Crystal structure of SiH4 at high pressure

Olga Degtyareva,1,2 Miguel Martinez Canales,3 Aitor Bergara,3,4 Xiao-Jia Chen,1 Yang Song,1,* Viktor V. Struzhkin,1 Ho-kwang Mao,1 and Russell J. Hemley1 1Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA 2Centre for Science at Extreme Conditions, School of Physics, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, United Kingdom 3Materia Kondentsatuaren Fisika, Zientzia Fakultatea, Euskal Herriko Unibertsitatea, 644 Posta Kutxatila, 48080 Bilbo, Spain 4Donostia International Physics Center (DIPC) and Centro Mixto CSIC-UPV/EHU, 1072 Posta Kutxatila, 20080 Donostia, Spain Received 12 March 2007; published 31 August 2007


INTRODUCTION
There is a considerable interest in producing metallic, and possibly superconducting, states of solid hydrogen at multimegabar pressures ͑several hundred gigapascals͒. [1][2][3] Although solid hydrogen becomes opaque in visible light near 300 GPa, much higher pressures ͑e.g., Ͼ400 GPa͒ are thought to be needed for metallization, which currently are a great challenge for hydrogen with high-pressure techniques. It has been recently suggested that hydrogen-rich compounds such as CH 4 , SiH 4 , and GeH 4 will require pressures far less than expected for pure hydrogen at equivalent densities to enter metallic states. 4 As is the case for pure hydrogen, these compounds are considered to be good candidates for hightemperature superconductors in their dense metallic forms. 4,5 Hydrogen can be viewed in these systems as being "chemically precompressed," which lowers the metallization pressure. While the conditions for forming metallic methane have not been reached experimentally, 6 recent theoretical studies predict that both SiH 4 ͑Refs. 5 and 7͒ and GeH 4 ͑Ref. 8͒ transform to a metallic state at much lower and accessible pressures. However, there is only limited high-pressure experimental data available on these compounds, where an evidence for an insulator to semiconductor transition has been reported for solid silane ͑SiH 4 ͒ at pressures around 100 GPa from optical absorption and reflection measurements. 9 At ambient pressures, solid CH 4 , SiH 4 , and GeH 4 consist of isolated high-symmetry tetrahedrally bonded molecules. For silane, two ambient pressure phases have been reported from the power x-ray diffraction, phase I forming on solidification below 88.5 K and phase II below 63.5 K. SiH 4 molecules form a noncubic crystal structure ͑probably with a tetragonal symmetry 10,11 ͒; however, the exact details could not be determined. Although several competitive crystal structures have been proposed for the high-pressure phases of silane in recent computational studies, 5,7,12 there is a complete lack of reliable experimental data for its molecular and crystal structure even at moderate pressures. Such information is important in understanding the changes in the electronic structure leading to metallization. In this paper, we report the crystal and molecular structure of solid silane at high pressures from a combined study using synchrotron x-ray diffraction and first-principles calculations.

EXPERIMENTAL AND COMPUTATIONAL DETAILS
Electronic grade ͑99.998+ % ͒ silane ͑SiH 4 ͒ from Aldrich was used without further purification. Loading was done cryogenically in a dry nitrogen purged glove box by precooling the diamond anvil cell ͑DAC͒ in a liquid nitrogen bath. Temperature was measured by a thermocouple attached to the DAC. SiH 4 solidified on the cooled rhenium gasket of the cell once the temperature of the cell body was a few degrees lower than the melting point of SiH 4 ͑88 K͒. The cell was then sealed and the solid SiH 4 was pressurized before warming up to room temperature. The entire loading process was monitored through an optical microscope mounted on the glove box. A ruby chip was inserted into an empty gasket prior to the loading of silane for pressure measurement. The pressure was determined from the shift of the R 1 ruby fluorescence line with an accuracy of ±0.05 GPa under quasihydrostatic conditions using the ruby scale from Ref. 13. After the loading was complete, the pressure was measured 11.3 GPa. Powder x-ray diffraction data were collected at beam line 16-ID-B ͑HPCAT͒ at the Advanced Photon Source. A focused monochromatic beam with wavelength = 0.3664 Å was used, and the data were recorded on a MAR image plate. The diffraction data were collected at room temperature from 11 to 40 GPa on pressure increase and down to 10 GPa on pressure decrease. Diffraction data were integrated azimuthally using FIT2D 14 and structural information was obtained using JANA2000. 15 Theoretical calculations were performed within the framework of density functional theory, 16,17 as implemented by the VASP package. 18,19 The Perdew-Burke-Ernzerhof generalized gradient approximation 20 has been used for the exchangecorrelation functional and, additionally, the all-electron problem has been simplified by means of ultrasoft pseudopotentials. 21 Plane waves have been chosen as the basis set, and several structures have been studied in the pressure range of interest. For each structure and for every volume, both the lattice parametres and the atomic positions have been optimized via a two step process: Parametres are preoptimized via a low precision relaxation, and this is followed by a high precision relaxation. The final energy has been obtained in a static calculation using a 10ϫ 10ϫ 10 Monkhorst-Pack 22 grid and a 600 eV cutoff. It should be noted that in cases with large length differences between lattice vectors, the grid has been modified accordingly.

III. RESULTS AND DISCUSSION
X-ray diffraction of silane at 11.3 GPa revealed a high quality pattern with numerous reflections ͑Fig. 1͒. At these conditions, phase V of silane is stable, as found by our Raman and infrared measurements, while silane solidifies at 4 GPa, phase III exists in a narrow pressure range from 4 to 6.5 GPa, and phase IV is stable from 6.5 to 10 GPa. 23 With pressure increase, the diffraction peaks become significantly broader, but the diffraction pattern does not change significantly. Above 25 GPa, additional peaks appear ͑Fig. 1͒, which signals a phase transformation. The transition pressure of 25 GPa is in agreement with that obtained in our optical measurements. 23 Phase V of silane is indexed with a monoclinic cell with lattice parameters a = 6.161͑1͒ Å, b = 4.0722͑2͒ Å, c = 6.122͑1͒ Å, and ␤ = 104.36͑1͒°at 11.3 GPa, giving a unit cell volume of 148.78͑1͒ Å 3 at this pressure. All diffraction patterns up to 25 GPa could be indexed with this unit cell. Above this pressure, when the additional peaks appear, all the diffraction peaks, except the additional peaks, could still be fitted with the monoclinic cell. This indicates that above 25 GPa, the structure of phase V either becomes more complex or coexists with an appearing new high-pressure phase. The fit of the pressure dependence of the unit cell volume for phase V ͑Fig. 2͒ with the Birch-Murnagham equation of state gives the following parameters at ambient pressure: V 0 = 250͑9͒ Å 3 and K 0 = 7.8͑9͒ GPa with K 0 Ј= 4 assumed. The estimation using the ambient volume of the SiH 4 molecule of 73 Å 3 gives four molecules in the unit cell. The space group is determined as P2 1 / c ͑No. 14͒. Phase V of SiH 4 appears to be isostructural with SnBr 4 and shares this structure type with several tetrahalides, where the Si atom is tetrahedrally coordinated by four hydrogen atoms ͑Fig. 3͒. Table I  obtained from our ab initio calculations. As can be seen in Fig. 2 ͑inset͒, the positions and intensities of the diffraction peaks of the SiH 4 V are fitted well within the proposed structures. The resulting R factors from the Rietveld refinement are R p = 12.7% and R wp = 9.9%. Although the diffraction rings appeared to be relatively smooth, a strong preferred orientation was present. The March-Dollase preferred orientation model has been used to fit the diffraction pattern, which gave the best fit in the ͓0,1,−1͔ direction. The refined lattice parameters at this pressure are a = 5.699͑1͒ Å, b = 3.814͑1͒ Å, c = 5.643͑1͒ Å, and ␤ = 104.38͑1͒°.
The stability of the SiH 4 V structure is confirmed by our ab initio calculations. According to these ͑Fig. 4͒, in the pressure range from 2 to 27 GPa the monoclinic P2 1 / c structure is favored over other molecular solids ͓with SiH 4 molecules packed in bcc ͑GeF 4 -type͒, simple cubic, and fcc ͑CH 4 -type͒ arrangements considered in a recent theoretical work 5 ͔. It is also more stable than the Pmna structure suggested by Feng et al. 5 and the P2/c structure proposed by Pickard and Needs. 7 As shown in Table I, a very good agreement between calculated and measured structural parameters defining the SiH 4 V structure is also obtained. Additionally, the calculated compressibility of the P2 1 / c phase agrees well with experiment. Fixing K 0 Ј to 4 and fitting only calculated data of SiH 4 V between 5 and 27 GPa to a Birch-Murnagham equation of state gives V 0 = 255͑±2͒ Å 3 and K 0 = 6.1͑±2͒ GPa. This result compares well with the experimental equation of state ͑Fig. 2͒, considering the approximations made for molecular crystals.
Ab initio calculations of the evolution of the Si-H distance with pressure provide additional insight into the modification of its bonding properties. At ambient pressure, the Si-H bond length in the P2 1 / c structure is 1.488 Å, which is identical to that of the free molecule. With increasing pressure, this distance decreases, though perhaps not as much as expected. At 27 GPa, although the volume has suffered a 2.4-fold reduction, corresponding to a 1.34 factor in linear dimensions, the Si-H distance is still 1.466 Å ͑a reduction of only 1.5%͒. On the other hand, the six H-Si-H angles in the tetrahedra range between 107.3°and 111.8°, which is close to the correspond-    ing angle in a regular tetrahedron ͑109.47°͒. Additionally, the band gap decreases rapidly on compression from 6.36 eV at ambient pressure to 2.86 eV at 27 GPa. All these features indicate the prevalence of repulsive intermolecular forces in silane under these conditions and that below 27 GPa silane is an insulating molecular solid. Figure 4 shows that with increasing pressure, both the P2/c structure 7 and the Pmna structure 5 are favored over P2 1 / c. Remarkable differences arise when these structures are compared with SiH 4 V ͑P2 1 / c͒. The P2/c structure is characterized by zigzag chains of H-bonded Si atoms along the c axis, while the Pmna structure can be described as sheets of octahedrally coordinated Si atoms sandwiched between layers formed by H double sheets. Therefore, around 27 GPa covalentlike structures with a higher coordination number begin to be preferred over P2 1 / c. However, we remark that the observed x-ray diffraction pattern above 27 GPa ͑Fig. 1͒ cannot be reproduced considering either P2/c nor Pmna structures with the atomic positions obtained from ab initio calculations. The observed diffraction patterns indicate that SiH 4 transforms to a new structure in this pressure range. In this regime, major changes in electronic properties are predicted. 5 In conclusion, the crystal structure of SiH 4 between 10 and 25 GPa has been solved from powder diffraction data and confirmed by ab initio calculations. In this pressure range, SiH 4 is found to be a molecular insulator with a monoclinic unit cell containing four tetrahedrally bonded molecules ͑space group P2 1 / c͒. The x-ray diffraction patterns observed above 27 GPa are more complex than predicted by the P2 1 / c structure and are not compatible with either P2/c or Pmna structures. Further experimental and theoretical work is underway to investigate the higherpressure phases.