Observation of high pressure o-GeTe phase at ambient pressure in Si-Te- Ge glasses

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I. INTRODUCTION
Chalcogenide glasses (ChGs) are being extensively investigated, both experimentally and theoretically, in view of their possible applications, including rewritable, digital versatile disks and nonvolatile phase-changes random access memories. 1Further, tellurium alloys are used for optical data storage in commercial rewritable phase change media.][4] On the other hand, few reports are available on silicon-telluride glasses, which are efficient acousticoptic materials and can also be used in memory-type switching diodes 5,6 and phase change memory devices. 7,8 n view of various potential applications of ChGs, a precise knowledge of crystalline phases is necessary for the development of suitable phase-change, erasable electrical/optical storage media.In the present work, the thermal behavior of Si 15 Te 85-x Ge x glasses of a wide composition range (1 ≤ x ≤ 11) has been investigated using Alternating Differential Scanning Calorimetry (ADSC).X-ray diffraction (XRD) studies have been performed to confirm the amorphous nature of as-prepared glasses and to identify the stable crystalline phases presented at different temperatures.Raman scattering studies have been undertaken at different crystallization temperatures for structural information.The results obtained have been found to throw light on anomalous o-GeTe crystalline phase transformation in SiTe 2 phase percolated glasses.

II. EXPERIMENTAL
Bulk Si-Te-Ge glasses of about 1.5 g have been synthesized by vacuum sealed melt-quenching method.See Ref. 8 for experimental details of sample preparation, thermal analysis and Raman scattering studies.In the present work, Raman experiments are performed on powder samples using 50× objective.The amorphous nature and homogeneity of the quenched samples are confirmed by XRD and EDAX respectively.
a Authors to whom correspondence should be addressed.Electronic mail: srinivasaraogunti@gmail.com (S.R. Gunti), sasokan@isu.iisc.ernet.in(A.Sundarrajan) 2158-3226/2012/2(1)/012172/6 C Author(s) 2012 2, 012172-1 FIG. 1.The total heat flow curves of selected Si 15 Te 85-x Ge x glasses in the ADSC scans.Here T c1 and T c2 represent the first and second crystallization temperatures respectively; whereas, T c1 represents the crystallization temperature at which all crystallization phases formed at T c1 and T c2 present.SiTe 2 crystalline phase percolation can be seen at T c3 for x = 6.

III. RESULTS AND DISCUSSION
ADSC scan of all Si-Te-Ge glasses (Fig. 1) show one endothermic glass transition peak and different exothermic crystallization peaks.In the composition range 1 ≤ x < 3, Si 15 Te 85-x Ge x glasses show two crystallization reactions at temperatures T c1 and T c2 respectively whereas, in the composition range 3 ≤ x ≤ 11 two crystallization reactions take place at single crystallization temperature, say, T c1 .It is also seen from Fig. 1 that there is a new crystalline phase percolation at T c3 in the composition range 6 ≤ x ≤ 11.For all samples, the values of glass transition and crystallization temperatures have been provided in supplementary material in table format. 9We have performed XRD studies to identify the crystallization phases presented in Si-Te-Ge glasses.
Before discussing about XRD studies, we adopt the following nomenclature to describe two different orthorhombic-GeTe (o-GeTe) crystalline phases.It has been confirmed from the XRD studies that the phase presented at T c1 (Fig. 2(b)) is hexagonal-Te (h-Te) (JCPDS PDF # 361452) with a unit cell defined by a = 4.457 Å, c = 5.927 Å; whereas, at T c2 (Fig. 2(c)) there are two crystalline phases presented which are identified as GeTe-I and cubic-GeTe 4 (c-GeTe 4 ) (JCPDS PDF # 330585) with a unit cell define by a = 11.09Å.However, in the composition range 3 ≤ x ≤ 11, all these three phases (h-Te, GeTe-I and c-GeTe 4 ) found to percolate at single crystallization temperature T c1 (see Fig. 2(c) and 2(d)).Further, the phase percolated at T c3 in the composition range 6 ≤ x ≤ 11 is identified as SiTe 2 (JCPDS PDF # 270610) (Fig. 2(e)).Now an interesting observation made from XRD studies is the transformation of GeTe-I phase, which is observed in all the samples at T c2 (or T c1 ), into GeTe-II phase in only SiTe 2 phase percolated samples at T c3 .In other words, both GeTe-I and GeTe-II phases present in SiTe 2 phase percolated samples (6 ≤ x ≤ 11) but GeTe-I can be seen only at T c1 beyond which, this phase is transforming into GeTe-II at T c3 .According to Shimada et al. 11 the later phase can be observed at temperature about 300 • C or above and at the pressure about 100 kbar.First time we have observed this phase at ambient pressure in ternary Si-Te-Ge glasses.The observed phase transition with temperature can be understood based on the concept of commensurate-incommensurate (C-IC) transition.The bulk-Te forms trigonal crystal consisting of parallel helical chains stacked on a triangular lattice, with their axes parallel to the crystalline c-axis.The stability of the Te structures depends on the Te-Te-Te bond angle and coordination number of Te atoms.Based on Wannier function analysis of the chemical bonding in Te helix, the helical structure is stabilized and closed loops of helices were more stable than the open helical chains.Among all the Te n (n = 2 to 12) clusters, Te 8 with a closed-ring like helical structure is the most stable one. 12n many of the chalcogenide systems, the coordination numbers of Si, Ge and Te obey the Mott's 8-N rule, where N is number of valance electrons, 13 therefore, in the present Si-Te-Ge glassy system, the coordination numbers of Si, Ge and Te can be safely assumed to be 4, 4 and 2 respectively.If we consider Si 15 Te 74 Ge 11 sample for the highest dopant concentration (x = 11), there will be a total of (15+11) × 4 = 104 Si + Ge bonds available in glassy network but at the same time 74 × 2 = 148 Te bonds available to form glassy network with Si and Ge atoms and among themselves.Since the glass samples studied in the present work are Te-rich, there will be a large number of Te-Te chains presented in the glassy network.Therefore, there are two factors involved in observed phase separations and phase transformation in Si-Te-Ge glasses.First one is Ge dopant concentration which is highly affecting Si-Te-Ge network, particularly, at higher dopant concentrations and second one is temperature.The later one is responsible for phase transformation of GeTe-I into GeTe-II due to the local C-IC Te-atom ordering mechanisms which would affect GeTe lattice in SiTe 2 phase percolated samples at about T c3 .We also observed the impact of Te chain ordering on GeTe network from Raman studies.

GeTe-I
Figure 3 shows the Raman spectra recorded at different temperatures on different samples.Band positions of different bands are as follows: a low intense hump at A ∼ 93 cm -1 and Raman bands B ∼ 122.9 -124.8 cm -1 , C ∼ 42.3 -143.6 cm -1 , D ∼ 106 cm -1 , E ∼ 133.6 cm -1 , F ∼ 177.6 cm -1 , G ∼ 80 cm -1 , H ∼ 125 cm -1 , J ∼ 160 cm -1 , K ∼ 173 cm -1 , X ∼ 155.8 cm -1 and Y ∼ 165.8 cm -1 .The peak B can be attributed to the A 1 mode, peaks A and C to the E TO modes and peak D to the E LO mode of Te-Te chain. 14The peaks J, 15 G and H (which is merged in peak B) can be assigned to the amorphous-GeTe phase. 16The peak F can be assigned to the symmetric stretching mode of the edge-sharing GeTe 4 tetrahedra. 16According to Norban et al. 17 the band at E ∼ 133.6 cm -1 can be assigned to the disordered chains of Te and the peak K ∼ 173 cm -1 is due to clusters composed of SiTe 4/2 tetrahedral units which intrinsically occur in the virgin Si x Te 1-x glasses.Also, the blue-shift observed in bands B and C (at T c3 , see Fig. 3(d  units in the composition range 6 ≤ x ≤ 11.Because, in this composition range due to cleaving of Te n chains upon application of 2 mW laser power, blue-shifts the optical-absorption edge and leaves a Si-rich bulk glass in which metastable clusters of tetrahedral SiTe 4/2 units predominate.The details of Raman band positions observed at different temperatures in Si 15 Te 85-x Ge x (1 ≤ x ≤ 11) glasses are presented in supplementary material in table format. 9All these bands are well reported in literature, however, there is no evidence of Raman bands observed at X and Y. Interestingly, the observed two sharp Raman bands X and Y arise in the region where low intense hump of amorphous-GeTe (band J) is observed at T c1 (Fig. 3(c)).One possible reason for observed sharp bands X and Y may be due to the effect of Te-Te chain ordering on GeTe lattice in SiTe 2 phase percolated samples at T c3 , however, so far there is no experimental/theoretical evidence.

IV. CONCLUSIONS
In conclusion, Si 15 Te 85-x Ge x (1 ≤ x ≤ 11) glasses are found to exhibit an anomalous phase separations.The SiTe 2 phase percolation is observed in the composition range 6 ≤ x ≤ 11.The XRD studies have shown the structural transformation of o-GeTe (GeTe-I) into high pressure o-GeTe (GeTe-II) in SiTe 2 phase percolated samples at T c3 .Raman scattering studies indicate that the ordering mechanism of 1D Te-Te chain is greatly depending on Ge dopant concentration and temperature.Further, the observation of Raman bands at E, X and Y indicates that there would be a strong correlation between 1D Te-Te structural chain ordering and GeTe structural modification at T c3 in Si 15 Te 85-x Ge x (6 ≤ x ≤ 11) glasses.