Indian Journal of Chemistry Vol. 43A, January 2004, pp. 11 - 17

Synthesis and characterization of AgBi03 with the cubic KSb03 structure

Ramesh Sharma", T K MandaI", K Rameshab & J Gopalakrishnan"*

"Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India

bMaterials Department, University of California, Santa Barbara, CA93 106-5050 USA

E-mail: gopal@sscu.iisc.ernet.in

Received 17 Novell/ber 2003

AgBi01, isotypic with cubic (111/-3 ) KBiO). has been prepared by K+/Ag+ ion-exchange starting from KBi03. Y3 H20 . Rietveld refinement of the structure from powder X-ray diffrac tion data shows that AgBi03 [a = 9.7852(2) A; Z = 12; space group 111/-3] consists of pairs of edge-shared Bi06 octahedra con nected by common corners wherein the Ag+ reside in the tunnels along <III> direction. The Bi06 octahedra are di storted and the average Bi-O bond distance (2 .117 A) is longer than the Bi- O distance (2.099 A) in the parent. In addition , the Ag(2)-O(2) bond distance (2.429 A) is considerably short, signal ing covalent interac tion between Ag+ and BiO)·framework. These structural changes are reflected in the optical properties as well ; while the brick red parent shows an absorption edge ~orresponding to - 2 eV band gap, black AgBiO j

shows free carrier absorption in the 400-800 nm range. Similar difference is also seen in the IR spectra.

Mixed valent bismuth oxides exhibit interesting structures and properties 1-4 . For example, BaBi03 is a charge-ordered (Ba2BiIIlBiv06) insulating perovskite ' that becomes superconducting on substituting Pb for Bi (BaPb,Bi l_x0 3 where x = 0.75)2 and K for Ba (Bal _x KxBi03 where x = 0.4)4. Interestingly, the corresponding Bi(V) oxide, KBi03 does not adopt a perovskite type structure ; instead it has a cubic tunnel structure5

.6 similar to KSb03. The adoption of a tunnel

structure by KSb03/KBi03 consi sting of pairs of edge-shared M06 (M = Sb v, Biv) octahedra that have common corners has been ascribed t05

.7 a large

covalency of M-O bonds that inhibit - 1800 M-O-M angle for the d 'o electronic configuration. A decrease in M-O covalency through substitution of less electropositive cations (such as Ag+, Cu+, Tn for K+ could stabilize other structures for the M03

framework. Indeed AgSb03 prepared from KSb03 by ion exchange8 is different from the parent having a deformed Sb03 framework and strong Ag-O covalent bonds. AgSb03 is also known with the pyrochlore framework9

. AgBi03 with ilmenite-type structure has been prepared by ion exchange from NaBi03.nH20

1o

recently. Also an AgBi03, apparently isostructural with the cubic KBi03, has been prepared by K+/Ag+ ion-exchange, long ago " . However, its structure, stabi lity and electronic properties have not been investigated. Here we report the results of our investigations of AgBi03 which brings out the differences in structure and properties of this solid from KBi03.

Materials and Methods Reactants used were high purity grade KOH from

Merck, bromine and AgN03, also from Merck with purity> 99% and Bi20 3 from Fluka with purity> 99%. KBi03 was prepared according to the method of Brauerl 2. KBi03.Y3H20 was synthesized by the oxidation of Bi20 3 (3.3g) in 50% aqueous KOH (30 ~I) by Br2 (4 ml) at the boiling point. The bright red precipitate of KBi03.Y3 H20 so formed was filtered and washed with 40% KOH and cold water.

K+/Ag+ exchange in KBi03.Y3 H20 was carried out in aqueous 0.25 M AgN03 at room temperature to give AgBi03. The sample was filtered, washed with distilled water and dried over anhydrous CaCI2 in a desiccator. The amount of Bi5+ was determined by iodometric titration. For this purpose, a known weight of AgBi03 was added to a solution of KI in 2 M HCI and titrated against a previously standardized sodium thiosulphate solution of strength - 0.05 M usi ng 1% starch solution as indicator. To determine the amount of Ag+, a known weight of AgBi03 was treated with I M HN03 and heated for one hour and filtered . The filtrate was titrated against previously standardized NH4SCN solution of strength - 0.05 M using ferric sulphate solution as indicator.

The product was characterized by powder X-ray diffraction (XRD) (Siemens D-5005 powder diffractometer using CuKu radiation), infrared (lR) spectroscopy in KBr pellets (Perkin-Elmer Spectrum 1000 FT-JR Spectrometer), diffuse reflectance spectra (SP 8-100 UV/vis spectrophotometer using MgO as

12 INDIAN J CHEM, SEC A, JANUARY 2004

standard), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis (JEOL-JSM-5600LY scanning electron microscope at a 20KY accelerating voltage fitted with an Oxford Instruments Be window detector). Before imaging the samples were fixed to double sticky carbon tabs and sputter coated with - 150 nm of gold. Single phase materials were identified by comparison either with standard powder XRD patterns (JCPDS Files) or with simulated patterns using the program POWDERCELL' 3 and by least squares refinement of the powder data using the PROSZKI'4 program. The intensity data for the Rietveld analysis were collected by using Ni-filtered CuKa radiation in the 28 range 5-100° with a step size of 0.02° and 7 sec per step. The Rietveld refinement was carried out using the program FULLPROF' 5. Thermal stability of AgBi03

was investigated by thermogravimetric (TG) analysis in argon at a heating rate of 2°C per minute up to 600°C in aCAHN TG-13l system.

0-

.. D. U

5

e;

N

Au A<j

AI

wJ \. ~

s 10 [rw>rgy (k.V)

Ag : Bi 1 : 1

15

Fig. 1- SEM image (top) and EDX spectrum (bottom) of AgBiOJ .

Results and Discussion AgBiOJ was synthesized as reported in the

literature", by ion exchange of KBi03: YJH20 with aqueous (- 0.25 M) AgNOJ at room temperature. Smoolh exchange occurred forming AgBi03 in 24 h. There is a distinct colour change during the ion exchange from brick red (KBi03. YJ H20) to black (AgBi03). Thermogravimetric (TG) analysis showed that AgBiOJ was anhydrous decomposing in air at -250°C (see later). lodometric titration of AgBiOJ gave 98.188% of the expected value and precipitation titration of Ag+ against NH.jSCN solution gave 98.585% of the expected value considering the stoichiometry of the compound as AgBi03.

Examination of the product by SEM and EDX (Fig. I) showed that it is single phase (with no detectable potassium) having I: I atomic ratio of Ag:Bi.

Powder XRD pattern of AgBi03 (Fig. 2a) is similar to that of KBi03, being indexable on a cubic cell with a = 9.7427(5) A. The pattern also compares very well

-::l

..!! >. ~ I/)

c ~ c

10

o o ~

20 30 40

o

'"

50

29 (degrees)

60

(e)

(b)

(a)

70 80

Fig. 2--(a) Powder XRD pattern of AgBiOJ . (b) Simulated powder pattern of AgBiOJ assuming AgSbOJ structure as mode l. In (c) powder XRD pattern of the parent KBiO,. YJ HzO is shown.

SHARMA el al.: SYNTHESIS OF AgBi03 WITH CUBIC KSbO.1 STRUCTURE 13

with the simulated pattern (Fig. 2b) obtained by assuming the structure of AgSb03

8 where Bi replaces Sb, indicating that the BiO) framework essentially remains intact during the K+/Ag+ exchange. To probe further into the details of the structure, we refined the structure of AgBi03 from powder XRD data using FULLPROF Rietveld refinement program l5

. The refinement was carried out in 1111-3 space group with the atomic positions of AgSb03 structure8 as the initial input, wherein the Ag was distributed at two different 16f sites, Bi at 12e, 0(1) at 12d and 0(2) at 24g crystallographic positions. At first, the scale factor, zero shift and the profile parameters (using the pseudo-Voigt profile function) were refined along with the cell parameter. In the second step, the heavy atom positions for Bi and Ag were refined keeping the occupancy and thermal parameters constant. Next, the

24000

21000

18000

15000

~

::l 12000

~ Q 'Vi 9000 c <)

.5 6000

3000

other positions and thermal parameters were refined . Finally the occupancy of Ag was refined. The final refinement converged to reasonable R-factors. The profile fit is shown in Fig. 3. The position and thermal parameters are listed in Table 1 and selected bond distances and angles are given in Table 2. The structure is drawn in Fig. 4. For comparison, the structure of KBi03 is also shown in the same figure.

The refinement shows that although the structure of AgBiOJ (Fig. 4a) is isotypic with KBi03 (Fig. 4b), consisting of edge-shared Bi20 6 dimers, which are in turn linked through corners to form the characteri stic cubic framework wherein the K+/Ag+ cations reside in the tunnels along the < III > directions , there are di stinct differences. The Bi06 octahedra are di storted in AgBi03 as compared to KBi03 (Fig. 4c and 4d; Table 2). Also the average Bi-O bond distance is

o I I I I II 11 111 11 / 11111111 1111 11 1111 1111 111 11 I

·3000

-6000

13 23 33 43 53 63 73 83 93 103

Fig. 3--Observed (+), calculated (-) and difference (bottom) Rietveld refined powder XRD profiles for AgBi03.

The vertical lines mark the Bragg reflections.

Table I-Atomic positions, site occupancies and isotropic temperature factors for AgBiOJu

Atom Position x y z B (A2) Site occupancy

Agl 16/ 0.1347(2) 0.1347(2) 0.1347(2) 1.12(4) 0.31(1) Ag2 16/ 0.2232(3) 0.2232(3) 0.2232(3) 1.22(3) 0.48(2) Bil 12e 0.8341(2) 0 0.5 0.68(3) 1.0 01 12d 0.3681 (2) 0 0 0.87(2) 1.0 02 24g 0 0.3196(3) 0.2732(2) 1.11(4) 1.0

uS .. pace group flll-3, a = 9.7852(2) A, Rp = 6.2%, RII'I' = 8. 1, RlJmgg = 7.0%, Rf = 7.6%.

14 INDIAN J CHEM, SEC A, JANUARY 2004

Table 2- Selected bond di stances (A) and bond angles (0) in AgBiOJ , KBiOJ and AgSbOJ

KBiO/'

BOlld diSTances Bi-O( I) = 2.074(2) (x2)

Bi-O(2) = 2.054(3) (x2)

Bi-O(2) = 2.224(2) (x2)

<Bi-O> = 2.117(2)

Bi-O(l ) = 2.108(7) (x2) Sb-O(l) = 1.956 (x2)

Sb-0(2) = 2.006 (x2)

Sb-0(2) = 2.279 (x2)

<Sb-O> = 2.0736

Bi-O(2) = 2.053(9) (x2)

Bi-O(2) = 2. 136(9) (x2)

<Bi-O> = 2.099(8)

Ag( 1)-0(1) = 2.948(2) (x3)

Ag(l)-O(2) = 2.617(3) (x3)

<Ag( 1 )-0> = 2.782(3)

K(I )-O( I) = 3.039(8) (x3)

K(I)-0 (2) = 2.735(7) (x3)

<K( I)-O> = 2.887(8)

Ag( 1 )-O( I) = 2.858 (x3)

Ag( I )-0(1) = 2.644 (x3)

<Ag(J)-O> = 2.751

Ag(2)-O(2) = 2.429(3) (x3)

Ag(2)-O(2) = 2.741 (3) (x3)

<Ag(2)-0> = 2.585(3)

K(2)-0 (2) = 2.685(3) (x6) Ag(2)-0(1) = 3:014 (x3)

Ag(2)-0(2) = 2.260 (x3)

<Ag(2 )-0> = 2.637 <K(2)-0> = 2.685(3)

K(3)-O( I) = 3.637( 10) (x6)

<K(3)-0> = 3.637(10)

<Ag-O> = 2.684(3) <K-O> = 3.069(8) <Ag-O> = 2.694

Bond al/gles O(2)-Bi-O(2) 172.68(7) 0 (2)- Bi ·0(2) 179.32 0 (2)-Sb-0(2) 156.59

0(1 )-Bi-0(2) 159.23(7) O( I )-Bi-0(2)

0(2)-B i -0(2) 88.13( I) 0(2)-Bi·0(2)

O( 1 )-B i-O(2) 82.26(6) O( I )-Bi-O(2)

O( I )-B i-O(2) 92.86( I) O( I )-Bi-O(2)

O( 1 )-Bi-O( I) 76.97(1) O( I )-B i-O( I)

Bi-O(I)-Bi 103.03( 1) Bi-O( I)·Bi

"Data from reference 6. h Data from reference 8.

larger (2.11 7 A) in AgBiO.1 than the correspond ing distance (2.099 A) in KBi03, c learly indicating that the Bi03 framework is more ionic in AgBi03 than in KBi03. A hi gher ionicity/larger Bi-O bond length for the BiO) framework in AgBiOJ is consistent with the replacement of basic K+ by ac idic Ag+. A similar increase in bond distances occurs in AgSbO, as

8 compared to KSbO,/NaSbO, . The K+IAg+ replacement results in an increased

covalent binding of Ag+ to the BiO) framework. This increased covalent binding is reflected in the Ag-O bond distances. There are two types of silver, Ag(l) and Ag(2) in the structure, giving average bond distances of 2.782 A for Ag(l )-0 and 2 .585 A for Ar.(2)-O. These bond dist:--<lCes are considerab ly shorter than the K- O distances in KBi03 (Table 2). Moreover, the Ag(2)-O(2) distance is unu <; uall y short (2.429 A) reflecting a strong covalent binding of Ag\2) to the Bi03-framework. Similar short Ag- O bonds also occur in the AgSbOJ structures.

168.86 O( I )-Sb-0(2) 161.32

89.79 0 (2)-Sb-0(2) 83.73

87.73 O( I )-Sb-0(2) 83.8 1

90.26 O( I )-Sb-O(2) 99.10

8 1. 13 0(1 )-Sb-O(l) 77.51

98.87 Sb-O( I )-Sb 102.49

The changes in the structu re and bonding of AgBiO, as compared to KBi03 are also reflected in the properties. The distinct colour change from brick red (KBi03. Y3 H20) to black (AgBi03) is a clear indication of a decrease in the band gap. The optical absorption spectrum of KBi03 . Y3H ~O [Fig. 5A(a)] shows a distinct absorption edge corresponding to a band gap of - 2 eV in KBi03.\-,m2o. AgBiO, does not show an absorption edge [Fig. 5A(b)] ; instead, a continuous absorption throughout the range (400-800 nm) signals the presence of free carriers arising from a small band gap. We cou ld not obtain sintered pellets of AgBiO) suitable for electrical res istIVIty measurement because of low thermal stab ility of the material. However, lR absorption spectra (Fig. 5B) corroborate optical absorption data. Whil e KBi0 3. Y3H ~O shows distinct IR absorption bands [Fig. 5B(a)] corresponding to the Bi03 framework, similar absorption bands are not seen in the IR spectru m of AgBiO, [Fig. 5B(b)]. Again the resul t suggests the

SHARMA et al.: SYNTHESIS OF AgBi03 WITH CUBIC KSb03 STRUCTURE 15

b o Ag+1 b • K+1

\-a • Bi+5 r a {} Bi+5

I $ 0 -2 • 0 -2 c c

(c) (d)

Fig. 4-Crystal structure of (a) AgBi03 and (b) KBiO.1 ' In (el and (d) , the edge-shared Bi20 s double oelahedralu nil s in bOlh lhe structures are compared.

presence of free carriers, which suppresses absorption of IR radiation due to bond vibrati ons I6

.17

•

The differences between the structu re and properties of AgB i03 and KBi03. YJH 20 find a natural explanation in te rms of the K+/Ag+ binding to the Bi03 framework. The basic K+ forms essenti ally ionic K-O bonds, rendering the Bi03 framework hi ghly covalent with short Bi v-O bonds. This results in a hi ghly stab ilized 0 :2p (HOMO) and destabili zed Bi (V):6s (LUM O) states with a definite gap (band gap) in between l8 (Fig. 5C). In contrast, the acidic Ag+ (wi th a polarisable 4io core due to 4.:-5s mixi ng) forms covalent bonds with the Bi03 framework , which in turn render the Biv - 0 bonds less covalent (larger Biv-O bond distances). Thi s has the effect of destabilizing the 0:2p and stabilizing Bi(V):6s states

in AgBi03, thus decreasing the band gap that accoun ts for the changes that we see in the structure and properties.

We in ves ti gated the thermal stabi lity of AgBi03 by thermogravimetry (Fig.6). In argon, AgBiO) loses weight in two stages corresponding to:

2S0-280°C 300-550°C

AgBiO-, ---4 AgBi02 ---4 Ag + Bi

Powder XRD pattern of the TGA product obtained at 550°C showed the presence of metallic silver and bis muth (Fig. 7a). The decomposition of AgBiO) in air follows the expected pathway yielding a mixture o f metalli c Ag and Bi 20 3 at 600°e. We separately

16 INDIAN J CHEM, SEC A, JANUARY 2004

E (eV)

1·75 2.00 2.50 3.0 3 .5

/----_._---

~ o (0 )

~-.-----'"

-- (b)

700 600 500 1.00 3500 3000 2500 2000 1500 1000 500

Wavelength (nm) Wavenumber (em")

(A) (B)

012p)

O(2p)

(0) (II )

(C)

Fig. 5--(A) Diffuse reflectance spectra of (a) KBiO) . YJHp and (b) AgBiO) . (B) IR absorption spectra of (a) KBiO). Y3 H20 and (b) AgBiO). (C) Schematic energy level diagram showing the O(2p) and Bi(6s) states in (a) KBi03 and (b) AgBiO).

c: Q)

e Q)

Cl.

:E co .0;

~

100

96

92

88

100 200 300 400 500 600

Temperature('C)

prepared the intermediate AgBi02 by heating AgBi03

in argon at 250°C for 12 h. The powder XRD pattern of the product (Fig. 7b) shows a similarity to that of delafossite (CuLa02) (JCPDS PDF File 35-1403) and is indexable on a hexagonal cell with a - 3.82 and c -17.68 A.

Conclusion

Fig. &-TG curve for the decomposition of AgBi03 in argon.

We have prepared AgBi03 from KBi03.YJ H20 by K+/Ag+ exchange in aqueous AgN0 3. While AgBiO j

retains the BiO j framework of the parent KBiO j , there are distinct changes in the structure and properties that include: distorted Bi06 octahedra with longer Bi­o bonds, short Ag-O bonds, lack of optical absorption edge and IR absorption bands in AgBiO j .

These changes find a natural explanation in terms of a

SHARMA et al .: SYNTHESIS OF AgB iOJ WITH CU BIC KSbOJ STRUCTU RE 17

(b)

(a) x

20 30

+ xX

40 50

28 ()

• - unidentified impurity

60

1- Bi +-Ag

70

Fig. 7-Powder XRD pattern of decomposi tion products of AgB iO.l : (a) Ag + Bi (b) AgBi02

covalent bonding of Ag+ to the Bi03-framework which in turn decreases the covalency of Bi-O bonds.

Acknowledgement We thank the CSIR, New Delhi and the Department

of Science and Technology, Government of India, for support of thi s work_ RS thanks the CSJR and TKM thanks the UGC, New Delhi fo r the award o f research fe llowships.

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