Structure of copper tellurite and borotellurite glasses by neutron diffraction, Raman, 11 B MAS-NMR and FTIR spectroscopy

for the structural studies of tellurite glasses and crystals. Te–O co-ordination is an important structural parameter that also influences the glass forming ability (GFA) of the tellurite materials. Copper can exist as Cu + , Cu 2+ and Cu 0 in oxide glasses. (20–22) At low concentration of CuO doping in Li 2 O–MoO 3 –B 2 O 3 glasses, it is reported that mostly Cu 2+ are present and that on increasing CuO concen - tration beyond 0·6 mol%, the Cu 2+ reduce to Cu + . (21) The increase in CuO concentration in the glass series: (35− x )Pb 3 O 4 – x CuO–65Li 2 B 4 O 7 is found to exhibit the structural transformation: BO 4 Æ BO 3 . Glasses that contain CuO have interesting electrical and mag - netic properties. (23) x CuO–(100− x )TeO 2 glasses show


Introduction
Tellurite glasses have several attractive properties such as high refractive indices, low melting points, low phonon energies, high dielectric constants and high transmittance from visible to the near-infrared range of the electromagnetic spectrum; and have applications in nonlinear optical devices for second and third harmonic generation and in optical waveguides for light communication. (1)(2)(3)(4) Tellurite glasses and glass ceramics that contain transition metal oxides such as CuO and V 2 O 5 are semiconducting and show significant electronic conduction and have potential electrochemical applications as cathode materials in secondary batteries. (5)(6)(7) TeO 4 and TeO 3 are the basic structural units of the tellurite glass network and both these structural units contain a lone pair of electrons at the equatorial sites. (8,9) The addition of transition metal oxides in tellurite glasses modifies the structure by forming new ionic bonds and creates nonbridging oxygens that affect the electrical conductivity, thermal stability and optical properties of glasses. (10)(11)(12)(13) The addition of metal ions elongates one oxygen-tellurium linkage in TeO 4 and forms TeO 3+1 and TeO 3 structural units. (14)(15)(16) It is important to determine the structural properties of tellurite glasses; in particular the Te 4+ and the modifier ion co-ordination environment, bond lengths, nearest neighbour distances and the bond angle distributions to model the mechanical, optical, thermal, electrical and magnetic properties of glasses. Several techniques such as x-ray and neutron diffraction, (9,14,17) x-ray photoelectron spectroscopy(XPS), (18) ab initio calculations, (19) and Raman spectroscopy (1,9) have been used for the structural studies of tellurite glasses and crystals. Te-O co-ordination is an important structural parameter that also influences the glass forming ability (GFA) of the tellurite materials.
Copper can exist as Cu + , Cu 2+ and Cu 0 in oxide glasses. (20)(21)(22) At low concentration of CuO doping in Li 2 O-MoO 3 -B 2 O 3 glasses, it is reported that mostly Cu 2+ are present and that on increasing CuO concentration beyond 0·6 mol%, the Cu 2+ reduce to Cu + . (21) The increase in CuO concentration in the glass series: (35−x)Pb 3 O 4 -xCuO-65Li 2 B 4 O 7 is found to exhibit the structural transformation: BO 4 AEBO 3 . Glasses that contain CuO have interesting electrical and magnetic properties. (23) xCuO-(100−x)TeO 2 glasses show antiferromagnetic and ferromagnetic interactions among the Cu 2+ ions. (24,25) Like gold nanoparticles, Cu 2+ and Cu + can be used to produce ruby-glass, Cu + in glasses can be oxidised to Cu 2+ by the photochemical reaction and this property can be used to synthesise photochromatic materials. (26) Therefore CuO doping in oxide glasses has important optical applications. The addition of Cu 2+ in lead and zinc borate glasses is reported to enhance the packing density, glass transition temperature and mechanical hardness and the elastic moduli. (27) Similar effects of doping with Cu 2+ is reported on the optical and thermal properties of (50−x/2)Na 2 O-xCuO-(50−x/2) P 2 O 5 glasses. (28) Cu 2+ ions are expected to have octahedral coordination with oxygens in crystalline compounds, complexes and glasses, however, due to Jahn-Teller (JT) distortion, the Cu-O coordination decreases from 6 to 4. (28)(29)(30) For instance, in [Cu(H 2 O) 6 ] 2+ the two Cu-O linkages elongate from a length of 2·00 to 2·45 Å and the Cu-O site transforms from the elongated octahedral to tetrahedral. It is well known that the metal ions with coordination number of three or four can act as a network formers whereas the cations with higher coordination numbers of 6 or more act as network modifiers. (31) It is therefore very interesting to study the speciation (coordination environment) of Cu 2+ in oxide glasses. Electron paramagnetic resonance (EPR) studies on sodium phosphate glasses containing Cu 2+ showed that these ions have tetrahedrally elongated octahedral sites in certain phosphate glasses. (32) Neutron diffraction is however the direct and the most suitable method to study the coordination environment of the metal ions in oxide glasses. TeO 2 is a conditional glass former that forms glassy phase only at high melt quenching rates of ~10 5 K/s, (9,33) however when it is mixed with alkali, alkaline earth, transition and heavy metal oxides it forms glasses rather easily at moderate quenching rates of ~10 2 -10 3 K/s. (3) The other good glass formers such as SiO 2 or B 2 O 3 form glasses with CuO only in the presence of a third component such as alkali and heavy metal oxides. (27,32,34) TeO 2 , on the contrary forms binary glasses in the wide composition range with a variety of metal oxides including CuO, (3,24,35) therefore in the present work, TeO 2 is used as a network former to prepare glasses from the xCuO-(100−x)TeO 2 system with the primary aim to study the changes in Cu-O and Te-O speciation with glass composition. Glasses from this system are reported to be semiconducting with an electrical conductivity that is three to four orders of magnitude higher than that of copper phosphate glasses with equal CuO concentration. (36) The incorporation of a second glass former such as B 2 O 3 into the tellurite network leads to a complex glass structure that consists of borate, tellurite and mixed structural units. B 2 O 3 is an excellent glass former which increases the GFA, thermal stability and the ultraviolet-visible transmittance of the tel-lurite glasses. (37,38) On increasing B 2 O 3 concentration in tellurite glasses, it is reported that the tetrahedral borons transform into the triangular boron units. (39) In the present study, copper tellurite glass and glass-ceramic samples were prepared and their short range and medium range order properties were studied by neutron diffraction and Raman spectroscopy. Reverse Monte-Carlo (RMC) simulations of the neutron scattering data of two copper tellurite glasses were performed and the partial pair correlation functions, Te-O and Cu-O coordinations and the bond angle distributions of the cation-oxygen linkages were determined. Further, the effects of the addition of the second network former (B 2 O 3 ) on B-O and Te-O speciation, and on the glass thermal properties were studied by 11 B magic angle spinning nuclear magnetic resonance (MAS-NMR), Fourier transform infrared (FTIR), Raman spectroscopy and differential scanning calorimetry (DSC) techniques, respectively. Finally, the short range structure of one copper tellurite glass sample and its devitrified (crystallised) sample are compared.

Glass preparation and devitrification
Copper tellurite and copper borotellurite glasses from the two systems (i) xCuO-(100−x) TeO 2 (where x=15 and 20 mol%) and (ii) 20CuO-yB 2 O 3 -(100−y) TeO 2 (y=10, 20 and 30 mol%) were prepared by melt quenching using CuO (Central Drug House, India, 98%), H 3 BO 3 (Aldrich, India, 99·9%) and TeO 2 (Aldrich, India, 99%) as starting materials. Appropriate amounts of these materials were weighed, mixed together and transferred in a platinum crucible and melted at 900°C for 30 min in a muffle furnace. Samples were prepared by splat quenching in which the melt was pressed between two steel plates and dark green colored samples were obtained.
Copper tellurite glass with 15 mol% CuO (15CuTe) was annealed at 380°C (higher than its glass transition temperature (T g )) for 4 h in air to devitrify it. The composition and density of the samples are given in Table 1.

X-ray diffraction (XRD)
XRD studies were performed on powdered glass samples on Bruker D8 Focus x-ray diffractometer with Cu K α radiation (λ=1·54056 Å) in the 2θ range of 10-65°. The x-ray tube was operated at 40 kV and 30 mA and the scattered x-ray intensity was measured with the scintillation detector.

Density measurement
The densities of glass samples was measured by Archimedes method using dibutylphatalate (DBP) as the immersion fluid and are given in Table 1. The maximum error in the densities is ±0·05 g/cm 3 .

Differential scanning calorimetry (DSC)
DSC studies were carried out using SETARAM SETSYS 16 TG-DSC system in the temperature range of 200-800°C at a heating rate of 10°C/min. Measurements were performed on the powdered samples kept in platinum pans. Samples amounts of 20-50 mg were used for DSC analysis. The maximum uncertainty in the measurement of glass transition (mid-point), crystallisation (peak point) and melting temperatures (peak point) is ±1°C ( Table 2).

Neutron diffraction
Neutron diffraction studies were carried out on two copper tellurite glass samples using monochromatic neutrons of de-Broglie wavelength, λ=0·783 Å at the Dhruva reactor of Bhabha Atomic Research Centre, Trombay, Mumbai, India. These studies were done on the pulverised samples kept in vanadium cans in the momentum transfer, Q-range: 1·0 to 14·3 Å −1 . RMC simulations were performed on the neutron scattering data using RMC ++ software, (40,41) and the partial pair correlation functions, bond lengths/nearest neighbour distances, Te-O and Cu-O coordinations and bond angle distributions were found from the RMC analysis.

RMC simulations
The neutron scattering data were corrected for background, multiple scattering and absorption and normalized with vanadium. (42)(43)(44)(45)(46) The experimental neutron structure factor S(Q) data was obtained and simulated by the RMC method using RMC ++ Version 1.5.1 software package to generate the partial pair correlation functions, coordination numbers

and the O-Te-O, O-Cu-O and O-O-O bond-angle
distributions. (40,47) The RMC technique minimises the squared difference between the experimental S(Q) and the simulated one from a three-dimensional atomic configuration and partial correlation functions g ij (r) and the neutron scattering weight factors for different correlations were calculated by the following formulas: (48)(49)(50)(51)(52)(53) ij ij , where δ ij is the Kroneker delta function; c i , c j are the molar fractions of the i-th and j-th atoms in the sample, respectively; b i , b j are the corresponding neutron coherent scattering lengths; and k is the total number of elements in the sample and calculated data is given in Table 3. The RMC simulation calculates the one-dimensional partial atomic pair correlation functions g ij (r), and these are Fourier transformed to calculate the partial structure factors, S ij (Q). The disordered atomic configuration was first built up to run the RMC program with a simulation box that contained 10000 atoms. The atomic number density values were 0·0653 and 0·0665 Å −3 and the RMC model box lengths were 26·753 Å and 26·588 Å for the two samples: 15CuTe and 20CuTe, respectively (Table 1). During the RMC simulations, the minimum interatomic distances constraints (cut-off distances) were used to fit the model with experimental S(Q). No other constraints (such as co-ordination constraints) were applied during RMC simulations. Repeated RMC runs were performed by modifying the values of cut-off distances in such a way that it produced reliable data for each pair correlation function, g ij (r) and coordination number, CN ij . The final cut-off distances used in the RMC program for Cu-Cu, Cu-Te, Cu-O, Te-Te, Te-O and O-O correlations were 2·30, 2·60, 1·60, 2·20, 1·60 and 2·35 Å, respectively, for the two copper tellurite samples and, the average coordination numbers of Te, Cu and O with O were obtained from RMC analysis. The r max and r min values of the partial pair correlation functions that were used for

Raman spectroscopy
Raman scattering studies were performed on the samples with Renishaw In-Via Reflex micro-Raman spectrometer using 514·5 nm argon ion laser (50 mW) as the excitation source, a diffraction grating having 2400 lines/mm, an edge filter for Stokes spectra and a Peltier cooled CCD detector. Measurements were carried out in an unpolarised mode at room temperature in the backscattering geometry, in the wave-number range: 30 to 1000 cm −1 at a spectral resolution of better than 1 cm −1 .

11
B MAS-NMR studies were performed on copper borotellurite glasses on Bruker AVIII HD NMR spectrometer operating at a magnetic field of 11·74 T with a 4 mm Bruker MAS probe at Larmor frequency of 160·5299 MHz for 11 B nuclei. Sample spinning rate was 14 kHz. Short RF pulses (<15°) with recycle delay of 20 s were used. Spectra were collected after 4096 scans and referenced to solid NaBH 4 at −42·16 ppm.

FTIR
The FTIR spectra of copper tellurite and borotellurite glasses were recorded on Bruker Vertex FTIR spectrophotometer using KBr disk technique in the wavenumber range: 400 to 2000 cm −1 at room temperature. The mixture of powdered glass sample and spectroscopic grade KBr (1:100 by weight) was subjected to a pressure of 10 ton/cm 2 to prepare thin pellets. The FTIR absorption spectra were measured immediately after preparing the pellets.

X-ray diffraction
The XRD pattern of the sample that contained 15 mol% CuO shows a broad hump and several sharp peaks centered at 26·4, 28·9, 30·0, 37·4, 48·7, 54·0, 55·4, 61·0, 62·4 and 63·0° which match with the tetragonal α-TeO 2 phase. (54) This sample is a mostly glassy phase with a small concentration of crystalline precipitates of tetragonal TeO 2 . After annealing this sample at 380°C in air for 4 h (Sample Code: 15CuTe-CR), crystallinity increases significantly and several new sharp peaks were detected in the XRD pattern; which match with the tetragonal TeO 2 and the monoclinic CuTe 2 O 5 phases, (55) the later phase shows peaks at 22·8, 27·5, 29·2, 36·7, 42·8, 46·9, 51·0 and 57·7° ( Figure  1). Crystalline CuTe 2 O 5 and TeO 2 phases are produced by the following crystallisation (devitrification) reaction with heat treatment: It may be noted that the devitrified sample (15CuTe-CR) is also not fully crystalline, but contains a significant amount of the remnant glassy phase as evidenced by the broad hump in its XRD pattern (Figure 1). Copper tellurite sample with 20 mol% CuO (20CuTe) and all the copper borotellurite samples show only broad humps in the 2θ range of 20° to 30° without any sharp peaks (Figure 2). The absence of sharp peaks confirms the amorphous nature of these samples. Clearly, the glass-forming ability (GFA) of the xCuO-(100−x)TeO 2 system is enhanced with an increase in CuO concentration from 15 to 20 mol%. The enhancement of GFA is also confirmed by the steady increase in thermal stability, ∆T=(T c −T g ) with increase in both CuO and B 2 O 3 mol% (Table 2), as found by DSC analysis (discussed below).

Thermal properties
The DSC thermograms of copper tellurite and copper borotellurite glasses are shown in Figure 3. The glass transition temperature, T g of copper tellurite glasses decreases from 306 to 301°C on increasing CuO concentration from 15 to 20 mol% due to lower bond enthalpy of Cu-O bonds (343 kJ/mol) as compared to that of Te-O bonds (391 kJ/mol). (59) The DSC scan of copper tellurite glass (20CuTe) shows single T g and two crystallisation temperature peaks at 380°C (T c1 ) and 457°C (T c2 ) which can produce precipitates of CuTe 2 O 5 and TeO 2 phases by the following thermal decomposition reaction: The incorporation of B 2 O 3 (10, 20 and 30 mol%) into copper tellurite glass (20CuTe) enhances the value of T g from 325 to 366°C due to the formation of stronger B-O-Te linkages (B-O bond dissociation energy =807 kJ/mol) in place of the weaker Te-O-Te linkages (Te-O bond energy=391 kJ/mol). (59) The addition of B 2 O 3 into 20CuTe glass sample suppresses the crystallisation tendency and the exothermic peaks at 464, 503 and 520°C are observed in glass samples: 20Cu10BTe, 20Cu20BTe and 20Cu30BTe, respectively. The thermal stability (resistance to crystallisation) of the copper borotellurite glasses enhances with increase in B 2 O 3 concentration from 0 to 30 mol% (Table 2).
T g shows a linear correlation with the average single bond energy (E B ) of glasses and the latter increases from a value of 381 to 506 kJ/mol ( Figure 4 and Table  2) with increase in B 2 O 3 from 0 to 30 mol%, E B was calculated by the following formula: where  (59) The important parameter that determines the value of glass transition temperature, is the average single bond dissociation energy, E B , of the glass network. (60,61) DSC scans of copper tellurite and borotellurite glasses show a linear correlation between E B and T g as also found in other tellurite glass systems. (15,62,63) The

Short range structure by neutron diffraction and RMC simulations
The (S(Q)−1) functions of the two copper tellurite glass samples calculated from RMC simulations matched well with the experimental ones and are shown in Figure 5. Although the copper tellurite sample with 15 mol% CuO has some crystalline precipitates of the tetragonal α-TeO 2 phase, it is predominantly amorphous and therefore the neutron diffraction studies provide reliable information about the structural transformations that occur with increase in CuO concentration from 15 to 20 mol% in the xCuO-(100−x)TeO 2 glass series. The partial atomic pair correlation distributions g Cu-O (r), g Te-O (r) and g O-O (r) were obtained from the RMC modelling and these are displayed in Figure  6. The first peak is at 1·98±0·02 Å in both g Cu-O (r) and g Te-O (r) and the oxygen-oxygen pair correlation distribution, g O-O (r) shows maxima at 2·71±0·02 Å. It is known from neutron diffraction and molecular dynamics studies on amorphous TeO 2 that there are two types of Te-O linkages, the shorter ones at 1·880 Å are due to the Te-O linkages in the equatorial plane, while the longer ones at 2·121 Å are due to Te-O axial linkages and the average bond length is 2·000 Å. (9,14) Neutron diffraction studies on the alkali and heavy metal oxide tellurite glasses found that the Te-O bond length distributions have a peak value at 1·95 Å, (43,64,65) which is in good agreement with the value of 1·96-1·98±0·02 Å for Te-O bond lengths found in the two copper tellurite glass samples. The average coordination number of Te with oxygen (N Te-O ) decreases from a value of 3·51±0·03 to 3·27±0·04 on increasing CuO concentration from 15 to 20 mol%. The Cu-O bond lengths are in the range: 1·96-1·98±0·02 Å ( Table  5) and are in good agreement with the previous findings, (66) however, Cu 2+ in tellurite glasses have tetra- Bhogi et al (20) and Rao et al (21) reported that Cu 2+ ions in glasses can be in tetrahedral, octahedral and square planar coordination and that at high concentration of CuO (>0·6 mol%) mostly Cu + ions exist. Duran et al reported that Cu 2+ ions in oxide glasses produce green coloured samples. (67) In the present tellurite glass series, which contain a higher concentration of CuO (15 and 20 mol%) dark green coloured samples were formed, which indicates that Cu 2+ , and not Cu + exist in the glass network, although the exact determination of the oxidation state of copper will require an experimental measurement by XPS or EPR techniques.
The RMC studies on neutron data show that the Cu ions are mostly tetrahedrally coordinated with oxygen (N Cu-O =3·94±0·03 in 15CuTe and 3·77±0·02 in 20CuTe). The Jahn-Teller distortion, therefore, modifies CuO 6 units into CuO 4 units, as found earlier by the EPR studies on phosphate glasses containing CuO. (32) The bond angle distributions, θ O-Cu-O , θ O-Te-O and θ O-O-O have maxima at 87·2±0·5°, 87·7±0·2° and 58·0±1·2°, respectively, and these curves are shown in Figure 8. It is known from the molecular dynamics simulations on the glassy-TeO 2 , that the O-Te-O bond angle distributions have two peaks; the first weaker peak is in the range: 150-170° and is due to O ax -Te-O ax linkages while the second stronger peak in the range of 70-105° is due to O eq -Te-O eq and O eq -Te-O ax bonds. (16) The maxima at 87·7±0·2° in θ O-Te-O distribution show that the majority of the linkages are O eq -Te-O eq and O eq -Te-O ax in TeO 3 units. (

8). Similar bond lengths, coordination numbers, and bond angle distributions of Cu-O and Te-O linkages
reveal that the two metal cations (Te 4+ and Cu 2+ ) exist in the structural units of very similar geometry and size. The short range structural properties data of the tellurite samples are summarised in Table 5.

Te-O speciation by Raman spectroscopy
The Raman spectra of copper tellurite and copper borotellurite glasses are shown in Figure 9. These spectra show one strong peak at low phonon wave-

Figure 8. Bond angles distributions for (a) O-Cu-O, (b) O-Te-O and (c) O-O-O linkages in copper tellurite glasses. Plots for the sample 20CuTe are shifted by 0·2 units for clarity [Colour available online]
calculated by the following formula: (15,72) 4 Te-O(Raman) 3 4 3 where I 4 is the sum of intensities of the peaks due to TeO 4 units (616 and 659 cm −1 ), and I 3 is the sum of intensities of peaks due to TeO 3 and TeO 3+1 units (711 and 765 cm −1 ). In case of xCuO-(100−x)TeO 2 samples, N Te-O(Raman) decreases from a value of 3·46±0·02 to 3·41±0·02 on increasing CuO concentration from 15 to 20 mol%. This result is consistent with RMC simulations of the neutron diffraction data, which showed that the Te-O coordination decreases from a value of 3·51±0·03 to 3·27±0·04.
In the case of copper borotellurite glasses, N Te-O(Raman) decreases from 3·39±0·01 to 3·31±0·01 on increasing the B 2 O 3 concentration from 10 to 30 mol% (Table 6). This property of decrease in N Te-O with an increase in metal oxide concentration is a well known effect in tellurite glasses and it occurs with the addition of alkali, alkaline earth, rare earth and heavy metal oxides. (72)(73)(74) The glass sample that was devitrified at 380 o C for 4 h (15CuTe-CR), shows sharp Raman peaks at 53, 116 144, 172, 423, 649, 680, 817 cm −1 and a shoulder at 615 cm −1 . The peaks at 53, 116 144 and 172 cm −1 are due to the longitudinal optical vibrations of bridging oxygens of TeO 2 (62,75) and peak at 423 cm −1 is due to the bending vibrations of Te-O-Te linkages. The shoulder at ~615 cm −1 shows the single degenerate symmetric vibrations of O-Cu-O linkages along with vibrations in TeO 4 units. (73,76) After the annealing treatment, the sharp peak at 683 cm −1 which corresponds to TeO 4 units becomes very prominent while the intensity of the band: 710-770 cm −1 (due to TeO 3 and TeO 3+1 units) decreases significantly compared to its intensity in the parent glass sample (20CuTe). Therefore the devitrified sample (that contains a mixture of tetragonal TeO 2 , monoclinic Cu 2 TeO 5 and glassy phases) has a lower concentration of TeO 3 units and has higher average Te-O coordination compared to that in the parent glass sample. Figure 11 shows the 11 B MAS-NMR spectra of the three borotellurite glasses in the full chemical shift range of 150 to −150 ppm. The spectra show two central resonant peaks; the first peak at ~9 ppm and the second at ~0·5 ppm due to BO 3 and BO 4 structural units, respectively. (72,77,78) The two NMR peaks are broad and could not be resolved even at high magnetic field of 11·7 T due to the magnetic interactions of Cu 2+ in the glasses. (21,22) These spectra also contain the two spinning side bands centred at approximately 95 ppm and −88 ppm. With the addition of B 2 O 3 in copper tellurite glasses, the central resonance peak at 9 ppm becomes more intense, compared to the peak at ~0·5 ppm. This confirms the formation of more BO 3

B-O speciation by FTIR
The FTIR absorption spectra of copper tellurite and copper borotellurite glasses are shown in Figure 13. Copper tellurite glasses show only one characteristic broad band from 520 to 900 cm −1 while the copper borotellurite glasses show two additional broad bands in the range: 1130-1550 cm −1 and 820-1130 cm −1 along with the band from 440-820 cm -1 . The broad band from 440-900 cm −1 is due to the stretching network were also determined from FTIR studies, for this purpose the spectra in the wavenumber range: 820 to 1500 cm −1 was normalised, baseline corrected and fitted with Gaussian peaks centered at 875, 911, 961, 1226, 1337 and 1431 cm −1 (Figure 14). The areas under these peaks were used for the quantitative estimate of the fraction of tetrahedral borons, N 4 by the following formula: (39) where, A 4 and A 3 are the areas under the Gaussian peaks in the wavenumber range: 820 to 1130 cm −1 and from 1130 to 1550 cm −1 , respectively. The value of N 4 decreases from 0·31 to 0·14 on increasing B 2 O 3 concentration from 10 to 30 mol%. This is due to the structural transformation: BO 4 AEBO 3 in xCuO-yB 2 O 3 -(100−x−y)TeO 2 glasses. The variation of N B-O with B 2 O 3 mol% in copper borotellurite glasses is shown in Figure 15.

Conclusions
The short range structure of copper tellurite and borotellurite glasses was studied by multiple techniques. Raman spectroscopy shows the structural transformation: TeO 4 AETeO 3 on increasing CuO and