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Raman signatures of intermediate phase in quaternary Ge15Te80 − xIn5Agx glasses
Journal of Non-Crystalline Solids 387 (2014) 143–147
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Raman signatures of intermediate phase in quaternary Ge15Te80 − xIn5Agx glasses G. Sreevidya Varma a, D.V.S. Muthu b, A.K. Sood b, S. Asokan a,⁎ a b
Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore- 560 012, India Department of Physics, Indian Institute of Science, Bangalore- 560 012, India
a r t i c l e
i n f o
Article history: Received 19 November 2013 Received in revised form 31 December 2013 Available online 24 January 2014 Keywords: Chalcogenide glasses; Intermediate phase; Reversibility window; Chemical threshold
a b s t r a c t Micro-Raman studies are conducted on as-quenched and annealed Ge15Te80 − xIn5Agx glasses to probe the structural network and its evolution with composition. These studies reveal the presence of tetrahedral GeTe4 structural units in as-quenched samples. Specific signatures of the intermediate phase (IP) are observed in the composition dependence of Raman frequencies and corresponding intensities of different modes in the composition range, 8 ≤ x ≤ 16. In addition, the Raman peak positions are found to shift with silver doping. Apart from the Raman results, the compositional dependence of density, molar volume and thermal diffusivity, observed in the present study, confirms the presence of the intermediate phase. In thermally annealed samples, a unique variation of Raman wave-numbers in the intermediate region is observed due to the retention of some of the local structure even after the sample is crystallized. The observed Raman peaks are attributed to crystalline tellurium and silver lattice vibrational modes. Based on our present and earlier studies, we propose the occurrence of three thresholds in Ge15Te80 − xIn5Agx glasses, namely percolation of rigidity, percolation of stress and the onset of chemical phase separation on a nanoscale at 8%, 16% and 20% of silver concentration respectively. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Systematic studies of local structure of amorphous chalcogenide glasses have led to the development of newer phase change alloys with desirable properties. The earliest known attempt to understand local atomic structure of chalcogenide network glasses is the continuous random network (CRN) model [1]. The random covalent network (RCN) model based on the hierarchy of bonds, the chemically ordered covalent network (COCN) model favoring heteropolar bonds and the topological threshold model which explains the evolution of the glassy network with composition, are derived from the CRN model [2,3]. Considering the short-range order and using the constraint counting analysis, Philips and Thorpe predicted that the RCN structure has maximum stability at a critical average co-ordination number b rc N = 2.4 called the rigidity percolation threshold (RPT) or the stiffness threshold (ST), at which the glassy system undergoes a percolative transition from a floppy polymeric glass to a rigid amorphous solid; in the last three decades, the signature of RPT has been observed in a variety of properties in many different chalcogenide systems [4–10]. Based on rigorous experimental and theoretical investigations, it has been conclusively shown by Boolchand et al. [11–13] that certain glassy systems exhibit rigidity percolation over a range of compositions with the presence of ⁎ Corresponding author. Tel.: + 91 80 22933195, + 91 80 22932271; fax: + 91 80 23608686. E-mail address: [email protected] (S. Asokan). 0022-3093/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.006
an isostatically rigid phase in between floppy polymeric glass and stressed rigid phase, which is known as the intermediate phase (IP). In systems exhibiting an extended stiffness transition, two elastic phase transitions occur, namely a “rigidity transition” followed by a “stress transition”. The optimally constrained glass compositions in the intermediate phase exhibit remarkable properties, including the near vanishing of non-reversing enthalpy at glass transition which suggests that these glasses are configurationally closer to their liquid counterparts [11,13–15]. The glasses in the intermediate phase, referred to as “reversibility window” are supposedly non-ageing, stress free and selforganized nano-structured functional materials optimized by nature [16]. The recent electrical switching studies reveal that there is strong correlation between the reversibility window and the electrical switching behavior of chalcogenide glasses [4–7,17,18]. Further, the width of intermediate phase indicates the structural variability, i.e., the variation in bonding patterns in the system [19]. In addition to stiffness and extended stiffness transitions, it is also possible to observe the effects of chemical ordering in chalcogenide glasses. According to COCN model, there exists a chemically ordered glass consisting of only heteropolar bonds and the corresponding composition is referred as chemical threshold (CT) of the glassy system [6,20]. Introduction of silver in chalcogenide glasses increases their crystallization rate and electrical conductivity by several orders of magnitude, which are important from the view point of phase change memory
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applications [21–23]. In this work, the structural changes in amorphous Ge15Te80 − xIn5Agx glasses with composition, have been studied in the range x = 2 to x = 24. The structural studies have also been carried after thermally annealing the samples to understand the crystallization processes. In addition, measurements have been made on the thermal diffusivity, density and molar volume of Ge15Te80 − xIn5Agx glasses of different compositions. The main motivation behind these studies is to understand the manifestation of stiffness/extended stiffness transition on structural, thermal and other properties of these glasses. It is interesting to note in this context that binary GexTe100 − x glasses exhibit a sharp rigidity transition at the composition, x = 20 and the CT at x = 33 [5]. However, ternary Ge15Te85 − xInx glasses show an extended rigidity transition, with the intermediate phase (IP) observed in the composition range x = 3 to 7; these glasses also show the effect of the CT at x = 9 [24–26]. The recent alternating differential scanning calorimetric (ADSC) studies on Ge15Te80 − xIn5Agx glasses reveal an extended stiffness transition in this system, with the reversibility window seen in the composition range x = 8 to16; it is also proposed that there is a possible coinciding of CT with the upper boundary of the IP at x = 16 [27].
2. Experimental Bulk Ge15Te80 − xIn5Agx glasses (2 ≤ x ≤ 24), where x denotes the atom %, are prepared using the melt quenching technique. High purity elements of Ge, Te, In and Ag are taken in appropriate quantities in batch sizes of 1 g in a quartz ampoule of 5 mm diameter and are sealed at 10−6 Torr. The sealed ampoules are kept in a horizontal rotary furnace and the temperature is increased in steps of 100 °C/h till it reaches 1040 °C. The ampoule is rotated in the furnace for 30 h and quenched in ice water and NaOH mixture. XRD studies are performed on bulk samples to confirm their amorphous nature using a Bruker powder diffractometer with CuKα radiation by scanning 2θ from 10° to 90° at a rate of 3°/min. The samples are also annealed for 2 h in ampoules sealed at 10−6 Torr at their respective crystallization temperatures to understand the crystallization phases. Micro-Raman studies have been carried out using a Horiba Jobin Yvon (LabRAM HR) laser Raman Spectrometer in the back scattering mode. The backscattered light is analyzed using a triple monochromator and detected by a CCD cooled at − 70 °C. The sample is illuminated by the 514.5 nm line of an argon ion laser focused using a 100 × objective. The spectral resolution for the recorded Stokes-side Raman is ~ 0.6 cm− 1. The calibration of the wave-number scale to take into account possible shifts of the monochromator, has been performed by measuring the silicon spectra. All the spectra have been recorded using less than 1 mW of laser power in order to avoid undesired irradiation-induced heating and to avoid the formation of secondary products related to photo-decomposition in silver alloys. The acquisition time of about 60 s has been used to obtain a good signal to noise ratio. Thermal diffusivity is measured using a custom built photo thermal deflection setup [28]. This consists of a probe beam (632.8 nm He–Ne laser at 5 mW), pump beam (514.5 nm Ar-ion laser at 12 mW) which is intensity modulated using a mechanical chopper, a quartz cuvette filled with CCl4 liquid in which samples of thickness 0.42 mm are placed. The pump beam falls normally on the surface of the sample. The probe beam skimming the sample surface is detected using a position sensitive detector. In the present experiments, the chopping frequency is kept in the range 0–200 Hz due to the better penetration depth of the thermal waves into the sample at lower frequencies. The thermal diffusivity (α) is determined from the slope of the plot of logarithm of the signal amplitude and the square root of frequency from the relation, α = π (l/slope) 2, where l is the thickness of the sample. The experiments are conducted on a minimum of three specimens for each composition and the mean value is reported.
The density is measured by Archimedes principle using ethanol as the liquid medium at a constant temperature using a Mettler Toledo microbalance for three samples per composition and average values are plotted. Molar volume (Mv) is estimated from density (D) and molecular weight (M) of each sample by the formula Mv = M/D. 3. Results and discussions Raman spectra of representative as-quenched Ge15Te80 − xIn5Agx samples are shown in Fig. 1(a). Fig. 1(b) shows the Raman spectrum of a representative as-quenched Ge15Te64In5Ag16 sample, after peak fitting (Gaussian), indicating five bands in the spectrum in the wavenumber range 50–300 cm−1. The approximate peak positions are 68, 99, 127, 158 and 206 cm−1. It is interesting to note here that the earlier work on the room temperature Raman spectrum of a-GeTe, has shown peaks at ~ 65, ~ 88, ~111, ~ 127, ~145, 162, ~ 182 and ~ 217 cm−1 [29]. Based on this, the assignment of peaks in the present Raman spectra of the present Ge15Te80 − xIn5Agx samples can be assigned to different vibrations of GeTe4 tetrahedra as follows: a) Peak at 68 cm−1 to symmetric bending mode, υ4(F2)]; b) Peak at 127 cm−1 to υ1(A1), n (n = 1,2 corner sharing) symmetric stretching mode; c) Peak at 158 cm− 1 to υ1(A1), n (n = 0, edge sharing) symmetric stretching mode; d) Peak at 206 cm−1 peak to υ3 (F2) anti symmetric modes. The shift in the corresponding peak positions observed in the present study can be attributed to silver addition in Ge15Te80 − xIn5Agx glasses. The variation of Raman modes and corresponding intensities relative to 158 cm−1 mode with composition for amorphous as-
Fig. 1. (a): Raman spectra of representative as-quenched Ge15Te80 − xIn5Agx samples. (b): Raman spectrum of a representative as-quenched Ge15Te64In5Ag16 sample, after Gaussian peak fitting. Correlation coefficient is 0.99892.
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Fig. 2. (a): The variation of Raman modes with composition (spline connected) for amorphous as-quenched Ge15Te80 − xIn5Agx glasses. (b): The variation of relative intensities of different modes relative to 158 cm−1 mode with composition (spline connected) for amorphous as-quenched Ge15Te80 − xIn5Agx glasses.
quenched Ge15Te80 − xIn5Agx glasses are given in Fig. 2(a) and (b) respectively. Here, we can observe a clear signature of intermediate phase in the composition range 8 ≤ x ≤ 16 as depicted in our ADSC studies reported earlier [24]. A trough is seen in the composition dependence of Raman modes, in the reversibility window. It may be noted here that a correlation between the Raman modes and domains of IP has been clearly demonstrated by Boolchand et al. [16]. It has been shown that the mode-wave-number variation of corner sharing (CS) and edge sharing (ES) tetrahedral units display thresholds opening an intermediate phase which correlates well with the reversibility window in specially prepared homogeneous GexSe100 − x glasses [30]. Further, the relative increase in intensity of the peak at 99 cm−1and 127 cm−1 with silver concentration is due to the silver lattice vibrational modes. The silver lattice vibrational modes observed in different silver compounds are ~95 cm−1 and ~146 cm−1 respectively [31]. Fig. 3(a) shows the micro-Raman spectra of representative Ge15Te80 − xIn5Agx glasses after annealing at their respective crystallization temperatures and Fig. 3(b) shows the peak fit for a representative annealed Ge15Te64In5Ag16 glass. The Lorentzian peak fitting of the spectra of annealed Ge15Te64In5Ag16 samples shows peaks at 67, 93, 102, 122, 134, 141, 164, 183, 198, 224, 244 and 271 cm− 1. These modes match well with the Raman spectrum of Ge15Te85 − xIn5 glasses as reported earlier [24]. It is seen that the intensities of peaks at 67, 93, 141, 164, 198, 244 and 271 cm−1 increase with silver addition; especially, a significant increase in intensity at 93 and 141 cm−1 is observed. The peaks 93, 122 and 141 cm−1 are attributed to crystalline tellurium [32–34].
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Fig. 3. (a): Raman spectra of representative annealed Ge15Te80 − xIn5Agx samples. (b): Raman spectrum of a representative annealed Ge15Te64In5Ag16 sample, after peak Lorentzian fitting; correlation coefficient is 0.99871.
The X-ray diffraction studies on annealed samples reveal the presence of c-Ag8GeTe6 and AgTe crystalline phases and their intensity increase with silver addition [27]. When tellurium is progressively replaced by silver in Ge15Te80 − xIn5Agx glasses, the observed increase in intensity at 93 and 141 cm−1 can be due to an increase in silver lattice vibration modes.
Fig. 4. The composition dependence of Raman modes with silver composition (spline connected) in Ge15Te80 − xIn5Agx glasses annealed at their respective crystallization temperatures.
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Fig. 4 shows the variation of Raman modes with composition in annealed samples. Even though the topological thresholds are very much confined to amorphous samples, we can clearly see the unique variation of Raman wave-numbers in the intermediate region. This can be due to the retention of some of the local structure even after the sample is crystallized. Comparison of representative Raman spectra of as-quenched and annealed Ge15Te64In5Ag16 glasses is given in Fig. 5. For as-quenched Ge15Te64In5Ag16 glasses, the width and intensity of the most prominent peak centered at 158 cm−1 are 23 cm−1 and 237 arbitr. units respectively. For thermally annealed Ge15Te64In5Ag16 samples, the most prominent peak is centered at 122 cm−1 having width and intensity 8 cm−1 and 858 arbitr. units respectively. The spectra of all amorphous samples (Fig. 1(a)) are broad with the prominent peak at ~ 158 cm−1 which resemble a-Te peaks (156 cm−1) and of less intensity; the shift in the peak positions are due to the replacement of Te–Te bonds by Ag–Te bonds. On the other hand, the spectra of all annealed samples (Fig. 3(a)) are sharp and intense which resemble c-Te with prominent peak at ~ 122 cm− 1. As the silver concentration increases beyond x = 16, the Chemical Threshold, silver phase separation occurs, leading to the increase in intensity of peaks at 93 cm−1 and 141 cm−1. The variation of density and molar volume with the silver composition as well as average coordination number (brN) in Ge15Te80 − xIn5Agx glasses are shown in Fig. 6. The standard deviation in density determination is found to be less than 0.03 gcm−3. The molar volume decreases and density increases with silver addition, with a plateau exhibited in both the parameters, in the composition range x = 8 to 16, which is the reversibility window identified on the basis of ADSC studies, which is also confirmed by the present Raman scattering investigations. It has been realized before that the intermediate phase forms efficiently packed space filling networks with low molar volumes [12]. Fig. 7 depicts the variation of thermal diffusivity, α with composition and the inset figure shows the variation of PTD signal amplitude with square root of modulation frequency, f. The standard deviation is found to be less than 0.0065 cm2s−1. The α increases with composition initially. It exhibits a minimum around x = 12. The thermal diffusivity is a parameter, important not only due to its physical interest but also due to its technological applications like device modeling and design. The reciprocal of α is a measure of the time required to establish thermal equilibrium in a given material. It depends on the compositional and micro-structural variables [35]. It is very sensitive to the structure of solids [36]. It is also used as a tool to study imperfections, dislocations and voids as the carrier mean free path is affected by lattice defects [37]. In general, the thermal diffusivity, α, increases with the increase in average co-ordination number and increasing structural rigidity, due to the decrease in floppy modes which scatter the thermal waves as mechanical threshold is approached [38]. A maximum is seen in α in many glassy systems which exhibit a sharp stiffness transition [35,39–42].
Fig. 5. Comparison of Raman shift of as-quenched and annealed Ge15Te64In5Ag16 glasses.
Fig. 6. The variation of density and molar volume with the silver composition as well as mean coordination number brN (B-spline connected) in Ge15Te80 − xIn5Agx glasses.
Further, the thermal diffusivity experiments on As-Te–Ga glasses which exhibit an extended rigidity transition indicate that α, after the initial increase, starts decreasing at the onset of extended stiffness transition; the decrease in α leads to a local minima at completion [38]. Further, a maximum is seen in the thermal diffusivity of As–Te–Ga glasses, at the chemical threshold, which is understood on the basis of the minimal scattering of the thermal waves at this composition [38,39]. It is interesting to note that in the present Ge15Te80 − xIn5Agx glasses also, there is an initial increase in thermal diffusivity with the average coordination number. Around x = 6, there is a maximum in thermal diffusivity, followed by a broad minimum around x = 12, the centroid composition of the intermediate phase. There is an increase in α beyond x = 12, leading to a maximum at the composition x = 14. The observed effects in the thermal diffusivity above x = 6 can be connected with the intermediate phase occurring in the range 8 ≤ x ≤ 16 seen in earlier ADSC and the present Raman scattering, density and molar volume experiments. However, there is a marginal shift in the boundaries of the intermediate phase and non observance of a minimum in the composition corresponding to completion of stiffness transition, in the thermal diffusivity measurements. Though the reason for shifting of the lower boundary is not clear, the occurrence of the minimum at the completion could have been masked by the possible chemical threshold which coincides with the upper boundary at x = 16. The decrease in α at compositions above x = 14 is due to the presence of additional vibrational modes characteristic of a rigid elastic
Fig. 7. The variation of thermal diffusivity with the silver composition and as well as brN in Ge15Te80 − xIn5Agx glasses; the inset figure shows the variation of PTD signal amplitude with square root of modulation frequency, f; the black solid line in the figure is a guide to the eye.
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namely percolation of rigidity, percolation of stress and the onset of chemical phase separation on a nanoscale at 8%, 16% and 20% of silver concentration respectively.
References
Fig. 8. The elastic phases of Ge15Te80 − xIn5Agx glasses in terms of their topology.
network, scattering away the thermal waves, resulting in reduction in α [43]. The recent studies on the compositional variation of physical properties in binary GexSe100 − x [30] and ternary GexSixTe100 − 2x [44] glasses indicate the occurrence of three thresholds: percolation of rigidity, percolation of stress and the onset of chemical phase separation on a nanoscale. Based on [44], and from the present micro Raman, thermal diffusivity, molar volume and the earlier thermal studies [27], the schematic representation of evolution of glass structure, in terms of topology in the Ge15Te80 − xIn5Agx glass system is depicted in Fig. 8; When silver is added to Ge15Te80In5 base glass (a composition in the IP), glasses become flexible at first as the coordination number of Ag is lesser than In. As the silver concentration increases from 8% to 16%, opening of an IP is evidenced. From 16% to 20%, a stressed rigid phase and from 20% and above, nanoscale phase separation is observed. As said in literature, [45] the width of the IP increases in multi-component glasses, with the number of components. 4. Conclusions Micro-Raman studies on amorphous Ge15Te80 − xIn5Agx glasses yield peaks at ~ 68, ~ 99, ~ 127, ~ 158 and ~ 217 cm−1 suggesting the presence of tetrahedrally bonded GeTe4 units. The compositional variation of Raman modes and corresponding intensities mark the opening of an “intermediate phase” that coincides well with the “reversibility window” seen in earlier ADSC studies. The micro-Raman spectra of annealed samples reveal the presence of c-Te and silver lattice vibrational modes. The increase in the intensity of peaks at 93 and 141 cm−1 with silver addition suggests an increase in silver lattice vibrational modes. Further, the compositional variation of density, molar volume and thermal diffusivity confirm the opening of the IP in the composition range 8 ≤ x ≤ 16. Based on our present and earlier studies, the occurrence of three thresholds in Ge15Te80 − xIn5Agx glasses is proposed,
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Raman signatures of intermediate phase in quaternary Ge15Te80 − xIn5Agx glasses G. Sreevidya Varma a, D.V.S. Muthu b, A.K. Sood b, S. Asokan a,⁎ a b
Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore- 560 012, India Department of Physics, Indian Institute of Science, Bangalore- 560 012, India
a r t i c l e
i n f o
Article history: Received 19 November 2013 Received in revised form 31 December 2013 Available online 24 January 2014 Keywords: Chalcogenide glasses; Intermediate phase; Reversibility window; Chemical threshold
a b s t r a c t Micro-Raman studies are conducted on as-quenched and annealed Ge15Te80 − xIn5Agx glasses to probe the structural network and its evolution with composition. These studies reveal the presence of tetrahedral GeTe4 structural units in as-quenched samples. Specific signatures of the intermediate phase (IP) are observed in the composition dependence of Raman frequencies and corresponding intensities of different modes in the composition range, 8 ≤ x ≤ 16. In addition, the Raman peak positions are found to shift with silver doping. Apart from the Raman results, the compositional dependence of density, molar volume and thermal diffusivity, observed in the present study, confirms the presence of the intermediate phase. In thermally annealed samples, a unique variation of Raman wave-numbers in the intermediate region is observed due to the retention of some of the local structure even after the sample is crystallized. The observed Raman peaks are attributed to crystalline tellurium and silver lattice vibrational modes. Based on our present and earlier studies, we propose the occurrence of three thresholds in Ge15Te80 − xIn5Agx glasses, namely percolation of rigidity, percolation of stress and the onset of chemical phase separation on a nanoscale at 8%, 16% and 20% of silver concentration respectively. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Systematic studies of local structure of amorphous chalcogenide glasses have led to the development of newer phase change alloys with desirable properties. The earliest known attempt to understand local atomic structure of chalcogenide network glasses is the continuous random network (CRN) model [1]. The random covalent network (RCN) model based on the hierarchy of bonds, the chemically ordered covalent network (COCN) model favoring heteropolar bonds and the topological threshold model which explains the evolution of the glassy network with composition, are derived from the CRN model [2,3]. Considering the short-range order and using the constraint counting analysis, Philips and Thorpe predicted that the RCN structure has maximum stability at a critical average co-ordination number b rc N = 2.4 called the rigidity percolation threshold (RPT) or the stiffness threshold (ST), at which the glassy system undergoes a percolative transition from a floppy polymeric glass to a rigid amorphous solid; in the last three decades, the signature of RPT has been observed in a variety of properties in many different chalcogenide systems [4–10]. Based on rigorous experimental and theoretical investigations, it has been conclusively shown by Boolchand et al. [11–13] that certain glassy systems exhibit rigidity percolation over a range of compositions with the presence of ⁎ Corresponding author. Tel.: + 91 80 22933195, + 91 80 22932271; fax: + 91 80 23608686. E-mail address: [email protected] (S. Asokan). 0022-3093/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2014.01.006
an isostatically rigid phase in between floppy polymeric glass and stressed rigid phase, which is known as the intermediate phase (IP). In systems exhibiting an extended stiffness transition, two elastic phase transitions occur, namely a “rigidity transition” followed by a “stress transition”. The optimally constrained glass compositions in the intermediate phase exhibit remarkable properties, including the near vanishing of non-reversing enthalpy at glass transition which suggests that these glasses are configurationally closer to their liquid counterparts [11,13–15]. The glasses in the intermediate phase, referred to as “reversibility window” are supposedly non-ageing, stress free and selforganized nano-structured functional materials optimized by nature [16]. The recent electrical switching studies reveal that there is strong correlation between the reversibility window and the electrical switching behavior of chalcogenide glasses [4–7,17,18]. Further, the width of intermediate phase indicates the structural variability, i.e., the variation in bonding patterns in the system [19]. In addition to stiffness and extended stiffness transitions, it is also possible to observe the effects of chemical ordering in chalcogenide glasses. According to COCN model, there exists a chemically ordered glass consisting of only heteropolar bonds and the corresponding composition is referred as chemical threshold (CT) of the glassy system [6,20]. Introduction of silver in chalcogenide glasses increases their crystallization rate and electrical conductivity by several orders of magnitude, which are important from the view point of phase change memory
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applications [21–23]. In this work, the structural changes in amorphous Ge15Te80 − xIn5Agx glasses with composition, have been studied in the range x = 2 to x = 24. The structural studies have also been carried after thermally annealing the samples to understand the crystallization processes. In addition, measurements have been made on the thermal diffusivity, density and molar volume of Ge15Te80 − xIn5Agx glasses of different compositions. The main motivation behind these studies is to understand the manifestation of stiffness/extended stiffness transition on structural, thermal and other properties of these glasses. It is interesting to note in this context that binary GexTe100 − x glasses exhibit a sharp rigidity transition at the composition, x = 20 and the CT at x = 33 [5]. However, ternary Ge15Te85 − xInx glasses show an extended rigidity transition, with the intermediate phase (IP) observed in the composition range x = 3 to 7; these glasses also show the effect of the CT at x = 9 [24–26]. The recent alternating differential scanning calorimetric (ADSC) studies on Ge15Te80 − xIn5Agx glasses reveal an extended stiffness transition in this system, with the reversibility window seen in the composition range x = 8 to16; it is also proposed that there is a possible coinciding of CT with the upper boundary of the IP at x = 16 [27].
2. Experimental Bulk Ge15Te80 − xIn5Agx glasses (2 ≤ x ≤ 24), where x denotes the atom %, are prepared using the melt quenching technique. High purity elements of Ge, Te, In and Ag are taken in appropriate quantities in batch sizes of 1 g in a quartz ampoule of 5 mm diameter and are sealed at 10−6 Torr. The sealed ampoules are kept in a horizontal rotary furnace and the temperature is increased in steps of 100 °C/h till it reaches 1040 °C. The ampoule is rotated in the furnace for 30 h and quenched in ice water and NaOH mixture. XRD studies are performed on bulk samples to confirm their amorphous nature using a Bruker powder diffractometer with CuKα radiation by scanning 2θ from 10° to 90° at a rate of 3°/min. The samples are also annealed for 2 h in ampoules sealed at 10−6 Torr at their respective crystallization temperatures to understand the crystallization phases. Micro-Raman studies have been carried out using a Horiba Jobin Yvon (LabRAM HR) laser Raman Spectrometer in the back scattering mode. The backscattered light is analyzed using a triple monochromator and detected by a CCD cooled at − 70 °C. The sample is illuminated by the 514.5 nm line of an argon ion laser focused using a 100 × objective. The spectral resolution for the recorded Stokes-side Raman is ~ 0.6 cm− 1. The calibration of the wave-number scale to take into account possible shifts of the monochromator, has been performed by measuring the silicon spectra. All the spectra have been recorded using less than 1 mW of laser power in order to avoid undesired irradiation-induced heating and to avoid the formation of secondary products related to photo-decomposition in silver alloys. The acquisition time of about 60 s has been used to obtain a good signal to noise ratio. Thermal diffusivity is measured using a custom built photo thermal deflection setup [28]. This consists of a probe beam (632.8 nm He–Ne laser at 5 mW), pump beam (514.5 nm Ar-ion laser at 12 mW) which is intensity modulated using a mechanical chopper, a quartz cuvette filled with CCl4 liquid in which samples of thickness 0.42 mm are placed. The pump beam falls normally on the surface of the sample. The probe beam skimming the sample surface is detected using a position sensitive detector. In the present experiments, the chopping frequency is kept in the range 0–200 Hz due to the better penetration depth of the thermal waves into the sample at lower frequencies. The thermal diffusivity (α) is determined from the slope of the plot of logarithm of the signal amplitude and the square root of frequency from the relation, α = π (l/slope) 2, where l is the thickness of the sample. The experiments are conducted on a minimum of three specimens for each composition and the mean value is reported.
The density is measured by Archimedes principle using ethanol as the liquid medium at a constant temperature using a Mettler Toledo microbalance for three samples per composition and average values are plotted. Molar volume (Mv) is estimated from density (D) and molecular weight (M) of each sample by the formula Mv = M/D. 3. Results and discussions Raman spectra of representative as-quenched Ge15Te80 − xIn5Agx samples are shown in Fig. 1(a). Fig. 1(b) shows the Raman spectrum of a representative as-quenched Ge15Te64In5Ag16 sample, after peak fitting (Gaussian), indicating five bands in the spectrum in the wavenumber range 50–300 cm−1. The approximate peak positions are 68, 99, 127, 158 and 206 cm−1. It is interesting to note here that the earlier work on the room temperature Raman spectrum of a-GeTe, has shown peaks at ~ 65, ~ 88, ~111, ~ 127, ~145, 162, ~ 182 and ~ 217 cm−1 [29]. Based on this, the assignment of peaks in the present Raman spectra of the present Ge15Te80 − xIn5Agx samples can be assigned to different vibrations of GeTe4 tetrahedra as follows: a) Peak at 68 cm−1 to symmetric bending mode, υ4(F2)]; b) Peak at 127 cm−1 to υ1(A1), n (n = 1,2 corner sharing) symmetric stretching mode; c) Peak at 158 cm− 1 to υ1(A1), n (n = 0, edge sharing) symmetric stretching mode; d) Peak at 206 cm−1 peak to υ3 (F2) anti symmetric modes. The shift in the corresponding peak positions observed in the present study can be attributed to silver addition in Ge15Te80 − xIn5Agx glasses. The variation of Raman modes and corresponding intensities relative to 158 cm−1 mode with composition for amorphous as-
Fig. 1. (a): Raman spectra of representative as-quenched Ge15Te80 − xIn5Agx samples. (b): Raman spectrum of a representative as-quenched Ge15Te64In5Ag16 sample, after Gaussian peak fitting. Correlation coefficient is 0.99892.
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Fig. 2. (a): The variation of Raman modes with composition (spline connected) for amorphous as-quenched Ge15Te80 − xIn5Agx glasses. (b): The variation of relative intensities of different modes relative to 158 cm−1 mode with composition (spline connected) for amorphous as-quenched Ge15Te80 − xIn5Agx glasses.
quenched Ge15Te80 − xIn5Agx glasses are given in Fig. 2(a) and (b) respectively. Here, we can observe a clear signature of intermediate phase in the composition range 8 ≤ x ≤ 16 as depicted in our ADSC studies reported earlier [24]. A trough is seen in the composition dependence of Raman modes, in the reversibility window. It may be noted here that a correlation between the Raman modes and domains of IP has been clearly demonstrated by Boolchand et al. [16]. It has been shown that the mode-wave-number variation of corner sharing (CS) and edge sharing (ES) tetrahedral units display thresholds opening an intermediate phase which correlates well with the reversibility window in specially prepared homogeneous GexSe100 − x glasses [30]. Further, the relative increase in intensity of the peak at 99 cm−1and 127 cm−1 with silver concentration is due to the silver lattice vibrational modes. The silver lattice vibrational modes observed in different silver compounds are ~95 cm−1 and ~146 cm−1 respectively [31]. Fig. 3(a) shows the micro-Raman spectra of representative Ge15Te80 − xIn5Agx glasses after annealing at their respective crystallization temperatures and Fig. 3(b) shows the peak fit for a representative annealed Ge15Te64In5Ag16 glass. The Lorentzian peak fitting of the spectra of annealed Ge15Te64In5Ag16 samples shows peaks at 67, 93, 102, 122, 134, 141, 164, 183, 198, 224, 244 and 271 cm− 1. These modes match well with the Raman spectrum of Ge15Te85 − xIn5 glasses as reported earlier [24]. It is seen that the intensities of peaks at 67, 93, 141, 164, 198, 244 and 271 cm−1 increase with silver addition; especially, a significant increase in intensity at 93 and 141 cm−1 is observed. The peaks 93, 122 and 141 cm−1 are attributed to crystalline tellurium [32–34].
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Fig. 3. (a): Raman spectra of representative annealed Ge15Te80 − xIn5Agx samples. (b): Raman spectrum of a representative annealed Ge15Te64In5Ag16 sample, after peak Lorentzian fitting; correlation coefficient is 0.99871.
The X-ray diffraction studies on annealed samples reveal the presence of c-Ag8GeTe6 and AgTe crystalline phases and their intensity increase with silver addition [27]. When tellurium is progressively replaced by silver in Ge15Te80 − xIn5Agx glasses, the observed increase in intensity at 93 and 141 cm−1 can be due to an increase in silver lattice vibration modes.
Fig. 4. The composition dependence of Raman modes with silver composition (spline connected) in Ge15Te80 − xIn5Agx glasses annealed at their respective crystallization temperatures.
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Fig. 4 shows the variation of Raman modes with composition in annealed samples. Even though the topological thresholds are very much confined to amorphous samples, we can clearly see the unique variation of Raman wave-numbers in the intermediate region. This can be due to the retention of some of the local structure even after the sample is crystallized. Comparison of representative Raman spectra of as-quenched and annealed Ge15Te64In5Ag16 glasses is given in Fig. 5. For as-quenched Ge15Te64In5Ag16 glasses, the width and intensity of the most prominent peak centered at 158 cm−1 are 23 cm−1 and 237 arbitr. units respectively. For thermally annealed Ge15Te64In5Ag16 samples, the most prominent peak is centered at 122 cm−1 having width and intensity 8 cm−1 and 858 arbitr. units respectively. The spectra of all amorphous samples (Fig. 1(a)) are broad with the prominent peak at ~ 158 cm−1 which resemble a-Te peaks (156 cm−1) and of less intensity; the shift in the peak positions are due to the replacement of Te–Te bonds by Ag–Te bonds. On the other hand, the spectra of all annealed samples (Fig. 3(a)) are sharp and intense which resemble c-Te with prominent peak at ~ 122 cm− 1. As the silver concentration increases beyond x = 16, the Chemical Threshold, silver phase separation occurs, leading to the increase in intensity of peaks at 93 cm−1 and 141 cm−1. The variation of density and molar volume with the silver composition as well as average coordination number (brN) in Ge15Te80 − xIn5Agx glasses are shown in Fig. 6. The standard deviation in density determination is found to be less than 0.03 gcm−3. The molar volume decreases and density increases with silver addition, with a plateau exhibited in both the parameters, in the composition range x = 8 to 16, which is the reversibility window identified on the basis of ADSC studies, which is also confirmed by the present Raman scattering investigations. It has been realized before that the intermediate phase forms efficiently packed space filling networks with low molar volumes [12]. Fig. 7 depicts the variation of thermal diffusivity, α with composition and the inset figure shows the variation of PTD signal amplitude with square root of modulation frequency, f. The standard deviation is found to be less than 0.0065 cm2s−1. The α increases with composition initially. It exhibits a minimum around x = 12. The thermal diffusivity is a parameter, important not only due to its physical interest but also due to its technological applications like device modeling and design. The reciprocal of α is a measure of the time required to establish thermal equilibrium in a given material. It depends on the compositional and micro-structural variables [35]. It is very sensitive to the structure of solids [36]. It is also used as a tool to study imperfections, dislocations and voids as the carrier mean free path is affected by lattice defects [37]. In general, the thermal diffusivity, α, increases with the increase in average co-ordination number and increasing structural rigidity, due to the decrease in floppy modes which scatter the thermal waves as mechanical threshold is approached [38]. A maximum is seen in α in many glassy systems which exhibit a sharp stiffness transition [35,39–42].
Fig. 5. Comparison of Raman shift of as-quenched and annealed Ge15Te64In5Ag16 glasses.
Fig. 6. The variation of density and molar volume with the silver composition as well as mean coordination number brN (B-spline connected) in Ge15Te80 − xIn5Agx glasses.
Further, the thermal diffusivity experiments on As-Te–Ga glasses which exhibit an extended rigidity transition indicate that α, after the initial increase, starts decreasing at the onset of extended stiffness transition; the decrease in α leads to a local minima at completion [38]. Further, a maximum is seen in the thermal diffusivity of As–Te–Ga glasses, at the chemical threshold, which is understood on the basis of the minimal scattering of the thermal waves at this composition [38,39]. It is interesting to note that in the present Ge15Te80 − xIn5Agx glasses also, there is an initial increase in thermal diffusivity with the average coordination number. Around x = 6, there is a maximum in thermal diffusivity, followed by a broad minimum around x = 12, the centroid composition of the intermediate phase. There is an increase in α beyond x = 12, leading to a maximum at the composition x = 14. The observed effects in the thermal diffusivity above x = 6 can be connected with the intermediate phase occurring in the range 8 ≤ x ≤ 16 seen in earlier ADSC and the present Raman scattering, density and molar volume experiments. However, there is a marginal shift in the boundaries of the intermediate phase and non observance of a minimum in the composition corresponding to completion of stiffness transition, in the thermal diffusivity measurements. Though the reason for shifting of the lower boundary is not clear, the occurrence of the minimum at the completion could have been masked by the possible chemical threshold which coincides with the upper boundary at x = 16. The decrease in α at compositions above x = 14 is due to the presence of additional vibrational modes characteristic of a rigid elastic
Fig. 7. The variation of thermal diffusivity with the silver composition and as well as brN in Ge15Te80 − xIn5Agx glasses; the inset figure shows the variation of PTD signal amplitude with square root of modulation frequency, f; the black solid line in the figure is a guide to the eye.
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namely percolation of rigidity, percolation of stress and the onset of chemical phase separation on a nanoscale at 8%, 16% and 20% of silver concentration respectively.
References
Fig. 8. The elastic phases of Ge15Te80 − xIn5Agx glasses in terms of their topology.
network, scattering away the thermal waves, resulting in reduction in α [43]. The recent studies on the compositional variation of physical properties in binary GexSe100 − x [30] and ternary GexSixTe100 − 2x [44] glasses indicate the occurrence of three thresholds: percolation of rigidity, percolation of stress and the onset of chemical phase separation on a nanoscale. Based on [44], and from the present micro Raman, thermal diffusivity, molar volume and the earlier thermal studies [27], the schematic representation of evolution of glass structure, in terms of topology in the Ge15Te80 − xIn5Agx glass system is depicted in Fig. 8; When silver is added to Ge15Te80In5 base glass (a composition in the IP), glasses become flexible at first as the coordination number of Ag is lesser than In. As the silver concentration increases from 8% to 16%, opening of an IP is evidenced. From 16% to 20%, a stressed rigid phase and from 20% and above, nanoscale phase separation is observed. As said in literature, [45] the width of the IP increases in multi-component glasses, with the number of components. 4. Conclusions Micro-Raman studies on amorphous Ge15Te80 − xIn5Agx glasses yield peaks at ~ 68, ~ 99, ~ 127, ~ 158 and ~ 217 cm−1 suggesting the presence of tetrahedrally bonded GeTe4 units. The compositional variation of Raman modes and corresponding intensities mark the opening of an “intermediate phase” that coincides well with the “reversibility window” seen in earlier ADSC studies. The micro-Raman spectra of annealed samples reveal the presence of c-Te and silver lattice vibrational modes. The increase in the intensity of peaks at 93 and 141 cm−1 with silver addition suggests an increase in silver lattice vibrational modes. Further, the compositional variation of density, molar volume and thermal diffusivity confirm the opening of the IP in the composition range 8 ≤ x ≤ 16. Based on our present and earlier studies, the occurrence of three thresholds in Ge15Te80 − xIn5Agx glasses is proposed,
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