Communication pubs.acs.org/crystal
Search for the Mysterious SiTeAn Examination of the Binary Si−Te System Using First-Principles-Based Methods Simon Steinberg,† Ralf P. Stoffel,† and Richard Dronskowski*,†,‡ †
Institute of Inorganic Chemistry and ‡Jülich-Aachen Research Alliance (JARA-FIT and -HPC), RWTH Aachen University, Landoltweg 1, 52056, Aachen, Germany S Supporting Information *
ABSTRACT: A1B1-type tellurides of group 14 elements are of great interest due to their applications as data and energy storage materials. While the features of ATe (A = Ge, Sn, Pb) have been determined, there is no report on SiTe in the solid state. Herein, we review a preexisting controversy in the literature regarding the Si−Te system and provide a feasible approach to SiTe.
T
as well as dynamic stability among diverse plausible models, is the most favorable to compare to an observed structure model,9 we critically revisited the controversy between Si2Te3 and SiTe2 based on evaluations of their total energies and electronic and vibrational properties. In addition, the total energies and the electronic as well as vibrational properties of diverse “SiTe” models in plausible structures were explored to identify a feasible synthetic route to SiTe in the solid state. The crystal structures of Si2Te3 and SiTe2 are both composed of hexagonal closest packed layers of Te atoms; yet, their crystal structures differ in the amounts and distributions of the Si atoms.7b,12 In SiTe2 (CdI2-type), the Si atoms occupy all octahedral voids of every second layer between the Te slabs (Figure 1c),7b whereas in Si2Te3, the Si atoms form dumbbells that are disordered and reside in 2/3 of all octahedral vacancies of every second layer sandwiched by the Te atoms (Figure 1f).12 Because the properties of Si2Te3 depend widely on the long-range order in its crystal structure,13 the electronic structures were examined for different “Si2Te3” models, for which the numbers of the face-sharing Te octahedra enclosing Si dumbbells were successively increased (Figure S1). Two “SiTe” models were derived from the crystal structures of SiTe2 and Si2Te3 through assignments of Si atoms and dumbbells, respectively, to empty, octahedral voids (Figure 2), while two other “SiTe” models were generated based on the structures of α- and β-GeTe (distorted (rhombohedral) and ordered (cubic) NaCl-types, respectively),14 as a cubic lattice was proposed for SiTe.7a
he binary A1B1-type tellurides of the tetrels Ge, Sn, and Pb are of great interest due to the applications in data and energy storage technologies. In particular, GeTe is a phasechange data-storage material and serves as a parental compound for several phase-change alloys,1 SnTe is a topological crystalline insulator,2 and PbTe is one of the best materials presently available for thermoelectric energy conversion.3 Albeit the structures and the properties of these economically relevant tetrel-monotellurides have been determined, still, there is no report on SiTe in the solid state. Explorations on gaseous SiTe allowed determination of the electronic states and thermodynamic properties for SiTe molecules in the vapor phase, yet not for SiTe in the solid state.4 Also, there has been a lot of controversy5 about the composition and the structure of a solid-state material in the Si−Te system: previous examinations indicated Si2Te3 as the only phase for this system,4a,6 while other explorations established another phase, i.e., SiTe2.7 Indicators about the tendencies to form a certain material may be gained from the thermochemical data of the reactions and, for instance, explorations on the reaction quantities for the tellurides in the Si−Te system revealed a negative free energy of formation for Si2Te3.4a,8 To identify the factors that promote or complicate the formation of a material at the atomic scale, it will be beneficial to know the electronic and vibrational properties for a composition of interest.9 Thus, the knowledge of the electronic and vibrational properties for a material of interest enables its accelerated discovery and a rational planning of its synthesis.9,10 Our impetus to discover the stability trends for an optimal synthesis of the solid-state material in the Si−Te system was stimulated by recent research on nanoplates and -ribbons of this telluride for applications as light-emitting devices or near-infrared photodetectors.11 Because the structure, which shows the lowest total energy and electronic © XXXX American Chemical Society
Received: August 4, 2016 Revised: September 23, 2016
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DOI: 10.1021/acs.cgd.6b01163 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Communication
Figure 1. (a,d) DOS, (b,e) −pCOHP, and (c,f) representations of the crystal structures for SiTe2 and “Si2Te3”-I, respectively. DOS and −pCOHP curves of two other “Si2Te3” models are provided in the Figures S17 and S18.
Figure 2. (a,b,c,d) Representations of the structures and (e,f,g,h) electronic DOS curves for the “SiTe” models I, II, III, and IV, respectively. The structure of “SiTe”-II comprises Si dumbbells with direct, through space Si−Si contacts. (i) E(V) curves of “SiTe”-III and a modulated structure of “SiTe”-III. (j) Relative enthalpies of “SiTe” and a mixture of Si and “Si2Te3”-I as functions of pressure at absolute zero.
be a semiconductor. The different widths of the gaps at EF in all “Si2Te3” models (Table S9) underline the influence of the preferred orientation of the Si dumbbells on the material
A comparison of the electronic densities-of-states (DOS) curves for all “Si2Te3” models (Figures 1 and S16−S18) reveals that the Fermi levels, EF, fall in band gaps suggesting Si2Te3 to B
DOI: 10.1021/acs.cgd.6b01163 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Table 1. Average − IpCOHP/Bond Values (eV) of the Si−Si, Si−Te, and Te−Te Contacts in SiTe2 and All “SiTe” and “Si2Te3” Modelsa Si−Si Si−Te Te−Te a
“SiTe”-I
“SiTe”-II
“SiTe”-III
“SiTe”-IV
SiTe2
“Si2Te3”-I
“Si2Te3”-II
“Si2Te3”-III
d. a. 5.4986 0.2641
4.5663 3.0560 0.1091
d. a. 1.9933 0.0080
d. a. 1.8058 0.0813
d. a. 2.8408 0.2614
4.6498 4.1449 0.1189
4.6538 4.0322 0.1071
4.6052 4.1077 0.1110
Si−Si − IpCOHP values were included for models with Si dumbbells. d. a. = does not apply.
properties (see Figure S20 for further details). EF in SiTe2 falls at a maximum in the DOS, which typically15 points to an electronic instability. Thus, the characteristics at EF in all “Si2Te3” models approximating the structure reported by Ploog et al.12 can not only explain the physical properties observed for the silicon telluride, as, e.g., red color and a band gap of 1.0 eV,7,11 but also show an electronically more favorable situation in contrast to the features at EF in SiTe2. As the Fermi levels fall in pseudogaps in the DOS of “SiTe”-I to III and a gap in the DOS of “SiTe”-IV, electronically favorable situations are predicted for all “SiTe” models (further details regarding the opening of the gap in “SiTe”-IV are provided in Figure S21). Below EF, the DOS of the “SiTe” and “Si2Te3” models as well as SiTe2 stem basically from the Si-p and the Te-p orbitals and, for the states near EF in all “SiTe” models, to a minor extent from the Si-s orbitals (see Figures S2−S5, S14, and S16−S18 for orbital-projected DOS). Bonding analyses for all “SiTe” and “Si2Te3” models and SiTe2 were completed based on the projected crystal orbital Hamilton populations (pCOHP) and their respective integrated values (−IpCOHP; Tables 1, and S10, S11). From the −IpCOHP values it is clear that the majority of the bonding populations reside between the Si−Te interactions and, for the case in which Si dumbbells are present in a particular structure, the Si−Si contacts. In “SiTe”-II, SiTe2, and all “Si2Te3” models, the Te−Te interactions in the empty layers between Te slabs are weakly bonding and show much smaller −IpCOHP values relative to those of the Si−Te (and Si−Si) separations. This outcome correlates well to the observation that the Te layers enclosing Si dumbbells in Si2Te3 are mechanically separated from each other without the slightest efforts due to the weak nature of the dispersion interactions between these layers.11 Examinations of the phonon band structures for SiTe2 and the “SiTe” models I to III (Figures S6−S8, S15) revealed the presence of negative (imaginary) modes, which are indicative of dynamic instabilities.16 Such large, imaginary modes are not evident in the phonon band structures of the “SiTe” and “Si2Te3” models with the lowest total energies, i.e., “SiTe”-IV and “Si2Te3”-I, respectively (Figures S9, S19, Table S12); yet, the difference between the total energies of “SiTe”-IV (αGeTe-type) and “SiTe”-III (β-GeTe-type) is rather small (Figure 2). To identify a feasible phase transition from a highto a low-temperature modification akin to that observed for GeTe,14,17 the electronic and vibrational properties were examined for modulated structures derived from the cubic “SiTe”-III. A comparison of the energy-volume, E(V), curves (Figures 2 and S13) for the original and distorted structures of “SiTe”-III reveals two minima in each E(V) curve belonging to a modulated structure of “SiTe”-III. The minima linked to the larger unit cell volumes are lower in energy than that of the undistorted “SiTe”-III variant and point to the presence of a feasible transition between “SiTe”-IV and “SiTe”-III. In summary, the examinations on the electronic and vibrational properties for SiTe2 and the models approximating
the structure of Si2Te3 reveal that an electronically and dynamically favorable situation is attained for the latter rather than the Si-poorer compound. Among the diverse “SiTe” models, the evaluations of the total energies and the vibrational properties indicate that a rhombohedral (α-GeTe-type) structure is preferred. The calculated relative enthalpies of “SiTe” and a mixture of “Si2Te3”-I and Si as functions of pressure at absolute zero (Figure 2) imply that the application of a pressure ≥7.2 GPa leads to the formation of “SiTe”. An auspicious hint to the crystal structure of the mysterious “SiTe” may come from the previously reported (GeTe)0.9(SiTe)0.1 that is isostructural to α-GeTe.18 Thus, synthetic efforts are on the way to confirm that finding and, furthermore, to detect ternary materials with higher Si-contents in the Si−Ge−Te system and SiTe in the solid state.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01163. Computational details, densities-of-states (DOS), projected DOS and −pCOHP curves, tables with integrated −pCOHP/Bond (−IpCOHP), average −IpCOHP/ Bond and cumulative −IpCOHP/Interactions values for all “SiTe” and “Si2Te3” models as well as SiTe2; phonon band structures and densities-of-states (PhDOS) of all “SiTe” models, “Si2Te3”-I and SiTe2; overviews of the locations of the Fermi levels in the DOS curves and of the total energies/formula unit for all “SiTe” and “Si2Te3” models as well as SiTe2; representations of the crystal structures for all “SiTe” and “Si2Te3” models as well as SiTe2 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by the IT center at RWTH Aachen (JARA-HPC partition) through the allocation of compute time and the Deutsche Forschungsgemeinschaft (SFB 917 “Nanoswitches”). We also thank V. L. Deringer for helpful discussions and suggestions.
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DEDICATION Dedicated to Prof. Klaus Ploog on the Occasion of his 75th Birthday C
DOI: 10.1021/acs.cgd.6b01163 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.cgd.6b01163 Cryst. Growth Des. XXXX, XXX, XXX−XXX
