Hydrothermal Synthesis of Transition Metal Oxides



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3.2 Experimental Procedure




3.2.1 Synthesis of KV3O8

Single crystals of KV3O8 were obtained from a reaction mixture of KVO3 (552.4 mg, 4 mmol), PbCl2 (556.9 mg, 2 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 170oC for 3 days and then slowly cooled to room temperature.

After cooling, the resulting mixture was opened, and solid products were filtered, washed several times with distilled water and acetone, and finally dried in air at room temperature. The hexagonal plate-shaped orange crystals were obtained in an orange solution as reaction products (Figure 3.5). These hexagonal orange crystals (KV3O8) were examined with EDX, and K (12.25%), V(47.15%), O(40.60%) atoms were found in these crystals. ICSCD database was used to check this data. The SEM EDX peaks of KV3O8 crystals and the SEM EDX results (weight %) were shown in Figure 3.6 and in Table 3.1, respectively.

Figure 3.5 Crystal pictures of KV3O8 (Nikon Eclips L150 Optic Microscope, 10 X magnitudes).

Figure 3.6 The SEM EDX peaks of KV3O8.

Tablo 3.1 EDX results of orange plate-shaped crystals (KV3O8).



Element Weight % Atomic %


K 12.25 8.30

O 40.60 67.19

V 47.15 24.51

Total 100.00 100.00

In order to synthesize new crystals, many different reactions were performed and two different crystals were obtained. The reaction (1) was the first reaction that was done for this crystal (2:1:15 ratio). The quality of the crystals was not good and yield of the reaction (approximately 10%) was also not good. In order to increase the yield and the quality of the crystals, similar reactions were tried under the same conditions (at 170oC for 3 days) with different ratios such as (3:1:15), (10:1:30), (5:1:15), (5:2:15). All these reactions gave the same products with better and different yields. Among these reactions the best yield was obtained from the reaction with 3:1:15 ratios (approximately 20%).


1,66M B(OH)3

(
170 oC for 3 days
2:1:15) KVO3 (s) + PbCl2 (s) KV3O8 (1)
After these reactions, we tried to increase the reaction time to obtain the better crystal quality. In order to do this, the reaction (2) with 3:1:15 mole ratio was done at 170oC for 5 days. The increase in the reaction time gave crystals with better quality. After all these reactions, the increase in the reaction temperature was tried, but it did not change either the crystal quality or the reaction yield. After this experiment we saw that the increase in the reaction time was more effective on producing good quality single crystals than the increase in the reaction temperature.


1,66M B(OH)3

(
170 oC for 5 days
3:1:15) KVO3 (s) + PbCl2 (s) KV3O8 (2)
The best reaction yield (approximately 30%) and the best crystal quality were obtained from the reaction (2) with 3:1:15 ratios and 170oC for 5 days.

As mentioned before these crystals were synthesized by Oka et.al with more complicated procedure in 2 days. We wondered if we could obtain these crystals in 1 or 2 days. In order to do this, we tried the following reactions (3:1:15 mole ratio):


1,66M B(OH)3

(
170 oC for 1 day


3:1:15) KVO3 (s) + PbCl2 (s) KV3O8 (3)

1,66M B(OH)3

(
170 oC for 2 days


3:1:15) KVO3 (s) + PbCl2 (s) KV3O8 (4)
Both reactions (3) and (4) gave the same product with different yield. There were a few orange crystals in the reaction (3), however many orange crystals were obtained from the reaction (4).

When looking at the compound, there are not Pb, and Cl atoms in the compound. In order to search the effect of PbCl2 on obtaining the orange crystals reactions (5), and (6) were done.



1,66M B(OH)3

(
170 oC for 1 days


7:15) KVO3 (s) KV3O8 (5)

1,66M B(OH)3

(
170 oC for 2 days


7:15) KVO3 (s) KV3O8 (6)

There were not any crystals in the reaction (5), however many orange crystals were obtained from the reaction (6).

Many reactions have made to get KV3O8 crystals by using hydrothermal method. As it is seen when reaction (3) was tried at 170oC for 1 day, a few orange crystals were able to be obtained. However, when the reaction (6) was done without using PbCl2 at 170oC for 2 days, many orange crystals were obtained. In reaction (3) PbCl2 might have acted as mineralizer. It can be said that KV3O8 crystals can be synthesized in 1 day with PbCl2, however if PbCl2 does not want to be used due to its being expensive, these crystals cane be obtained in 2 days without using PbCl2 with this method.

3.2.2 X-ray Crystallographic Analyses

A single crystal of KV3O8 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, 0.3o oscillations for 3 different values of ). Monochromatic MoK (=0.71073 Ao) 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 3.2.



Table 3.2 Crystallographic Data for KV3O8


Formula KV3O8

Fw 446.55

Crystal system Monoclinic

Space group P2(1)/m

Z 2


a, Å 4.9664(10)

b, Å 8.3600(17)

c, Å 7.5982(15)

= deg 90

= deg 96.643(3)

V, (Å) 3 313.35(11)

Dcalc, g/cm3 2.366

, mm-1 3.211

2 range, deg 2.70– 27.53
Atomic coordinates and equivalent isotropic displacement coefficients, and anisotropic displacement coefficients were given in Table 3.3 and Table 3.4, respectively. The bond angles were given in Table 3.5. All bond distances and bond valence were given in Table 3.6.

Table 3.3 Atomic Coordinates (x104) and Equivalent Isotropic Thermal Parameters of

KV3O8.


Atom X Y Z Ueq

V1 0.58113 0.25000 0.41967 0.0042

V2 1.06012 0.05337 0.30764 0.0041

K1 0.5444 0.25000 -0.06579 0.0121

O1 0.9031 0.25000 0.2325 0.0049

O2 0.3776 0.25000 0.5698 0.0083

O3 0.8114 0.0855 0.4942 0.0061

O4 1.3818 0.0971 0.2537 0.0059

O5 0.9303 -0.0706 0.1609 0.0079



Table 3.4 Anisotropic Displacement Coefficients (Å2x 103) of KV3O8.


Atom U11 U22 U33 U23

V1 0.0026(5) 0.0034(5) 0.0067(5) 0.000

V2 0.0033(5) 0.0029(5) 0.0063(5) 0.00020(19)

K1 0.0097(6) 0.0157(7) 0.0099(6) 0.000

O1 0.0031(16) 0.0041(15) 0.0097(16) 0.000

O2 0.0068(16) 0.0079(17) 0.0097(17) 0.000

O3 0.0045(11) 0.0049(11) 0.0077(12) 0.0005(8)

O4 0.0052(11) 0.0045(11) 0.0080(12) 0.0005(9)

O5 0.0099(14) 0.0057(13) 0.0094(12) 0.0007(9)

Table 3.5 All Bond Angles (degrees) of KV3O8.


O2 V1 O3 101.62(13)x2 O3 V1 O3 97.03(17)

O2 V1 O4 98.20(14)x2 O3 V1 O4 87.73(11)x2

O3 V1 O4 158.20(12)x2 O4 V1 O4 80.50(16)

O2 V1 O1 173.95(18) O3 V1 O1 74.53(11)x2

O4 V1 O1 86.40(10)x2 O2 V1 V2 139.62(9)x2

O3 V1 V2 38.25(9)x2 O3 V1 V2 90.66(9)x2

O4 V1 V2 79.93(8)x2 O4 V1 V2 120.82(9)x2

O1 V1 V2 37.20(5)x2 V2 V1 V2 64.26(3) O2 V1 V2 67.78(12)x2 O3 V1 V2 162.22(8)x2

O3 V1 V2 99.08(8)x2 O4 V1 V2 80.07(8)x2

O4 V1 V2 30.95(8)x2 O1 V1 V2 117.13(8)x2

V2 V1 V2 148.30(4)x2 V2 V1 V2 106.67(3)x2

V2 V1 V2 64.04(3) O5 V2 O4 106.24(15) O5 V2 O1 103.58(14) O4 V2 O1 96.05(14) O5 V2 O3 102.81(13) O4 V2 O3 94.57(12) O1 V2 O3 147.43(14) O5 V2 O3 110.34(13) O4 V2 O3 143.10(12) O1 V2 O3 80.19(14) O3 V2 O3 72.95(12) O5 V2 V1 105.96(10) O4 V2 V1 135.46(10) O1 V2 V1 46.61(11) O3 V2 V1 107.32(8) O3 V2 V1 34.57(8) O5 V2 V1 142.19(11) O4 V2 V1 35.95(9) O1 V2 V1 85.62(10) O3 V2 V1 85.15(8) O3 V2 V1 107.34(8) V1 V2 V1 106.67(3) V2 O1 V2 121.96(19) V2 O1 V1 96.18(12)x2 V1 O3 V2 145.19(16) V1 O3 V2 107.18(14) V2 O3 V2 107.05(12) V2 O4 V1 113.10(14)



3.2.3 Bond Valence Calculations



Table 3.6 Bond Lengths (Å) and Bond Valence (italic) in KV3O8.

Environment of V(1) Environment of V(2)

V(1) – O(1) 2.260(4) 0.290(3) V(2) – O(1) 1.880(2) 0.812(39)

V(1) – O(2) 1.609(4) 1.690(2) V(2) – O(3) 1.949(3) 0.674(6)

V(1) – O(3) 1.836(3) 0.914(7) V(2) – O(3) 2.003(3) 0.582(4)

V(1) – O(3) 1.836(3) 0.914(7) V(2) – O(4) 1.734(3) 1.205(10)

V(1) – O(4) 1.979(3) 0.621(5) V(2) – O(5) 1.602(3) 1.721(13)

V(1) – O(4) 1.979(3) 0.621(5)



1.916(20) 1.834(14)

∑ij sij = 5.05(29) ∑ij sij = 4.99(72)

Environment of K(1) Environment of O(1)

K(1) – O(1) 2.716(4) 0.206(2) O(1) – V(2) 1.880(2) 0.812(39)

K(1) – O(2) 2.797(4) 0.166(2) O(1) – V(2) 1.880(2) 0.812(39)

K(1) – O(4) 2.938(3) 0.113(1) O(1) – V(1) 2.260(4) 0.290(3)

K(1) – O(4) 2.938(3) 0.113(1) O(1) – K(1) 2.716(4) 0.206(2)

K(1) – O(5) 2.817(3) 0.157(1)

K(1) – O(5) 2.817(3) 0.157(1)

K(1) – O(5) 3.164(3) 0.061

K(1) – O(5) 3.164(3) 0.061

K(1) – O(4) 3.273(3) 0.046(1)



K(1) – O(4) 3.273(3) 0.046(1)

2.990(32) 2.184(12)

∑ij sij = 1.125(10) ∑ij sij = 2.12(83)

Environment of O(2) Environment of O(3)

O(2) – K(1) 2.797(4) 0.166(2) O(3) – V(2) 1.949(3) 0.674(6)

O(2) – V(1) 1.609(4) 1.690(2) O(3) – V(2) 2.003(3) 0.582(4)

O(3) – V(1) 1.836(3) 0.914(7)

< O(2) – K> 2.203(8) < O(2) – K> 1.929(9)

∑ij sij = 1.856(4) ∑ij sij = 2.17(17)

Environment of O(4) Environment of O(5)

O(4) – V(1) 1.979(3) 0.621(5) O(5) – K(1) 2.817(3) 0.157(1)

O(4) – V(2) 1.734(3) 1.205(10) O(5) – K(1) 3.164(3) 0.061

O(4) – K(1) 3.273(3) 0.046(1) O(5) – V(2) 1.602(3) 1.721(13)



O(4) – K(1) 2.938(3) 0.113(1)

< O(4) – K> 2.481(12) < O(5) – K> 2.528(9)

∑ij sij = 1.985(17) ∑ij sij = 1.939(14)


The parameters needed to calculate bond valences from bond lengths have been determined for 750 atom pairs by using Inorganic Structure Database. The 141 most reliable values are listed in Ref 52.

The results refer to the equation ∑s(M-L)= ∑exp[(ro-r)/0.37] with ro= 1.803(3)Å, 2.132(4)Å for V5+-O, K1+-O, respectively [57]. V5+-O, K1+-O distances are from the table in reference 52. This equation is given which allows the calculation of the remainder as well as the calculation of parameters for over a thousand other bond types.

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

3.2.4 Results and Discussion

The compound having the formula KV3O8 was prepared from the reaction of KVO3: PbCl2 in boric acid as orange hexagonal plate-shaped crystals. KV3O8 adopts a layered structure with V3O8 layers consisting of VO6 octahedra and VO5 square pyramids. In figure 3.7, the unit cell of KV3O8 is viewed down the a-axis. The title compound has two unique vanadium atoms (V1, V2) with different coordination environment.

The V1 has a distorted octahedral environment and it coordinates to six O atoms at distances in the range of 1.609(4)– 2.260(4)Å. The V2 has a square pyramidal coordination environment with the apical oxygen at a distance of 1.602(3)Å and four basal oxygens at distances in the range of 1.734(3)– 2.003(3)Å. In figure 3.8, coordination environment of V1 and V2 atoms with O atoms were shown. V(1)O6 octahedra shares four corners and two edges with V(2)O5 square pyramids. In addition to this, V(2)O5 shares one corner and one edge with V(2)O5 square pyramid, and two corners, one edge with V(1)O6 octahedra. In figure 3.9, polyhedral representations of VO6 octahedras and VO5 square pyramids were shown.

Valence sum calculations indicate that all V are pentavalent, all oxygens are divalent, and K atom is monovalent. This oxidation state is consistent with the overall charge balance of the compound. The square pyramidal vanadium and octahedral vanadium have an oxidation state of +5. As shown from Table 3.6, this assignment of oxidation state is confirmed by the valence sum calculations which gave value of 5.05(29) and 4.994(72) for V(1), and V(2) atoms , respectively. .As mentioned before, the pentavalent vanadium atoms can adopt various environments such as octahedral, pyramidal, and tetrahedral. Two configurations are observed in VvO6, either 2+2+2 or 4+1+1. In the title compound, VvO6 octahedra has 4+1+1 configuration as shown in figure 3.1d, and VvO5 polyhedra has also 4+1 configuration as shown in figure 3.1b. Interlayer K atoms are sandwiched by V(1)O6 octahedral faces (Figure 3.10).



Figure 3.7 Unit cell view of KV3O8 running along a axis.


Figure 3.8 Representation of V-O bonds running along c axis in KV3O8 structure.


Figure 3.9 Polyhedral representations of VO6 octahedras and VO5 square pyramids.




Figure 3.10 Representation of KV3O8 showing K atoms



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