稀有金属(英文版) 2019,38(08),754-763

Temperature-dependent Raman scattering and photoluminescence in YBa₂Cu₃O₇ doped with SiO₂ and Zn_0.95Mn_0.05O nanoparticles:comparative study

Munirah A.Al Messiere

Department of Physics,College of Science,Imam Abdulrahman Bin Faisal University

作者简介：*Munirah A.Al Messiere e-mail: malmessiere@iau.edu.sa;

收稿日期：13 August 2017

Temperature-dependent Raman scattering and photoluminescence in YBa₂Cu₃O₇ doped with SiO₂ and Zn_0.95Mn_0.05O nanoparticles:comparative study

Munirah A.Al Messiere

Department of Physics,College of Science,Imam Abdulrahman Bin Faisal University

Abstract：

A combined study examining the temperature dependencies of Raman scattering and photoluminescence(PL)of a YBa₂ Cu₃ O₇(YBCO)matrix doped with SiO2(12 nm;0.01 wt%.,0.10 wt%)and Zn_0.95Mn_0.05O(20 nm;0.02 wt%,0.10 wt%)nanoparticles was presented.X-ray diffraction(XRD)analysis confirms that both YBCO types exhibit aperovskite structure with the orthorhombic Pmmm phase.The microstructure was examined using environmental scanning electron microscopy(ESEM).Raman scattering and photoluminescence measurements as functions of temperature were conducted in the 77-837 K range.The photoluminescence intensity is observed to decrease for the doped YBCO than for the pure YBCO,because of localized defects.The photoluminescence spectrum is primarily composed of three bands at 1.60,1.88,and 2.40 eV.A clearly pronounced correlation is observed between electronic and structural changes in the doped YBCO,which is due to the temperature,illumination,added oxygen or metal ions,and spectral parameters.The PL integrated intensity as a function of the inverse temperature was simulated using the Arrhenius model.This analysis reveals that the energy exchange between the different levels in the pure and doped YBCO was conducted via two vibration modes only,which are strongly linked to the oxygen and copper atoms in the YBCO matrix.The temperature dependencies of the modes at 340 and 500 cm^-1 exhibit softening with temperature increase,resulting from microstructure control,which may be due to small concentrations of Si,Zn,and Mn substitutions at the chain Cu(1)and plane Cu(2)sites.

Keyword：

Matrix YBa₂Cu₃O₇; Nanoparticles; Raman scattering; Raman modes; Photoluminescence spectra;

Received： 13 August 2017

1 Introduction

Photoinduced superconductivity is commonly used to investigate h i gh-temperature superconductors (HTS);however,understanding of the general behaviours of these materials remains incomplete.Therefore,in HTS materials in which metal-insulator transitions occur,photoexcitation involving different charge-carrier concentrations has attracted considerable research interest [ 1, 2] .In such investigations of superconductivity,photodoping is typically used to induce layered cuprates and to adjust both the superconducting-and normal-state properties of these materials [ 1] .Recently,interest in superconducting electronics applications has increased,leading to identification of singularity features for these materials.In order to recognize the behaviours of photoinduced effects in such materials,extensive experiments have been conducted to measure the resistivity,magnetization,mobility,and optical and structural characteristics of copper oxides,e.g.YBa₂Cu₃O_6+x (YBCO) [ 3] .In addition,many explanations of the photoconductivity mechanism have been proposed.The photoconductivity effect has been shown to affect the structural properties of YBCO by controlling the redistribution of oxygen in this material [ 4] ,and by decreasing the c-axis lattice parameter [ 5] .Consequently,the oxygen content and its distribution exert an influence on the photoinduced superconducting behaviour.

For YBCO compounds,it is well known that a threedimensional (3D) transition-metal substitution for chain Cu(l) and plane Cu(2) sites causes reduction in the superconducting transition temperature (T_c) with increasing doping concentration [ 6] .Furthermore,substitution of palentmetals on Cu-plane sites generally induces an intense reduction in T_c,whereas substitution of trivalent metals on the Cu-chain sites has a lesser effect [ 6] .Therefore,the substitution of palent metals on Cu-plane sites is mostly interesting regarding the various 3D transition-metal substitutions in the YBCO compounds,because of the resultant strong decrease in T_c without alteration of the orthorhombic symmetry of the crystal structure [ 7] .Remarkably,there is no significant qualitative difference in the suppression of T_c caused by substitution of the magnetic or nonmagnetic elements.The T_c reduction may alter the position of the superconducting gap because of the phonon energies;thus,both the frequency and linewidth can change rapidly,once the superconducting gap crosses the energy of a certain phonon.Therefore,measurement of the temperature (t) dependence of this phonon may elucidate the value and symmetry of the superconducting gap [ 7] .Recently,it was reported that the addition of a nanomaterial such as SiO₂ or Zn_0.95Mn_0.05O during the final sintering stage of YBCO superconductor preparation does not affect the crystal structure but adjusts the local microstructure by generating defects such as twin boundaries,dislocations,columnar defects,planar defects,precipitates,inhomogeneities,and poor superconducting phases [ 8, 9] .

As regards the material characterization,the T-dependent photoluminescence (PL) and Raman scattering are some of the most functional and powerful optical characterization techniques for materials.These methods have been used to study the optical characteristics of various materials,such as their band structures [ 10] ,impurity luminescence [ 11] ,and point defects [ 12, 13] .Based on these optical characterization techniques,a systematic and mature theory has been developed to determine the material defects related to nonradiative processes [ 14, 15] ,localized states [ 16] ,and carier transport dynamics.It has been found that variations in the oxygen content of YBa₂Cu₃O₇ generate significant changes in the physical properties [ 3] ,and examinations of the resonant and nonresonant excitations are the two primary methods of exploring this behaviour.With this objective and as a result of the excellent enhancement of the electric and magnetic properties of the YBCO matrix achieved in a previous work via addition of SiO₂ and Zn_0.95Mn_0.05O [ 9, 17] ,this study extends the existing work by investigating the impact of SiO₂ and Zn_0.95Mn_0.05O nanometre-sized particle addition in YBCO.This effect of this addition is determined via examination of the experimentally observed T-dependent PL and Raman scattering behaviours of these materials.

To best of our knowledge,little information is currently available on the influence of adding nanosized particles on the optical properties of YBCO compounds.In this paper,a comparative study of the effect of adding SiO₂ (12 nm) and Zn_0.95Mn_0.05O (20 nm) nanoparticles during the final sintering cycles of YBCO on the resultant luminescence was reported.The T dependencies of the Raman modes and PL for pure and foped YBCO specimens (with 0.01 wt%or0.10 wt%SiO₂ or 0.02 wt%or 0.10 wt%Zn_0.95Mn_0.050nanoparticles) were investigated to explore the effects of these additive schemes on both modes with respect to T.The T dependence of the spectra was also discussed.

2 Experimental

Pure and doped YBCO specimens were prepared using the conventional solid-state reaction method.Single-phase YBCO was fabricated by mixing high-purity Y₂O₃ (99.9%,Sigma-Aldrich),Ba₂CO₃ (99.9%,Sigma-Aldrich),and CuO (99.9%,Sigma-Aldrich) according to the chemical formula of Y:Ba:Cu=1:2:3.This powder mixture was compressed to form pellets and then calcined at 950℃for12 h in air to produce an oxide precursor (YBCO) with no carbonate residue [ 17] .

During the final processing stage,SiO₂ (99.9%,SigmaAldrich) orZno.95Mno.o5O nanoparticles with sizes of approximately 12 and 20 nm shown in Fig.1 [ 9, 17] were added to the YBCO precursor powder by mixing and grinding both powders in an agate mortar.The aadded SiO₂and Zn_0.95Mn_0.05O constituted 0.01 wt%or 0.10 wt%and0.02 wt%or 0.10 wt%of the total sample mass,respectively.The mixed powders were compressed into pellets again,in the form of circular discs with 13 mm in diameter.The pellets were sintered at 950℃for 8 h in air and then cooled to room temperature at a rate of 4℃.min^-1.

The structure of the doped YBCO was analysed at room temperature using X-ray powder diffractometer (XRD,XRD-6100,Shimadzu Corp.) with Cu Kαradiation(λ=0.1 540 nm).The diffraction data were collected over a diffraction angle range of 2θ=20°-60°via step scanning with a width of 0.02°.XRD results were refined through Rietveld refinement using the Fullprof and Match 3 software (Version 3.2.1).The morphology was characterized using Philips XL-30 FEG environmental scanning electron microscopy (ESEM).ESEM was operated at 20 kV,high vacuum mode and 10 mm working distance.ESEM backscattered and secondary electron morphological images were acquired from different parts of the samples at magnifications ranging at 5000 times to get insight into micro structural information of the samples.The Raman and PL measurements were conducted using a micro-Raman spectrometer (Horiba LabRam ARAMIS) with the473-nm excitation line of a He-Cd laser.LowT measurements were conducted in a cryogenic cell (Linkam 600) in the 77-837 K range.

Fig.1 TEM images of a SiO₂ and b Zn_0.95Mn_0.05O nanoparticles

3 Results and discussion

3.1 XRD analysis

Figures 2 and 3 show XRD patterns of the pure YBCO samples and those doped with SiO₂ (Fig.2) and Zn_0,95Mn_0.05O (Fig.3) nanoparticles,with the main indexed diffraction signals being indicated.Predominantly,single-phase materials with orthorhombic Pmmm symmetry and minor secondary phases are obtained,confirming the efficacy of the proposed synthesis method regarding acquisition of the crystalline phase.No effects on the orthorhombicity of the structure of Y-123 were detected by adding a small concentration of SiO₂ and Zn_0.95Mn_0.05O because it looks that it does not enter the YBCO structure.

The sample structural parameters were determined via Rietveld refinement using the Fullprof and Match 3 software,and the lattice parameters (the constants a,b,and c;the cell volume;and the lattice anglesα,β,andγ) for the pure YB CO (both tables) and the SiO_2-and Zn_0.95Mn_0.05O-doped YBCO are listed in Table 1,respectively.It may be noted that,for the pure and doped samples,a,b,and c are almost unchanged,to within an estimated 0.0005-nm precision.Figures 2b and 3b show the observed and fitted profiles for samples sintered with 0 wt%,0.01 wt%,and0.10 wt%SiO₂ and 0 wt%,0.02 wt%,and 0.10 wt%Zn_0.95Mn_0.05O nanoparticles,respectively.

3.2 Morphological study

Morphological analysis was carried out by cross-sectional samples mounted on ESEM sample holders using doublesided conductive tape to reduce the vibration during analyses.The obtained ESEM surface topographical data in Fig.4a-e show that the samples have particles with irregular shape and size as a result of adding SiO₂ and Zn_0.95Mn_0.05O nanoparticles in comparison with pure YBCO.Additionally,particle agglomerations are evident in the samples.Furthermore,ESEM morphological characterizations show porosity in the samples with degree varying from one sample to another.The obtained ESEM will assist in identifying the impact of adding SiO₂ and Zn_0.95Mn_0.05O nanoparticles with different concentrations on the microstructure of the samples.

Fig.2 a XRD patterns of pure YBCO and YBCO doped with 0.01 wt%and 0.10 wt%SiO₂ nanoparticles and b XRD patterns of pure YBCO and YBCO doped with SiO₂ after Rietveld (R_P being quality of refinement)

Fig.3 a XRD patterns of pure YBCO and YBCO doped with 0.02 wt%and 0.10 wt%Zn_0.95Mn_0.05O nanoparticles and b XRD patterns of pure YBCO and YBCO doped with Zn_0.95Mn_0.05O after Rietveld refinements (RP being quality of refinement)

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Table 1 Structural and statistical parameters of pure YBCO and YBCO doped with SiO₂ or Zn_0.95Mn_0.05O 0btained via Rietveld refinement

Fig.4 ESEM images of a pure YBCO,b YBCO doped with 0.01 wt%SiO₂,c YBCO doped with 0.10 wt%SiO₂,d YBCO doped with0.02 wt%Zn_0.95Mn_0.05O,and e YBCO doped with 0.10 wt%Zn_0.95Mn_0.05O

3.3 PL spectroscopy

PL spectra for the pure and doped YBCO specimens are obtained within a T range of 77-837 K in increments of20 K under 473-nm laser excitation wavelength,corresponding to an energy of 2.62 eV.For simplicity,in Fig.5,only three PL spectra are present (T=77,97,and 297 K),corresponding to the materials in superconducting,transition and normal states,respectively.

PL spectra shown in Fig.5 reveal luminescence peaks located at 1.44,1.66,1.88,2.30,and 2.40 eV,fluctuating from narrow to broad upon the addition of Zn_0.95Mn_0.05O and SiO₂ nanoparticles with different concentrations.PL intensity of the pure YBCO is approximately four times those of the Zn_0.95Mn_0.05O-and SiO₂-doped YBCO samples and increases gradually from the normal to the superconducting states.As shown in Fig.5a,PL spectrum at 77 K is composed of three bands located at 1.60,1.88,and 2.40 eV.The maximum intensity is close to the laser energy excitation and sharp peaks appear in this region,which correspond to the resonant Raman modes.For T=97 and 297 K,the band intensity decreases at 1.6 eV.The band located at 2.4 eV is strongly correlated with the oxygen stoichiometry in CuO chains and CuO₂ planes and corresponds to transitions from oxygen vacancies and photoexcitation in the CuO₂ plane [ 18, 19] .Further,the band located at 1.88 eV is influenced by the photogenerated electrons,which are transferred to the CuO chains and may become trapped at an oxygen-vacancy chain defect [ 3] .

In Fig.5b-e,PL spectra of the YBCO specimens doped with SiO₂ and Zn_0.95Mn_0.05O exhibit a decrease in intensity compared to that of pure sample with convolution among T=77,97 and 297 K bands.The YBCO doped with0.01 wt%SiO₂ (Fig.5b) exhibits almost the same behaviour as the pure specimen.However,for the sample sintered with 0.10 wt%SiO₂ (Fig.5c),PL spectrum at 77 K is composed of three bands located approximately at 1.44,1.88,and 2.30 eV.For this specimen,inverse behaviour is obtained from the normal to superconducting states compared with the pure specimen.This new effect occurs because the added nanosized SiO₂ particles effectively control the microstructure by generating finer twin boundaries and microscopic inhomogeneities rich with Si,which are uniformly incorporated within the YBCO matrix;hence,superconducting properties are produced [ 8] .Thus,microscopic inhomogeneities can generate local variation of the oxygen stoichiometry,which may induce changes in the charge carriers.The band located at 1.44 eV disappears when T increases to 97 K.

Fig.5 PL intensity as function of energy at different temperature for a pure YBCO and YBCO doped with b 0.01 wt%SiO₂,c 0.10 wt%SiO₂,d 0.02-wt%Zn_0.95Mn_0.05O,and e 0.10 wt%Zn_0.95Mn_0.05O

As for the Zn_0.95Mn_0.05O-doped specimens,PL spectrum of YBCO doped with 0.02 wt%Zn_0.95Mn_0.05O(Fig.5d) is composed of three bands located at approximately 1.80,2.30,and 2.50 eV,and the behaviour reflected the transition from the superconducting to normal states,in contrast with that of the pure YBCO case.Finally,the intensities of the bands of the YBCO doped with 0.10 wt%Zn_0.95Mn_0.05O (Fig.5e) increase gradually from the normal state to the superconducting state.This sample shows the same behaviour as the pure YBCO sample,except a new band appears at 1.7 eV in PL spectrum measured at T=97 K.This behaviour may be attributed to the effect of substituting small amounts of Zn at Cu sites in the CuO₂planes,which has been found to suppress the superconductivity in cuprate systems [ 20] .However,it is well known that Mn occupies Cu(1) chain sites with orthorhombic structure retention [ 9] .It may be noted that PL spectra of the YBCO specimens doped with Zn_0.95Mn_0.05O exhibit opposite behaviour to those of SiO₂-doped YBCO [ 21] .

The vibration density in the crystal and the carrier diffusion rates due to the defects increase with increasing T,and as a consequence,the integrated PL intensities can gradually increase with increasing T.This effect can be attributed to the exchange of photocarriers between different excited levels and the interaction of the photocarriers with the crystal vibration modes and defects [ 2] .Figure 6shows the integrated PL intensity against the inverse of temperature for YBCO samples sintered without and doped with SiO₂ and Zn_0.95Mn_0.05O.

The integrated PL intensity results were analysed in the range of 77-817 K using the Arrhenius formula [ 22, 23] for two activation energies (e₁ and e₂),in which the fundamental state (F) interacts with two excited levels (L₁ and L₂) with n₁(T) and n₂(T) state densities at T.The carrier creation at L₁ and L₂ is generated by the optical excitation pump with rate G,and the carrier exchange proceeds between these two levels via the radiative and nonradiative relaxation processes.The integrated PL intensity related to L₁ as a function of T can be expressed as:

where is the integrated PL intensity at T=0 K related to L₁; is the quotient between the number of carriers in L₁ and L₂ created by G; are the quotients between the nonradiative and radiative rates of L₁ and L₂,respectively;R₁ and R₂ are the radiative carriers of levels L₁ and L₂,and Nr₁ and Nr₂ are nonradiative carriers of levels L₁ and L₂;and k_B is the Boltzmann constant.

Table 2 present a summary of the best-fit parameters obtained from the best fit of the integrated PL intensity as a function of T for the five samples.The estimated activation energies are 40.91 and 60.75 meV for the pure YBCO;44.41 and 61.37 meV,and 42.15 and 62.42 meV for the0.01 wt%and 0.10 wt%SiO₂-doped YBCO samples,respectively;and 35.00 and 65.00 meV,and 40.00 and65.00 meV for the 0.02 wt%and 0.10 wt%Zn_0.95Mn_0.05O-doped YBCO samples,respectively.Those energies correspond practically to the mean energies of the vibration modes presented in Fig.7.

This result confirms that the energy exchange between the different levels in the pure and doped YBCO samples occurs via an increase in the defects that act as carrier traps.Through the I(0) parameter,the model confirms that the PL intensity decreases as the SiO₂ and Zn_0.95Mn_0.05O concentrations increase.In addition,the a₁ term,which corresponds to the quotient between the nonradiative and radiative relaxations,increases with the SiO₂ and Zn_0.95Mn_0.05O concentrations.This behaviour can beexplained by recognizing that the SiO₂ and Zn_0.95Mn_0.05O addition in YBCO matrix increases the density of the twin boundaries and the inhomogeneities,whereas the radiative relaxation process is reduced [ 24, 25] .

Fig.6 Inverse T dependence of integrated PL intensity of a pure YBCO and YBCO doped with 0.01 wt%and 0.10 wt%SiO₂ and b pure YBCO and YBCO doped with 0.02 wt%and 0.10 wt%Zn_0.95Mn_0.05O

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Table 2 Parameters calculated from the best fit of pure YBCO and YBCO doped with Si0₂ or Zn_0.95Mn_0.05O

Fig.7 Raman spectra at various T values of a pure YBCO,YBCO doped with b 0.01 wt%SiO₂,c 0.10 wt%SiO2,d 0.02 wt%Zn_0.95Mn_0.05O,and e 0.10 wt%Zn_0.95Mn_0.05O

3.4 Raman scattering analysis

The Raman spectrum measurements for the pure,SiO_2-and Zn_0.95Mn_0.05O-doped YBCO specimens were performed in the range of 77-837 K.For clarity,the results of measurements performed at T values of 77,97,and 297 K are presented in Fig.7 The characteristic features of the Raman spectrum are clearly dependent on the xx (or yy),xy(or yx),x'x'(or y'y'),and x'y'(or y'x') polarizations of the incident and scattered light fields,respectively,after excitations of A_g,B₁g,and A_g+B₁g symmetries [ 25] and with absenteeism of any additional line,that usually activated via oxygen disorder or impurity atoms.In superconducting YBCO,the influence of the chain oxygen on the spectra is typically considered [ 26] .It is clearly apparent from Fig.7 that the Raman spectra of the doped YBCO samples are sharper and the intensities are reduced compared to those of the pure specimen.Further,the strongest vibration bands increase gradually from the normal to superconducting state.

The Raman spectra in Fig.7 consist of several vibration bands located at 334,435,500,564,and 634 cm^-1.The mode at 564 cm^-1 exhibits softening,whereas the other bands exhibit hardening.The band at 500 cm^-1 in the Raman spectrum is due to a stretching vibration of the CuO bond along the crystal c-axis.The bands at 334 and435 cm^-1 have been detected in rare-earth superconducting materials and are associated with the bending vibrations of oxygen in the CuO₂ planes [ 27] .In particular,the334-cm^-1 mode becomes a totally symmetric A_g mode and is associated with the z-displacements of the O(2) and O(3)atoms,whereas the 435-and 500-cm^-1 vibrations which have orthorhombic distortion are a mix of the tetragonal B₁g (O(2)-O(3)) and A₁g (O(2)+O(3)) modes associated with the same A_g symmetry in the D_2h structure [ 28] .Moreover,the 564-cm^-1 mode is associated with the A_g symmetry of the Ba-O_1,2,3 out-of-plane motion.The634-cm^-1 mode is frequently observed in numerous types of YBCO and is attributed to a Raman-forbidden phonon [ 26] .The source of this mode is typically activation of the mode itself,resulting from a local change of the Cu(2)-O(1) in-plane stretching,which is associated with B₂g or B₃g symmetry [ 28] .It is notable that the mode at 634 cm^-1is split into two modes (603 and 634 cm^-1) in the YBCO specimen doped with 0.10 wt%Zn_0.95Mn_0.05O at T=77and 97 K.This behaviour may be associated with the substitution of Mn at the Cu(1) chain site with orthorhombic structure retention [ 9] .

The variations of the Raman mode frequencies at 340and 500 cm^-1 with T shown in Figs.8 and 9 were determined using the Lorentzian function line fit of the Balkanski model,which is expressed as

The T-dependent terms in Eq.(2) indicate the real part of the proper phonon self-energy,and the constants A₂ and A₃define the cubic and quartic contributions⊿₁ and⊿₂,respectively [ 29] ;the coefficients⊿₁(T) and⊿₂(T) are as follows:

where A₁=ω₀ is a phonon frequency at the T=0 and h is Planck's constant.The results of fitting of the experimental high-T Raman data and the anharmonic coefficients A₂ and A₃ are shown in Table 3.

The Raman modes are presented in Figs.8 and 9.It is apparent that they decrease gradually with T increasing,which is due to the anharmonic as of the oxygen atoms'potential.In the majority of studies which examine the superconductivity effects on the Raman spectrum of YBCO,the 340-and 500-cm^-1 modes are the main targets,because they are sensitive to oxygen disorder.Figure 8shows that the 340-cm^-1 phonon is slightly shifted towards higher frequencies for the pure and SiO_2-doped YBCO samples,whereas the Zn_0.95Mn_0.05O-doped samples exhibit a slight shift to lower frequencies for this phonon.However,below 200 K,the magnitude of the superconductivity-induced phonon softening and phonon stiffening of the B₁g (A_g) modes at 340 cm^-1 is suppressed significantly in the doped samples [ 29, 30] .

Fig.8 T dependence of 340 cm^-1 modes for a YBCO and YBCO doped with b 0.01 wt%SiO₂,c 0.10 wt%SiO₂,d 0.02 wt%Zn_0.95Mn_0.05O,and e 0.10 wt%Zn_0.95Mn_0.05O

Fig.9 T dependence of 500 cm^-1 modes for a YBCO and YBCO doped with b 0.01 wt%SiO₂,c 0.10 wt%SiO₂,d 0.02 wt%Zn_0.95Mn_0.05O,and e 0.10 wt%Zn_0.95Mn_0.05O

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Table 3 Fitting constants described in Eq.(1)

The phonon modes for the doped samples exhibit hardening at T values lower than 700 K,as a result of microstructure control that may be due to small concentrations of Si and Mn substitutions at the chain Cu(1) sites [ 7] .Moreover,the 500-cm^-1 mode (Fig.9) exhibits a random distribution with T increasing in the case of the pure YBCO sample;this behaviour is observed because this mode broadens and ultimately disappears from the spectrum at higher temperatures.In contrast,the doped samples exhibit softening that begins above T_c,which is related to apical oxygen displacements along the z-axis [ 9] .

4 Conclusion

In summary,the effects of adding SiO₂ and Zn_0.95Mn_0.05O nanoparticles on YBCO matrix were examined considering the temperature dependence of PL and Raman scattering in the range of 77-837 K.Hence,it is observed that the PL intensity of the YBCO is four times greater than that of the YBCO with addition.The PL spectra exhibit fluctuating from narrow to broad with the addition of nanoparticles in different concentrations and temperature.The Arrhenius model with two activation energies was used to fit the temperature-dependent integrated PL intensity for pure and doped YBCO;these energies correspond practically to the mean energies of the vibration modes at 340 and500 cm^-1.This effect is attributed to the oxygen vacanciesin the Cu-O chains,which trap at chain defect and prohibit their recombination with photogenerated holes.

Furthermore,the T dependence of the Raman frequencies at 340 and 500 cm^-1 for both the pure and doped YBCO shows that the Raman modes decrease nonlinearly with temperature increase;this behaviour is associated with the oxygen vibrations of the CuO₂ planes along the z-axis.For the 340 cm^-1 mode,a softening with additive concentration increase is observed.However,the phonon at500 cm^-1 in the pure and doped YBCO softens with temperature increase.These results are related to the adjustment of the local microstructure due to defect generation.Thus,examination of the temperature-dependent PL and Raman spectra constitutes a sensitive method of monitoring microstructural changes that may cause oxygen disorders in the Zn_0.95Mn_0.05O samples.

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