Vibrational and electronic spectra of copper(II) chromate

SpectrochimicaActa,

Pergamon 0584435~94)JDOO1-Q

Vol. SOA,No. 14, pp. 2385-2389, 1994 Copyright@ 1994 ElsevierScienceLtd Printedin Great Britain. All rightsreserved 05&t-8539/M$7.00+ 0.00

Vibrational and electronic spectra of copper(I1) chromate ENRIQUE J. BARAN Quimica Inorganica (QUINOR), Facultad de Ciencias Exactas, Universidad National de La Plata, C. Correo %2, l!?OO-LaPlata, Argentina (Received 13 September

1993)

Abstract-The infrared, Raman and electronic (reflectance) spectra of CuCrO, were recorded. The internal vibrations of the CrO:- ions were assigned with the aid of a factor group analysis. The ‘d-d’ transitions of the Cu”06 moieties and one of the 0-01 charge transfer bands could also be assigned. Some comparisons with other structurally related compounds are made.

CHROMATE, CuCrO,, is a precursor for the preparation CuO/CuCrz04, a very active and selective catalyst (often called Adkins’ Although the structural 141, magnetic ]5,6] and thermal 16-81__properties _ are well known, it is only poorly characterized from the spectroscopic Therefore, we have investigated its vibrational (infrared and Raman) (reflectance) spectra.

COPPER

of the system catalyst) [l-3]. of this material point of view. and electronic

EXPERIMENTAL at 22o”C, starting from prepared hydrothermally were Samples of CuCr04 CuC03. Cu(OH)r/Cr03 mixtures [6, 91. The reaction time was about 24 h. The resulting redbrown powder was filtered off, washed several times with water and finally dried at 100°C. It was characterized by means of its X-ray powder diagram [S] and by weight loss during heating (2CuCr04-* CuO + CuCr204 + 1SO*). The infrared spectra were recorded with a Perkin Elmer 580 B spectrophotometer, using the KBr pellet technique. Raman spectra were scanned on a Brucker FRA 106 instrument mounted on an IFS 66 Fourier transform optical bench. A Nd/YAG laser (power = 100 mW) was used for excitation. The electronic (reflectance) spectra were measured with a Shimadzu UV-308 instrument, using MgO as a standard.

RESULTS AND

DISCUSSION

Structural characteristics CuCr04 belongs to the CrVO, structural type, being orthorhombic, of space group Cmcm and with 2 = 4 [9, lo]. The structure is built up by infinite chains of truns-edge sharing Cu06 octahedra, linked by Cr04 tetrahedra. A recent structural refinement shows that the CuOGoctahedra present an important tetragonal distortion [Cu-0 (axial) = 2.400 A and Cu-O(equatoria1) = 1.965 A] [4]. Vibrational spectrum In order to analyse the vibrational spectrum of this material, and taking into account the condensed nature of its building units, we have undertaken a factor group analysis of the internal vibrations of the CrO:- anions [ll, 121, correlating the point group of the ‘free’ ion (Z’,,), its site symmetry (Cr,) and its factor group (&,). The results are shown in Table 1.

E. J. BARAN

2386

Table 1. Factor group analysis of the internal vibrations of the CrO:- ion in CuCr04 (Cmcm-D$ Z= 4/2) ‘Free ion’ Td (VIM

I

K2 v4

2

D2h

Al A,+=42

(v2W

Factor group

Site symmetry C&1

A,+B,+B2 A, + B, + B,

A,

+

B2u

A,+B,+A.+B,,

A,+ B,,+ B,+ B,,+ B,,+ B,. A, + B,, + B, + B,u + B2, + B3,

Activity: ‘Free ion’: A,, E, F2: Raman; F2: IR. Site symmetry: A,, AZ. B,, B2: Raman; A,, B,, B2: IR. Factor group: A,, B,,, B,,, B,: Raman; B,,, B,, B,,: IR; A,: ia.

The IR and Raman spectra of CuCr04 are shown in Figs 1 and 2, respectively. The proposed assignment is given in Table 2. The IR spectrum shows a very similar pattern to that measured for other structurally related chromates [13]. The Raman spectrum appears very well defined. It is the first Raman spectrum of a chromate belonging to the CrV04 structural type so far published. Only Raman data for the structurally related CdCrO, were previously reported, but they were not presented graphically and their assignments are incorrect [14]. The analysis of the spectra shows that the expectations of the factor group analysis are fulfilled to a great extent in the stretching region of the CrO$- groups. In the bending region, the assignment is not so straightforward, as will be discussed below. The great splitting in the stretching region, probably related to the presence of two pairs of very different Cr-0 bond lengths (1.599 and 1.731 A [4]), repeats the behaviour previously observed in structurally related vanadates [15, 161 and phosphates [15, 171. The unusual intensity of one of the y3 components in the Raman effect, suggests that this

/

A

I; . : J 1 i

1000

660

660

Fig. 1. Infrared spectrum of CuCrO,.

Lb0

’

[cm-t]

2387

Vibrational and electronic spectra of copper(H) chromate

I

I

1000

800

I

I

I

I

600

,500

I

ZO!

[cm-l]

._

Fig.2. Ramanspectrum of CuCrO,. line (944 cm-‘) can probably be assigned to the A, species predicted by the factor group analysis. The vI(AIg) component is also seen as a very strong Raman line, whereas its IR counterpart is missing. It is surely overlapped by the broad band centred at 785 cm-‘. One of the expected v,-Raman components could also not be identified with certainty, although the very weak peak located at 928 cm-’ is tentatively assigned to this mode. The difference in frequency values between corresponding IR and Raman bands is in agreement with their different phononic origin (Table 1) and these differences clearly point to important coupling effects between the Cr04 vibrators in the unit cell [18, 191. Below 500 cm-’ assignment is more difficult because, as discussed earlier [15], coupling between the CrO, bendings and Cu06 motions are expected to occur in this region. In fact, the somewhat high frequency value of the IR band assigned to v.,, in comparison with the 375 cm-’ solution value [20], also suggests some couplings with this mode. Notwithstanding this, in the assignment proposed in Table 2 we have supposed the existence of practically ‘pure’ v4 and v2 bendings. Interestingly, this latter mode Table 2. Assignment of the vibrational spectrum of CuCrO, (band positions in cm-‘) Infrared

956 vs -805 sh 785 vs, br 475 vs 406w 370 s 320 s 290m 237 s

Raman

Assignment

966m

v3

-

v3

944 vs 928 vs

v3 v3

-

v3

806vs

VI

-

v4

412m 386 m 342w

v2

1

\

CuO, modes

254w 1

vs, very strong; s, strong; m. medium; w, weak; vw, very weak; sh, shoulder; br, broad.

E.J.BARAN

2388

I

I

I

400

I

600

I

I

800

Inml

Fig. 3. Electronic (reflectance) spectrum of CuCrO,.

shows, as expected, one IR and two Raman components. The intensity of these Raman lines is predicted to be higher than those of the vq components [21, 221, which are not observed in this spectrum. The position of all these v2 components is also appreciably higher than the solution value (347 cm-’ [20]). Finally, all the IR and Raman bands located below the v2 components can be assigned to vibrational modes of the Cu06 chains. Electronic spectrum Information on the electronic spectra of M”Cr04 compounds belonging to the CrVO, structural type is very scarce. Data for MgCrO, [23] and ZnCrO, [24] were the only so far available. The reflectance spectrum of CuCrO, is shown in Fig. 3. The higher energy band could ’ be clearly resolved into two Gaussian components by a standard deconvolution procedure. The other band is not so well structured, due to instrumental limitations. The band positions and the proposed assignments are shown in Table 3. There is general agreement that the order of the MO energy levels in CrOi- and other tetroxometallates is [25, 261: [(0)2u, < (a)2t2< (n)le] < (z)3t2< (n)ltl < (d)2e< (d)4t2. The main bonding characteristics are in parentheses and square brackets indicate orbitals for which transitions have not been identified with certainty [26]. In the case of the do CrO:- anion, the higher occupied level, in the ground state, is the (.n) It,, which is a MO essentially localized over the oxygen atoms. Therefore, the first transition can be assigned to the ltl--,2e transition and is found at 26.8 kK in the ‘free’ CrO:- ion [26]. The other two expected transitions, 3t2+2e (36.6 kK) and t,-4t2 (39.2 kK) [26], lie outside our measured range. Table 3. Assignment of the electronic (reflectance) spectrum of CuCrOl Transition nm

kK

448

22.3 17.9 12.6

557 795

Half-width (kK) 10.0 3.4 -4.5

Assignment 11,+2e (CrO:-) 2*+x*-y* (CUO,) (XY), (x2. yz)+xZ-Y2 (CuO,)

Vibrational and electronic spectra of copper(B) chromate

2389

According to this analysis, the first band (22.3 kK) observed in the CuCrO., spectrum can be assigned with confidence to the lt,+2e transition of the CrOi- ion. In the isostructural ZnCrO, this transition has been found at 23, 0 kK [24]. The two remaining bands are related to ‘d-d’ transitions in the CuO, octahedra. As noted above, there is a marked tetragonal Jahn-Teller distortion in these Cu06 moieties [4]. This implies that the ep level will be split in such a way that z2 lies appreciably below dx2+

Therefore, the band at lower energy is assigned to the z2+x2 -y2 transition. The other band, centred at 17.9 kK, can be assigned to the xy+x2 -y2 and (xz, yz)+x2-y2 transitions. Evidently, the tetragonal distortion is not sufficient to separate appreciably the t2gcomponents. Assuming that both transitions present nearly the same energy and using known relations for the calculation of g factors [27], we have estimated a figure of 2.18 for the isotropic g value, which is in excellent agreement with the experimentally derived values, reported to lie around 2.15 [S, 61. Acknowledgemenb-This

work was supported by CONICET (Argentina).

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