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Novel ZrO2 based ceramics stabilized by Fe2O3, SiO2 and Y2O3
Accepted Manuscript Research paper Novel ZrO2 based ceramics stabilized by Fe2O3, SiO2 and Y2O3 S. Rada, E. Culea, M. Rada PII: DOI: Reference:
S0009-2614(18)30135-0 https://doi.org/10.1016/j.cplett.2018.02.049 CPLETT 35461
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
3 November 2017 13 February 2018 15 February 2018
Please cite this article as: S. Rada, E. Culea, M. Rada, Novel ZrO2 based ceramics stabilized by Fe2O3, SiO2 and Y2O3, Chemical Physics Letters (2018), doi: https://doi.org/10.1016/j.cplett.2018.02.049
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Novel ZrO2 based ceramics stabilized by Fe2O3, SiO2 and Y2O3 S. Rada1, 2, E. Culea1, M. Rada2 1
Department of Physics & Chemistry, Technical University of Cluj-Napoca, 400020, Romania
2
Nat. Inst. For R&D of Isotopic and Molec. Technologies, Cluj-Napoca, 400293, Romania
Abstract Samples in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 were synthetized and investigated by XRD, SEM, density measurements, FTIR, UV-Vis, EPR and PL spectroscopies. X-ray diffraction patterns confirm the presence of the tetragonal and cubic ZrO2 crystalline phases in all samples. The IR data show the overlaps of absorption bands assigned to Zr-O-Zr and SiO-Si linkages in samples. UV-Vis and PL data indicate higher concentrations of intrinsic defects by doping with Y2O3 concentrations. The EPR spectra are characterized by two resonance lines situated at about g~4.3 and g~2 for lower Y 2O3 contents.
Keywords: iron-silicate-yttria-zirconia, tetragonal and cubic zirconia, XRD, FTIR, UV-Vis, PL and EPR spectroscopies.
*Corresponding Author: E-mail: [email protected], [email protected] (S. Rada) [email protected] (E. Culea)
1. Introduction The use of ceramic materials such as zirconia, ZrO 2, alumina, Al2O3 and glassbased ceramics, in dental applications is highly desirable because of their excellent properties, including improved biocompatibility, wear resistance and chemical durability in addition to aesthetics. Biocompatibility is one of the most important advantages of zirconium based ceramics. At atmospheric pressure, pure ZrO2 is recognized to adopt three different crystalline structures [1]. Pure zirconia has monoclinic structure below 16700C, between 1670 and 23000C has tetragonal structure and at high temperature (>2300 0C) has cubic structure. Most stable phase of monoclinic ZrO2 has limited practical applications due to its 3-5% volume expansion during cooling from the tetragonal phase [2]. To stabilize the cubic or tetragonal ZrO2 crystalline phases at room temperature was paid much attention due to their high performances as solar cells, sensors, dental applications including the field of biomaterials, novel catalyst, inert fuel matrix inside the core of nuclear reactors and accelerators [3-10]. In this context, trivalent and tetravalent dopants were incorporated into the host monoclinic ZrO2 crystalline phase [7-9]. Tetragonal zirconia is unstable and can be easily translated into monoclinic zirconia crystalline phase at room temperature. The addition of yttria, Y 2O3 can not only stabilize tetragonal zirconia, but also a right amount of yttrium has protective effects on cells and can improve the biocompatibility of materials. The addition of trivalent cation dopants, such as yttrium, stabilizes the cubic phase of zirconia down to room temperature and results in the presence of oxygen
vacancies. Cubic zirconium dioxide exists down to room temperatures when the concentration of yttria is higher than 10mol%. In yttria-stabilized zirconia ceramics the oxygen vacancies are generated in the material to maintain electrical neutrality, since that the tetravalent zirconium ions are replaced by trivalent yttrium ions. Thus, two Y3+ ions correspond to an anionic oxygen vacancy, which it is responsible for the oxygen ion conductivity The ceramics based on ZrO2 must stabilize by two main methods: i. with other oxides which stabilizes total or partially, the tetragonal and/or cubic phases and ii. by the reduction of the crystallite sizes [11]. This paper presents for the first time the stabilization of the high temperature zirconia phase with 5mol% Fe2O3 content in the presence of SiO2 and Y2O3 contents at room temperature after sintering. The aim of the work was to determine correlations between changes in microstructure with spectroscopic properties of the stabilized zirconia by investigations of X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy, UltraViolet-Visible (UV-Vis) spectroscopy, Electron Paramagnetic Resonance (EPR) spectroscopy, Photoluminescence (PL) spectroscopy and density measurements. Accordingly, the paper will contribute to the understanding of the associated mechanisms with the formation of tetragonal or/and cubic zirconia crystalline phase which are highly desirable in technological applications.
2. Materials and Methods Samples were synthesized through a high temperature solid state reaction process using commercial zirconium (IV) oxide, yttrium (III) oxide, silicon dioxide and iron (III)
oxide of high purity in suitable proportion. The mechanically homogenized mixtures were uniaxial compacted under pressure in the form of disks (2 mm thick, 9 mm diameter) and thus were sintered in alumina crucibles at 14000C in an electric furnace, in air atmosphere. The samples were put into the electric furnace direct at this temperature for two hours. After that, the disks material was put on a stainless-steel plate at room temperature. The samples were analyzed by means of X-ray diffraction using a XRD-6000 Shimadzu diffractometer, with a monochromator of graphite for Cu-Kα radiation (λ=1.54Å) at room temperature. The FT-IR spectra of the samples were obtained in the 350-1500cm-1 spectral range with a JASCO FTIR 6200 spectrometer using the standard KBr pellet disc technique. The spectra were carried out with a standard resolution of 2cm-1. UV-VIS absorption spectra of the prepared samples were investigated with a JASCO V-550 spectrometer, in the wavelength range of 200-2000nm having a resolution of 2nm. The fluorescence spectra were obtained using an ABLE & Jasco V 6500 spectrometer with xenon lamp of 150 W. For comparison, all emission spectra were measured at room temperature with the same instrumental parameters (λex = 220nm, band with (Ex) 3nm, band with (Em) 5nm, response 2s, data pitch 2nm, scanning, speed 200 nm/min). EPR measurements were performed at room temperature using an ADANI portable EPR PS 8400-type spectrometer, in the X frequency band and a field modulation of 100kHz. The microwave power was 5mW.
The samples density has been measured by immersion in water at room temperature using Archimedes method.
3. Results and discussion 3.1. XRD analysis Figure
1
shows
the
XRD
patterns
of
three
samples
in
the
5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3. In all samples the formation of the cubic and tetragonal zirconia crystalline phases were detected. The shift of XRD peaks towards higher angle side also authenticates well incorporation of iron, silicon and yttrium ions into ZrO2 matrix. The small quantities of monoclinic zirconia crystalline phase with an orientation (-111) at 28.240 and (111) at 31.50 were detected in prepared samples. A smaller diffraction peaks situated at about 20.01, 26.98, 35.63 and 53.480 attributed to the ZrSiO4 crystalline phase with tetragonal structure (PDF no. 00-006-0266) can be observed in sample with x=5mol% Y2O3. The ZrSiO4 structure consists of chains of edge-sharing, alternating [SiO4] tetrahedral units and [ZrO8] triangular dodecahedral units extending parallel to the crystallographic c axis [12]. A rigorous analysis of the tetragonal and cubic ZrO2 phases shows that satellite peaks belong to the tetragonal phase should appear before and after the 200 (50.570), 004 (73.070) and 311 (87.510) reflections in the XRD pattern [13]. In our samples, XRD diffractograms show peaks at 50.03, 73.75 and 84.040 corresponding to the cubic ZrO2 crystalline phase. Accordingly, the stabilization of cubic ZrO2 crystalline phase may be due to the presence of excess oxygen vacancies [14].
Cubic zirconia has the ideal calcium fluorite (CaF2) structure. The zirconium atoms in a cubic structure are coordinated with eight oxygen atoms while in a monoclinic structure with Zr cations 7-fold coordinated by oxygen anions. Table 1 shows the lattice parameters of the cubic, tetragonal and monoclinic zirconia crystalline phases. The lattice constant of cubic and tetragonal zirconia phases increase from a=4.989Å and a=3.592Å, respectively in the samples with x=5mol% to a=5.033Å and a=3.598Å in the samples with x=15mol% Y2O3. For cubic zirconia phase the effective crystallite mean sizes, Deff decreases from 107nm to 37.2nm and the unit cell volume, V was also decreased with the amount of yttrium trioxide. The decrease of unit cell volume results in a better packing, respectively a more compact network. The contraction of the cell volume can be associated to the substitution of bigger Zr+4 ions by smaller Si+4 ions. Due to the difference between ionic radii the reaction between ZrO2 and SiO2 results in the formation of oxygen vacancies. Moreover, the incorporation of the Y+3 dopant ion with large ionic radius into zirconia lattice creates oxygen vacancies in ZrO2 crystal. The fractions of the cubic ZrO2 crystalline phase increase by the diminishing the monoclinic ZrO2 phase by addition of Fe2O3 content. A progressive stabilization effect of the tetragonal and cubic ZrO2 crystalline phases was observed with an increase in Y2O3 concentration. 3.2. SEM The scanning electron image of the samples in the 5Fe2O3·10SiO2·xY2O3·(85x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 are shown in Fig. 2. It can be seen that irregular and agglomerated shapes like morphology was formed in all samples.
Figures 2a) and 2b) are illustrations of the SEM images of the samples with x=10 and 15mol% Y2O3, respectively. These images show irregular shapes and aggregations with the presence of grains having polyhedral shapes with different sizes. Sample with x=15mol% Y2O3 reveals a slightly higher crystal density and a more homogeneous when compared with the sample with x=10mol% Y2O3. Moderate aggregation of zirconia particles seems to be present in the SEM image of the sample with x=5mol% Y2O3 indicated in Fig. 2c). The morphology of the particles was found to be less uniform and the particles are randomly distributed in the case of the sample with x=5mol% Y2O3.
3.3. FTIR spectroscopy FTIR spectra of the samples in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 are shown in Fig. 3. The covalence bond becomes stronger with the decrease of the difference of electronegativity between cation and oxygen ions. Since the values of electronegativity for Zr, Y, Si, Fe and O are 1.4, 1.2, 1.8, 1.8 and 3.5, respectively, the covalency of the SiO and Fe-O bonds are stronger than that of Zr-O and Y-O, respectively. As a result, the higher affinity of the silicon and iron ions to attract oxygens atoms results in the formation of higher coordination silicon and iron structural units. Then, the excess of non-bridging oxygen ions will coordinate with zirconium ions and after that with yttrium ions only after the silicon and iron atoms attain the maximum of their coordination number.
In the FTIR spectrum of Fe2O3, the bands situated at about 470 and 510 cm-1 are attributed to the Fe-O vibrations. IR band situated at about 580 cm-1 is associated to Fe-O vibrations from [FeO4] structural units [15]. The IR bands centered at about 465 and 627 cm-1 are due to the Zr–O stretching vibrations and indicates the formation of cubic ZrO2 crystalline phase [10]. All IR spectra exhibit bands due to various silicate structural units centered at about 430 cm-1 and in the region between 850 and 1200 cm-1. The IR band situated at about 430 cm-1 can be ascribed to asymmetric Si-O-Si bending vibrations in the [SiO4] structural units. IR bands situated between 850 and 1200 cm-1 can be attributed to asymmetric stretching vibrations of O-Si-O bonds of [SiO4] structural units. The IR absorption bands centered at 970 cm-1 is attributed to Si-O-Si stretching of bridging oxygen. By increasing of Y2O3 content up to 15 mol% in the host matrix, the intensity of these bands was gradually increased and the IR band centered at about 970 cm-1 becomes more formed and intense. By increasing of Y2O3 content in the host matrix, the IR data reveal some structural features which can be summarized as follows: i) The prominent and broader IR bands situated in region between 400 and 700cm-1 can be attributed to overlaps of contributions provided by the stretching vibrations of Zr-O bond, the stretching vibrations of Fe-O bond and the creation of Y-OY bonding (peak centered at about 407cm-1) [16]. By increasing of Y2O3 content up to 15mol%, the position of the IR peak centered at about 407 cm-1 was shifted to longer wavenumbers (417cm-1) and its intensity was increased. A new IR band appears at
580cm-1 corresponding to the Fe-O vibrations in the [FeO4] structural units. The intensity of this band increases with the Y2O3 addition in the host matrix. The intense IR band centered at about 480cm-1 is attributed to Zr-O-Zr bond, indicating the formation of Zr-O-Zr network. The intensity of this band was enhanced in the sample with x=10 and 15mol% Y2O3 suggesting that the number of cubic/tetragonal zirconia crystallite was increased, in agreement with SEM data. This is due to the structural conversion of Zr cations seven-fold coordinated by oxygen anions in monoclinic structure into Zr cations eight-fold coordinated as the content of the Y2O3 modifier oxide was increased up to 15mol%. ii) The second region of IR bands situated between 850 and 1250cm-1 is due to the overlaps of contributions of the stretching vibrations of Zr-O bonds from monoclinic ZrO2 crystalline phase and the stretching vibrations of O-Si-O and Si-O-Si bonds of [SiO4] structural units [17]. The intensity of these IR bands was enhanced by increasing the Y2O3 contents. For the sample with x=5mol% Y2O3, a new IR band appears situated at about 1020cm-1 which can be assigned to the Si-O stretching band of ZrSiO4 crystalline phase. In brief, IR data suggests that the fractions of the tetragonal and the cubic zirconia phase and of bridging oxygen ions as well as Si-O-Si and Si-O-Zr linkages were increased with increasing the Y2O3 content.
3.4. UV-Vis spectroscopy UV-Vis spectra of the samples in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 are shown in Fig. 4.
The pure ZrO2 shows the UV-Vis absorption bands at a lower wavelength of 243 nm [18]. In our case, two UV-Vis bands centered at about 220 and 262nm are observed in all the spectra. This evolution can be explained by considering that the first band can be assigned to the O2--Zr4+ charge transfer transition [19] and the former UV-Vis band situated at about 262nm is attributed to the cubic zirconia crystalline phase. The intensity of last UV-Vis band was increased by adding of higher Y2O3 content in the host matrix. The absorption UV-Vis band centered at about 330nm is due to charge transfer transition O2- 2p – 5Fe3+ 3d. In the region of UV-Vis bands situated between 270 and 440nm there exists an increasing trend in intensity and a broadening of the bands with increases gradually up to 15mol% Y2O3. UV-Vis bands becomes more formed and intense for sample with x=15mol% Y2O3. The broad bands in this range between 300 and 450nm can be assigned to inter-band transitions, Zr+4-O-2 transitions and also a number of defects due to strong interactions with Fe2O3 sites. The Fe+2 ions have two absorption bands situated in the region between 450 and 550nm (due to the Fe+2-O-2 charge transfer) and another band centered at about 1100nm. The Fe+3 ions produce an intense band in the ultraviolet region at 310-380nm range and one in the visible region at about 700nm [20]. The intensity of the band centered at about 1100nm increased abruptly by doping with x=10mol% Y2O3 and after that its intensity remains almost unchanged for higher doping levels. The absorption bands situated in region between 310 and 380nm with regard to Fe+3 ions increased distinctly by increasing of Y2O3 content up to 15mol%.
Intrinsic defects such as dangling bonds, oxygen deficiency or oxygen excess can cause optical transitions of absorption and photoluminescence. These optical active centers are due to some local atomic rearrangements that differ from a perfect matrix. Optical transitions associated with the non-bridging oxygen holes centers (NBOHC) and oxygen deficient centers (ODC) can be evidenced by the presence of the UV bands centered at about 260 and 220nm, respectively [21]. The intensity of the UV-Vis band centered at about 220nm increases gradually up to x=15 mol% Y2O3. The characteristic feature of the UV-Vis band situated at about 260nm increases abruptly in strength and intensity with adding of higher Y2O3 concentrations up to 15 mol%. By doping with higher Y2O3 contents a reaction mechanism can be represented as follows: Fe+2 + NBOHC → Fe+3 + NBO After that the content of NBO can be coordinated with Si+4 and/or Zr+4 and/or Y+4 and/or Fe+3 ions. The IR band located at about 940cm-1 is assigned to stretching vibration of Si-O with two non-bridging oxygens (Si-O-2NBO) per [SiO4] tetrahedral units. The IR band located at 1030cm-1 observed only in sample with x=5mol% Y2O3 is characteristic of asymmetric stretching vibrations of the Si-O-Si bonds with three dimensional network structure [22]. By doping the structural rearrangement can take place in the Si-O-Si environment due to creation of NBOs. As a result, the prominent role of Y+3 ions as network former in the matrix host can be illustrated in a net decrease of gap energy by doping with higher levels. These changes can be attributed to structural changes which are associated with variation of inter-tetrahedral band angles caused by Y2O3 [22].
The gradual increase of the Y2O3 content up to 15mol% in the host network leads to a decrease of the UV-Vis bands situated between 700 and 2000nm. This structural evolution indicates that the number of electronic transitions was decreased slightly in this region. 3.5. Gap energy Among the wide band gap (Eg) metal oxides, ZrO 2 is an active material and has two direct band to band transitions at 5.2 and 5.79 eV [23]. The relation between the absorption coefficient α and the incident photon energy hν can be written as αhν = A(hν-Eg)n, where the photon energy is hν, h being the Planck constant,
is the absorption coefficient in cm-1, Eg is the band gap, A is the energy
dependent constant a and n is an integer that can take different values depending on the type of electronic transition, for a permitted direct transition n = ½ and indirect transition n = 2. The optical band gap of ceramics is estimated by using Tauc’s plot. The direct and indirect band gap value is obtained by extrapolating the linear portions of the plots versus (αhν)2 and
versus (αhν) ½ to the energy axis as shown in Fig. 5.
From Fig. 5, it can be seen that the compositional evolution of gap energy for both direct and indirect transitions increases with the increasing of Y 2O3 content up to 15mol%. The values for direct gap energy are lying gradually from 2.01 to 2.07eV whereas for indirect gap the values are ranging from 2.50 to 2.66eV. The increase in the value of optical band gap energy on increasing of Y 2O3 concentration in the host matrix can be understood in terms of the structural modifications that are taking place in the host ceramics. These effects can be attributed to
lattice defects. Lattice defects such as NBOHC and ODC usually introduce extrinsic energy levels between bands and hence reduce the band gap [24]. In this manner the lattice defects added to the host matrix by the addition of Fe 2O3 content are taken up by the excess of Y2O3 content and as a result, the band gap was increased. This observation is in agreement with the one made by the authors [25] based on the study of the (ZrO2)0.9(Y2O3)0.1-x(Fe2O3)x system where x≤ 2mol% Fe2O3 and the authors [26] based on the study of Y2O3- ZrO2 and Fe2O3- ZrO2 systems where Y2O3 and Fe2O3 increase up to 30 mol%. Thus, concerning the two additives used in case (Y2O3 and Fe2O3), the authors [25] concluded that only Y2O3 is a stabilizer of ZrO2, while Fe2O3 may only promote, under certain conditions, the stabilizing efficiency of Y2O3. The authors [26] demonstrated that Y2O3 play the stabilizer role even for low contents (1-2 mol% Y2O3) while Fe2O3 may reach this role for higher contents (over 15 mol%). Finally, the increase of Y2O3 content of the studied samples has caused the significant depolymerization of silicate network, the creation of more BOs in the host matrix due to more Si-O(2NBO) units and hence results in an increase of gap energy.
3.6.
Photoluminescence (PL) Spectroscopy
The photoluminescence spectra of the samples in the 5Fe2O3·10SiO2·xY2O3·(85x)ZrO2 composition where x=5, 10 and 15mol% Y 2O3 are shown in Fig. 6. The excitation spectra of samples are formed by broader and intense bands in the range between 300 and 475nm and a moderate band centered at about 540nm. The PL band centered at about 340nm is attributed to compensating oxygen vacancies [27]. This band attains the maximum value for sample with x=5mol% Y 2O3.
This can be explained considering the presence of small quantities of impurities (such as Fe2O3 crystalline phases) in the cubic ZrO2 crystalline phase which creates oxygen deficiency in the host matrix and thus, the decrease of the number of compensating oxygen vacancies. It is well-known that the Zr+4 ions are non-luminous and the observed luminescence from the ZrO2 samples must be due to non-stoichiometry created by the oxygen deficiency in the system [23]. The Fe2O3 does not show photoluminescence (PL) due to the local forbidden d-d transition resonant energy transfer between cations and efficient lattice and magnetic relaxations. A simple inspection of photoluminescence spectra shows a notable change in the intensity of the samples suggesting that the host matrix accommodates more defects. The PL intensity was decreased abruptly in the sample with x=10mol% ZrO2. This decrease in PL intensity can be expected due to the change in the crystallite size, in agreement with SEM data or/and accumulated oxygen vacancies in the zirconia lattice. The intense PL band centered at about 390nm can be related to the ionized oxygen vacancies existing in the ZrO2 lattice. Emission occurs due to the radioactive recombination of photo generated hole with an electron occupying the oxygen vacancy [28]. The PL intensity of the sample with x=15mol% Y2O3 was found to be maximum when compared to the samples with x=5 and 10mol% Y2O3. The fact that the PL intensity of the sample with x=15 mol% Y2O3 is the highest suggests that this sample accommodates the higher amount of oxygen vacancies in comparison to the other two ones.
The intense PL band centered at about 440 nm can be due to the transitions from the surface trap states in the conduction band to lower energy levels close to the valance band of ZrO2. The PL intermediate band centered at about 540nm can be due to the involvement of mid-gap trap states, such as surface defects and oxygen vacancies. The intensities of these bands were decreased in the sample with x=10mol% Y2O3 and shows a large emission for the sample with x=15mol% Y2O3. It means that the wide band gap of ZrO2 seems to accommodate more new energy levels or electron traps, showing variety of emissions. Accordingly, large amounts of surface defects exist on the sample with x=10mol% Y2O3. The relationship between the luminescence situated at about 650nm and the absorption UV-Vis bands centered at about 260 and 620nm in vitreous SiO2 are correlated in agreement with a model for understanding the structural defects in which they are assigned to induced intrinsic defects such as non-bridging oxygen holes centers, NBOHC [28]. The NBOHC represents the oxygen dangling bond with a structure of O3Si-O· where silicon is bonded with three oxygen atoms and another O· which denotes an unpaired electron. Then, the oxygen deficient centers, ODC can be assigned to the PL band situated at about 500nm. An ODC center can be described as O3Si-SiO3. These defects can be optically active and may have sensitizing properties. All intensities of these bands were enhanced for the sample with x=15mol% Y2O3. As a result, we come to the conclusion that with the increase of intrinsic defects concentration, the luminescent peak intensity increases. In the given situation, the sample with higher Y2O3 (x=15%) concentration contains the higher concentration of intrinsic defects.
3.7. The
EPR spectroscopy features
from
the
EPR
spectra
of
the
samples
in
the
5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 presented in Fig. 7 are characterized by two resonance lines situated at about g~4.3 and g~2 [29, 30]. The first resonance line located at g~4.3 characterized of high crystal fields is due to the isolated Fe+3 ions disposed in rhombical distorted octahedral sites [31, 32]. The second resonance line situated at about g~2 is attributed either to Fe+3 ions associated in clusters or/and disposed in sites of less distorted octahedral symmetry with low crystal fields [33, 34]. For the sample with x=15mol% Y2O3, the resonance line situated at about g~2 do not appear in the EPR spectrum. For samples with x= 5 and 10mol% Y2O3, the EPR spectra have both resonance signals located at about g~2 and 4.3. The line intensity of the resonance centered at about g~4.3 increases with increasing the Y2O3 content of the samples from x=5 up to x=15mol% Y2O3. The increase of this resonance line intensity is correlated with the increase of the amount of isolated Fe+3 ions. In the sample with x=15mol% Y2O3, the Fe+3 ions involved in the randomly distorted structural units have the ability to impose a certain order in their neighborhood. The gradual decreasing of the Y2O3 in the ceramic leads to the progressive formation of non-bridging oxygen atoms and destroys the local ordering of the Fe +3 ion vicinities. The decrease of the resonance line intensity situated at about g~4.3 at the same time with the increasing of the resonance line situated at about g~2 for samples with x= 5 and 10mol% Y2O3 is due to the destruction of the configuration from the iron ions
vicinities, which assures their magnetic isolation. For these samples the structural units of the Fe+3 ions become less represented because they will interact by dipole-dipole interactions or the super-exchange coupled pairs. In brief, the analysis of the EPR data indicates that the shape and the values of resonance lines depend on the composition of the sample. In the x=15% Y2O3 sample, Fe+3 ions are homogeneously distributed, impose a certain order in their neighborhood and act as network formers. While with decrease in the Y2O3 content, the EPR spectra show the evolution of the Fe+3 ions from isolated species located in sites of strongly distorted octahedral symmetry (rhombic or tetragonal) to species interacting by dipoledipole interactions and finally to those associated in clusters. Accordingly, this evolution is correlated with the increasing disorder in the sample structure and with the role plays as network modifier. 3.8.
Density measurements
The density is a tool in revealing the degree of change in the structure with the sample composition. Fig. 8 shows the composition dependence of the density (d), the oxygen packing density (dO) and the molar volume (V m) of the studied samples in 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3. It can be observed that the values of density and oxygen packing density decrease with Y2O3 addition in the host matrix and have minimum values at the concentration with x=10mol% Y2O3. The results show that the density was decreased from 5.18g/cm3 to 4.77g/cm3. The values of the molar volume were changed from 226.21 cm3/mol to 225.74 cm3/mol.
The decrease in the density values for the samples with x=5 and x=10mol% Y2O3 was possibly due to the formation of non-bridging oxygen atoms around the doping ions or/and silicon ions (in agreement with IR data) or/and iron ions (in agreement with IR and EPR data). For sample with x=15mol% Y2O3, the value of density is 4.80 g/cm3. The variation in density for two concentrations (x=10 and x=15mol% Y 2O3) can be due to the clustering of iron ions, in agreement with the EPR data. The study on the silica glass [35] indicated a strong correlation between the PL intensities centered at about 540nm and 590nm, respectively and the effect of glass density. These emissions become more efficient when the glass gets a more compact structure. In our case, PL data demonstrated that the intensity of emission bands situated at about 540nm and 590nm were clearly enhanced when the sample exhibits a higher density (for x=15mol% Y2O3). In brief, the incorporation of Y2O3 content into the samples with x≤10mol% Y2O3 induces: (i) the availability of more non-bridging oxygen on structural units hence decreasing of cross-link density, (ii) increasing the size of interstitial space, and therefore an open structure is expected leading to a smaller density. In the x=10mol% Y2O3 sample, both the mechanisms produce abruptly decrease of density. The decrease in density may be attributed to comparatively open network due to formation of [FeO 4] tetrahedral units created in the structure in place of other types of modified tetrahedral units. The decrease in molar volume is ascribed to a decrease in the number of nonbridging oxygens [36]. The creation of non-bridging oxygens atoms will break the bonds
of the host matrix and increase free space in the network. This decrease in molar volume indicates that yttrium ions have a contracting effect. Combining all of the outputs from different characterizations, we can conclude that the addition of 5mol% Fe2O3 in the host matrix with higher Y2O3 contents (x=15 mol%) produces the following improvements: i. enricher cubic ZrO 2 content; ii. higher concentration of oxygen vacancies; iii. smaller concentration of NBO due to coordination with silicon and/or iron ions; iv. the highest gap energy value; v. a large light emission; vi. homogeneous distributed Fe+3 ions which impose a certain order in his neighborhood.
4. Conclusions The zirconia samples in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition with x=5, 10 and 15mol% Y2O3 were synthesized and characterized by means of XRD, SEM, IR, UV-Vis, PL and EPR spectroscopies. The samples show diffraction peaks corresponding to the tetragonal and the cubic ZrO2 crystalline phases. The analysis of the FTIR spectra indicates that the fractions of tetragonal and cubic zirconia phases and bridging oxygen ions as well as Si-O-Si and Zr-O-Si linkages were increased with increasing the Y2O3 content for all studied samples. The UV-Vis, PL and EPR data show that the increase of the Y 2O3 content of the samples leads to the increase of amounts of intrinsic defects such as non-bridging oxygen holes centers and the oxygen deficient centers leading to important modifications concerning the band gap energy, the intensity of PL bands and the capacity of iron ion clusters.
The increasing disorder in the sample structure by doping with higher Y 2O3 concentrations can be supported by the ability of the Fe+2 ions to create non-bridging oxygen ions which in the next stage will be coordinated with the Si+4, Fe+3 and Zr+4 ions. Taking into account the behavioral properties of the studied materials we conclude that they could be a candidate for biomedical applications.
Acknowledgements This research was supported by the Bridge Program Projects 2016 (PN-III-P2-2.1BG-2016-0077). The authors are gratefully acknowledged. The measurements by the SEM, UV-Vis and PL investigations were performed at INCDTIM.
References 1. S. Kumar, S. Bhunia, A. K. Ojha, Chem. Phys. Lett. 644 (2016) 271-275. 2. A. Sharma, M. Varshney, H. J. Shin, Y. Kumar, S. Gautam, K. H. Chae, Chem. Phys. Letters 592 (2014) 85-89. 3. S. Shukla, S. Seal, Int. Mater. Rev. 50 (2005) 45-64. 4. D. R. Clarke, C. G. Levi, Annu. Rev. Mater. Res. 33 (2003) 383-417. 5. J. Y. Thompson, B. R. Stoner, J. R. Piascik, R. Smith, Dental Materials 27 (2011) 71-82. 6. V. Fiorentini, G. Gulleri, Phys. Rev. Lett. 89 (2002) 266101(1-4). 7. P. Li, I. W. Chen, J. E. Penner-Han, J. Am. Ceram. Soc. 77 (1994) 118-128. 8. P. Li, I. W. Chen, J. E. Penner-Han, J. Am. Ceram. Soc. 77 (1994) 1281-1288.
9. D. Ysoutsou, G. Apostolopoulos, S. Galata, A. Sotiropoulos, G. Mavrou, Y. Panayiotatos, A. Dimoulas, Microelect. Engg. 86 (2009) 1626-1628. 10. S. Manjunatha, M. S. Dharmaprakash, J. Luminescence 180 (2016) 20-24. 11. M. Taguchi, A. Matsushita, T. Uchikoshi, Y. Sakka, S. Takami, T. Funazukuri, T. Naka, Nano-Structures & Nano-Objects 1 (2015) 1-6. 12. A. Meldrum, S. J. Zinkle, L. A. Boatner, Phys. Rev B 59(6) (1999) 3981-3992. 13. I.O. Fabregas, M. Reinoso, E. Otal, M. Kima, J. European Ceramic Society 36 (2016) 2043-2049. 14. S. Kumar, A. K. Ojha, J. Alloys Comp. 644 (2015) 654-662. 15. R. Chelcea, S. Rada, E. Culea, M. Rada, Spectrochim. Acta A 79 (2011) 481485. 16. P. Vomacka, O. Babushkin, J. Eur. Ceram. Soc. 15 (1995) 921-928. 17. J. H. Ryu, H. S. Kil, J. H. Song, D. Y. Lim, S. B. Cho, Powder Technology 221 (2012) 228-235. 18. Q. Lin, K. Shimizu, A. Satsuma, Applied Catalysis A: General 365 (2009) 55-61. 19. Y. Cong, B. Li, S. Yueand, D. Fan, J. Phys. Chem. C 113 (2009) 13974-13978. 20. L. Rus, S. Rada, V. Rednic, E. Culea, M. Rada, A. Bot, N. Aldea, T. Rusu, J. Non-Cryst. Solids 402 (2014) 111-115. 21. Y. Sakurai, J. Non-Cryst. Solids 352 (2006) 5391–5398. 22. E. A. Mahdy, S. Ibrahim, J. Molec. Structure 1027 (2012) 81–86. 23. L. Renuka, K. S. Anantharaju, S. C. Sharma, H. Nagabhushana, Y. S. Vidya, H. P. Nagaswarupa, S. C. Prashantha, J. Alloys and Compounds 695 (2017) 382-395. 24. S. Miyazaki, Appl. Surf. Sci. 190 (2002) 66-74.
25. A. G. Belous, E.V. Pashkova, K.V. Kravchyk, Oleg V'yunov, J. Chem. Phys. C 112(10) (2008) 3914-3919. 26. M. Lajavardi, D. J. Kenney, Sheng Hsien Lin, J. Chinese Chem. Societ. 47 (2000) 1065-1075. 27. G. da Silva Sousa, L. Santos Cavalcante, M. R. de Morais Chaves Santos, J. M. Elias de Matos, Materials Science Forum 869 (2016) 35-39. 28. I. J. Berlin, K. Joy, Phys. B 457 (2015) 182-187. 29. S. Mitrici, S. Rada, E. Culea, L. David, A. Bot, J. Non-Cryst. Solids 458 (2017) 34-40. 30. R. Stefan, P. Pascuta, A. Popa, O. Raita, E. Indrea, E. Culea, J. Phys. Chem. Solids 73 (2012) 221-226. 31. M. Cannas, L. Vaccaro, B. Boizot, J. Non-Crystalline Solids 352 (2006) 203-208. 32. A. Dehelean, A. Popa, S. Rada, E. Culea, J. Magn. Magn. Mater. 381 (2015) 131-137. 33. S. Rada, A. Dehelean, M. Stan, R. Chelcea, E. Culea, J. Alloys Compds 509 (2011) 147–151. 34. P. Pascuta, Gh. Borodi, A. Popa, V. Dan, E. Culea, Mater. Chem. Phys. 123 (2010) 767–771 35. N. Ollier, K. Piven, C. Martinet, T. Billotte, V. Martinez, D.R. Neuville, M. Lancry, J. Non-Cryst. Solids 476 (2017) 81-86. 36. M. Rada, E. Culea, S. Rada, A. Bot, N. Aldea, V. Rednic, J. Non-Cryst. Solids 358 (2012) 3129-3136.
Figures and Table Caption Fig. 1: XRD difractograms of the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 2: SEM images of the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 3: FTIR spectra of the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 4: UV-Vis spectra of the 5Fe2O3·10SiO2·xY2 O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 5: Plots of a) (αhν)2 and b) (versus hν for 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3 and linear portion behavior for the indication of the optical gap energy. Fig. 6: Photoluminescence (PL) spectra of the 5Fe2 O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 7: EPR spectra of the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 8: Yttrium (III) oxide composition dependence on a) density, d, b) molar volume, V m and c) the oxygen packing density, dO, for 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Table 1: Unit cell volume (V), effective crystallite mean size (Deff) and the root mean square microstrain ((ε2)1/2hkl) of zirconia ceramics in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3
5Fe2O3·10SiO2· 5Y2O3·80ZrO2 ; x=5%Y2O3 Lattice Parameters Unit cell volume a ± Δa b ± Δb [Å] c ± Δc β V [Å3] [Å] [Å] 5.1776± 5.2733± 5.2098± 99.15 140.517 0.278 0.278 0.278 72
Deff [Å]
<2>1/2hkl x103
127.3
436
134.935
107
479
3.592± 3.592± 3.592± 66.97 0.302 0.302 0.302 6.6039± 6.6039± 6.6039± 260.91 0.192 0.192 0.192 5Fe2O3·10SiO2· 10Y2O3·75ZrO2 ; x=10%Y2O3 Lattice Parameters Unit cell volume a ± Δa b ± Δb c ± Δc β V [Å3] [Å] [Å] [Å] 134.935 5.0080± 5.0080± 5.0080± 0.359 0.359 0.359
65.7
596
590.7
81
Deff [Å]
<2>1/2hkl x103
53.9
269
3.5946± 3.5946± 3.5946± 66.961 0.302 0.302 0.302 5Fe2O3·10SiO2· 15Y2O3·70ZrO2 ; x=15%Y2O3 Lattice Parameters Unit cell volume a ± Δa b ± Δb c ± Δc β V [Å3] [Å] [Å] [Å] 132.335 5.033± 5.033± 5.033± 0.362 0.362 0.362
348.7
129
Deff [Å]
<2>1/2hkl x103
37.2
114
3.598± 0.352
66.4
289
Crysta lline phase mZrO2
Proportio n [vol. %] 5.62
c-ZrO2
93.8
4.9891± 0.308
t-ZrO2
0.58
tZrSiO4
0
Crystal line phase
Proportio n [vol. %]
c-ZrO2
99.42
t-ZrO2
0.58
Crystal line phase
Proportio n [vol. %]
c-ZrO2
100
t-ZrO2
0
4.9891± 0.308
3.598± 0.352
4.9891± 0.308
3.598± 0.352
-
-
67.103
Highlights The formation of tetragonal and cubic zirconia crystalline phase · The formation of Zr-OSi and Si-O-Si linkages · Higher concentrations of oxygen vacancies with addition of Y2O3 contents · The EPR resonance line situated at g~4.3 and g~2.
Graphical abstract
S0009-2614(18)30135-0 https://doi.org/10.1016/j.cplett.2018.02.049 CPLETT 35461
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
3 November 2017 13 February 2018 15 February 2018
Please cite this article as: S. Rada, E. Culea, M. Rada, Novel ZrO2 based ceramics stabilized by Fe2O3, SiO2 and Y2O3, Chemical Physics Letters (2018), doi: https://doi.org/10.1016/j.cplett.2018.02.049
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Novel ZrO2 based ceramics stabilized by Fe2O3, SiO2 and Y2O3 S. Rada1, 2, E. Culea1, M. Rada2 1
Department of Physics & Chemistry, Technical University of Cluj-Napoca, 400020, Romania
2
Nat. Inst. For R&D of Isotopic and Molec. Technologies, Cluj-Napoca, 400293, Romania
Abstract Samples in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 were synthetized and investigated by XRD, SEM, density measurements, FTIR, UV-Vis, EPR and PL spectroscopies. X-ray diffraction patterns confirm the presence of the tetragonal and cubic ZrO2 crystalline phases in all samples. The IR data show the overlaps of absorption bands assigned to Zr-O-Zr and SiO-Si linkages in samples. UV-Vis and PL data indicate higher concentrations of intrinsic defects by doping with Y2O3 concentrations. The EPR spectra are characterized by two resonance lines situated at about g~4.3 and g~2 for lower Y 2O3 contents.
Keywords: iron-silicate-yttria-zirconia, tetragonal and cubic zirconia, XRD, FTIR, UV-Vis, PL and EPR spectroscopies.
*Corresponding Author: E-mail: [email protected], [email protected] (S. Rada) [email protected] (E. Culea)
1. Introduction The use of ceramic materials such as zirconia, ZrO 2, alumina, Al2O3 and glassbased ceramics, in dental applications is highly desirable because of their excellent properties, including improved biocompatibility, wear resistance and chemical durability in addition to aesthetics. Biocompatibility is one of the most important advantages of zirconium based ceramics. At atmospheric pressure, pure ZrO2 is recognized to adopt three different crystalline structures [1]. Pure zirconia has monoclinic structure below 16700C, between 1670 and 23000C has tetragonal structure and at high temperature (>2300 0C) has cubic structure. Most stable phase of monoclinic ZrO2 has limited practical applications due to its 3-5% volume expansion during cooling from the tetragonal phase [2]. To stabilize the cubic or tetragonal ZrO2 crystalline phases at room temperature was paid much attention due to their high performances as solar cells, sensors, dental applications including the field of biomaterials, novel catalyst, inert fuel matrix inside the core of nuclear reactors and accelerators [3-10]. In this context, trivalent and tetravalent dopants were incorporated into the host monoclinic ZrO2 crystalline phase [7-9]. Tetragonal zirconia is unstable and can be easily translated into monoclinic zirconia crystalline phase at room temperature. The addition of yttria, Y 2O3 can not only stabilize tetragonal zirconia, but also a right amount of yttrium has protective effects on cells and can improve the biocompatibility of materials. The addition of trivalent cation dopants, such as yttrium, stabilizes the cubic phase of zirconia down to room temperature and results in the presence of oxygen
vacancies. Cubic zirconium dioxide exists down to room temperatures when the concentration of yttria is higher than 10mol%. In yttria-stabilized zirconia ceramics the oxygen vacancies are generated in the material to maintain electrical neutrality, since that the tetravalent zirconium ions are replaced by trivalent yttrium ions. Thus, two Y3+ ions correspond to an anionic oxygen vacancy, which it is responsible for the oxygen ion conductivity The ceramics based on ZrO2 must stabilize by two main methods: i. with other oxides which stabilizes total or partially, the tetragonal and/or cubic phases and ii. by the reduction of the crystallite sizes [11]. This paper presents for the first time the stabilization of the high temperature zirconia phase with 5mol% Fe2O3 content in the presence of SiO2 and Y2O3 contents at room temperature after sintering. The aim of the work was to determine correlations between changes in microstructure with spectroscopic properties of the stabilized zirconia by investigations of X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy, UltraViolet-Visible (UV-Vis) spectroscopy, Electron Paramagnetic Resonance (EPR) spectroscopy, Photoluminescence (PL) spectroscopy and density measurements. Accordingly, the paper will contribute to the understanding of the associated mechanisms with the formation of tetragonal or/and cubic zirconia crystalline phase which are highly desirable in technological applications.
2. Materials and Methods Samples were synthesized through a high temperature solid state reaction process using commercial zirconium (IV) oxide, yttrium (III) oxide, silicon dioxide and iron (III)
oxide of high purity in suitable proportion. The mechanically homogenized mixtures were uniaxial compacted under pressure in the form of disks (2 mm thick, 9 mm diameter) and thus were sintered in alumina crucibles at 14000C in an electric furnace, in air atmosphere. The samples were put into the electric furnace direct at this temperature for two hours. After that, the disks material was put on a stainless-steel plate at room temperature. The samples were analyzed by means of X-ray diffraction using a XRD-6000 Shimadzu diffractometer, with a monochromator of graphite for Cu-Kα radiation (λ=1.54Å) at room temperature. The FT-IR spectra of the samples were obtained in the 350-1500cm-1 spectral range with a JASCO FTIR 6200 spectrometer using the standard KBr pellet disc technique. The spectra were carried out with a standard resolution of 2cm-1. UV-VIS absorption spectra of the prepared samples were investigated with a JASCO V-550 spectrometer, in the wavelength range of 200-2000nm having a resolution of 2nm. The fluorescence spectra were obtained using an ABLE & Jasco V 6500 spectrometer with xenon lamp of 150 W. For comparison, all emission spectra were measured at room temperature with the same instrumental parameters (λex = 220nm, band with (Ex) 3nm, band with (Em) 5nm, response 2s, data pitch 2nm, scanning, speed 200 nm/min). EPR measurements were performed at room temperature using an ADANI portable EPR PS 8400-type spectrometer, in the X frequency band and a field modulation of 100kHz. The microwave power was 5mW.
The samples density has been measured by immersion in water at room temperature using Archimedes method.
3. Results and discussion 3.1. XRD analysis Figure
1
shows
the
XRD
patterns
of
three
samples
in
the
5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3. In all samples the formation of the cubic and tetragonal zirconia crystalline phases were detected. The shift of XRD peaks towards higher angle side also authenticates well incorporation of iron, silicon and yttrium ions into ZrO2 matrix. The small quantities of monoclinic zirconia crystalline phase with an orientation (-111) at 28.240 and (111) at 31.50 were detected in prepared samples. A smaller diffraction peaks situated at about 20.01, 26.98, 35.63 and 53.480 attributed to the ZrSiO4 crystalline phase with tetragonal structure (PDF no. 00-006-0266) can be observed in sample with x=5mol% Y2O3. The ZrSiO4 structure consists of chains of edge-sharing, alternating [SiO4] tetrahedral units and [ZrO8] triangular dodecahedral units extending parallel to the crystallographic c axis [12]. A rigorous analysis of the tetragonal and cubic ZrO2 phases shows that satellite peaks belong to the tetragonal phase should appear before and after the 200 (50.570), 004 (73.070) and 311 (87.510) reflections in the XRD pattern [13]. In our samples, XRD diffractograms show peaks at 50.03, 73.75 and 84.040 corresponding to the cubic ZrO2 crystalline phase. Accordingly, the stabilization of cubic ZrO2 crystalline phase may be due to the presence of excess oxygen vacancies [14].
Cubic zirconia has the ideal calcium fluorite (CaF2) structure. The zirconium atoms in a cubic structure are coordinated with eight oxygen atoms while in a monoclinic structure with Zr cations 7-fold coordinated by oxygen anions. Table 1 shows the lattice parameters of the cubic, tetragonal and monoclinic zirconia crystalline phases. The lattice constant of cubic and tetragonal zirconia phases increase from a=4.989Å and a=3.592Å, respectively in the samples with x=5mol% to a=5.033Å and a=3.598Å in the samples with x=15mol% Y2O3. For cubic zirconia phase the effective crystallite mean sizes, Deff decreases from 107nm to 37.2nm and the unit cell volume, V was also decreased with the amount of yttrium trioxide. The decrease of unit cell volume results in a better packing, respectively a more compact network. The contraction of the cell volume can be associated to the substitution of bigger Zr+4 ions by smaller Si+4 ions. Due to the difference between ionic radii the reaction between ZrO2 and SiO2 results in the formation of oxygen vacancies. Moreover, the incorporation of the Y+3 dopant ion with large ionic radius into zirconia lattice creates oxygen vacancies in ZrO2 crystal. The fractions of the cubic ZrO2 crystalline phase increase by the diminishing the monoclinic ZrO2 phase by addition of Fe2O3 content. A progressive stabilization effect of the tetragonal and cubic ZrO2 crystalline phases was observed with an increase in Y2O3 concentration. 3.2. SEM The scanning electron image of the samples in the 5Fe2O3·10SiO2·xY2O3·(85x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 are shown in Fig. 2. It can be seen that irregular and agglomerated shapes like morphology was formed in all samples.
Figures 2a) and 2b) are illustrations of the SEM images of the samples with x=10 and 15mol% Y2O3, respectively. These images show irregular shapes and aggregations with the presence of grains having polyhedral shapes with different sizes. Sample with x=15mol% Y2O3 reveals a slightly higher crystal density and a more homogeneous when compared with the sample with x=10mol% Y2O3. Moderate aggregation of zirconia particles seems to be present in the SEM image of the sample with x=5mol% Y2O3 indicated in Fig. 2c). The morphology of the particles was found to be less uniform and the particles are randomly distributed in the case of the sample with x=5mol% Y2O3.
3.3. FTIR spectroscopy FTIR spectra of the samples in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 are shown in Fig. 3. The covalence bond becomes stronger with the decrease of the difference of electronegativity between cation and oxygen ions. Since the values of electronegativity for Zr, Y, Si, Fe and O are 1.4, 1.2, 1.8, 1.8 and 3.5, respectively, the covalency of the SiO and Fe-O bonds are stronger than that of Zr-O and Y-O, respectively. As a result, the higher affinity of the silicon and iron ions to attract oxygens atoms results in the formation of higher coordination silicon and iron structural units. Then, the excess of non-bridging oxygen ions will coordinate with zirconium ions and after that with yttrium ions only after the silicon and iron atoms attain the maximum of their coordination number.
In the FTIR spectrum of Fe2O3, the bands situated at about 470 and 510 cm-1 are attributed to the Fe-O vibrations. IR band situated at about 580 cm-1 is associated to Fe-O vibrations from [FeO4] structural units [15]. The IR bands centered at about 465 and 627 cm-1 are due to the Zr–O stretching vibrations and indicates the formation of cubic ZrO2 crystalline phase [10]. All IR spectra exhibit bands due to various silicate structural units centered at about 430 cm-1 and in the region between 850 and 1200 cm-1. The IR band situated at about 430 cm-1 can be ascribed to asymmetric Si-O-Si bending vibrations in the [SiO4] structural units. IR bands situated between 850 and 1200 cm-1 can be attributed to asymmetric stretching vibrations of O-Si-O bonds of [SiO4] structural units. The IR absorption bands centered at 970 cm-1 is attributed to Si-O-Si stretching of bridging oxygen. By increasing of Y2O3 content up to 15 mol% in the host matrix, the intensity of these bands was gradually increased and the IR band centered at about 970 cm-1 becomes more formed and intense. By increasing of Y2O3 content in the host matrix, the IR data reveal some structural features which can be summarized as follows: i) The prominent and broader IR bands situated in region between 400 and 700cm-1 can be attributed to overlaps of contributions provided by the stretching vibrations of Zr-O bond, the stretching vibrations of Fe-O bond and the creation of Y-OY bonding (peak centered at about 407cm-1) [16]. By increasing of Y2O3 content up to 15mol%, the position of the IR peak centered at about 407 cm-1 was shifted to longer wavenumbers (417cm-1) and its intensity was increased. A new IR band appears at
580cm-1 corresponding to the Fe-O vibrations in the [FeO4] structural units. The intensity of this band increases with the Y2O3 addition in the host matrix. The intense IR band centered at about 480cm-1 is attributed to Zr-O-Zr bond, indicating the formation of Zr-O-Zr network. The intensity of this band was enhanced in the sample with x=10 and 15mol% Y2O3 suggesting that the number of cubic/tetragonal zirconia crystallite was increased, in agreement with SEM data. This is due to the structural conversion of Zr cations seven-fold coordinated by oxygen anions in monoclinic structure into Zr cations eight-fold coordinated as the content of the Y2O3 modifier oxide was increased up to 15mol%. ii) The second region of IR bands situated between 850 and 1250cm-1 is due to the overlaps of contributions of the stretching vibrations of Zr-O bonds from monoclinic ZrO2 crystalline phase and the stretching vibrations of O-Si-O and Si-O-Si bonds of [SiO4] structural units [17]. The intensity of these IR bands was enhanced by increasing the Y2O3 contents. For the sample with x=5mol% Y2O3, a new IR band appears situated at about 1020cm-1 which can be assigned to the Si-O stretching band of ZrSiO4 crystalline phase. In brief, IR data suggests that the fractions of the tetragonal and the cubic zirconia phase and of bridging oxygen ions as well as Si-O-Si and Si-O-Zr linkages were increased with increasing the Y2O3 content.
3.4. UV-Vis spectroscopy UV-Vis spectra of the samples in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 are shown in Fig. 4.
The pure ZrO2 shows the UV-Vis absorption bands at a lower wavelength of 243 nm [18]. In our case, two UV-Vis bands centered at about 220 and 262nm are observed in all the spectra. This evolution can be explained by considering that the first band can be assigned to the O2--Zr4+ charge transfer transition [19] and the former UV-Vis band situated at about 262nm is attributed to the cubic zirconia crystalline phase. The intensity of last UV-Vis band was increased by adding of higher Y2O3 content in the host matrix. The absorption UV-Vis band centered at about 330nm is due to charge transfer transition O2- 2p – 5Fe3+ 3d. In the region of UV-Vis bands situated between 270 and 440nm there exists an increasing trend in intensity and a broadening of the bands with increases gradually up to 15mol% Y2O3. UV-Vis bands becomes more formed and intense for sample with x=15mol% Y2O3. The broad bands in this range between 300 and 450nm can be assigned to inter-band transitions, Zr+4-O-2 transitions and also a number of defects due to strong interactions with Fe2O3 sites. The Fe+2 ions have two absorption bands situated in the region between 450 and 550nm (due to the Fe+2-O-2 charge transfer) and another band centered at about 1100nm. The Fe+3 ions produce an intense band in the ultraviolet region at 310-380nm range and one in the visible region at about 700nm [20]. The intensity of the band centered at about 1100nm increased abruptly by doping with x=10mol% Y2O3 and after that its intensity remains almost unchanged for higher doping levels. The absorption bands situated in region between 310 and 380nm with regard to Fe+3 ions increased distinctly by increasing of Y2O3 content up to 15mol%.
Intrinsic defects such as dangling bonds, oxygen deficiency or oxygen excess can cause optical transitions of absorption and photoluminescence. These optical active centers are due to some local atomic rearrangements that differ from a perfect matrix. Optical transitions associated with the non-bridging oxygen holes centers (NBOHC) and oxygen deficient centers (ODC) can be evidenced by the presence of the UV bands centered at about 260 and 220nm, respectively [21]. The intensity of the UV-Vis band centered at about 220nm increases gradually up to x=15 mol% Y2O3. The characteristic feature of the UV-Vis band situated at about 260nm increases abruptly in strength and intensity with adding of higher Y2O3 concentrations up to 15 mol%. By doping with higher Y2O3 contents a reaction mechanism can be represented as follows: Fe+2 + NBOHC → Fe+3 + NBO After that the content of NBO can be coordinated with Si+4 and/or Zr+4 and/or Y+4 and/or Fe+3 ions. The IR band located at about 940cm-1 is assigned to stretching vibration of Si-O with two non-bridging oxygens (Si-O-2NBO) per [SiO4] tetrahedral units. The IR band located at 1030cm-1 observed only in sample with x=5mol% Y2O3 is characteristic of asymmetric stretching vibrations of the Si-O-Si bonds with three dimensional network structure [22]. By doping the structural rearrangement can take place in the Si-O-Si environment due to creation of NBOs. As a result, the prominent role of Y+3 ions as network former in the matrix host can be illustrated in a net decrease of gap energy by doping with higher levels. These changes can be attributed to structural changes which are associated with variation of inter-tetrahedral band angles caused by Y2O3 [22].
The gradual increase of the Y2O3 content up to 15mol% in the host network leads to a decrease of the UV-Vis bands situated between 700 and 2000nm. This structural evolution indicates that the number of electronic transitions was decreased slightly in this region. 3.5. Gap energy Among the wide band gap (Eg) metal oxides, ZrO 2 is an active material and has two direct band to band transitions at 5.2 and 5.79 eV [23]. The relation between the absorption coefficient α and the incident photon energy hν can be written as αhν = A(hν-Eg)n, where the photon energy is hν, h being the Planck constant,
is the absorption coefficient in cm-1, Eg is the band gap, A is the energy
dependent constant a and n is an integer that can take different values depending on the type of electronic transition, for a permitted direct transition n = ½ and indirect transition n = 2. The optical band gap of ceramics is estimated by using Tauc’s plot. The direct and indirect band gap value is obtained by extrapolating the linear portions of the plots versus (αhν)2 and
versus (αhν) ½ to the energy axis as shown in Fig. 5.
From Fig. 5, it can be seen that the compositional evolution of gap energy for both direct and indirect transitions increases with the increasing of Y 2O3 content up to 15mol%. The values for direct gap energy are lying gradually from 2.01 to 2.07eV whereas for indirect gap the values are ranging from 2.50 to 2.66eV. The increase in the value of optical band gap energy on increasing of Y 2O3 concentration in the host matrix can be understood in terms of the structural modifications that are taking place in the host ceramics. These effects can be attributed to
lattice defects. Lattice defects such as NBOHC and ODC usually introduce extrinsic energy levels between bands and hence reduce the band gap [24]. In this manner the lattice defects added to the host matrix by the addition of Fe 2O3 content are taken up by the excess of Y2O3 content and as a result, the band gap was increased. This observation is in agreement with the one made by the authors [25] based on the study of the (ZrO2)0.9(Y2O3)0.1-x(Fe2O3)x system where x≤ 2mol% Fe2O3 and the authors [26] based on the study of Y2O3- ZrO2 and Fe2O3- ZrO2 systems where Y2O3 and Fe2O3 increase up to 30 mol%. Thus, concerning the two additives used in case (Y2O3 and Fe2O3), the authors [25] concluded that only Y2O3 is a stabilizer of ZrO2, while Fe2O3 may only promote, under certain conditions, the stabilizing efficiency of Y2O3. The authors [26] demonstrated that Y2O3 play the stabilizer role even for low contents (1-2 mol% Y2O3) while Fe2O3 may reach this role for higher contents (over 15 mol%). Finally, the increase of Y2O3 content of the studied samples has caused the significant depolymerization of silicate network, the creation of more BOs in the host matrix due to more Si-O(2NBO) units and hence results in an increase of gap energy.
3.6.
Photoluminescence (PL) Spectroscopy
The photoluminescence spectra of the samples in the 5Fe2O3·10SiO2·xY2O3·(85x)ZrO2 composition where x=5, 10 and 15mol% Y 2O3 are shown in Fig. 6. The excitation spectra of samples are formed by broader and intense bands in the range between 300 and 475nm and a moderate band centered at about 540nm. The PL band centered at about 340nm is attributed to compensating oxygen vacancies [27]. This band attains the maximum value for sample with x=5mol% Y 2O3.
This can be explained considering the presence of small quantities of impurities (such as Fe2O3 crystalline phases) in the cubic ZrO2 crystalline phase which creates oxygen deficiency in the host matrix and thus, the decrease of the number of compensating oxygen vacancies. It is well-known that the Zr+4 ions are non-luminous and the observed luminescence from the ZrO2 samples must be due to non-stoichiometry created by the oxygen deficiency in the system [23]. The Fe2O3 does not show photoluminescence (PL) due to the local forbidden d-d transition resonant energy transfer between cations and efficient lattice and magnetic relaxations. A simple inspection of photoluminescence spectra shows a notable change in the intensity of the samples suggesting that the host matrix accommodates more defects. The PL intensity was decreased abruptly in the sample with x=10mol% ZrO2. This decrease in PL intensity can be expected due to the change in the crystallite size, in agreement with SEM data or/and accumulated oxygen vacancies in the zirconia lattice. The intense PL band centered at about 390nm can be related to the ionized oxygen vacancies existing in the ZrO2 lattice. Emission occurs due to the radioactive recombination of photo generated hole with an electron occupying the oxygen vacancy [28]. The PL intensity of the sample with x=15mol% Y2O3 was found to be maximum when compared to the samples with x=5 and 10mol% Y2O3. The fact that the PL intensity of the sample with x=15 mol% Y2O3 is the highest suggests that this sample accommodates the higher amount of oxygen vacancies in comparison to the other two ones.
The intense PL band centered at about 440 nm can be due to the transitions from the surface trap states in the conduction band to lower energy levels close to the valance band of ZrO2. The PL intermediate band centered at about 540nm can be due to the involvement of mid-gap trap states, such as surface defects and oxygen vacancies. The intensities of these bands were decreased in the sample with x=10mol% Y2O3 and shows a large emission for the sample with x=15mol% Y2O3. It means that the wide band gap of ZrO2 seems to accommodate more new energy levels or electron traps, showing variety of emissions. Accordingly, large amounts of surface defects exist on the sample with x=10mol% Y2O3. The relationship between the luminescence situated at about 650nm and the absorption UV-Vis bands centered at about 260 and 620nm in vitreous SiO2 are correlated in agreement with a model for understanding the structural defects in which they are assigned to induced intrinsic defects such as non-bridging oxygen holes centers, NBOHC [28]. The NBOHC represents the oxygen dangling bond with a structure of O3Si-O· where silicon is bonded with three oxygen atoms and another O· which denotes an unpaired electron. Then, the oxygen deficient centers, ODC can be assigned to the PL band situated at about 500nm. An ODC center can be described as O3Si-SiO3. These defects can be optically active and may have sensitizing properties. All intensities of these bands were enhanced for the sample with x=15mol% Y2O3. As a result, we come to the conclusion that with the increase of intrinsic defects concentration, the luminescent peak intensity increases. In the given situation, the sample with higher Y2O3 (x=15%) concentration contains the higher concentration of intrinsic defects.
3.7. The
EPR spectroscopy features
from
the
EPR
spectra
of
the
samples
in
the
5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3 presented in Fig. 7 are characterized by two resonance lines situated at about g~4.3 and g~2 [29, 30]. The first resonance line located at g~4.3 characterized of high crystal fields is due to the isolated Fe+3 ions disposed in rhombical distorted octahedral sites [31, 32]. The second resonance line situated at about g~2 is attributed either to Fe+3 ions associated in clusters or/and disposed in sites of less distorted octahedral symmetry with low crystal fields [33, 34]. For the sample with x=15mol% Y2O3, the resonance line situated at about g~2 do not appear in the EPR spectrum. For samples with x= 5 and 10mol% Y2O3, the EPR spectra have both resonance signals located at about g~2 and 4.3. The line intensity of the resonance centered at about g~4.3 increases with increasing the Y2O3 content of the samples from x=5 up to x=15mol% Y2O3. The increase of this resonance line intensity is correlated with the increase of the amount of isolated Fe+3 ions. In the sample with x=15mol% Y2O3, the Fe+3 ions involved in the randomly distorted structural units have the ability to impose a certain order in their neighborhood. The gradual decreasing of the Y2O3 in the ceramic leads to the progressive formation of non-bridging oxygen atoms and destroys the local ordering of the Fe +3 ion vicinities. The decrease of the resonance line intensity situated at about g~4.3 at the same time with the increasing of the resonance line situated at about g~2 for samples with x= 5 and 10mol% Y2O3 is due to the destruction of the configuration from the iron ions
vicinities, which assures their magnetic isolation. For these samples the structural units of the Fe+3 ions become less represented because they will interact by dipole-dipole interactions or the super-exchange coupled pairs. In brief, the analysis of the EPR data indicates that the shape and the values of resonance lines depend on the composition of the sample. In the x=15% Y2O3 sample, Fe+3 ions are homogeneously distributed, impose a certain order in their neighborhood and act as network formers. While with decrease in the Y2O3 content, the EPR spectra show the evolution of the Fe+3 ions from isolated species located in sites of strongly distorted octahedral symmetry (rhombic or tetragonal) to species interacting by dipoledipole interactions and finally to those associated in clusters. Accordingly, this evolution is correlated with the increasing disorder in the sample structure and with the role plays as network modifier. 3.8.
Density measurements
The density is a tool in revealing the degree of change in the structure with the sample composition. Fig. 8 shows the composition dependence of the density (d), the oxygen packing density (dO) and the molar volume (V m) of the studied samples in 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3. It can be observed that the values of density and oxygen packing density decrease with Y2O3 addition in the host matrix and have minimum values at the concentration with x=10mol% Y2O3. The results show that the density was decreased from 5.18g/cm3 to 4.77g/cm3. The values of the molar volume were changed from 226.21 cm3/mol to 225.74 cm3/mol.
The decrease in the density values for the samples with x=5 and x=10mol% Y2O3 was possibly due to the formation of non-bridging oxygen atoms around the doping ions or/and silicon ions (in agreement with IR data) or/and iron ions (in agreement with IR and EPR data). For sample with x=15mol% Y2O3, the value of density is 4.80 g/cm3. The variation in density for two concentrations (x=10 and x=15mol% Y 2O3) can be due to the clustering of iron ions, in agreement with the EPR data. The study on the silica glass [35] indicated a strong correlation between the PL intensities centered at about 540nm and 590nm, respectively and the effect of glass density. These emissions become more efficient when the glass gets a more compact structure. In our case, PL data demonstrated that the intensity of emission bands situated at about 540nm and 590nm were clearly enhanced when the sample exhibits a higher density (for x=15mol% Y2O3). In brief, the incorporation of Y2O3 content into the samples with x≤10mol% Y2O3 induces: (i) the availability of more non-bridging oxygen on structural units hence decreasing of cross-link density, (ii) increasing the size of interstitial space, and therefore an open structure is expected leading to a smaller density. In the x=10mol% Y2O3 sample, both the mechanisms produce abruptly decrease of density. The decrease in density may be attributed to comparatively open network due to formation of [FeO 4] tetrahedral units created in the structure in place of other types of modified tetrahedral units. The decrease in molar volume is ascribed to a decrease in the number of nonbridging oxygens [36]. The creation of non-bridging oxygens atoms will break the bonds
of the host matrix and increase free space in the network. This decrease in molar volume indicates that yttrium ions have a contracting effect. Combining all of the outputs from different characterizations, we can conclude that the addition of 5mol% Fe2O3 in the host matrix with higher Y2O3 contents (x=15 mol%) produces the following improvements: i. enricher cubic ZrO 2 content; ii. higher concentration of oxygen vacancies; iii. smaller concentration of NBO due to coordination with silicon and/or iron ions; iv. the highest gap energy value; v. a large light emission; vi. homogeneous distributed Fe+3 ions which impose a certain order in his neighborhood.
4. Conclusions The zirconia samples in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition with x=5, 10 and 15mol% Y2O3 were synthesized and characterized by means of XRD, SEM, IR, UV-Vis, PL and EPR spectroscopies. The samples show diffraction peaks corresponding to the tetragonal and the cubic ZrO2 crystalline phases. The analysis of the FTIR spectra indicates that the fractions of tetragonal and cubic zirconia phases and bridging oxygen ions as well as Si-O-Si and Zr-O-Si linkages were increased with increasing the Y2O3 content for all studied samples. The UV-Vis, PL and EPR data show that the increase of the Y 2O3 content of the samples leads to the increase of amounts of intrinsic defects such as non-bridging oxygen holes centers and the oxygen deficient centers leading to important modifications concerning the band gap energy, the intensity of PL bands and the capacity of iron ion clusters.
The increasing disorder in the sample structure by doping with higher Y 2O3 concentrations can be supported by the ability of the Fe+2 ions to create non-bridging oxygen ions which in the next stage will be coordinated with the Si+4, Fe+3 and Zr+4 ions. Taking into account the behavioral properties of the studied materials we conclude that they could be a candidate for biomedical applications.
Acknowledgements This research was supported by the Bridge Program Projects 2016 (PN-III-P2-2.1BG-2016-0077). The authors are gratefully acknowledged. The measurements by the SEM, UV-Vis and PL investigations were performed at INCDTIM.
References 1. S. Kumar, S. Bhunia, A. K. Ojha, Chem. Phys. Lett. 644 (2016) 271-275. 2. A. Sharma, M. Varshney, H. J. Shin, Y. Kumar, S. Gautam, K. H. Chae, Chem. Phys. Letters 592 (2014) 85-89. 3. S. Shukla, S. Seal, Int. Mater. Rev. 50 (2005) 45-64. 4. D. R. Clarke, C. G. Levi, Annu. Rev. Mater. Res. 33 (2003) 383-417. 5. J. Y. Thompson, B. R. Stoner, J. R. Piascik, R. Smith, Dental Materials 27 (2011) 71-82. 6. V. Fiorentini, G. Gulleri, Phys. Rev. Lett. 89 (2002) 266101(1-4). 7. P. Li, I. W. Chen, J. E. Penner-Han, J. Am. Ceram. Soc. 77 (1994) 118-128. 8. P. Li, I. W. Chen, J. E. Penner-Han, J. Am. Ceram. Soc. 77 (1994) 1281-1288.
9. D. Ysoutsou, G. Apostolopoulos, S. Galata, A. Sotiropoulos, G. Mavrou, Y. Panayiotatos, A. Dimoulas, Microelect. Engg. 86 (2009) 1626-1628. 10. S. Manjunatha, M. S. Dharmaprakash, J. Luminescence 180 (2016) 20-24. 11. M. Taguchi, A. Matsushita, T. Uchikoshi, Y. Sakka, S. Takami, T. Funazukuri, T. Naka, Nano-Structures & Nano-Objects 1 (2015) 1-6. 12. A. Meldrum, S. J. Zinkle, L. A. Boatner, Phys. Rev B 59(6) (1999) 3981-3992. 13. I.O. Fabregas, M. Reinoso, E. Otal, M. Kima, J. European Ceramic Society 36 (2016) 2043-2049. 14. S. Kumar, A. K. Ojha, J. Alloys Comp. 644 (2015) 654-662. 15. R. Chelcea, S. Rada, E. Culea, M. Rada, Spectrochim. Acta A 79 (2011) 481485. 16. P. Vomacka, O. Babushkin, J. Eur. Ceram. Soc. 15 (1995) 921-928. 17. J. H. Ryu, H. S. Kil, J. H. Song, D. Y. Lim, S. B. Cho, Powder Technology 221 (2012) 228-235. 18. Q. Lin, K. Shimizu, A. Satsuma, Applied Catalysis A: General 365 (2009) 55-61. 19. Y. Cong, B. Li, S. Yueand, D. Fan, J. Phys. Chem. C 113 (2009) 13974-13978. 20. L. Rus, S. Rada, V. Rednic, E. Culea, M. Rada, A. Bot, N. Aldea, T. Rusu, J. Non-Cryst. Solids 402 (2014) 111-115. 21. Y. Sakurai, J. Non-Cryst. Solids 352 (2006) 5391–5398. 22. E. A. Mahdy, S. Ibrahim, J. Molec. Structure 1027 (2012) 81–86. 23. L. Renuka, K. S. Anantharaju, S. C. Sharma, H. Nagabhushana, Y. S. Vidya, H. P. Nagaswarupa, S. C. Prashantha, J. Alloys and Compounds 695 (2017) 382-395. 24. S. Miyazaki, Appl. Surf. Sci. 190 (2002) 66-74.
25. A. G. Belous, E.V. Pashkova, K.V. Kravchyk, Oleg V'yunov, J. Chem. Phys. C 112(10) (2008) 3914-3919. 26. M. Lajavardi, D. J. Kenney, Sheng Hsien Lin, J. Chinese Chem. Societ. 47 (2000) 1065-1075. 27. G. da Silva Sousa, L. Santos Cavalcante, M. R. de Morais Chaves Santos, J. M. Elias de Matos, Materials Science Forum 869 (2016) 35-39. 28. I. J. Berlin, K. Joy, Phys. B 457 (2015) 182-187. 29. S. Mitrici, S. Rada, E. Culea, L. David, A. Bot, J. Non-Cryst. Solids 458 (2017) 34-40. 30. R. Stefan, P. Pascuta, A. Popa, O. Raita, E. Indrea, E. Culea, J. Phys. Chem. Solids 73 (2012) 221-226. 31. M. Cannas, L. Vaccaro, B. Boizot, J. Non-Crystalline Solids 352 (2006) 203-208. 32. A. Dehelean, A. Popa, S. Rada, E. Culea, J. Magn. Magn. Mater. 381 (2015) 131-137. 33. S. Rada, A. Dehelean, M. Stan, R. Chelcea, E. Culea, J. Alloys Compds 509 (2011) 147–151. 34. P. Pascuta, Gh. Borodi, A. Popa, V. Dan, E. Culea, Mater. Chem. Phys. 123 (2010) 767–771 35. N. Ollier, K. Piven, C. Martinet, T. Billotte, V. Martinez, D.R. Neuville, M. Lancry, J. Non-Cryst. Solids 476 (2017) 81-86. 36. M. Rada, E. Culea, S. Rada, A. Bot, N. Aldea, V. Rednic, J. Non-Cryst. Solids 358 (2012) 3129-3136.
Figures and Table Caption Fig. 1: XRD difractograms of the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 2: SEM images of the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 3: FTIR spectra of the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 4: UV-Vis spectra of the 5Fe2O3·10SiO2·xY2 O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 5: Plots of a) (αhν)2 and b) (versus hν for 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3 and linear portion behavior for the indication of the optical gap energy. Fig. 6: Photoluminescence (PL) spectra of the 5Fe2 O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 7: EPR spectra of the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Fig. 8: Yttrium (III) oxide composition dependence on a) density, d, b) molar volume, V m and c) the oxygen packing density, dO, for 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 samples where x=5, 10 and 15mol% Y2O3. Table 1: Unit cell volume (V), effective crystallite mean size (Deff) and the root mean square microstrain ((ε2)1/2hkl) of zirconia ceramics in the 5Fe2O3·10SiO2·xY2O3·(85-x)ZrO2 composition where x=5, 10 and 15mol% Y2O3
5Fe2O3·10SiO2· 5Y2O3·80ZrO2 ; x=5%Y2O3 Lattice Parameters Unit cell volume a ± Δa b ± Δb [Å] c ± Δc β V [Å3] [Å] [Å] 5.1776± 5.2733± 5.2098± 99.15 140.517 0.278 0.278 0.278 72
Deff [Å]
<2>1/2hkl x103
127.3
436
134.935
107
479
3.592± 3.592± 3.592± 66.97 0.302 0.302 0.302 6.6039± 6.6039± 6.6039± 260.91 0.192 0.192 0.192 5Fe2O3·10SiO2· 10Y2O3·75ZrO2 ; x=10%Y2O3 Lattice Parameters Unit cell volume a ± Δa b ± Δb c ± Δc β V [Å3] [Å] [Å] [Å] 134.935 5.0080± 5.0080± 5.0080± 0.359 0.359 0.359
65.7
596
590.7
81
Deff [Å]
<2>1/2hkl x103
53.9
269
3.5946± 3.5946± 3.5946± 66.961 0.302 0.302 0.302 5Fe2O3·10SiO2· 15Y2O3·70ZrO2 ; x=15%Y2O3 Lattice Parameters Unit cell volume a ± Δa b ± Δb c ± Δc β V [Å3] [Å] [Å] [Å] 132.335 5.033± 5.033± 5.033± 0.362 0.362 0.362
348.7
129
Deff [Å]
<2>1/2hkl x103
37.2
114
3.598± 0.352
66.4
289
Crysta lline phase mZrO2
Proportio n [vol. %] 5.62
c-ZrO2
93.8
4.9891± 0.308
t-ZrO2
0.58
tZrSiO4
0
Crystal line phase
Proportio n [vol. %]
c-ZrO2
99.42
t-ZrO2
0.58
Crystal line phase
Proportio n [vol. %]
c-ZrO2
100
t-ZrO2
0
4.9891± 0.308
3.598± 0.352
4.9891± 0.308
3.598± 0.352
-
-
67.103
Highlights The formation of tetragonal and cubic zirconia crystalline phase · The formation of Zr-OSi and Si-O-Si linkages · Higher concentrations of oxygen vacancies with addition of Y2O3 contents · The EPR resonance line situated at g~4.3 and g~2.
Graphical abstract