Applications of the variable-composition structure prediction

Transcription

1 Applications of the variable-composition structure prediction Chaohao Hu School of Materials Science and Engineering Guilin University of Electronic Technology August 7, 2013, GUET, China

2 Contents Why do we use the variable-composition structure (VCS) prediction? The variable-composition structure prediction on B-H system

3 Why do we use the VCS prediction Enthalpy of formation a b c f d e A x B

4 Why do we use the VCS prediction Building the high-pressure phase of A-B system from the fixed composition search (FCS) A m B (m =1, 2, 3, 4, 5, ), 1~4 formula unit (f.u.) FCS AB n (n =1, 2, 3, 4, 5, ), 1~4 f.u. FCS A m B n (m = 1, 2, 3, 4, 5, ; n =1, 2, 3, 4, 5, ), 1-4 f.u. FCS W 2 B 5, Fe 3 O 4, Cr 3 B 4, Ta 3 N 5, Nb 2 O 5, Fe 5 C 2 m =? n=?

5 Why do we use the VCS prediction Fixed composition search on Na-H: NaH n (n = 6-12) P. Baettig and E. Zurek, Phys. Rev. Lett 106, (2011) NaH 7, NaH 9, NaH 11 are energetically favorable in 25~300 GPa But are the searches on NaH 6, NaH 8, NaH 10, NaH 12 necessary?

6 Why do we use the VCS prediction Advantages of the VCS prediction: Enhancing the efficiency of structure prediction Being easier to explore new stable compounds with special chemical composition

7 B-H system: Background High-pressure effects - on chemical bonding - on coordination number -etc. Influencing the physical and chemical properties of matters Uncovering many novel phenomena: - structural transition - decomposition - superconductivity -etc. H 2, SiH 4, GeH 4,

8 B-H system: Background Diborane at ambient conditions Diborane (B 2 H 6 ) is an electrondeficient molecule, containing four terminal and two bridging H atoms. B H H H B d B-H_bridge = 1.33 Angst. d B-H_terminal = 1.19 Angst. B-H-B three-center two-electron bonds (3c-2e) are formed in B 2 H 6.

9 B-H system: Background Diborane under high pressure (previous work) Phase I, Liquid-solid transition around 4 GPa; Phase II, Phase transition above 6.9 GPa; Phase III, above 14.7 GPa Y. Song, C. Murli, and Z. Liu, J. Chem. Phys. 131, (2009) C. Murli and Y. Song, J. Phys. Chem. B 113, (2009)

10 B-H system: Background Diborane under high pressure (recent work) Above 36 GPa, an extended structure constructed from chains of BH 3 stoichiometry becomes in our calculations the most stable phase and remains so to at least 100 GPa Y. Yao and R. Hoffmann, J. Am. Soc. Chem. 133, (2011)

11 B-H system: Background Diborane under high pressure (recent work) Although B 2 H 6 is thermodynamically unstable to phase separation into B and H in the intermediate pressure range, it is restabilized beyond 350 GPa. K. Abe and N. W. Ashcroft, Phys. Rev. B 84, (2011)

12 B-H system: Computational details Structure search using USPEX Variable composition search (VCS) + fixed composition search (FCS) Some main input parameters: calculationtype: 1 (FCS) or 4 (VCS) bestfrac: 0.70 fracgene: 0.40 fracrand: 0.20 fractrans: 0.20 fracatomsmut: 0.20 Pressures: 10, 20, 50, 100, 200, and 300 GPa

13 B-H system: Computational details Total-energy calculations using VASP Exchange-correlation functional: GGA-PBE Electron-ion interaction: PAW Kinetic energy cutoff: 540 ev Relaxation: full optimization K-points mesh: less than 2π*0.03 Å -1 Convergence criteria: energy difference < 0.1 mev; force < 1 mev/å -1

14 B-H system: Computational details Lattice dynamics and EPC using QE Pseudopotential: ultrasoft, GGA- PBE, Vanderbilt-type Kinetic energy cutoff: 180 Ry Monkhorst-Pack k-point grid: 24*24*16 for the selfconsistent calculations q-point mesh during EPC calculations: 6*6*4

15 B-H system: Current results Complete convex-hulls built from the VCS

16 B-H system: Current results More accurate convex-hulls from the FCS α-b 12 Pbcm P-1 P6 3 /mc γ-b 28 P6 3 /mc Ibam P2 1 /c α-ge-type Ibam P2 1 /c P6 3 /mc α-ge-type P6/mmm P2 1 /c C2/c

17 B-H system: Current results Enthalpy-pressure relation of BH 3

18 B-H system: Current results Two energetically favorable B 2 H 6 containing H 2 Cm (H 2 ) P2 1 /c (H 2 )

19 B-H system: Current results Enthalpy-pressure relation of BH Structural Transitions: Pbcm -> Ibam, 22 GPa Ibam -> P6/mmm, 168 GPa After ZPE correction: Ibam -> P6/mmm, 159 GPa

20 B-H system: Current results Equation of states (EOS) of BH

21 B-H system: Current results Energetically favorable phases of BH Pbcm, 20 GPa Ibam, 150 GPa P6/mmm, 200 GPa 2D structures with puckered BH layers 3D structure with flat close-packed B layers connected by H

22 B-H system: Current results Transformation barrier between Ibam and P6/mmm Ibam -> P6/mmm is two-stage Energy barrier: Ibam Pbcm P6/mmm 0.32 ev/f.u. for the first stage (Ibam -> Pbcm) 0.19 ev/f.u. for the second stage (Pbcm -> P6/mmm) Energy barrier calculated from the VC-NEB method

23 B-H system: Current results Pressure-induced decomposition of B 2 H 6 B 2 H 6

24 B-H system: Current results High-pressure phase diagram of B-H system

25 B-H system: Current results Band structure of the Ibam phase of BH at 150 GPa Eg = 0.19 ev

26 B-H system: Current results Band structure and DOS of the P6/mmm BH

27 B-H system: Current results Fermi surface of the P6/mmm BH at 175 GPa

28 B-H system: Current results ELF of the P6/mmm and Ibam phases of BH Ibam, 150 GPa P6/mmm, 200 GPa

29 B-H system: Current results Phonon related properties of the P6/mmm BH

30 B-H system: Current results T c from the Allen-Dynes equation At 175 GPa, Tc = 14.1 K (u*=0.13) or 21.4 K (u*=0.10) T c decreasing with pressure in an approximate rate of K/GPa

31 B-H system: Discussion BH (P6/mmm) Vs. MgB 2 Vs. BH (P6/mmm) MgB 2 (P6/mmm) (H + ) 2 B 2 Vs. (Mg 2+ )B 2

32 B-H system: Acknowledgments Thanks: 1. Financial support from NSFC, Guangxi NSF, and NSF 2. Co-authors: Artem R. Oganov, G. Frapper, and others 3. Computational facilities from CFN-BNL and GUET

33 Summary 1. In USPEX code, variable composition search not only enhances the efficiency in predicting crystal structures, but also improves the capability in exploring new stable compounds. 2. A complete high-pressure phase diagram of B-H system has been built from the variable composition search in combination with the fixed composition search. 3. High pressure leads diborane to decompose into BH and H2, not into B and H2. Especially, the hexagonal P6/mmm phase of BH is a new phonon-mediated supconductor.

34 Thank you for your attention!