Comparison of the Vibrational Spectra of Copper Polysilicate, CuSiO₃, with Those of the Prototypic Copper Polygermanate, CuGeO₃

Orthorhombic copper polysilicate, CuSiO ₃, is isotypic to the spin-Peierls compound CuGeO ₃ and represents a further example of a quasi-one-dimensional spin = 1/2 antiferromagnetic Heisenberg chain system. This is a representation of the first Raman and IR/FIR spectra for CuSiO ₃, measured at room temperature on polycrystalline samples. A comparison of the optical phonons, predicted by a factor group analysis, with those observed for the CuGeO ₃ prototype, is presented. A mode assignment for the silicate is given. Surface effects due to a very small crystallite size may cause additional broad bands observed in the Raman spectrum of CuSiO ₃. From the analysis of the Davydov doublet an intralayer-to-interlayer bond strength of about 40 is derived for the silicate, which is about 20% lower than for the isotypic germanate, allowing for different magnetic responses at low temperature.

• IR/FIR Spectra
• Raman Spectra
• Factor Group Analysis
• Davydov Analysis
• Copper Polysilicate
• CuSiO₃
• Copper Polygermanate
• CuGeO₃
• Low-Dimensional Compounds
• Spin-Peierls Transition

1. Introduction

Low-dimensional quantum spin systems are of considerable theoretical and experimental interest. Some years ago a spin dimerization transition (spin-Peierls transition), which is one of the quantum phenomena in a S = 1/2 antiferromagnetic Heisenberg linear chain, was observed in inorganic compounds and extensively studied in quasi-one-dimensional (1D) copper polygermanate, CuGeO₃ [1] [2] respectively α’ − NaV₂O₅ [3] .

The crystal structure of copper polygermanate consists of two types of chains which running down the shortest translation period: einer single chains of GeO₄ tetrahedra are connected by chains of edge-sharing CuO₄₊₂ “octahedra” [4] [5] , which are in reality due to the Jahn-Teller effect strongly elongated and distorted tetragonal dipyramids. Because of this the magnetic chains actually are formed by ladders of CuO₂ squares.

Until now CuGeO₃ was the prototypic compound showing such tetrahedral single chains with a repeat sequence of only one GeO₄ unit. The substitution of Si⁴⁺ for Ge⁴⁺ in copper polygermanate according to CuGe_1−xSi_xO₃ was only possible in the limited concentration range x ≤ 0.5 for polycrystals by hydrothermal synthesis [6] , and the existence of pure CuSiO₃ was considered non-existent [7] [8] . However, we succeeded in the preparation of polycrystalline CuSiO₃, isotypic to CuGeO₃, by thermal decomposition of the mineral dioptase, Cu₆Si₆O₁₈・6H₂O, and investigated its crystal and magnetic structure [9] [10] [11] [12] . Strong bonds in the ac-plane form zigzag layers in the crystal structure (Figure 1), but the layer-like character is less pronounced for the silicate than for the germanate, because its interlayer distance is reduced about 3.6% and the intralayer chain distance enlarged about 3.5% in comparison to the germanate. Lattice parameters and CuO₂ chain separations of both compounds are compared in Table 1 and individual bond lengths summarized in Table 2 to complete the data used for the further analysis [10] [13] .

Here we report on a first investigation of the room temperature vibrational spectra of a polycrystalline CuSiO₃ sample and compare these measurements with that for CuGeO₃ using previously reported data as well as our own. While the synthesis of CuGeO₃ can easily be performed as a polycrystalline powder by sintering of equimolar amounts of CuO and GeO₂ at 800˚C [11] , or as single crystals by the floating zone method [14] , the synthesis of pure CuSiO₃ remains a worthwhile task. This paper is an extended (and corrected) version of a contribution that has been pre-published some years ago [15] .

Table 1

Lattice parameters, unit-cell volume and separations of CuO 2 chains for isotypic CuSiO 3 [ 10 ] [ 11 ] [ 12 ] and CuGeO 3 [ 13 ] , respectively. Space group Pmma (No. 51, standard setting)

Compound	a (Å)	b (Å)	c (Å)	V (Å3)
CuSiO3	8.7735 (11)	2.8334 (4)	4.6357 (6)	115.24 (5)
CuGeO3	8.4749 (3)	2.9431 (1)	4.8023 (2)	119.78 (2)
CuO2 chain separations in the unit cell
	a/2 (Å) intralayer		c (Å) interlayer
CuSiO3	4.3868 (5)		4.6357 (6)
CuGeO3	4.2375 (2)		4.8023 (2)

Table 2

Bond lengths for CuSiO 3 and CuGeO 3

Compound	Atoms	Bond length (Å)	Reference
CuSiO3	Cu − O (1)	2.926 (7) x 2	[10]
	Cu − O (2)	1.941 (4) x 4
	Si − O (1)	1.640 (7) x 2
	Si − O (2)	1.582 (7) x 2
CuGeO3	Cu − O (1)	2.7549 (8) x 2	[13]
	Cu − O (2)	1.9326 (7) x 4
	Ge − O (1)	1.7730 (8) x 2
	Ge − O (2)	1.7322 (10) x 2

2. Experimental

The Raman and IR spectra of CuSiO₃ and CuGeO₃ were measured on polycrystalline samples. The copper polysilicate specimen has been obtained by thermal decomposition of the mineral dioptase, Cu₆Si₆O₁₈・6H₂O, as a multi-phase educt containing about 76 wt-% CuSiO₃, 14 wt-% CuO (tenorite) and 10 wt-% amorphous SiO₂ [10] . Single-phase powder of CuGeO₃ has been synthesized by sintering of pellets of a thoroughly homogenized CuO and GeO₂ mixture at 1200 K and a repeated grinding and sintering procedure.

The Raman spectrum of CuSiO₃ was recorded with a DILOR LabRAM spectrometer, using a Nd-YAG-laser with frequency doubler (l = 532 nm) as an exciting source, and the spectrum of CuGeO₃ with a Raman FRA 106 modul attached to a BRUKER IFS 66v interferometer, using once again a Nd-YAG-laser, but with l = 1064 nm. Infrared spectra of both compounds were obtained with the above mentioned interferometer using CsI pellets of the powdered samples.

The IR and Raman spectra of the minor phases in the phase mixture (CuO as tenorite and amorphous SiO₂) were also recorded with the same experimental equipment in order to estimate the possible distortion of the CuSiO₃ spectra and are given by Meibohm (1999) [11] . Only few strong bands were observed (IR: 470 and 1108 cm⁻¹ for amorphous SiO₂ and 530 cm⁻¹ for CuO; Raman: 450 cm⁻¹ for amorphous SiO₂ and 300 cm⁻¹ for CuO) that would be in a position to introduce a marginal distortion of the main phase spectra. Only the background of the Raman spectrum near 300 cm⁻¹ as well as the left (high frequency) shoulder of the IR band near 500 cm⁻¹ may be influenced by the presence of the minor phases.

3. Crystallographic Data

Relevant crystallographic data such as lattice parameters, bond lengths and chain separations are summarised in Table 1 and Table 2 for both CuSiO₃ and CuGeO₃. The Figures 1-3 illustrate crystal structural details in the standard setting (space group Pmma).

4. Factor Group Analysis

For the factor group analysis (f.g.a.) the unit-cell, containing two non-translationally equivalent CuSiO₃ units and thus n = 10 atoms, were adapted to the standard setting of the Pmma space group in order to apply the tables given by Rousseau [16] and Adams [17] and to compare with the data already reported for CuGeO₃ [18] - [23] . From the total number of vibrations (N_T = 3n = 30) the 3 acoustic and 2 silent modes were subtracted, leaving 25 remaining optic modes, 12 of which are Raman-active and 13 IR-active modes. The results are summarized in Figure 4 displaying the active modes in frames. The following are features of the analysis that will make the mode assignment easier: a) the copper atoms do not contribute to the Raman spectrum; b) coupling between the two non-translationally equivalent silicate chains of C_2v site symmetry in the unit-cell with D_2h symmetry leads to Davydov pairs [24] ; the chain modes split into crystal modes of type A_g − B_1u, B_2g − B_3u and B_3g − B_2u, respectively [19] . The obtained doublets involving both Raman- and IR-active modes are indicated in Figure 4 by vertical lines.

5. Results and Discussion

The room temperature Raman and infrared spectra of both CuSiO₃ and CuGeO₃ are shown in Figure 5 and Figure 6, and the extracted vibrational frequencies are summarized in Table 3 along with their intensities and the mode assignments. At a rough estimate the spectra of the silicate shift on average by a factor of F = F_m*・F_bl = 1.23 to higher frequencies in comparison to that of the germanate. In this way the mode assignment for the germanate, confirmed by a normal coordinate analysis [13] , can be applied to the silicate spectra. In particular is

Table 3

Vibrational frequencies (cm −1 ) and mode assignment for CuSiO 3 and CuGeO 3

CuGeO3			Mode Assignment			CuSiO3
Raman		IR	Raman		IR	Raman		IR
880 w			B2g			1019 w
858 s	-	857 s	Ag	-	B1u	998 m	-	1092 m
		776 vs			B3u			959 s
712 w	-	718 vs	B3g	-	B2u	854 vw	-	859 vs
592 vs	-	619 m	Ag	-	B1u	719 m	-	737 s
						634 m ?
						590 m, b ?
		528 vs			B2u			576 vs
		478 m, sh			B1u			558 m
430 vw	-	380 s	B2g	-	B3u	480 vw	-	465 m, b
[411] vw			B3g			-
[388] vw			B1g			-
330 w	-	347 m	Ag	-	B1u	429 w	-	465 m, b
		289 s			B3u			395 w
								353 vw?
222 m	-	215 s	B2g	-	B3u	349 w	-	317 m
184 vs			Ag			300 vs
		166 m			B2u			227 m
		132 w			B1u			160 w
113 m			B2g			255 m
110 m			B3g			233 m
		[48] vw			B3u			*)

vs = very strong, s = strong, m = medium, w = weak, vw = very weak, sh = shoulder, b = broad, O_t = terminal oxygen atom, O_br = bridging oxygen atom, vw? = very weak peak on the shoulder of a medium peak, ⁻ Davydov pairs, *) outside of measurement range. Frequencies in square brackets were not observed in the powder spectra, but have been reported for single crystal spectra.

F_m* = (m*_germanate/m* _silicate)^1/2 = 1.13 , where m* is the reduced mass, and F_bl = _germanate/ _silicate = 1.08; the mean bond lengths (bl) were applied as a measure of the stiffness of the compounds with = 1.747 Å for the germanate and = 1.611 Å for the silicate ( Table 2).

The frequency quotient of silicate to germanate modes versus the germanate frequency affords some information about the source of the vibrations and is displayed in Figure 7. In this drawing three regions of different slope can be made out, beginning with a low frequency region below 250 cm⁻¹ (350 cm⁻¹ for the silicate) originating from Cu and Ge/Si atom vibrations, then a region between 250 cm⁻¹ and 550 cm⁻¹ (350 to 600 cm⁻¹) with Cu-O and Ge/Si-O vibrations, and finally the region above 550 cm⁻¹ showing modes with a dominant contribution of oxygen vibrations. For the minimum values of the curve it is allowed to neglect F_m* thus obtaining a value, which actually reflects the force constant ratio. In Figure 7 distinction is also made between Raman and IR data in order to demonstrate the difference of their low frequency modes. Because the copper atom does not contribute to the Raman spectrum (Table 3), the large Ge to Si mass ratio is decisive for the large frequency ratio, while for the IR spectrum the contribution of Cu to the reduced mass level out the frequency difference between germanate and silicate.

The number of modes found in the spectra are in agreement with the space group symmetry and f.g.a. taking into account the following points. Three very weak bands in the vibrational spectra of CuGeO₃, given in Table 3 in squares, which have been observed in polarized single crystal spectra [19] , are absent in the spectra of powder samples of both CuGeO₃ and CuSiO₃ due to very low intensity. For the silicate, the peak with the lowest Raman frequency obviously lies outside of the measurement range. A very broad band at 465 cm⁻¹ in the IR-spectrum of CuSiO₃ is interpreted as a double peak. Two very broad bands in the Raman spectrum of CuSiO₃, showing no counterpart in the CuGeO₃ spectrum, may be caused by surface effects due to the very small crystallite size, which is concluded from the broad X-ray diffraction peaks observed [10] . Finally, a very weak side band at 353 cm⁻¹ observed in the IR spectrum of CuSiO₃

may belong to interference fringes caused by the parallelism of the faces of the pressed CsI disk [22] .

Once the correct mode assignment has been found, it is sufficient to apply to the silicate the results of the normal coordinate analysis for the germanate, already given by Popović et al. [19] , in order to find out what the origin of an individual vibration is.

With regard to the crystal symmetry, the spectroscopic data as well as X-ray diffraction results are supported by a further experiment. According to NQR measurements of Cu in CuSiO₃ there exists, even down to 4.2 K, only one Cu site [25] .

Following Popović et al. [19] , we can analyse the chain mode doublets (Davydov pairs) for CuSiO₃, like that for CuGeO₃, as the result of the vibration of a pair of weakly coupled identical oscillators by the relation n_± = (n₀² ± D²)^1/2, where n₀ is the frequency of the isolated oscillators and D² is proportional to the coupling force constant. Then (n₀/D)² gives a measure of the ratio of intralayer-to-interlayer bond strength. Table 4 summarizes the phonon doublet analysis (n₊ = n_Raman, n₋ = n_IR), which yields the mean value of (n₀/D)² for the silicate about 20 % lower than for the germanate [18] , in accordance with the changed chain separations that have been derived from the crystal structure analysis (Table 1).

The more 3D-like character of the silicate may be responsible for the different spin-phonon coupling in comparison to the germanate leading at low temperature to a long-range antiferromagnetic Néel state order instead of a spin-Peierls order.

6. Conclusions

The vibrational spectra of orthorhombic CuSiO₃ from powder samples are presented. The observed Raman respectively IR modes are assigned based on single crystal results for CuGeO₃ previously given. A chain mode analysis of Davydov

Table 4

Analysis of Raman and infrared active phonon doublets for CuSiO 3 . Frequencies ν and Δ are given in cm −1

νRaman	νIR	ν0	Δ	(ν0/Δ)2
Ag	B1u
429	465*)	447	128	12
719	737	728	115	40
998	1092	1045	313	11
B2g	B3u
349	317	333	105	10
480	465*)	473	85	31
B3g	B2u
853	859	856	72	143
<(ν0/Δ)2> = 41

*) This broad band is interpreted as a double peak.

pairs suggested that the intralayer-to-interlayer bond strengths for the silicate are about 20% lower than for the germanate, reflecting the different magnetic behaviour of the silicate and the germanate.

Single crystals of CuSiO₃ are required to improve the obtained vibration spectra and extend the measurement to low temperatures and to perform a meaningful lattice dynamical calculation. However, the generation of thin CuSiO₃ layers may be a promising task [26] .

The negligible mismatch between certain lattice parameters of tenorite (CuO) and CuSiO₃ suggests the possibility of a metastable formation of CuSiO₃ by the way of a topotactically induced reaction on the tenorite surface. It is proposed to use a tenorite substrate or at least a tenorite buffer layer for an epitaxial growth of well crystallized copper polysilicate. Like straw crystals of TiO₂ (rutile) [27] or CuGeO₃ nanorods [28] , also CuSiO₃ may show photocatalytic activity for the degradation of wastewater or toxic NO_x exhaust gases under sun light exposure. Therefore, dd-excitations and the energy gap of CuSiO₃ were empirically estimated quite recently, due to the lack of pure crystalline material or even single crystals to experimentally measure the spectra [29] [30] .

Acknowledgements

The authors would like to thank the late Prof. Dr. W. Brockner, Institute of Inorganic and Analytical Chemistry at the TU Clausthal, for the opportunity to measure the spectra.

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