— physics pp. 945–953
Growth and high pressure studies of zirconium sulphoselenide single crystals
K R PATEL1,∗, R D VAIDYA1, M S DAVE2and S G PATEL3
1Ashok & Rita Patel Institute of Integrated Study & Research in Biotechnology and Allied Sciences (ARIBAS), New Vallabh Vidyanagar 388 121, India
2N.V. Patel College of Pure and Applied Sciences, Vallabh Vidyanagar 388 120, India
3Department of Physics, Sardar Patel University, Vallabh Vidyanagar 388 120, India
∗Corresponding author. E-mail: kparibas@yahoo.co.in
MS received 14 November 2008; revised 12 May 2009; accepted 11 June 2009
Abstract. Transition metal trichalcogenides are well suited for extreme pressure lu- brication. These materials being semiconducting and of layered structure may undergo structural and electronic transition under pressure. In this paper authors reported the de- tails about synthesis and characterization of zirconium sulphoselenide single crystals. The chemical vapour transport technique was used for the growth of zirconium sulphoselenide single crystals. The energy dispersive analysis by X-ray (EDAX) gave the confirmation about the stoichiometry of the as-grown crystals and other structural characterizations were accomplished by X-ray diffraction (XRD) study. The variation of electrical resis- tance was monitored in a Bridgman opposed anvil set-up up to 8 GPa pressure to identify the occurrence of any structural transition. These crystals do not possess any structural transitions upto the pressure limit examined.
Keywords. Single crystals; chemical vapour transport (CVT) technique; high pressure;
Bridgman anvil; electrical properties.
PACS Nos 61.50.Nw; 61.10.Nz; 78.30.Fs
1. Introduction
Transition metal trichalcogenides MX3(M is a transition metal of group IVB, VB and VIB, X is a chalcogen S, Se and Te) constitute structurally and chemically well-defined family of compounds. These trichalcogenides are thin fibrous ribbons and offer several interesting phenomena originating from their anisotropy. The electronic structure of these compounds is of considerable interest both from the experimental and theoretical points of view. The transition metal trichalcogenides belong to the family of materials with low-dimensional structure which possess typical physical properties. Their crystal structure simultaneously gives evidence of chain-like and layered characters. The basic structural elements are prismatic
columns of MX6 trigonal prisms, linked together to build layers, which themselves are bound by weak forces.
The simplest structure is found in the semiconducting compounds of the type ZrSe3[1,2]. The unit cell with two formula units per cell has monoclinic symmetry.
The non-metallic character of the crystals is mostly due to the strong chalcogen–
chalcogen bonds within the prisms which retain two electron pairs per unit cell [3].
This nearly di-atomic bond plays a determining role in the dynamical properties of the crystals [4]. It involves the phonon mode with the highest Raman active vibra- tional frequency. The mutual separation of each chalcogen pair leads to a dynamical representation of the structure settled on a distorted octahedral coordination of the cation [5].
It is interesting to note that the distorted coordination yields semi-metallic char- acter. Since pressure typically favours higher symmetry/coordination and met- allization in contrast to the distorted coordination and metallic nature of these materials, it is interesting to investigate the effect of pressure on the structure and resistivity of these materials.
In this paper authors reported the preparation, characterization and results of room-temperature electrical resistance measurements as a function of pressure on zirconium sulphoselenide layered single crystals.
2. Experimental
Single crystals of zirconium sulphoselenide were grown by chemical vapour transport technique using iodine as a transporting agent. The size of the crystals depends on the temperatures at the two ends of the quartz ampoule and the concentration of the transporting agent. The ampoule used has 250 mm length and 22 mm inner diameter. The optimum growth conditions and physical parameters of zirconium sulphoselenide single crystals are shown in table 1. The chemical composition of the grown crystals was confirmed from energy dispersive analysis by X-ray (EDAX) Table 1. Growth parameters of ZrSxSe3−xsingle crystals grown using chem- ical vapour transport technique.
Physical characteristics of the crystals Reaction Growth Growth Plate
temperature temperature time area Thickness Colour and
Sample (K) (K) (h) (mm2) (mm) appearance
ZrSe3 873 1023 370 20 0.09 Gray shining
ZrS0.5Se2.5 993 1043 370 15 0.09 Black shining
ZrSSe2 1013 1063 370 13 0.08 Black shining
ZrS1.5Se1.5 1033 1083 370 15 0.07 Silver black shining
ZrS2Se 1053 1103 370 11 0.07 Red shining
ZrS2.5Se0.5 1073 1123 370 10 0.06 Red shining
ZrS3 1093 1143 370 10 0.06 Red shining
Table 2. The EDAX data for zirconium sulphoselenide single crystals.
Stoichiometric proportion (wt%) EDAX results (wt%)
Sample Zr S Se Zr S Se
ZrSe3 27.80 – 72.20 25.26 – 74.74
ZrS0.5Se2.5 29.94 5.26 64.80 27.78 3.45 68.67
ZrSSe2 32.44 11.40 56.16 31.19 7.96 60.86
ZrS1.5Se1.5 35.39 18.66 45.95 34.22 14.67 51.11
ZrS2Se 38.90 27.40 33.70 45.41 24.15 30.45
ZrS2.5Se0.5 43.26 38.02 18.72 50.43 30.09 19.48
ZrS3 48.67 51.33 – 48.65 51.35 –
as shown in table 2. The structural characterization of these crystals was done by X-ray diffraction analysis. For X-ray diffraction study, several small crystals were finely ground with the help of an agate mortar and filtered through a 100- micron sieve to obtain grains of nearly equal size. The powder obtained during the growth process was studied by X-ray diffraction. X-ray diffractometer (Make:
Philips, Model: X’PERT MPD) was used to obtain the diffraction pattern in which the wavelength used was 1.542 ˚A and Cu target X-ray tube was used as a source and all the measurements were taken with an accuracy upto±0.0025. The X-ray diffractograms of these crystals are shown in figure 1.
The optical band gaps of the as-grown crystals were obtained by optical ab- sorption. The optical absorption spectra were taken by means of UV–VIS–NIR spectrophotometer (Make: Perkin Elmer, Model: Lambda-19) in the wavelength range of 200–1450 nm. The absorption spectra of the as-grown crystals are shown in figure 2. For obtaining the absorption spectra using UV–VIS–NIR spectrophotome- ter from single crystal specimens, thin flakes of as-grown crystals were used. These flakes were pasted on a thick black paper with a cut exposing the crystal flake to the incident light. The reference used was a replica of the black paper, having the cut at exactly the same position as the crystal flake. This arrangement was necessary because the crystal size was smaller than that of the sample compartment. For the determination of band gap for semiconducting materials, absorption of incident photon by semiconducting material is an important technique. In this technique, photons of selected wavelengths were bombarded on the sample and their relative transmission was observed. The photons with energies greater than the band gap were absorbed while photons with energies less than the band gap were transmitted.
For the determination of energy band gap, the spectral variations of (αhν)1/2 vs.
hν (for indirect band gap) and (αhν)2 vs. hν (for direct band gap) were studied and graphically the values of the direct and indirect band gaps were found out.
A careful study of these spectra reveals the presence of absorption edges in the spectral range studied.
In order to achieve higher pressure, Bridgman suggested the use of two opposed anvils [6]. In the basic design of the anvil, the truncated anvils of tungsten car- bide are supported by steel binding rings with an interference-fit to apply inward acting radial stresses. This design uses Bridgman’s principle of ‘massive support’.
Figure 1. The X-ray diffractrogram for zirconium sulphoselenide crystal.
Bridgman anvil can be readily used up to about 10 GPa [7]. The sample is in the form of a thin disk surrounded by a gasket, normally of pyrophyllite material [8] with talc as a pressure transmitting medium. A four-probe method was used to measure the resistance of zirconium sulphoselenide single crystals up to 8 GPa pressure.
3. Results and discussion
The chemical vapour transport technique was most suitable for the growth of good quality needle-shaped layered single crystals of zirconium sulphoselenide. The lat- tice parameter, unit cell volume and X-ray density of the as-grown crystals have
Figure 2. The absorption spectra of the as-grown crystals of zirconium sulphoselenide.
been calculated using X-ray diffraction analysis and listed in table 3. From this analysis it has been found that the crystals possess monoclinic structure.
Figure 3 shows the spectral variation of (αhν)1/2vs. hν. Since the curve indicates a discontinuous straight line it is quite possible that it represents indirect interband transition involving the emission or absorption of phonon. The band gap was obtained by extrapolating the linear part to zero on energy axis.
Table 3. The crystallographic data of zirconium sulphoselenide single crystals.
X-ray density Optical band Sample a(˚A) b(˚A) c(˚A) β V (˚A)3 ρ(gm/cc) gapEg (eV)
ZrSe3 5.452 3.7874 9.4514 97.46 193.5 5.6304 1.10
ZrS0.5Se2.5 5.378 3.725 9.3913 97.44 185.54 5.423 1.25
ZrSSe2 5.301 3.675 9.2778 97.42 179.21 5.212 1.35
ZrS1.5Se1.5 5.227 3.6425 9.2111 97.38 173.92 4.9211 1.50 ZrS2Se 5.182 3.6288 9.1597 97.34 170.83 4.5543 1.55 ZrS2.5Se0.5 5.125 3.6051 9.09 97.31 166.57 4.3205 1.77
ZrS3 5.082 3.5823 8.975 97.28 162.06 3.8393 2.00
Table 4. The values of electrical band gap at atmospheric pressure, 2 GPa, 4 GPa, 6 GPa and 8 GPa pressure.
Band gap (eV) at
Atmospheric 2 GPa 4 GPa 6 GPa 8 GPa
Crystal pressure pressure pressure pressure pressure
ZrSe3 1.10 0.92 0.84 0.77 0.74
ZrS0.5Se2.5 1.25 1.15 1.06 0.99 0.91
ZrSSe2 1.35 1.22 1.14 1.07 1.02
ZrS1.5Se1.5 1.50 1.38 1.23 1.09 0.99
ZrS2Se 1.55 1.27 1.19 1.08 0.95
ZrS2.5Se0.5 1.77 1.68 1.59 1.54 1.44
ZrS3 2.00 – – – –
The graph of logR vs. pressure for the as-grown single crystals using Bridgman anvils are shown in figure 4. As shown in figure 4, resistance decreases continuously as pressure increases. No phase transition occurs in as-grown crystals up to 8 GPa.
However, the samples are becoming more conducting in nature at higher pressure.
But in ZrS3, the resistivity increases with the increase in pressure, which shows resistive behaviour of the grown crystal up to 8 GPa. This result is completely opposite to those obtained with respect to temperature. A possible explanation for the increase in resistance for ZrS3might be attributed to the detachment of sulphur atom from the unit cell with increasing pressure [9].
The electrical resistance decreases by an order of magnitude when pressure in- creases from atmospheric pressure to 8 GPa pressure. The optical band gaps at different pressures are given in table 3. The conductivity obeys Arrhenius law at all pressures: R=R0exp(−Eg/2kBT) andR0 =R0exp(−E0g/2kBT), whereR0 is the resistance at high pressure andEg0 is the band gap at high pressure. Accordingly, ln(R0/R) = [(Eg0−Eg)/2kBT] and from the Arrhenius law, the band gaps of the as- grown crystals are found to be decreasing with increase in pressure, which indicate the semiconducting nature of the as-grown crystals. But for ZrS3the Arrhenius law is not applicable. The variations of band gap vs. pressure of the as-grown crystals
Figure 3. The graph of (αhν)1/2vs.hνfor the as-grown crystal of zirconium sulphoselenide.
are shown in figure 5. The variations in values of band gap with pressure are listed in table 4. The decrease in band gaps of these crystals is due to the change in interatomic distance within layers that affect the value of band gap.
Figure 4. The graph of logRvs. pressure for zirconium sulphoselenide single crystals.
4. Conclusion
Chemical vapour transport technique (CVT) was found suitable for the growth of large size single crystals of ZrSxSe3−xcompounds. The crystals possess monoclinic layered crystal structure. The band gap varies as a function of pressure and it is pressure-dependent. Measurement of electrical resistance up to 8 GPa does not indicate the occurrence of any structural transition in them. The samples become
Figure 5. The variation in energy band gap with pressure for zirconium sulphoselenide single crystals.
more conducting as pressure increases. But in ZrS3 single crystals the resistance increases with pressure up to 8 GPa because of the detachment of sulphur atom from the unit cell with increasing pressure.
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