Iron Doped-ZrSiO₄: Structural, Microstructural and Vibrational Characterization

Abstract

Fe_x-ZrSiO₄ is known for the applications in the ceramic industry such as ceramic pigment. In this article, we focus our attention to the structural, microstructural and vibrational changes of Fe_x-ZrSiO₄ from free-mineralizer precursors, treated at different temperatures in the range of 1100-1600 °C. The refinements of X-ray diffraction patterns show that Fe³⁺ cations were distributed into tetrahedral sites replacing Si⁴⁺. The evolution of the shape distribution analyzed by transmission electron microscopy, reveal a polyhedral morphology at 1100 °C during 3h. In comparison, well-rounded and homogeneous particle size was determined in the sample heated at 1600 °C during 24 h. On the other hand, it was observed that increase the content of iron and the increased heat treatment (temperature and time) both plays an important effect on the observed Raman results. The profile line deconvolution applied through the ν₃ vibration of the SiO₄ group shows a spectral change similar to that seen in radiation-damaged zircon: a decrease in frequency and increase in bandwidth.

Keywords:
ceramic pigments; zircon; Rietveld; Raman

1 Introduction

Electronic and vibrational properties of nanometric doped zirconium silicate (ZrSiO₄₎ are of considerable interest because of the large number of applications of this compound. ZrSiO₄, commonly called zircon is widely used in the ceramic, foundry and refractory industries, Zircon adopts a garnet-related structure (Figure 1) having lattice parameters a = 6.607(1) Å; c = 5.982(1) Å, Z = 4^[1-3]. The zircon structure can be described as built of chains of alternating edge-sharing SiO₄ tetrahedra and ZrO₈ triangular dodecahedra extending parallel to the c-axis, which are joined laterally by edge-sharing dodecahedra^4-6. This oxide exhibits low thermal expansion⁷, low thermal conductivity, and high resistance to thermal shock⁷, as well as good corrosion resistance (for example, against glass melts, slag and liquid metal alloys). Furthermore, a large variety of transition and/or rare-earth metal cations can be incorporated into the host lattice. It is for this reason that zircon is often considered as a natural host material for the storage of radioactive waste material^8-11. ZrSiO₄ is also considered as a promising alternative to silicon oxide as the gate dielectric material in metal-oxide semiconductor devices because of its high permittivity¹².

Figure 1
A schematic representation of garnet-type crystal structure of ZrSiO₄. The chains of alternating edge-sharing SiO₄ tetrahedra and ZrO₈ dodecahedra projected on (001) and showing the edge sharing between dodecahedra. This representation was obtained by CIF file of Fullprof and it was plotted by Diamond 3.21 software.

For Fe doped-ZrSiO₄ compound, it is vital interest to gain a fundamental understanding of the influence of size and shape morphology; aggregation of several particles leading to defects in the structure of particles and improvement of crystallinity, which mediate the oxygen ion transport, on the functional properties. In the last decade, several authors have studied different topics referred to the structural and chemical characteristics of Fe-ZrSiO₄^[10-24]. The mechanism of formation, the chemical state, and location of Fe cation in the zircon structure, the actual presence of segregated hematite and dissociation of zircon^25-27 have been considered as main topics. In this context, our work over the last few years has been to develop Fe-containing zircon solid solution materials from free-mineralizer precursors with new potential applications, as for instance in electrocatalytic processes^28,29.

Nasdala et al.^30-33 have introduced Raman spectroscopy as a method to estimate quantitatively the degree of radiation damage in zircon. In particular, the peak profile of Raman ν₃ (SiO₄) band at approximately 1,000 cm^–1 has been used to predict the thermal history of zircon, and this one quantifies the degree of short order around the fourfold Si sites. Nasdala et al.³⁴ used the width to estimate the possible thermal history of natural zircons, whereas Geisler et al.³⁵ proposed the use of both the width and frequency to trace potential thermal alterations. They showed that Raman spectra clearly shift towards lower wavenumbers and broaden, which reflects the loss of short-range order and the general expansion of the lattice. The FWHM (full width at half maximum) of the internal ν₃ mode was found to increase most sensitively with increasing radiation damage. It increases from 2 cm^–1 for well-crystallized (i.e., undamaged) to well above 30 cm^–1 for strongly radiation-damaged zircon.

Based on these facts, the motivation of the present work is to investigate and demonstrate that the increase of Fe content in ZrSiO₄ prepared by the sol-gel technique plays an important role over size effects during the heat treatments and it has an important influence on the intensity, shift symmetry and bandwidth of the Raman results. The first goal was to prepare a series of Fe_x–ZrSiO₄ solid solutions from free-mineralizer precursors with increasing content of Fe. The gels were annealed at 1100 °C during 3h, 1200 °C during 3 h and 1600 °C during 24 h. The last temperature and holding time was selected to improve the densification of the material. Our second purpose was to estimate the solubility limit of iron in the zircon structure (in particular in the sample heated at 1600 °C) and compare and elucidate the crystal parameters of the samples by the X-ray diffraction (XRD) Rietveld refinements. The particle size evolution was monitored by transmission electron microscopy (TEM) as the Fe concentration increased. In the third goal, the Raman investigations are motivated by the close relation between the asymmetric broadening of Raman peaks with the crystal-chemical effects such as the tetrahedral distortion determined by the XRD Rietveld refinements.

2 Experimental Procedure

2.1 Samples preparation

Gels with stoichiometry, Fe_x–ZrSiO₄, where x = 0, 0.02, 0.05, 0.07, 0.1 and 0.125 were prepared by the sol–gel liquid phase route, using zirconium n-propoxide (ZnP, Zr(OC₃H₇)₄) tetraethylorthosilicate (TEOS, Si(OC₂H₅)₄) and iron acetylacetonate (Fe(AcAc)₃, FeC₁₅H₂₄O₆), as sources for Zr, Si and Fe, respectively. Details of synthesis and structure analysis can be found elsewhere^36,37.

2.2 Characterization techniques

X-ray powder diffraction analysis (Model AXS D5005 Bruker) was performed using a graphite monochromatic CuKα radiation. The diffractograms were run with a step size of 0.02° 2θ and a counting time of 10 sec. Lattice constants and other structural parameters of zircon phase were determined by refinement with the Rietveld technique using Fullprof98^[38] available in the software package Winplotr³⁹. The Rietveld refinement involved the following parameters: scale factor; zero displacement correction; unit cell parameters; peak profile parameters using a pseudo-Voigt function; and overall temperature factor. The starting structural parameters and atomic positions of tetragonal zirconia⁴⁰, monoclinic zirconia⁴¹, hematite⁴² and of zircon-type crystal structure^7-9, were taken from the literature. They were also refined and plotted with Diamond 3.21^[43] as shown in Figure 1. The morphology of iron-containing zirconia and zircon particles was also examined using transmission electron microscopy (Model 1010, Jeol Ltd., Tokyo, Japan) at an accelerating voltage of 100 kV. Samples were crushed and dispersed in absolute ethyl alcohol and drops of the dispersion were transferred to a specimen copper grid carrying a lacey carbon film. Particle size analyses were carried out by using the Image J software⁴⁴. The Raman spectra were recorder with spectral windows of 3500-100 cm^–1 in a high performance dispersive Raman Jasco NRS-3100 spectrometer. A L1200/B500 nm grating dispersed the radiation onto a TE-cooled CCD detector. The spectra were excited with a 632.8 nm radiation from a He-Ne ion laser. The laser beam was focused on the sample with an acquisition time of 10 sec. Profile line deconvolution was performed following the approach as described by Presser & Glotzbach⁴⁵. For the undoped sample we determined the true Raman bandwidth to be 3.3 ± 0.1 cm⁻¹. This value falls within the range of the reported values by Presser & Glotzbach⁴⁵ and Gucsik et al.⁴⁶.

3 Results and Discussion

3.1 X-ray powder diffraction and the Rietveld refinement

The first step of this study was to characterize the thermal stability of Fe_x-ZrSiO₄ reactions up to 1600 °C. Figure 2 shows the evolution of X-ray diffraction results for the powders of Fe_0.1-ZrSiO₄ heated at different temperatures. In panel (a) one can observe an important contribution of tetragonal zirconia (t-ZrO₂), hematite and zircon phases on the sample heated at 1100 °C during 3 h. The XRD pattern obtained at 1200 °C during 3 h shows the decrease in the intensity of the reflections attributed to the t-ZrO₂; the hematite phase still remains and it is indicated by an asterisk in panel (b). Panel (c) shows the XRD for the sample obtained at 1600 °C during 24 h. In this XRD pattern, one can observe the Rietveld refinement profile, the residual profile and the vertical lines (Bragg positions) related to zircon. According to Alahakoon et al.²⁶, the presence of monoclinic zirconia (m-ZrO₂) indicates that zircon is slightly dissociated at this time and temperature²⁵. At this stage, the reflection due to t-ZrO₂ disappearing, confirms the theory that zircon is formed from amorphous silica and m-ZrO₂^[26]. Furthermore, at this stage, relatively sharp reflections of hematite crystal phase are resolved in XRD. The detailed structural parameters and goodness of fit between the observed and calculated profiles at selected temperatures are given in Table 1. One can observe from the Rietveld refinement results that a small portion of Fe crystallizes as hematite.

Figure 2
(a) X-ray powder diffraction data for the sample with Fe_0.1-ZrSiO₄ heated at 1100 °C during 3 h (♦ is tetragonal ZrO₂). (b) XRD data for the same composition heated at 1200 °C during 3 h (_* is hematite). (c) Rietveld refined X-ray diffraction pattern for the sample with Fe_0.1-ZrSiO₄ powders obtained after the heat treatments at 1600 °C during 24 h. This figure shows the observed intensity (Yobs), the calculated intensity (Ycalc) and the difference between observed and calculated intensities (Yobs - Ycalc) and the vertical lines represents the Bragg positions, (↓ is monoclinic ZrO₂).

Table 1
Crystallographic data and results of Rietveld refinement of X-ray diffraction patterns for Fe_0.1-ZrSiO₄ heated at different temperatures.

	1100 °C / 3h	1200 °C / 3h	1600 °C / 24h
a(Å)	6.5993(2)	6.6003(5)	6.6030(7)
c(Å)	5.9777(1)	5.9773(3)	5.9793(4)
V(Å3)	260.335(4)	260.400(1)	260.702(5)
Rp(%)	8.8	8.6	9.2
Rwp(%)	11.6	10.7	12.1
Rwp (expected)(%)	7.9	8.1	7.5
χ2	1.5	1.7	1.6
tetragonal phase (wt %)	30.5	3.4	----
monoclinic phase (wt %)	2.5	1.9	3.5
zircon phase (wt %)	65.5	92.8	93.8
hematite phase (wt %)	1.4	1.9	2.7
Y atomic coordinate of O	0.6649(8)	0.6634(2)	0.6634(2)
z atomic coordinate of O	0.1964(9)	0.1982(3)	0.1993(6)

As suggested by Ocaña et al.^47,48 iron must play some role as catalyzer in the zircon formation. Thus, the complete formation of zircon takes place over the range of temperatures between around 1100 °C and 1600 °C depending on the nominal amount of iron. The Rietveld refinements of XRD patterns allowed us to monitor the local distortions of the oxygen around the cations. Table 2 summarized the selected interatomic distances for the samples obtain at 1600 °C during 24 h. In order to perform the calculation we used the structural parameters obtained from Rietveld refinements of the XRD data (CIF file) using Bond Str subprogram in Fullprof⁴⁹. One can observe in Table 2 that the selected interatomic distances reveal a gradual increase in the SiO₄ tetrahedra with increasing iron content up to the sample with 0.07 mol of iron per mol of zircon. On the other hand, one can observe an apparent anomalous variation of the O-O distances.

Table 2
Selected interatomic distances (A) for Fe_x-ZrSiO₄ powders heated at 1600 °C during 24 h.

Bond type / Fex	0	0.02	0.05	0.07	0.1
Zr - Si [2]*	2.9912	2.9895	2.9896	2.9895	2.9897
Zr - O [4] [4]	2.1311 2.2681	2.1117 2.2930	2.1110 2.2860	2.1053 2.2880	2.1147 2.2957
Si – O [4]	1.6221	1.6263	1.6318	1.6356	1.6222
O – O [1] [1] [2] [4] [2]	2.4302 2.4942 2.7522 2.8422 3.0711	2.4708 2.5103 2.7435 2.8335 3.0497	2.4707 2.4945 2.7565 2.8335 3.0499	2.4799 2.4903 2.7615 2.8321 3.0383	2.4669 2.5197 2.7355 2.8343 3.0550

• ^*
  Bracketed numbers are bond multiplicities.

Figure 3 shows the evolution of the lattice volume against the Fe nominal content in Fe_x–ZrSiO₄ solid solutions heated at 1600 °C during 24 h. This behavior is similar to the one reported previously for the samples heated at 1100 °C or 1200 °C^[36]. From these results, it can be inferred that the solubility limit is higher than 0.07 mol of iron per mol of zircon (~3.0 wt. % as Fe₂O₃). However, it must be lower than 0.1 mol of Fe (~4.2 wt. % as Fe₂O₃) because in the sample Fe_0.1–ZrSiO₄, hematite is detected as a secondary phase and also a small decrease in its lattice volume. The change of lattice parameters, as the iron content increases in the solid solution, is consistent with the idea of expansion of the lattice, as a larger ion substitutes a small ion. The observed increase in lattice parameters can be understood by assuming that Fe³⁺ replaces Si⁴⁺ in tetrahedral sites. The larger ionic radius of Fe³⁺ compared with Si⁴⁺ causes the progressive increase of the unit cell parameters with increasing substitution^36,50. However, it is to be noted that for reaching electroneutrality in the formation of the Fe-doped zircon solid solution, some oxygen vacancies should be produced. Thus, the probable mechanism could be written out as Si⁴⁺ → Fe³⁺ + ½O^2-. Furthermore, the simultaneous creation of anion vacancies would be in agreement with the small increase of lattice volume in the Fe-doped zircon series of samples. An alternative simple mechanism of solid solution formation involving the substitution of Zr⁴⁺ by Fe³⁺ does not seem to be operative because the difference of ionic radius of Zr⁺⁴ and Fe³⁺ and the simultaneous oxide vacancies creation would give rise to a large decrease in doped-zircon lattice parameters. Carreto et al.⁵¹ studied the preparation of Fe–zircon pigments by the ceramic method adding LiF as mineralizer and reported results on the Fe–zircon solid solution formation^51,52. They found that only a small fraction of iron, about 2.5 mol %, was hosted in the zircon structure. Strikingly, the trend in the lattice volume variation of Fe–ZrSiO₄ solid solutions with increasing the Fe nominal content found by those authors is opposite to the one reported in this work, that is, the lattice volume exhibits a contraction on incorporating the Fe into the zircon lattice. These changes in lattice parameters for samples heated at 1600 °C could be caused by the dissociation of zircon^25,26. This dissociation will be subjected for a further characterization work.

Figure 3
Evolution of lattice volume of Fe_x–ZrSiO₄ solid solutions heated at 1600 °C during 24 h as a function of the nominal content of Fe (x).

3.2 Transmission electron microscopy

As a second step, transmission electron micrographs of Fe_0.1-ZrSiO₄ powders without any etching to remove the amorphous silica were collected. Three representative micrographs are show in Figure 4a-c. In Figure 4a it can be observed that the powders heated at 1100 °C during 3 h, the non-aggregated particles shows a small quasi-spherical particles and polyhedral-shape particles. The particle size distribution was determined over the analysis (histograms) of five micrographs. The results show a bimodal particle size distribution center at 11 ± 2 nm and 35 ± 2 nm, respectively. TEM micrograph of Fe_0.1–ZrSiO₄ powders heated at 1200 °C during 3 h is display in Figure 4b. As can be seen an arrangement of quasi-spherical particles with the average size around 19 ± 2 nm is observed. At this stage, it seems that the amount of dopant plays an important role in the particle size and shape distribution. We also may assume that almost all previous non-crystalline parts of the sample are recrystallizing at this stage, in favor of the remnant ZrO₂. Figure 4c shows the TEM micrograph for the powders heated at 1600 °C during 24 h. The increase of the heat treatment gives rise to a decrease of zircon particle size. The results shows well-rounded particles exhibiting a particle size distribution center at 29 ± 1 nm.

Figure 4
TEM micrographs evolution of Fe_0.1–ZrSiO₄ powders obtained at different temperatures and their respective particle size histogram: (a) Powders heated at 1100 °C during 3 h. (b) Powders heated at 1200 °C during 3 h. (c) Powders heated at 1600 °C during 24 h.

3.3 Raman spectroscopy

The next objective was to characterize the Raman evolution for Fe_x-ZrSiO₄ powders in order to monitor the structural changes. Figure 5 shows the evolution of the Raman spectra obtained for Fe_0.1–ZrSiO₄ powders heated at different temperatures. Panel (a) of Figure 5 shows the Raman spectrum after a heat treatment at 1100 °C during 3 h, this result supports the interpretation of XRD showed in Figure 3a. The space group for zircon (ZrSiO₄) is I4₁/amd, Z=4, D¹⁹_4h, No. 141^[53] and from group theory considerations, 12 Raman active modes are predicted (2A_1g + 4 B_1g + B_2g + 5E_g)^54-61. According to Nicola & Rutt⁵⁴ and Dawson et al.⁵⁵, the most intensive Raman bands of zircon, which lie in the wavenumber range 350-450 cm^–1 and around 1000 cm^–1, must be interpreted as internal vibrations of the SiO₄ units. Intense external lattice vibrations (rotational and translational) occur in the range 200-230 cm^–1. On the other hand, zirconium oxide (ZrO₂) exists in three polymorphic forms, namely: (1) monoclinic, which has space group P2₁/b (C⁵_2h) and Z = 4. It has 18 Raman active modes 9A_g + 9B_g; (2) the tetragonal phase (ZrO₂), space group P4₂/mnc (D¹⁵_4h) and Z = 2. It has six Raman active modes, A_1g + 2 B_1g + 3E_g; and (3) the cubic phase, space group Fm3m (O⁵_h), Z = 4 with only one Raman active mode⁵⁶. Hematite belongs to the D⁶_3d crystal space group and seven phonon lines are expected in the Raman spectrum^55,56, namely two A_1g modes (225 and 498 cm^–1) and five E_g modes (247, 293, 299, 412 and 613 cm^–1). The positions of the characteristic Raman bands of this work are generally in good agreement with those reported in the literature^57-63. One can observe that the frequency of the ν₃(SiO₄) band shows a shift by about 2 cm^–1 toward lower wavenumbers, when compared with undoped crystalline zircon (Table 3).

Figure 5
Room temperature Raman spectra for Fe_0.1-ZrSiO₄ powders (a) heated at 1100 °C during 3 h (b) heated at 1200 °C during 3 h and (c) heated at 1600 °C during 24 h. (_* is zircon; ▼is hematite, ○ is monoclinic ZrO₂ and ♦ is tetragonal ZrO₂).

Table 3
Results obtained from the profile^* deconvolution of selected Raman peak positions and corrected bandwidth values (full width at half maximum, FWHM) for Fe_x-ZrSiO₄ powders heated at 1600 °C during 24 h.

Fex	0.00		0.02		0.05		0.07		0.1
Modes	Peak position (cm–1)	FWHM (cm–1)	Peak position (cm–1)	FWHM (cm–1)	Peak position (cm–1)	FWHM (cm–1)	Peak position (cm–1)	FWHM (cm–1)	Peak position (cm–1)	FWHM (cm–1)
B1g ν3(SiO4)	1007.9	3.3	1002.3	10.9	991.9	8.2	995.5	7.9	997.6	11.7
A1g ν1 (SiO4)	975.1	3.1	970.9	9.9	964.2	7.5	967.2	7.5	968.1	10.2
A1g ν2 (SiO4)	438.7	6.8	438.3	14.0	432.9	14.7	435.8	15.9	435.8	12.8
Eg	357.2	5.5	355.8	14.7	350.9	13.3	354.5	17.8	353.5	12.9

• ^*
  Averaged at least over five analysis.

Analogously, the Raman spectrum at 1200 °C (Figure 5b) show the bands of zircon, t-ZrO₂ and hematite as one can observe in the XRD pattern (Figure 3b). At this stage ν₃(SiO₄) FWHM shows a notable broadening by about 10.3 cm^–1 and a clear shift approximately of 4 cm^–1 from undoped ZrSiO₄. In panel (c) of Figure 6 shows the Raman spectrum for the sample heated at 1600 °C during 24 h, this one confirms the presence of zircon, m-ZrO₂ and hematite. This spectrum shows the gradual increase back to the initial value of 1008 cm^-1, however it shows an increase in the FWHM around 15.2 cm^–1. Considering these observations and the fact that Fe and heat treatments causes changes in the crystal size and structure; in the next part of this study we focus on the most intensive Raman bands of zircon (between 960-1020 cm^–1) in order to discuss the effects of Fe, in particular on the samples obtained at 1600 °C during 24 h. It can be observe from Figure 6 that the Raman peak position shows a systematic red shift on going from the undoped sample to the sample with x=0.05. On the other hand, as one increase the iron concentration from x=0.05 to x=0.1, a reverse trend in the peak position is observed, i.e. a systematic blue shift of the peak position. According to Nasdala et al.⁶⁴ the slight shift toward higher vibrational energies is due to the compressive strain in small ZrSiO₄ particles, which increases with decreasing particle size. Figure 6 also shows the deconvoluted spectra of ν₃(SiO₄) and ν₁(SiO₄) modes for the samples heated at 1600 °C during 24 h. All spectra were fitted with a Gaussian-Lorentzian type profile to estimate their peak position and its full width half maximum (FWHM). The estimated values of the peak positions and FWHMs are list in Table 3. It can be observe from this table that the peak position of the ν₃(SiO₄) Raman mode has the lowest value of 1007.9 cm^–1 at undoped sample and this value gradually shifted to lower wavenumbers with respect to this one. In addition, one can observe a pronounce asymmetry as one increases the iron content. Now we will discuss the mechanism of the wavenumber shift of the ν₃(SiO₄) mode. Several factors such as temperature, defects and stress can cause a significant shift in the Raman peak position. According to Balkanski et al.⁶⁵ and Verma et al.⁶⁶ temperature causes shift and broadening of Raman bands. In the present case, all measurements were performed at room temperature. It may be noted that high laser power can cause significant thermal effect, which can result in the broadening and shift of the Raman band⁶⁷. Defects in the structure also cause shifts of the Raman peaks and significant broadening of Raman line shape. The defects in Fe-ZrSiO₄ can in fact double the Raman line width. The shift of all main Raman bands (Figure 6, Figure 7 and Table 3) toward lower wavenumbers indicates that in general the average distances between atoms become somewhat larger. As already mentioned in the XRD section concerning the lattice is slightly expanded (Table 2). The increase in band full-width and the accompanying decrease of intensity can be interpreted in such a way that the distribution of bond lengths and bond angles within and between SiO₄ tetrahedra becomes increasingly irregular. The discrepancy between the Raman and XRD evaluation of lattice parameters is due to the fact the XRD is a predominantly probe of long order and Raman is a probe of short order. Also mathematical formalism behind each method that investigates the line broadening is different, resulting in different results.

Figure 6
(a-e) Raman spectra evolution for powders of Fe_x-ZrSiO₄ to demonstrate band deconvolution obtained at symmetric stretching (ν₁; A_1g mode at 974 cm^-1) and antisymmetric stretching (ν₃; B_1g mode at 1008 cm^–1) of SiO₄ tetrahedra. Individual's peaks were fitted by Gauss-Lorentz functions.

Figure 7
(a-e) Raman spectra evolution and the result of fitting Gauss-Lorentz functions for Fe_x-ZrSiO₄ powders in the region between 180-700 cm^–1.

4 Conclusions

Structural, microstructural and vibrational properties of Fe_x-ZrSiO₄ from free-mineralizer precursors were investigated as a function of heat treatments. XRD, TEM and Raman analyses demonstrated that iron plays an important role over the crystal size and shape distribution in Fe-ZrSiO₄. The increase in lattice parameters determined by the Rietveld refinement of XRD patterns is consistent with the idea that Fe³⁺ cations are distributed mainly into tetrahedral sites replacing Si⁴⁺. The solubility limits of Fe3+ in ZrSiO4 at 1600 °C are in the range 0.07-0.1 mol of iron per mol of zircon. The evolution of particle size and shape distribution reveals a polyhedral morphology at 1100 °C during 3 h. The well-rounded and homogeneous particle size and shape distribution was determined in the sample heated at 1600 °C during 24 h. The Raman spectra for Fe_x-ZrSiO₄ treated at different temperatures in the range 1100-1600 °C shows that, as the iron content increases, all Raman bands decrease in intensity becoming increasingly broader and show a notable shift toward lower wavenumbers. These changes suggest that the bond length of SiO₄ tetrahedra become increasingly irregular and the lattice is slightly expanded.

Acknowledgements

G. Herrera thanks Mexico-CONACyT for the student fellowship Grant No. 170588; Postdoctoral Research Scholarship No. 129569 and No. 172529. GH is also indebted with A. Moreno for the library facilities at PUEC-UNAM (Mexico) and L. R. Jiménez-Velasquez and G. Rojas-George. The author also thanks to M.Sc. A. Mestre (SCSIE-Universitat de Valencia) for their assistance in XRD measurements and M.Sc. M. Alcalde for Raman measurements performed at Universitat Jaume I.

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Publication Dates

• Publication in this collection
  3 Nov 2015
• Date of issue
  Nov-Dec 2015

History

• Received
  13 Aug 2015
• Reviewed
  01 Oct 2015