) 0.94(1)

Pb(1) - O(1) 2.464(4) 0.386(4) V(1) – O(1) 1.827(5) 0.94(1)

Pb(1) - Cl(1) 2.7912(19) 0.39(2) V(1) – O(1) 1.928(4) 0.713(8)

Pb(1) - Cl(1) 2.9794(11) 0.232(7) V(1) – O(1) 1.928(4) 0.713(8)

Pb(1) - Cl(1) 2.9794(11) 0.232(7) V(1) – O(2) 1.606(6) 1.70(3)

Pb(1) – Cl(1) 3.277 0.104

2.74 1.82

∑ij sij = 1.73(2) ∑ij sij = 5.00(2)
Environment of O(1) Environment of O(2)
O(1) – V(1) 1.928(4) 0.713(8) O(2) – V(1) 1.606(6) 1.70(3)

O(1) – V(1) 1.827(5) 0.94(1)

O(1) – Pb(1) 2.464(4) 0.386(4)



2.073(13) 1.606(6)

∑ij sij = 2.04(1) ∑ij sij = 1.70(3)

Environment of Cl(1)
Cl(1) – Pb(1) 2.7912(19) 0.39(2)

Cl(1) – Pb(1) 2.9794(11) 0.232(7)

Cl(1) – Pb(1) 2.9794(11) 0.232(7)

Cl(1) – Pb(1) 3.277 0.104

< Cl(1) – Pb> 3.01(4)

∑ij sij = 0.96(2)

The results refer to the equation ∑s(M-L)= ∑exp[(r_o-r)/0.37] with r_o= 1.803(3)Å, 2.112(4)Å, and 2.4395 Å for V⁵⁺-O, Pb²⁺-O, and Pb²⁺-Cl respectively [57]. V⁵⁺-O, Pb²⁺-O distances are from the table in reference 52. Pb²⁺-Cl distance was calculated using the formula in the same reference.

s= individual bond valences, r = bond distances in structure and r_o = empirically derived single M-L bond distance in angstrom.

Table 4.4 Anisotropic Displacement Coefficients (Ųx 10³) of PbVO₃CI.

Atom U11 U22 U33 U23

Pb1 0.00978 0.00893 0.00917 0.00000

V1 0.00864 0.00275 0.00475 0.00000

Cl1 0.01019 0.00597 0.00848 0.00000

O1 0.01154 0.00032 0.00727 -0.00208

O2 0.01063 0.00604 0.00426 0.00000

Table 4.5 All Bond Angles (degrees) of PbVO₃Cl.

O1-Pb1-O1 62.6(2) O2-V1-O1 108.7(2) x 2

O1-Pb1-Cl1 78.61(10) x 2 O1-V1-O1 96.6(3)

O1-Pb1-Cl1 135.14(11) x 2 O2-V1-O1 105.0(2) x 2

O1-Pb1-Cl1 77.72(11) x 2 O1-V1-O1 145.29(10) x 2

Cl1-Pb1-Cl1 73.65(4) x 2 O1-V1-O1 80.27(19) x 2

Cl1-Pb1-Cl1 125.08(7) O1-V1-O1 83.2(3)

3.2.3 Results and Discussion

The compound having a formula PbVO₃Cl was prepared from the reaction of NaVO₃, and PbCl₂ in boric acid as needle shaped yellow crystals. Structure of the lead chlorovanadate, PbVO₃Cl, consists of a chain of VO₅ and [PbCl] _n sheets running along b-axis as shown in Figure 4.3. In figure 4.3, the unit cell of PbVO₃Cl is viewed down the b-axis. The edge-sharing VO₅ pyramids with a trans configuration were formed by [VO₃] _n chains (Figure 4.4). Such chains have already been observed in the AV₃O₇ type of compounds [65], and also isostructural AVO₃Cl (A=Ba, Sr, Cd).

Each of the V centers has square pyramidal geometry coordinated by five O atoms. The VO₅ square pyramids of PbVO₃Cl exhibit one short apical V-O bond to O2 atom of 1.606(6) Å corresponding to the free apical site, and four longer bonds with basal oxygen that ranges from 1.827(5) to 1.924(4) Å corresponding to the oxygen atoms shared between the pyramids (Figure 4.4). The VO₅ square pyramid has a distorted square pyramidal geometry with the O1-V-O2 angles ranging from 108.7(2) to 105.0(2).

[PbCl]_n sheets reside between the chains of VO₅ square pyramids. Connection between VO₅ pyramids and PbCl sheets are shown in Figure 4.3. Each lead atom is coordinated by both O and Cl atoms. Lead has two equal bonds with oxygen atoms with a bond distance of 2.464(4) Å. In addition, lead has three bonds to chlorine atoms that range from 2.792(19) to 2.979(11) Å and one longer bond to fourth chlorine atom with a distance of 3.277 Å. Connections between PbCl sheets are made by long Pb-Cl bonds. In Ref. 40, Pb-O bonds range from 2.36(2) Å to 3.27(2) Å. All bonds for lead atom are in agreement with the articles reported by N. Henry [61] and by M. Cooper [63].

The relation between bond length and bond valence were studied to show formal charge on the atom in PbVO₃Cl using the empirical relationship developed by Brown and Alternatt [57]. The valence of a given atom is calculated from sum of the individual bond strengths of M-L bonds. The calculated bond valence sums were given in Table 4.3. Calculations show that vanadium is pentavalent, chloro is monovalent, and lead and oxygen are divalent. These results are in agreement with the expected oxidation states. The square pyramidal vanadium has an oxidation state of +5. This assignment of oxidation state is consistent with the overall charge balance of the compound and confirmed by the valence sum calculations which gave a value of 5.00(2) for V(1).



Figure 4.3 Unit cell view of PbVO₃Cl running along b axis.



Figure 4.4 Polyhedral projection of the structure of PbVO₃Cl along a axis showing the

edge-sharing VO₅ pyramids with a trans configuration.

Figure 4.5 View of VO₅ bonds in PbVO₃Cl structure running along a axis.

Vanadium has three oxidation states in minerals: 3+, 4+, 5+. Trivalent V has the electron configuration 3s²3p⁶3d² and occurs mainly in octahedral coordination. In an octahedral ligand field, the two unpaired d electrons occupy the t_2g orbitals and are responsible for the paramagnetic and optical properties of V³⁺ compounds. Many minerals containing V³⁺ as the only transition element are green (e.g., garnet) [66].

Tetravalent vanadium has the electron configuration 3s²3p⁶3d¹ and occurs in 5 or 6 coordination. In both coordination, the degenerate t_2g and e_g orbitals are further split and the d electron occupies one of the nonbonding orbitals (3d_xy): it is responsible for the paramagnetic and optical properties of V⁴⁺. The colors of minerals containing V⁴⁺ are in the green/blue range, but minor contents of V⁵⁺ or interaction with other transition metals or OMCT bands (oxygen-metal charge transfer) can produce different colors. For example, sincosite, Ca(VO)₂(PO₄)₂(H₂O)₅, and simplotite, Ca(VO)₂(VO₄)₂(H₂O)₅, are green, minasragrite, VOSO₄(H₂O)₅, and pentagonite, Ca(VO)(Si₄O₁₀)(H₂O)₄, are blue, and many mixed-valent V⁴⁺/V⁵⁺ minerals are black or dark colored (e.g. melanovanadinite, Ca₂V₈O₂₀(H₂O)₁₀) [66].

Pentavalent vanadium has the electron configuration 3s²3p⁶3d⁰ and forms different kinds of coordination polyhedra: tetrahedral coordination which occurs also in structures with other transition elements of d⁰ configuration (e.g. Ti⁴⁺, Cr⁶⁺, Mo⁶⁺, W⁶⁺), and 5 and 6 coordination. Minerals containing V⁵⁺ are almost always colored even though they have no d electrons. Interactions with other transition metals, minor amounts of V⁴⁺, and OMCT bands cause a broad range of color from red through brown [descloizite, ZnPb(VO₄)(OH)] and orange [schoderite, Al₂(PO₄)(VO₄)(H₂O)₈] to yellow [carnotite, K₂(UO₂)₂(V₂O₈)(H₂O)₃] and green [fernandinite, Ca_0.6(V₈O₂₀)(H₂O)₁₀] [66].

According to this, the title compound that synthesized in our lab has pentavalent vanadium and its electron configuration is 3s²3p⁶3d⁰.

Clark [67] defined a vanadyl bond as one which has a short bond length in the range 1.54-1.68Å. It is a multiple bond with a -component arising from electron flow from O(p) to V(d) orbitals. In 5 and 6 coordinated (V⁴+O_n) and (V⁵+O_n) polyhedra, equatorial bonds occur in a cis arrangement to the vanadyl bonds, and they are longer than the vanadyl bonds. A viewed in Table 4.3, PbVO₃Cl has one vanadyl bond with oxygen atom with a bond distance of 1.606(4) Å, and it can be seen this compound is in agreement with literature.

The number of vanadyl, equatorial, and trans bonds in a polyhedron may be indicated by using a multiple coordination number in which the numbers of bonds are listed in the order vanadyl+ equatorial (+ trans). Thus, 1+4 coordination indicates 5 coordination with one vanadyl bond and four equatorial bonds, and 2+2+2 coordination indicates 6 coordination with two vanadyl bonds, two equatorial bonds, and two trans bonds [66]. In the presence of this knowledge title compound has 1+4 coordination number.

Five and six coordinated V⁵⁺ are characterized by the occurrence of one and two strong vanadyl bonds, respectively. Their presence in mineral and synthetic structures causes a larger variation of vanadyl, equatorial, and trans bond lengths in (V⁵⁺O_n) polyhedra than in (V⁴⁺O_n) polyhedra. Figure 4.6 and 4.7 show the variation of vanadyl, equatorial bond lengths in different (V⁵⁺O_n) polyhedral geometries. There are distinct populations of V-O bond lengths, separated by ranges in which no, or only a few, bond lengths occur. This feature allow us to define different types of V⁵⁺-O bonds and different (V⁵⁺O_n) coordinations [66].

As it is seen in Figure 4.6 and 4.7, in five coordination (1+4 and 2+3), there is a minimum at 1.74-1.76Å for vanadyl bonds. In six coordination (1+4+1 and 2+2+2, there is also a minimum at 1.74-1.75 Å. Thus, we can define a vanadyl bond in five and six coordinations as a bond shorter than 1.74 Å [66].

In five coordination, the equatorial bond can be defined as a bond longer than 1.74 Å. Figure 4.6 shows the variation in length for 1+4 and 2+3 coordinations. The variation in equatorial bond length for 1+4 coordination is generally between 1.74 and 2.04 Å, with a maximal frequency at 1.88 Å (Figure 4.6a). Average bond lengths of V⁵⁺-O_Vanadyl and V⁵⁺-O_Equatorial for square pyramids have reported by Schindler as 1.59 Å and 1.89 Å, respectively [66]. As viewed in Table 4.3, bond length for Equatorial> and Vanadyl> are 1.877 Å, and 1.606 Å, respectively. This result is in agreement with the values reported by Schindler.

Figure 4.6 The distribution of individual bond lengths in (V⁺⁵O₅) polyhedra in mineral and inorganic crystal structures: (a) [1+4] coordination and (b) [2+3] coordination.






Figure 4.7 The distribution of individual vanadyl, equatorial, and trans bonds in (V⁵⁺O₆) polyhedra in mineral and inorganic crystal structures: (a) [1+4+1] coordination and (b) [2+2+2] coordination

Examination of reported crystals with an EDX-equipped Philips XL 30S FEG SEM gave results consistent with the stated compositions. Atomic percentages of the elements of O, Cl, V, and Pb are 38.33%, 15.67%, 14.85%, and 30.60%, respectively. According to the EDX results and peaks of the yellow needle crystals were given in Table 4.6 and in Figure 4.8, respectively.

Table 4.6 EDX results of yellow needle crystals (PbVO₃Cl).




Element Weight % Atomic %

O 7.52 38.88

Cl 6.72 15.67

V 9.14 14.85

Pb 76.62 30.60

Total 100.00 100.00

PbVO₃Cl was synthesized in high yield crystalline form and analyzed by powder X-ray diffraction. The resulting product had two phases that were yellow needle crystals and white crystals. White crystals were identified by powder X-ray diffraction. The X-ray diffraction pattern of white powder matched with PbCl₂ (Figure 4.9). X-ray powder peaks of yellow needle crystals (PbVO₃Cl) did not match with any compound in the XRD database (Figure 4.10).



Figure 4.8 The SEM EDX peaks of PbVO₃Cl.


Figure 4.9 Powder patterns of PbCl₂ and white product.


Figure 4.10 Powder pattern of PbVO₃Cl.
The infrared spectrum of PbVO₃Cl was run on a powder sample in the form of a pellet. In an infrared spectrum, the vanadyl bond gives rise to a very strong sharp V=O stretching bond at 985 ± 50 cm^-1 [66, 68]. The presence of a strong band at 962 cm^-1, which is indicative of the vanadyl V=O bond in the structure, suggests that PbVO₃Cl contain this bond. Thus, this explains why one of the bonds with oxygen is double bond at approximately 1.60 Å. Infrared spectrum of PbVO₃Cl is shown in Figure 4.11. Multiple features attributable to the bridging V-O-V groups are found in the 840-400 cm^-1 region [68]. The peaks at 739 cm^-1 and 519 cm^-1 show the V-O-V in-plane vibration and V-O-V out-of-plane vibration, respectively [68, 69].

The TGA curve of the compound shows one major weight loss between 368^oC and 510^oC. The weight loss was 0.539mg (10.55%). Molecular weight of PbVO₃Cl is 341.55g. 10.55% of this value is 34.15g. This shows that one major weight losses comes from the losses of one Cl element. The result of DSC thermal analysis shows one endothermic peak at the same range about at 480^oC. These two analyses are in agreement with each other and this can be attributed to the decomposition of the compound (Figure 4.12).

Figure 4.11 Infrared spectrum of PbVO₃Cl.


Figure 4.12 TGA and DSC curve of PbVO₃Cl.