Crystallization of ZrSiO4 from a SiO2âZrO2 Binary System: The

Article pubs.acs.org/crystal

Crystallization of ZrSiO4 from a SiO2−ZrO2 Binary System: The Concomitant Effects of Heat Treatment Temperature and TiO2 Additions Amit Kumar Yadav, V. Ponnilavan, and S. Kannan* Centre for Nanoscience and Technology, Pondicherry University, Puducherry 605 014, India ABSTRACT: The concomitant effects of adding varied amounts of TiO2 and temperature of heat treatment on the crystallization of ZrSiO4 from SiO2−ZrO2 binary systems were investigated. The results showed that the t-ZrO2 phase is stabilized in the amorphous SiO 2 network, and the simultaneous occurrence of SiO2 crystallization along with the tetragonal zirconia (t-ZrO2) → monoclinic zirconia (mZrO2) phase transition tends to activate the reaction between m-ZrO2 and SiO2 to yield ZrSiO4 at elevated temperatures. The formation of metastable ZrTiO4 is also witnessed in the intermediate temperatures that were dependent on TiO2 content, and its dissociation into their individual ZrO2 and TiO2 oxides resulted in ZrSiO4 formation. The Ti4+ occupancy at the Zr4+ lattice sites ensured enhanced crystallization of ZrSiO4, and the limit of Ti4+ occupancy is determined as 9%. Excess TiO2 discarded from the ZrSiO4 lattice gets crystallized into rutile TiO2 (r-TiO2). and TiO2.11,12 The onset temperature of ZrSiO4 formation was decreased to 1300 °C and to 1200 °C by adding Al2O3 to the yttria stabilized ZrO2−SiO2 system,9 or 5 wt % Fe2O3.10 Additions of TiO2 in its rutile form appeared to exert an opposite effect elevating the temperature required for the formation of ZrSiO4, which was attributed to the limited solubility of Ti4+ in ZrSiO4 structure where it replaces Si4+ sites.11−13 The positive aspect of such cationic replacement is the yield of a thermodynamically more stable component in the form of ZrTiO4. The structure of ZrSiO4 displays triangular dodecahedral ZrO8 groups that form edge-sharing chains parallel to the a-axis and SiO4 tetrahedral monomers that form an edge-sharing chain with alternate ZrO8 groups parallel to the c-axis. The influence of TiO2 on the formation of ZrSiO4 has not been addressed in detail so far. The aim of the present work is to conduct an insightful investigation in this specific area by adopting a bottom up approach to synthesize ZrSiO4 in the presence of various added amounts of TiO2, starting from solutions of the precursor reactants using the sol−gel technique. The concomitant effects of heat treatment temperature and adding various amounts of TiO2 precursor within the triphasic ZrO2−SiO2−TiO2 system were comprehensively investigated aiming at evaluating the suitability of ZrSiO4based ceramics as alternative candidate materials for biomedical applications in hard tissue replacements. The structural features

1. INTRODUCTION The common industrial application of ZrSiO4 as a pigment derives from its ability to host a variety of trace elements in its structure. This salient feature, associated with a high thermal stability, makes ZrSiO4 ceramics also attractive candidate materials for hard tissue replacements. It was shown that high purity ZrSiO4 ceramics exhibited enhanced mechanical properties.1 The body friendly features of individual ZrO2 and SiO2 oxides are well-known as they are common compounds in ceramics and glass formulations aimed at biomedical applications. The impressive mechanical features of t-ZrO2 polymorph justify its use in the fabrication of hard tissue substitutes and total hip joint replacements (THR), whereas SiO2 is a major component in bioactive glasses for health care.2,3 The failure of commercial ZrO2 implants in vivo due to its gradual t-ZrO2 → m-ZrO2 phase transformation and the associated volume expansion led to the search of alternative synthetic products for hard tissue replacements.4,5 The formation of zircon (ZrSiO4) from its constituent oxides has been the subject of extensive investigation during the past few decades. The kinetics of the reaction depends on the crystalline phase changes occurring within the constituent oxides in the system. The temperature of monoclinic ZrO2 (mZrO2) to tetragonal ZrO2 (t-ZrO2) phase transformation at ∼1170 °C augments the reaction rate between ZrO2 and SiO2 to yield ZrSiO4.6 The onset temperature of the solid state reaction between SiO2 and ZrO2 is usually observed at ∼1400 °C, and the formation of ZrSiO4 is boosted with increasing temperature7,8 or by adding impurities that decrease the onset temperature of the solid state reaction such as Al2O3,9 Fe2O3,10 © 2016 American Chemical Society

Received: June 23, 2016 Revised: July 26, 2016 Published: August 10, 2016 5493

DOI: 10.1021/acs.cgd.6b00959 Cryst. Growth Des. 2016, 16, 5493−5500

Crystal Growth & Design

Article

semiconductor diode laser (0.5% of power) source with an excitation wavelength of 785 nm has been used to obtain Raman spectra. FT-IR analysis of powders was performed in transmission mode using a FTIR spectrophotometer (Perkin-Elmer, USA) in the spectral range from 4000 to 400 cm−1 by the KBr method. The structural features of the selected compositions were also determined on a (JEOL TEM-2100) high resolution transmission electron microscope (HRTEM) with a tungsten filament at an accelerating voltage of 200 kV. The samples were prepared by placing a drop of prepared solution on the surface of a copper grid and dried at room temperature.

of the synthesized materials were assessed by several spectroscopic techniques including Fourier transform infrared (FT-IR) spectroscopy, Raman, and X-ray diffraction (XRD) coupled with Rietveld refinement.

2. MATERIAL AND METHODS 2.1. Powder Synthesis. Analytical grade ZrOCl 2 ·8H 2 O, (C2H5)4OSi, and Ti(OCH(CH3)2)4 procured from Sigma-Aldrich, India, were used as precursors for the powder synthesis through a sol− gel technique. The concentrations of ZrOCl2 and (C2H5)4OSi were maintained constant, whereas those of the Ti(OCH(CH3 ) 2 ) 4 precursor were varied to synthesize six different compositions. The molar concentrations of all the precursors and their sample codes are listed in Table 1. In brief, the solution mixtures comprising

3. RESULTS The concomitant effects of increasing the added amounts of TiO2 and the heat treatment temperature on the evolution of crystalline phase assemblage from the ternary ZrO2−SiO2− TiO2 system are displayed in Figure 1−5. The results displayed

Table 1. Molar Concentrations and Its Corresponding Measurement of the Precursors Used in the Synthesis precursor molar concentrations

sample code 10TZS 20TZS 40TZS 60TZS 80TZS 100TZS a

ZrOCl2 0.500 8.056 0.500 8.056 0.500 8.056 0.500 8.056 0.500 8.056 0.500 8.056

M ga M ga M ga M ga M ga M ga

(C2H5)4OSi Ti(OCH(CH3)2)4 0.500 5.582 0.500 5.582 0.500 5.582 0.500 5.582 0.500 5.582 0.500 5.582

M ga M ga M ga M ga M ga M ga

0.050 M 0.710 g 0.100 M 1.421 ga 0.200 M 2.842 ga 0.300 M 4.260 ga 0.400 M 5.684 ga 0.500 M 7.105 ga

Zr/Ti ratio

wt % of Ti with respect to Zr

1.0

10

1.0

20

1.0

40

1.0

60

1.0

80

1.0

100

a

Figure 1. XRD patterns of different ZrO2−SiO2−TiO2 triphasic systems at 900 °C.

Denotes the measured weight of precursors used for the synthesis.

in Figure 1 reveal that the samples heat treated at 900 °C consist essentially of t-ZrO2 as there is a good coincidence between the XRD patterns and its corresponding ICDD standard. The incipient formation of ZrSiO4 at this temperature is also apparent from its corresponding low intensity emerging peaks detected in all the compositions. A close observation of the XRD reflections indicated a minor shift of the 101 plane of t-ZrO2 toward higher 2θ angles with increasing added amounts of TiO2. The increase of the heat treatment temperature to 1100 °C promoted drastic changes in the evolution of the crystalline phase assemblage as shown in Figure 2, with the corresponding peaks intensity being also dependent on the added amounts of TiO2. The formation of t-ZrO2 and of ZrSiO4 as major and minor phases, respectively, can be inferred from the observed corresponding high and low intensity reflections. In cases of 40TZS and 60TZS, strong ZrSiO4 reflections together with low intensity peaks corresponding to srilankite (ZrTiO4) phase were also detected. For the samples 80TZS and 100TZS heat treated at 1100 °C, the most intense reflections observed pertain to ZrTiO4 and the less intense ones belong to ZrSiO4. For these TiO2-rich compositions rutile TiO2 (r-TiO2) phase could be also identified. The preferred formation of ZrSiO4 in 40TZS and 60TZS is due to a more extensive reaction between ZrO2 and SiO2. An optimal added amount of TiO2 to the ZrO2−SiO2 binary system is expected to reduce the Gibb’s free energy for the formation of ZrSiO4, thus explaining the observed results.

(C2H5)4OSi and Ti(OCH(CH3)2)4 were prepared in C2H5OH under constant stirring (250 rpm) using a magnetic stirrer followed by the addition of 5 mL of HNO3 as a catalyst. A discretely prepared ZrOCl2 in deionized water was then slowly added to the (C2H5)4OSi and Ti(OCH(CH3)2)4 mixtures under constant stirring conditions until the formation of semisolid gels. The as-obtained semisolid gels were dried at 120 °C overnight and then ground to fine powders for further analysis. 2.2. Characterization Methods. The phase analysis of the powders was determined using a high resolution X-ray diffractometer (RIGAKU, ULTIMA IV, JAPAN) with Cu Kα radiation (λ = 1.5406 nm) produced at 40 kV and 30 mA to scan the diffraction angles (2θ) between 10 and 80° with a step size of 0.02° 2θ per second. All the powder samples were heat treated at specific temperatures with a dwell time for 4 h in a high temperature muffle furnace. Phase determinations were made using Standard ICDD (International Centre for Diffraction Data) Card Nos. 01-083-1376 for ZrSiO4, 01079-1765 for t-ZrO2, 01-083-0944 for m-ZrO2, 01-078-4190 for rTiO2, 01-080-1783 for ZrTiO4, and 00-076-0941 for crystoballite (cSiO2). Quantitative phase analysis of the powders was performed by employing the Rietveld method using the GSAS-EXPGUI software package.14 All the standard crystallographic information file (CIF) data were obtained from American mineralogist crystal structure database. The standard crystallographic data used for the refinement of t-ZrO2, ZrSiO4, ZrTiO4, r-TiO2, and c-SiO2 were reported by Howard et al.,15 Hazen et al.,16 Troitzsch et al.,17 Meagher et al.,18 and Peacor et al.,19 respectively. The structural refinement was performed according to the procedure described elsewhere by the authors.20 The Raman spectra (Renishaw, United Kingdom) were gathered using backscattering geometry of the confocal Raman microscope. A 5494



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but the peak reflections are slightly higher intensity, indicating an improved crystallinity of the phases formed. Figure 5 depicts the phase evolution with the heat treatment temperature from 900 to 1400 °C of the 10TZS composition.


Further increasing the heat treatment temperatures to 1200 °C (Figure 3) led to the formation of ZrSiO4 as the major

Figure 5. XRD patterns of different 10TZS compositions at various temperatures.

The low crystalline and discrete t-ZrO2 phase detected at 900 °C transformed into highly crystalline t-ZrO2 along with the formation of a minor amount of ZrSiO4 at 1100 °C. Further increasing the heat treatment temperature stimulated the reaction between ZrO2 and SiO2 to yield ZrSiO4 as a major phase (together with some remaining t-ZrO2) at 1200 °C and as a discrete phase at 1400 °C. Raman spectroscopy is a useful technique to determine the vibrational modes and defects in inorganic components. The compositions heat treated at different temperatures were subjected to Raman analysis with the aim to better understand the mechanism of ZrSiO4 formation in the presence of added TiO2. Raman spectra of all the TZS compositions at 1200 and 1400 °C are displayed in Figures 6 and 7. The characteristic


phase, together with smaller amounts of r-TiO2 that are more easily noticeable with TiO2-enrichment of the compositions, as expected. The diffraction patterns obtained for samples heat treated at 1400 °C (Figure 4) exhibit good similarities with those reflections observed for samples heat treated at 1200 °C,

Figure 6. Raman spectrum of different ZrO2−SiO2−TiO2 triphasic systems at 1200 °C.

bands that correspond to ZrSiO4 were determined in the spectral range at 200, 223, 356, 386, 435, 977, and 1009 cm−1 for all the compositions. The bands determined at 1009 and 977 cm−1 correspond to asymmetric ν3, B1g stretching and symmetric symmetric ν1, A1g stretching of SiO4 group of ZrSiO4, whereas other minor bands located at a lower wavenumber corresponds to the bending vibration of the

Figure 4. XRD patterns of different ZrO2−SiO2−TiO2 triphasic systems at 1400 °C. 5495



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Figure 7. Raman spectrum of different ZrO2−SiO2−TiO2 triphasic systems at 1400 °C.

SiO4 group with respect to the zirconium atom.21,22 The Raman active Eg and A1g bands of r-TiO2 were detected at 440 and 620 cm−1, and these active modes belongs to anion motion with respect to stationary titanium cation either perpendicular or parallel to the c-axis, and the intensity of these bands gets enhanced as a function of TiO2 content in increasing order.23 The bending vibration of the SiO4 group observed at 435 cm−1 tends to broaden with enhanced intensity as a function of TiO2 content, and this effect is also observed at 1400 °C. The Raman ν3 band of ZrSiO4 indicated a red shift as a function of TiO2 content until a certain limit, and beyond this no significant shift was observed, and moreover this limit also gets enhanced with increasing temperature. In other words, the compositions at 1200 °C indicated a shift until 40TZS, and beyond this no shift was observed for other compositions; however, at 1400 °C the shift was observed until 60TZS. Rietveld refinement was performed for all the compositions at selective temperatures for a better understanding on the roles of TiO2 and temperature on the ZrO2−SiO2 systems. The weight fractions of the various phases formed at 1100, 1200, and 1400 °C determined from refinement are presented in Table 2. Rietveld plots of 10TZS at 1200 and 1400 °C and 40TZS annealed at 1400 °C are shown in Figure 8. 10TZS and 20TZS indicated the major presence of metastable t-ZrO2 and minor amount of ZrSiO4 at 1100 °C. The active role of TiO2 was found obvious in all the compositions ranging from 40TZS to 100TZS at 1100 °C in which the complete absence of t-ZrO2 is noticed. The optimal of TiO2 levels at this temperature correspond to the 40TZS and 60TZS compositions, leading to the preferential formation of ZrSiO4 along with small amounts of r-TiO2 and ZrTiO4 phases. For the samples with high TiO2 contents (80TZS and 100TZS), ZrTiO4 was detected as a

Figure 8. Refined powder diffraction patterns of (a) 10TZS at 1200 °C, (b) 10TZS at 1400 °C, and (c) 40TZS at 1400 °C.

major phase along with the presence of r-TiO2 and ZrSiO4 as minor phases. Thus, at 1100 °C, the phase fraction of ZrSiO4 was apparently reduced with the TiO2 additions, while those of r-TiO2 and ZrTiO4 were enhanced.

Table 2. Temperature-Dependent Phase Fractions of Six Different ZrO2−SiO2−TiO2 Triphasic Systems Determined from Rietveld Refinement heat treatment temperatures 1100 °C sample code

t-ZrO2

ZrSiO4

10TZS 20TZS 40TZS 60TZS 80TZS 100TZS

73.70 84.20

26.30 15.80 83.80 66.80 1.60 1.70

r-TiO2

11.40 11.90 9.90 7.40

1200 °C ZrTiO4

4.80 21.30 88.50 90.90

t-ZrO2

ZrSiO4

10.90

89.10 92.40 79.90 78.40 73.00 67.90 5496

r-TiO2 2.20 9.00 18.20 22.20 27.90

1400 °C ZrTiO4

ZrSiO4

r-TiO2

ZrTiO4

c-SiO2

5.40 11.10 3.40 4.80 4.20

100 87.50 80.90 76.90 67.60 57.60

6.70 15.70 19.00 30.30 38.20

4.00 2.30 2.20 2.10 2.50

1.80 1.10 1.90 1.70



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Table 3. Rietveld Agreement Factors, Lattice Parameter, Occupancy Factors and Selected Bond Lengths of ZrSiO4 Phase Obtained from Six Different ZrO2−SiO2−TiO2 Triphasic Systems Rietveld agreement factors 1200 °C

1400 °C

lattice parameters of ZrSiO4 (Å) 1200 °C

1400 °C

sample code

χ2

Rp

χ2

Rp

a = b-axis

c-axis

a = b-axis

10TZS 20TZS 40TZS 60TZS 80TZS 100TZS

1.94 1.98 1.81 1.66 1.94 1.56

6.3 6.8 5.9 5.58 5.84 6.74

3.03 2.86 2.27 2.67 2.5 1.86

6.49 6.51 6.68 6.23 6.59 6.08

6.6049(3) 6.6044(3) 6.6016(2) 6.6032(2) 6.6007(2) 6.6010(4)

5.9886(3) 5.9877(3) 5.9849(2) 5.9880(1) 5.9858(2) 5.9858(3)

6.6072(4) 6.6056(4) 6.6044(4) 6.6034(3) 6.6026(4) 6.6018(4)

The results of Rietveld refinement for samples heat treated at 1200 and 1400 °C indicated the predominance of ZrSiO4 for all the compositions. The r-TiO2 was not detected in the low TiO2-containing (10TZS and 20TZS) compositions. For the compositions ranging from 40TZS to 100TZS, the phase fractions of r-TiO2 were higher at 1400 °C than at 1200 °C, while those of ZrTiO4 underwent drastic reductions in comparison to those detected at 1100 °C. This enables us to infer about the poor thermal stability of ZrTiO4, which liberates Ti4+ that tends to oxidize and form titania, thus explaining the increasing trend of the r-TiO2 phase fractions noticed with the increase of heat treatment temperature. In order to simplify the analyzing process, the refined lattice data were only considered for the main phase of interest (ZrSiO4). The lattice data of ZrSiO4 at 1100 °C displayed irregular variations as a function of TiO2 content, namely, an increasing trend until 20TZS followed by a reduction tendency with further increasing TiO2 contents (40TZS to 100TZS). At 1100 °C the formation of ZrSiO4 is not favored, and Zr4+ is prone to react with Ti4+ to yield ZrTiO4, thus explaining its excess formation in high TiO2containing compositions. Table 3 presents the Rietveld agreement factors, lattice parameter, occupancy, and selected bond lengths of ZrSiO4 determined from refinement. The lattice data of ZrSiO4 at both 1200 and 1400 °C exhibited a reducing trend, which is mainly due to the influence of TiO2 content in the compositions. The lattice contraction occurs only when the host lattice is subjected to a certain sort of compression stress. It is known that in the ZrSiO4 structure, Zr4+ possesses 8-fold coordination with oxygen atoms, and hence the replacement of the Zr4+ (0.84 Å) by the lower sized Ti4+ (0.74 Å) is expected to cause significant contraction of the lattice parameters. The refined occupancy factor also confirms the replacement Zr4+ by Ti4+ at the ZrSiO4 lattice. It was also determined that the average replacement of Zr4+ by Ti4+ in the ZrSiO4 lattice is around 9% for all the compositions irrespective of TiO2 content. Thus, the indication of TiO2 phase fraction in an increasing order is due to the oxidation of unoccupied Ti4+ as r-TiO2. Selected bond lengths of Zr−O and Zr−Si determined from refinement were closely examined to understand the effect of replacement of Zr4+ by Ti4+ in ZrSiO4, and the results displayed a reducing trend as a function of TiO2 additions. This is consistent with the previous reports on the reduction in Zr−O bond lengths due to doping with lower sized V4+.24 FT-IR spectra of TZS compositions heat treated at 1400 °C (Figure 9) display sharp transmittance bands at 437 and 611 cm−1, a broad band in the range of 800 to 1400 cm−1, and many shoulder bands detected in the region at 798, 890, 1022, and 1083 cm−1. The shoulder bands at 798 and 1202 cm−1 are

1400 °C c-axis

Ti4+occupancy in ZrSiO4

Zr−Si bond distance (Å)

Zr−O bond distance (Å)

5.9880(4) 5.9871(4) 5.9868(4) 5.9863(3) 5.9856(4) 5.9854(3)

8.30 7.25 7.27 5.02 8.72 9.24

2.9940 2.9935 2.9933 2.9932 2.9928 2.9928

2.1370 2.1310 2.1295 2.1271 2.1350 2.1270

Figure 9. FT-IR spectrum of different ZrO2−SiO2−TiO2 triphasic systems at 1400 °C.

assigned to the symmetric and asymmetric stretching of SiO4. The stretching vibration of the Si−O bond is assigned at 1022 cm−1. The sharp bands at 433 cm−1 are assigned for either the stretching mode of the Zr−O bond or the bending mode of O−Si−O bond. The band corresponding to the Zr−O−Si bond is observed at ∼970 cm−1 for 10TZS and disappears for other compositions. This might be due to its merging with the broad Si−O−Si bond. The band observed at 1083 cm−1, assigned to the stretching mode of the Si−O bond, is expected to shift from the actual 1022 cm−1 position because of SiO2 segregation at high temperatures.25,26 The formation of ZrSiO4 is confirmed from the bands at 1022, 890, 611, and 437 cm−1.27−29 TEM is a useful technique to gain information on morphology, crystal structure, and phase composition of a material. TEM analysis were performed for selected compositions (10TZS, 60TZS, and 100TZS) at 1400 °C to gather information about the influence of TiO2 on ZrSiO4 formation, and the obtained results are shown in Figure 10. Morphology of samples did not yield any significant information on the structures. High resolution (HR) images of the compositions are depicted in Figure 10a−c. Fringes with an average spacing value of 3.29 Å, corresponding to the (200) plane of ZrSiO4, were determined for all compositions. HR image of 10TZS displayed fringes with diverse orientation with varied d-spacing values, which match well with the (112) and (101) planes of 5497



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Figure 10. TEM images (a−c) correspond to HR images of 10TZS, 60TZS, and 100TZS; (d−f) correspond to SAED patterns of 10TZS, 60TZS, and 100TZS; (g−i) indicate the marked areas from panels b and c.

ZrSiO4 and the (110) plane of r-TiO2. HR image of 60TZS explores only the (200) plane that corresponds to ZrSiO4. However, an in-depth observation of these fringes indicates a small distortion in the (200) plane of ZrSiO4. Similar dislocations were also observed for the 100TZS composition. The SAED pattern (Figure 10d−f) displayed the presence of planes that correspond to ZrSiO4 and r-TiO2. A critical observation is also made from the TEM results which indicate the presence of the m-ZrO2 plane in 10TZS composition. It is important to note here that m-ZrO2 was not confirmed either from XRD or Raman results, which is a key factor in ZrSiO4 formation. A comprehensive view of the marked portions is indicated by Roman letters I, II, and III in Figure 10b,c, where the occurred dislocation in fringes are shown in Figure 10g−i.

impurities, and these impurities are prone to destroy the crystal structure of ZrSiO4 that results in its dissociation to individual m-ZrO2 and SiO2.22,30,31 The authors reported the unintentional formation of ZrSiO4 during their attempt to stabilize tZrO2 by the addition of SiO2 and rare earth elements.32,33 X-ray diffraction results confirmed the pivotal role of TiO2 on ZrSiO4 formation. The observation of t-ZrO2 with less crystalline features at 900 °C is mainly due to the presence of amorphous SiO2 that restricts the phase degradation of zirconia from tZrO2 to m-ZrO2. The peak shift in t-ZrO2 toward a higher diffraction angle as a function of TiO2 content at 900 °C suggests that ZrTiO4 formation is favored due to the occupancy of lower sized Ti4+ at the higher sized Zr4+. Moreover, the ZrSiO4 formation is also witnessed in the high TiO2 contained compositions at 900 °C by virtue of their observed corresponding X-ray reflections. A literature report reveals that the ZrSiO4 formation originates from the reaction between m-ZrO2 and SiO2 in which m-ZrO2 is formed from the phase transition of t-ZrO2.28 t-ZrO2 → m-ZrO2 phase transition induced by the dopant

4. DISCUSSION ZrSiO4 mineral that comes under mesosilicate groups is abundantly available in the Earth’s crust with high wear resistant features. However, the retrieval of pure ZrSiO4 from the crust possesses a minor level of uranium and thorium as 5498



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concentration is also reported.28,34 In the present study, the presence of excess TiO2 is expected to activate ZrSiO4 formation by inducing t-ZrO2 → m-ZrO2 transition followed by the initiation of the reaction between Zr4+ and Si4+. However, there has been no evidence of t-ZrO2 degradation noticed in XRD patterns obtained at 900 °C. Observation of ZrSiO4 crystallization at 1100 °C for high TiO2 contained compositions indicates that the reaction should have occurred between completely converted m-ZrO2 and SiO2. In the case of low TiO2 contained compositions, ZrO2 is retained in its tetragonal form, and hence the reaction kinetics becomes slow. The XRD pattern of the compositions at 1200 °C indicates the complete formation of ZrSiO4 along with the presence of rTiO2 and ZrTiO4 phase. The formation of crystalline ZrTiO4 above the annealing temperature of 600 °C is already reported.35 However, in the present work, the metastable tZrO2 phase is preserved in the amorphous SiO2 network, and for these reactions the ZrTiO4 formation is delayed. Moreover, due to the excess SiO2 in the amorphous state it is rather difficult to differentiate ZrTiO4 from t-ZrO2. The degradation of ZrTiO4 to yield m-ZrO2 and r-TiO2 is reported in the literature.36 The detection of ZrTiO4 in trace levels at 1400 °C enables us to infer information about its dissociation into individual m-ZrO2 and r-TiO2 followed by the reaction between m-ZrO2 and SiO2 to form ZrSiO4, while TiO2 crystallizes into its rutile form. The occupancy of Ti4+ at the Zr4+ sites of ZrSiO4 is confirmed from the refinement results by the observed contraction in the lattice parameter as a function of TiO2 content. These results are expected by the occupancy of lower sized Ti4+ for the higher sized Zr4+ in ZrSiO4 lattice. The refined occupancy factors clearly depict the occupancy of Ti4+ at the ZrSiO4 lattice by replacing Zr4+. The maximum capacity of ZrSiO4 to hold Ti4+ is determined as 9% from the refined occupancy factors, and beyond this limit TiO2 is crystallized into its rutile form. The contraction in bond distance of ZrSiO4 due to the replacement of higher sized Zr4+ by lower sized Ti4+ also corroborates with the above observations. The results from Raman analysis confirms the ZrSiO4 formation and also the crystallization of r-TiO2 upon its excess additions, which is also justified from enhanced intensity of Raman active mode of rTiO2. The red shift observed inν3 band of ZrSiO4 in different TZS compositions on enhanced TiO2 additions is mainly due to the occupancy of lower sized Ti4+ at ZrSiO4 lattice, and this infers the observed shift in Raman spectra. However, such shift observed until certain limit indicates the minimum occupancy level of Ti4+ at ZrSiO4 lattice. FTIR results confirmed the ZrSiO4 formation due to TiO2 additions by the observed transmittance bands at 1022, 890, 611, and 437 cm−1. Identification of m-ZrO2 from SAED pattern of 10TZS reveals that ZrSiO4 formation started to occur after t-ZrO2 → m-ZrO2 transition which has not been detected either from XRD or Raman techniques. A few authors observed that with lower dopant concentration, the stability of t-ZrO2 is sustained at high temperature in comparison to the high dopant content.37 Hence, the observation of the m-ZrO2 plane in 10TZS is due to the low TiO2 content present in the ZrO2− SiO2 system that enhances the thermal stability of t-ZrO2, whereas in other compositions the stability becomes minimized due to the excess TiO2 content. Considerable dislocation in fringes of the (200) planes of ZrSiO4 observed from HRTEM image of 60TZS and 100TZS indicates the negative crystal orientation that occurs due to the migration of Ti4+.

5. CONCLUSION The results from the investigation enunciated the concomitant effects exerted by TiO2 content and heat treatment temperature on the formation of ZrSiO4 from the ZrO2−SiO2−TiO2 triphasic systems. Metastable t-ZrO2 is retained as a major phase along with evidence of ZrSiO4 crystallization at 900 °C. Low level of TiO2 additions still retained t-ZrO2 as a major phase at 1100 °C along with the ZrSiO4 formation. A medium level of TiO2 additions articulated ZrSiO4 crystallization at 1100 °C along with the complete absence of t-ZrO2. ZrTiO4 as a major phase is detected for a high level of TiO2 additions besides the formation of minor additions of r-TiO2 and ZrSiO4 at 1100 °C. Heat treatments at 1200 and 1400 °C signaled the invariable crystallization of ZrSiO4 as a major phase, and structural analysis confirmed the Ti4+occupancy at the Zr4+ sites of ZrSiO4. The limit of Ti4+ occupancy at the ZrSiO4 lattice is determined as 9%, and excess Ti4+ gets precipitated in its rutile form at elevated temperatures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 0091-413-2654973. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial assistance received from DST-SERB [Reference: EMR/2015/002200 dated 20.01.2016] India is acknowledged. The facilities availed from Central Instrumentation Facility (CIF) of Pondicherry University are also acknowledged.

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