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Lu Mengya, Huang Yanping, Tian Fubo, Li Da, Duan Defang, Zhou Qiang, Cui Tian. Ab initio studies on ammonium iodine under high pressure. Chinese Physics B, 2020, 29(5): 053104  
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Ab initio studies on ammonium iodine under high pressure
Lu Mengya1, Huang Yanping1, Tian Fubo1, †, Li Da1, Duan Defang1, Zhou Qiang1, Cui Tian1, 2, ‡
State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China
School of Physical Science and Technology, Ningbo University, Ningbo 315211, China

 

† Corresponding author. E-mail: tianfb@jlu.edu.cn cuitian@jlu.edu.cn

Project supported by the National Natural Science Foundation of China (Grant Nos. 11574109, 51632002, 51572108, 91745203, and 11574112), the National Key Research and Development Program of China (Grant Nos. 2016YFB0201204 and 2018YFA0305900), the Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT 15R23), and the National Fund for Fostering Talents of Basic Science of China (Grant No. J1103202)

Abstract

Ammonium iodine (NH4I) as an important member of hydrogen-rich compounds has attracted a great deal of attention owing to its interesting structural changes triggered by the relative orientations of adjacent ammonium ions. Previous studies of ammonium iodide have remained in the low pressure range experimentally, which we first extended to so high pressure (250 GPa). We have investigated the structures of ammonium iodine under high pressure through ab initio evolutionary algorithm and total energy calculations based on density functional theory. The static enthalpy calculations show that phase V is stable until 85 GPa where a new phase Ibam is identified. Calculations of phonon spectra show that the Ibam phase is stable between 85 GPa and 101 GPa and the Cm phase is stable up to 130 GPa. In addition, ammonium iodine dissociates into NH3, H2, and I2 at 74 GPa. Subsequently, we analyzed phonon spectra and electronic band structures, finding that phonon softening is not the reason of dissociation and NH4I is always a semiconductor within the pressure range.

PACS: ;31.15.A-;;61.50.Ah;;61.82.Fk;
Keyword:;hydrogen-rich compounds;;high pressure;;phase transition;
1. Introduction

Hydrogen has been a concern for ages due to its potential of being room-temperature superconductor.[1–5] Dias et al.[6] have reported metallization of solid H2 at 495 GPa, however, its reality and difficulty of reaching so high pressure compel us to turn our attention to hydrogen-rich compounds,[7] which are able to metallize at much lower pressures. In recent years, in addition to the breakthrough in H3S,[8,9] extensive hydrogen-rich compounds with high Tc have been uncovered consecutively, such as LaH10[10,11] and YH10.[12] Nevertheless, ammonium halides as another important member of hydrogen-rich compounds have received much less attention.

Research on structures has always been one of the vital and major scientific tasks, and is also of challenge due to their complexity and diversity. Ammonium halides have been grabbing researchers’ eyes owing to their similarity to alkali halides on the one hand and abundant phase transitions involving the relative orientations of adjacent ammonium ions[13] on the other hand. It is well known that there exist five phases in NH4Br and NH4I. An ordered tetragonal phase III does not exist in NH4Cl. The others are a disordered NaCl structure (phase I), a disordered CsCl structure (phase II), an ordered CsCl structure (phase IV) and a slightly distorted tetragonal CsCl structure (phase V). These have successively been proved experimentally[14–22] and theoretically.[23–25] At room temperature, phase I transforms to phase II at about 0.05 GPa with increasing pressure, subsequently a further transition to phase IV with a parallel ordering of ammonium ions occurs at 2.7 GPa.[21] As the pressure increases up to 5.4 GPa,[15] a phase transition to phase V occurs, interestingly the transition is caused by reorientational motions of the ions in phase IV. Nevertheless, numerous experiments have been conducted to determine the phase transition and to establish the crystallographic structures, such as x-ray,[26] neutron diffraction,[27–31] electron diffraction,[32] Raman,[14,15,17] and infrared studies.[33] Although extensive efforts have been made for NH4I, the structures, stability and electronic band structures of ammonium iodine at high pressure are still little known. Here we will study them up to 250 GPa at zero temperature.

2. Computational methods

The evolutionary algorithm performed with the USPEX code[34–36] is implemented to explore potential stable structures of NH4I at pressures 0–250 GPa, which is a potent tool of predicting all thermodynamically stable compounds in the case of given elements. The first generation is generated randomly, then 60 % of low-energy structures are passed on to the next generation. The process is terminated after about 40 generations. Our structure relaxations are carried out using the density functional theory within the Perdew–Burke–Ernzerhof functional (generalized gradient approximation)[37] and the projector-augmented wave method as implemented in the VASP code.[38,39] A cutoff energy of 900 eV and the Mokhorst–Pack k-points meshes with a reciprocal space resolution of 2π × 0.03 Å−1 are used to ensure that the total energy is well converged to less than 1 meV per atom. We also calculated phonon dispersion curves with the PHONOPY code[40,41] based on a supercell approach with the force-constant matrices. Supercell of each phase contains 100 atoms at least.

3. Results and discussion
3.1. Crystal structures and stability of NH4I

We performed fixed-composition structure prediction of NH4I with the cell size containing two, three and four formula units by the ab initio evolutionary algorithm up to 250 GPa. There is no doubt that these three phases (III, IV and V) have been found and confirmed experimentally by us.[14–22] Still, three novel structures with space group Ibam, Cm (in Fig. 2) and P1 have been uncovered for the first time. Before the pressure reaches 70 GPa, the structures and phase transformation sequence are almost identical to the previous results. We present the enthalpy differences of phases III and V (phase IV is the reference) at 0 K in Fig. 1(a), where we can easily see phases III and IV existing in narrow pressure ranges, 0–3.3 GPa and 3.3–9 GPa, respectively. Heyns and Hochheimer suggested the transition pressures are 0.45 GPa[14,15] and 4 GPa[14,17] at 100 K by measuring Raman spectroscopy. Balagurov uncovered that phase transition from phase IV to phase V takes place at 8.6 GPa[42,43] from neutron diffraction experiments at room temperature. The discrepancies between the experimental results and our calculated results are distinctly caused by the temperature effect. In addition, Fig. 1(b) shows the enthalpy difference diagram of those mentioned new phases relative to phase V, from which we can see that as pressure is enhanced to 85 GPa, the new phase with space group Ibam emerges. When pressure reaches 101 GPa the orthorhombic Ibam phase transforms to the monoclinic Cm phase. Another phase P1 is predicted to emerge above 154 GPa. Due to the dynamic instability, i.e., the phonon frequencies soften to negative value, the P1 phase will not be mentioned at all. However, considering the instability of HI[44] and precedent of NH4Br,[45] we think that NH4I has the possibility of dissociation under high pressure. Then, we compared the enthalpies of all predicted structures and above 50 GPa, finding that phase V persists up to 74 GPa, where NH4I dissociates into .

Fig. 1. (a) Enthalpy curves for phases III and V (relative to IV phase) as a function of pressure. The inset figures out the transition pressure points from III to IV, then to V at 0 K. (b) Enthalpy curves of our predicted phases Ibam, Cm and P1 as a function of pressure (relative to phase V).
Fig. 2. Structure of NH4I with space group P4/nmm (phases III and V) (a), (phase IV) (b), Ibam (c) and Cm (d). Ammonium ions take the tetrahedron shape with N3− ion (blue solid ball) at the center and H+ ion (grey solid ball) at the corner. The red solid balls represent iodine ions.

As shown in Fig. 2, the P4/nmm and structures have similar configurations, in which iodine ion is located in the body center and surrounded by eight neighbor ammonium ions. In addition, there are four nearest neighbor N–H…I bonds in both the structures. However, in the P4/nmm structure the orientation of each ammonium ion is determined by the rotation of its neighbor by 90°, resulting in the four nearest neighbor N–H…I bonds distributing on the same side. In the structure all the ammonium ions have the same orientation so the N–H…I bonds between the ammonium ions pointing directly toward the iodine ion are distributed on the diagonal lines. These prove that the transition is caused by the orientation change of ammonium ions.

We have calculated the phonon dispersion curves of NH4I along high symmetry directions as shown in Fig. 3. Only the phonon dispersion curves of 80 GPa for phase V, 90 GPa for Ibam structure, 120 GPa for Cm structure and 160 GPa for P1 structure are presented here. We can conclude from the calculated results that phase V exists from 9 GPa to 85 GPa, where no imaginary phonon frequency is observed in the whole Brillouin zone, indicating that the structure is dynamically stable. Subsequently, we calculated the phonon dispersion curves of the Ibam phase at 90 GPa and the Cm phase at 120 GPa and even higher pressures, finding that the Ibam phase is dynamically stable from 85 GPa to 101 GPa and the Cm phase is stable up to 130 GPa. Nevertheless, the P1 phase has been dynamically unstable in the pressure range we studied, because the phonon frequencies soften to negative values.

Fig. 3. The calculated phonon dispersion curves of NH4I for (a) phase V at 80 GPa, (b) the Ibam phase at 90 GPa, (c) the Cm phase at 120 GPa and (d) the P1 phase at 160 GPa.
3.2. Electronic properties of NH4I

To further study the electronic properties of NH4I, the electronic band structures and density of states (DOS) are calculated. Here we merely present electronic band structures and corresponding density of states of the Ibam phase at 90 GPa and the Cm phase at 120 GPa, respectively. An apparent gap is observed in both the band diagrams, implying the character of the semiconductor. In addition, it is at first a direct gap semiconductor, then turns to an indirect gap semiconductor. We also calculated the gap values of phase III, IV, V and new phases with pressure in Fig. 5, finding that the gap increases first, as the structure transforms into phase V the value begins to decrease. From the figure of band energy we can see that as pressure is enhanced, the distances among atoms decrease and the hybridization between N and I atoms increases. Furthermore, the conduction band (bottom) moves from the high-energy zone to the low-energy zone gradually, causing the variation of band gap.

Fig. 4. The electronic band structures and density of states for the Ibam phase at (a) 90 GPa and the Cm phase at (b) 120 GPa.
Fig. 5. The calculated band gaps of NH4I versus pressure.
4. Conclusions

We have searched the structures of NH4I up to 250 GPa by ab initio evolutionary algorithm based on DFT. The novel structures with space groups Ibam and Cm have been uncovered for the first time. The Ibam and Cm phases are stable in pressure ranges of 85–101 GPa and 101–130 GPa, respectively. However, considering the instability of HI, we compare the enthalpy values of all predicted structures and , and find that phase V dissociates into at 74 GPa. We also verify that the mechanism of III–IV to IV–V phase transitions is the ions rotating 90° around the c-axis from the point of microscopic view. The calculations of electronic band structure indicate that it is a semiconductor, and the gap increases first, as the structure transforms into phase V the value begins to decrease.

Parts of calculations were performed in the High Performance Computing Center (HPCC) of Jilin University.

Reference
[1] Duan D F Tian F B He Z Meng X Wang L C Chen C B Zhao X S Liu B B Cui T 2010 J. Chem. Phys. 133 074509
[2] Cudazzo P Profeta G Sanna A Floris A Continenza A Massidda S Gross E K U 2008 Phys. Rev. Lett. 100 257001
[3] McMahon J M Ceperley D M 2011 Phys. Rev. 84 144515
[4] Drozdov A Eremets M Troyan I 2015 arXiv:1508.06224vl
[5] Peng F Sun Y Pickard C J Needs R J Wu Q Ma Y M 2017 Phys. Rev. Lett. 119 107001
[6] Dias R P Silvera I F 2017 Science 355 715
[7] N W 2004 Phys. Rev. Lett. 92 187002
[8] Duan D F Huang X L Tian F B Li D Yu H Y Liu Y X Ma Y B Liu B B Cui T 2015 Phys. Rev. 91 180502
[9] Drozdov A P Eremets M I Troyan I A Ksenofontov V Shylin S 2015 Nature 525 73
[10] Liu H Y Naumov I I Geballe Z M Somayazulu M John S T Hemely R J 2018 Phys. Rev. 98 100102
[11] Somayazulu M Ahart M Mishra A K Geballe Z M Baldini M Meng Y Struzhkin V V Hemely R J 2019 Phys. Rev. Lett. 122 027001
[12] Heil C Simone D C Bachelet G B Boeri L 2019 Phys. Rev. 99 220502
[13] Pistorius C W 1976 Prog. Solid State Ch. 11 1
[14] Hochheimer H D Spanner E Strauch D 1976 J. Chem. Phys. 64 1583
[15] Heyns A M Hirsch K R Holzapfel W B 1980 J. Chem. Phys. 73 105
[16] Pistorius C W F T 1969 J. Chem. Phys. 50 1436
[17] Heyns A M Hirsch K R Holzapfel W B 1979 Solid. State. Commun. 29 351
[18] Stevenson R 1961 J. Chem. Phys. 34 346
[19] Press W Eckert J Cox D E Rotter C Kamitakahara W 1976 Phys. Rev. 14 1983
[20] Leung R C Zahradnik C Garland C W 1979 Phys. Rev. 19 2612
[21] Andersson P Ross R G 1987 J. Phys. C. Solid. State. 20 4737
[22] Huang Y P Huang X L Wang L Wu G Duan D F Bao K Zhou Q Liu B B Cui T 2015 RSC. Adv. 5 40336
[23] Yamada Y Mori M Noda Y 1972 J. Phys. Soc. Jpn. 32 1565
[24] Vaks V G Schneider V E 1976 Phys. Status Solidi (a) 35 61
[25] Hüller A 1974 Z. Phys. 270 343
[26] Jeon S J Porter R F Vohra Y K Ruoff A L 1987 Phys. Rev. B Condens. Matter 35 4954
[27] Seymour R T A W 1970 Acta Crystall B-Stru 26 1487
[28] Goyal P S Dasannacharya B A 1979 J. Phys. C. Solid. State 12 219
[29] Kozlenko D P Glazkov V P Savenko B N Somenkov V A Hull S 2000 Int. J. High Press. Res. 17 235
[30] Glazkov V P Kozlenko D P Savenko B N Somenkov V A 2000 J. Exp. Theor. Phys. 90 319
[31] Glazkov V P Kozlenko D P Savenko B N Somenkov V A Telepnev A S 2001 J. Exp. Theor. Phys. Lett. 74 415
[32] Kolomiichuk V N 1966 Sov. Phys. Crystallogr. 10 475
[33] Durig J R Antion D J 1969 J. Chem. Phys. 51 3639
[34] Oganov A R Glass C W 2006 J. Chem. Phys. 124 244704
[35] Oganov A R Lyakhov A O Valle M 2011 Acc. Chemical Research 44 227
[36] Lyakhov A O Oganov A R Stokes H T Zhu Q 2013 Comput. Phys. Commun. 184 1172
[37] Perdew J P Burke K Ernzerhof M 1996 Phys. Rev. Lett. 77 3865
[38] Kresse G Furthmüller J 1996 Phys. Rev. 54 11169
[39] Kresse G Joubert D 1999 Phys. Rev. 59 1758
[40] Togo A Tanaka I 2015 Scr. Mater. 108 1
[41] Togo A Oba F Tanaka I 2008 Phys. Rev. 78 134106
[42] Levy H A Peterson S W 1953 J. Am. Chem. Soc. 75 1536
[43] Balagurov A M Kozlenko D P Savenko B N Glazkov V P Somenkov V A Hull S 1999 Phys. B: Condens. Matter 265 92
[44] Binns J Liu X D Philip D S Afonina V Gregoryanz E Howie R T 2017 Phys. Rev. 96 144105
[45] Tian F B Li D Duan D F Chen C B He Z Sha X J Zhao Z L Liu B B Cui T 2014 Chin. Sci. Bull. 59 5272