Glass and glass-ceramics along the SrTiO 3-NaPO 3 line

The xSrTiO3-(1−x)NaPO3 (x = 0−0.20 mol%) glasses were prepared by the conventional melt-quenching method. The amorphous state of the samples was verified by X-ray diffraction. The glass-ceramic materials were obtained by the well known thermal controlled crystallization process. It is found that several physical properties such as the density, molar volume, and the glass transition temperature depend strongly on the chemical composition. Vickers test on the glasses showed that the micro-hardness increases with the SrTiO3 content. The structural approach of the glasses was realized by IR spectroscopy. This technique has highlighted the co-existence of different phosphate and titanium structural units in the glassy-matrix. Crystallization of the glasses was enhanced by heat treatments and followed by X-ray diffraction. A mechanism for this glass crystallization was proposed.


INTRODUCTION
Phosphate glasses are used in various applications such as bone transplantation, glass to metal seals, confining radioactive wastes, fast ion conductors, laser host materials, etc. . .[1].
Strontium titanate SrTiO 3 is considered a promising material for tunable microwave applications due to its high dielectric constant, high dependence of dielectric constant on the applied field, and low losses [2].
In the present work, our interest is focused on studying phosphate glasses inside the NaPO 3 -SrTiO 3 system for the first time.The experimental requirements for the synthesis were determined and the characterization by density measurements, thermal analysis, Vickers test and infrared spectroscopy were realized.

EXPERIMENTAL PROCEDURES
The (1−x)NaPO 3 -xSrTiO 3 glasses with 0 ≤ x ≤ 0.2 mol% were prepared from appropriate contents of vitreous NaPO 3 and SrTiO 3 phase.The glasses were prepared by the conventional melt-quenching route.The amorphous state of samples was verified by X-ray diffraction using a Philips X'Pert apparatus equipped with a CuK α X-ray source and a Ni filter (λ Cu =1.5406 Å).
The crystallization process is realized by submitting the glass samples to heat treatments at 600 • C in air for 3 hours.
The density measurements (D) of glasses are determined by the Archimedes method using diethyl orthophthalate as a fluid.Molar volume (V m ) of each glass is derived from the density.
DTA runs were carried out for ground glass batches of about 50 mg in nitrogen atmosphere at a heating rate of 10 • C min −1 using a Perkin Elmer DTA-7 analyzer.a e-mail: bihlahcen@yahoo.fr(L.Bih).
Vickers micro-hardness values were determined at room temperature in air using a Leitz instrument.Polished samples were subjected to a load of 100 g at an indentation time of 25 s.
IR spectra of the glass-ceramics are carried out by means of KBr pellets in the frequency range 1400-400 cm −1 using a FTIR TENSOR27 spectrometer.

XRD analysis
The width of the glassy region inside the system NaPO 3 -SrTiO 3 was checked by X-ray diffraction analysis and the transparency of the samples.The homogeneous glasses could be elaborated up to 20 mol% of SrTiO 3 .XRD patterns for different samples in the range of compositions (0 < x < 0.20) show only broad hallo around 25-35 • (2θ values) which confirm the glassy nature of these samples.

Density and molar volume
Results of the density measurements and the calculated molar volume of the studied glasses xSrTiO 3 -(1−x)NaPO 3 (x = 0-0.20 mol%) are represented at Fig. 1 and gathered in Table 1.One can observe that the density and the molar volume increases and decreases, respectively, with the SrTiO 3 content in the glass.The substitution of NaPO 3 by SrTiO 3 is accompanied with an increasing in molecular mass that's why probably the density variation shows the increasing trend with SrTiO 3 content.A decrease of the molar volume versus composition parameter (x) could not be interpreted in terms decreasing of the mean ionic radius related to the substitution of (Na + + P 5+ ) by (Sr 2+ + Ti 4+ ) since r(Ti 4+ ) = 0.61 Å, r(Sr 2+ ) = 1.18 Å, r(Na + ) = 1.02Å and r(P 5+ ) = 0.17 Å [3].However, the observed decrease of the molar volume versus (xSrTiO 3 ) could be associated to the decreasing number of non-bridging oxygen atoms.In other words, this decrease is due to the decrease of bond length or interatomic spacing among the atoms of glass network which causes closing of the structure in agreement with the fact that when non-bridging oxygen (NBO) is converted to bridging oxygen (BO) the average M-O distance decreases [4][5][6].Moreover, the variation V=f(x) suggests the formation of more covalent Sr-O and Ti-O bonds with short lengths, instead of ionic Na-O bonds, which reticules the phosphate network and lead to the close structure of the glasses [7].

DTA analysis and Microhardness (H v )
Table 1 shows the value of the glass transition temperature (T g ) of each glass.Figure 2 represents the variation of the glass transition temperature (T g ) as a function of the SrTiO 3 content.The substitution of NaPO 3 for SrTiO 3 leads to a large increase of T g in the glass and shows that the structure is strongly strengthened.We assume that in this case, when strontium and titanium cations substitute sodium ions, -P-O. . .Sr and -P-O. . .Ti more covalent bonds are formed instead of ionic Na-O links (the Sr and Ti electronegativities being larger than that of Na).This means that the glasses will show high glass transition temperature [7].
Values of microhardness, H v , for the glasses xSrTiO 3 − (1 − x)NaPO 3 are listed in Table 1.It is seen that H v increases with the increase of titanium content.According to the chemical composition of the studied glasses, one can consider that P-O-P, P-O-Ti, Ti-O-Ti are the principal covalent bonds which determine the cohesive energies  of the network [8].Therefore, the observed increase of H v with SrTiO 3 could be explained by the formation of P-O-Ti and/or Ti-O-Ti linkages in their glassy-matrix.According to the composition it is observed that mainly metaphosphates and pyrophosphates, Sr 3 P 4 O 13 and Na 2 TiO 3 are the crystallized phases.When the glass composition contains large amount of SrTiO 3 (x = 0.1 and 0.2), the ratio of the perovskite Na 2 TiO 3 phase increases and the Sr 3 P 4 O 13 phase transforms to Sr 2 P 2 O 7 and Sr(PO 3 ) 2 phases.

IR spectra
The IR spectra of the xSrTiO 3 -(1 − x)NaPO 3 glasses are shown in Fig. 4.
The absorption bands of the given materials can be assigned as follows: (i) a band at (1270 cm −1 ) is due to the asymmetric stretch vibrations (P=O) [8] and the band lying in the range 1200-1170 cm −1 is related 04013-p.2 to the stretching vibration of the (PO 2 ) − group [9,10], this band is intensity decreasing when NaPO 3 content decreases; (ii) the intensity of the band at ≈750 cm −1 , attributed to a stretching vibration of P-O-P bridges decreases with increasing SrTiO 3 content [9,10], but with further introduction of SrTiO 3 , this band is replaced by two new bands one at 735 cm −1 and another at 624 cm −1 .These new bands are assigned respectively to TiO 6 and TiO 4 structural units [11,12]; (iii) the bands at 1070 and 1116 cm −1 present in all the spectra are assigned to high and low frequency components of the asymmetric stretch of the (PO 3 ) 2− terminal group, respectively [10].The increase of SrTiO 3 content induces the appearance of a broad band near 990-1038 cm −1 which is attributed to stretching vibration of PO 4 [10].The broad bands around 542 cm −1 and 430 cm −1 are associated with the bending vibrations of basic structural units of the network [8].

CONCLUSION
The glasses inside the xSrTiO 3 -(1 − x)NaPO 3 system were prepared by melt-quenching route and their physical properties (density, Tg, H v ) increase with SrTiO 3 ratio.IR spectra indicate the formation of different phosphate units along with TiO 4 and TiO 6 groups in the glasses.The crystallization of the glasses is performed by heat treatments and the obtained glass-ceramics contain sodium titanate Na 2 TiO 3 (perovskite structure) phase rather than SrTiO 3 phase.

Figure 3
Figure 3 shows the obtained XRD patterns for the crystallized xSrTiO 3 -(1 − x)NaPO 3 samples.According to the composition it is observed that mainly metaphosphates and pyrophosphates, Sr 3 P 4 O 13 and Na 2 TiO 3 are the crystallized phases.When the glass composition contains large amount of SrTiO 3 (x = 0.1 and 0.2), the ratio of the perovskite Na 2 TiO 3 phase increases and the Sr 3 P 4 O 13 phase transforms to Sr 2 P 2 O 7 and Sr(PO 3 ) 2 phases.

Table 1 .
Some physical properties of the glasses.