This journal is© the Owner Societies 2019 Phys. Chem. Chem. Phys., 2019, 21, 10931--10938 | 10931

Cite this:Phys.Chem.Chem.Phys.,

2019, 21, 10931

First-principles study of thermal transportproperties in the two- and three-dimensionalforms of Bi2O2Se

Xue-Liang Zhu,ab Peng-Fei Liu, ac Guofeng Xie*bde and Bao-Tian Wang *acfg

Recently, an air-stable layered semiconductor Bi2O2Se has been synthesized [Nat. Nanotechnol., 2017,

12, 530; Nano Lett. 2017, 17, 3021]. It possesses ultrahigh mobility, semiconductor properties, excellent

environmental stability and easy accessibility. Here, we report on the thermal transport properties in

monolayer (ML), bilayer (BL), and bulk forms of Bi2O2Se using density-functional theory and the

Boltzmann transport approach. The results show that the ML exhibits better thermal transport properties

than the BL and bulk. The intralayer opposite phonon vibrations greatly suppress the thermal transport

and lead to an ultralow lattice thermal conductivity of B0.74 W m�1 K�1 in the ML, which has a large

band gap of B2.12 eV, a low value of average acoustic group velocity of B0.76 km s�1, low-lying

optical modes of B0.54 THz, strong optical-acoustic phonon coupling, and large Gruneisen parameters

of B5.69. The size effect for all three forms is much less sensitive due to their short intrinsic phonon

mean free path (MFP).

1 Introduction

Thermoelectric (TE) generators, which provide a promisingroute for converting thermal energy and waste heat into electricalenergy directly, play an important role in sustainable energyfuture.1–3 In general, the conversion efficiency of TE devices isrelatively low on account of their limitations measured by thethermoelectric figure of merit (ZT)4

ZT ¼ sS2T

ke þ kl; (1)

where S, T, s, ke and kl are the Seebeck coefficient, temperature,electric conductivity, electronic thermal conductivity and latticethermal conductivity, respectively. An outstanding TE materialshould exhibit low thermal conductivity and/or a large Seebeckcoefficient.5–9 However, optimizing one parameter without affecting

another is extremely difficult due to complex competition betweenthem. Several representative approaches to enhance ZT mainlyconcentrate on optimizing electrical transport properties by bandstructure engineering,10,11 electron energy barrier filtering,12 andquantum confinement effects13 or reduce the heat conductivityability through nanostructuring14 and phononic crystal patterning.15

These methods help people to simplify conflicting parameters andfocus on optimizing one with little effect on others.

The most popular TE materials are IV–VI (PbSe,16 PbTe,17

and SnSe18) semiconductors, all of which possess fairly low thermalconductivity. However, these TE materials possess some short-comings, such as having high production costs, being toxic,volatile, and having low conversion efficiency. Therefore, one ofthe main purposes in this field is to search environmentallyfriendly materials with high ZT values.5,19 Recent advances inseveral categories of pristine TE materials include but notlimited to silica-based,20 magnesium-based,21 and bismuth-based22 systems. Compared to silica-based and magnesium-based systems, bismuth-based materials possess relatively lowerthermal conductivity and higher carrier mobility. Among them,the bismuth oxide compounds are very popular due to theirextraordinary chemical stability and high TE performance.23–25

Recently, a new star material Bi2O2Se has attracted wide-spread attention in ferroelectric, semiconductor, and TE researchfields.26 It is a typical Bi-based oxychalcogenide material with alayered structure, belonging to the so-called Sillen compoundclass.27 Compared with other bismuth oxide members (Bi2O2S28

and Bi2O2Te29), the ultrathin Bi2O2Se crystals exhibit tunable

a Institute of High Energy Physics, Chinese Academy of Sciences (CAS),

Beijing 100049, China. E-mail: wangbt@ihep.ac.cnb School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, China.

E-mail: gfxie@xtu.edu.cnc Dongguan Neutron Science Center, Dongguan 523803, Chinad School of Materials Science and Engineering, Hunan University of Science and

Technology, 411201 Xiangtan, Chinae Hunan Provincial Key Lab of Advanced Materials for New Energy Storage and

Conversion, Xiangtan, Chinaf Institute of Applied Physics and Computational Mathematics, Beijing 100088,

Chinag Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan,

Shanxi 030006, China

Received 3rd April 2019,Accepted 29th April 2019

DOI: 10.1039/c9cp01867k

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band gap sizes, high Hall mobility values (up to 450 cm2 V�1 s�1),large current on/off ratios (4106) and near-ideal subthresholdswing values (65 mV dec�1) at room temperature.30,31 Theseoutstanding properties make Bi2O2Se a promising candidate forrealizing novel quantum phenomena, future logic transistors andoptoelectronics. Experimentally, this compound can be synthe-sized by replacing Se atoms with O atoms in Bi2Se3

27,32 and theultrathin Bi2O2Se crystals can also be obtained via facile chemicalvapor deposition. Similar to BiCuOSe,33 the distinctive alternationof layer structures with weak atomic bonding in Bi2O2Se can alsosignificantly suppress thermal transport due to the enhancement ofinterfacial scattering of phonons.34 According to the reportedultralow thermal conductivity (0.7–0.8 W m�1 K�1 at 800 K) andthe high mobility,35 Bi2O2Se was considered as a promisingcandidate for TE applications. Additionally, the ZT of bulk Bi2O2Secan reach 0.2 at 800 K35 and can be further improved to 1.42 at800 K by p-type doping.36 For the ML Bi2O2Se, a high ZT value of3.35 was obtained by theoretical calculation at 800 K.37

One of the main factors for the good TE performance ofBi2O2Se is its low lattice thermal conductivity. In this work, byutilizing first-principles calculations and the Boltzmann transportequation, we systematically investigate the change of the thermaltransport properties of Bi2O2Se from the two-dimensional (2D)limit to three-dimensional (3D) bulk. The kl is calculated using theself-consistent iterative approach. The results indicate that theintrinsic kl at 300 K is 0.74, 1.29, and 2.32 W m�1 K�1 for ML, BL,and bulk forms, respectively. Detailed analyses of the phononscattering curve, atomic vibration modes, phonon density of states,phonon velocity, Gruneisen parameters, phonon relaxation time,and three phonon scattering phase space are exhibited to helpexplain their ultralow kl. The size dependence of kl, which isbeneficial to design nanoscale TE devices, is investigated. Theseresults demonstrate that the ML form shows wonderful TEbehaviors and will greatly motivate further experimental effortsto synthesize it.

2 Computational details

First-principles calculations were performed by using the Viennaab initio simulation package (VASP)38 based on density functionaltheory (DFT). A plane wave cutoff of 600 eV, the projector aug-mented wave potentials for the core, and the generalized gradientapproximation39 in the Perdew–Burke–Ernzerhof40 form for theexchange–correlation functional were used. For calculations of theML and BL, a vacuum layer of 20 Å thickness along the z directionwas used to avoid the interaction between adjacent images. A 15�15 � 1 (17 � 17 � 5) k-mesh was used to sample the reciprocalspace of the unit cell for the 2D (3D) form. The geometricstructures were fully relaxed until the residual forces on atomsare less than 0.01 eV Å�1. The total energy convergence was set as10�6 eV Å�1. All the electronic structures were computed byusing the screened exchange hybrid density functional of Heyd–Scuseria–Ernzerhof (HSE06).41

Using the ShengBTE code,42 the thermal transport propertieswere obtained from the Boltzmann transport equation, utilizing

the harmonic second-order interaction force constants (2ndIFCs) and the anharmonic third-order IFCs (3rd IFCs) as inputs.The 2nd IFCs were calculated using the Phonopy code43 using a4� 4� 1 supercell with a 3� 3� 1 k-mesh for all the 2D and 3Dforms. The 3rd IFCs, reflecting the properties for the phonon–phonon scattering, were calculated using the 3� 3� 1 supercellwith the finite-difference method.44 The interactions includingthe sixth-nearest-neighbor atoms were taken into account forcalculating the 3rd IFCs. After testing the convergence of thelattice thermal conductivity with respect to the k-grid, a dense27� 27� 1 (15� 15� 4) k-mesh was used for the 2D (3D) form.

3 Results and discussion3.1 Atomic and electronic structures

Bulk Bi2O2Se crystallizes in a body-centered tetragonal lattice(space group I4/mmm, No. 199) and has 10 atoms in its unit cellas illustrated in Fig. 1(a). The layered structure consisted ofjagged Bi–O layers and planar Se layers alternately arrangedalong the c-axis.27 Fig. 1(b) and (c) show the structures of theML and BL Bi2O2Se, where, to balance the non-stoichiometry,hydrogen atoms are added to passivate the outermost Selayers.37 Unlike Bi2Se3

45 where the van der Waals (vdW) gap isvisible, Bi2O2Se did not exhibit a clear vdW gap.30 The weakelectrostatic interactions connect the layers together. Thus, inour present work, we never included dispersion corrections inall our calculations. Our optimized lattice constants for bulk

Fig. 1 Crystal structures of (a) bulk, (b) BL, and (c) ML Bi2O2Se. (d) Topview of the ML.

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Bi2O2Se are a = 3.92 Å and c = 12.39 Å, slightly larger than theexperimental values of a = 3.88 Å and c = 12.16 Å.30 Theoptimized lattice parameter for the ML form is 4.01 Å, whichis consistent with one previous theoretical report.37 Moredetails about the structural parameters are listed in Table 1.

To illustrate the features of the electronic structure, wepresent the orbital-resolved band structure and correspondingelectronic density of states (DOSs) for 2D and 3D Bi2O2Se inFig. 2. Similar to the band gaps of black phosphorus (2.0 to0.3 eV from the ML to bulk),46 strongly dependent on layerthickness due to the quantum-confinement effects, we observethat the values of the band gaps for Bi2O2Se are also thickness-dependent, specifically, 1.01, 1.08, and 2.12 eV for the ML, BL,and bulk. From our calculated band structures, we find thatBi2O2Se is an indirect band-gap semiconductor with the valenceband maximum (VBM) and the conduction band minimum(CBM) located at X and G points, respectively. In the vicinity ofthe Fermi level, the conduction band (CB) is mainly occupiedby the Bi-p orbitals, whereas the valence band (VB) is primarilyoccupied by the Se-p and O-p orbitals. The Se-p orbitals mainlyspread across the Brillouin-zone (BZ) in the energy range of �2to 0 eV while the O-p orbitals are mainly located below �2 eV,consisting of the DOSs. There is almost no contribution fromBi-p to the VB. This fact is mainly due to the weak bondingbetween the Se layers and the Bi–O layers.47 This can beexplained to some extent by the bond lengthes listed inTable 1. The Bi–Se bonds are greatly longer than the Bi–Obonds. The short Bi–O bonds are responsible for their strongcovalent bonding and bind their own electrons, making them

difficult as free electrons. The long Bi–Se bonds induce weakbonding and leave Se-p electrons easily as free electrons.

We note that twofold degeneracy appears at the X and Rpoints. Such band degeneracy has been shown to be critical forachieving high ZT48,49 and can be realized through properengineering.50 A mixture of light and heavy bands can beobserved near the VBM for all three considered forms. Specifically,the light band appearing around the X point can provide highelectronic mobility while the heavy band around the G point isbeneficial for good Seebeck coefficients.

3.2 Phonon spectrum

The phonon scattering curves and phonon density of states(PhDOS) of Bi2O2Se are shown in Fig. 3. The phonon modes aredefined according to the continuity of their eigenvectors51,52

|Sek,s1*( j )�ek+D,s2( j )| = |ds1,s2 � o(D)|, (2)

where ek,s*( j) is the displacement of the atom j in the eigen-vector of the (k, s) vibrational mode and D is a small wavevector. The phonon dispersion curves can guarantee our optimizedstructures located at the minima on their corresponding potentialenergy surfaces. No imaginary frequency is observed in the BZ,indicating the dynamically stable nature of all our consideredforms for Bi2O2Se. From the decompositions of the phonon curveswith respect to the Bi, O, and Se atomic vibrations, it is clear thatthe three acoustic branches and the optical modes below about5 THz are mainly the Bi and Se atomic vibrations while thevibrations of O atoms dominate in the high-frequency opticalmodes. The lowest frequencies of the optical branch, located atthe G point, for the ML, BL, and bulk Bi2O2Se are 0.54, 0.31, and0.72 THz, respectively, which are smaller than 1.0–1.5 THz forthe typical TE materials Bi2Se3, Bi2Te3, and Sb2Se3.53,54 Thiskind of optical mode softening is beneficial to slow down thephonon transport.55,56 For the optical branches, due to largemass difference between Bi(Se) and O atoms, there are largefrequency gaps of 1.2, 3.3, and 3.7 THz for the ML, BL, and bulkBi2O2Se, respectively. These gaps separate the optical phononmodes into high-energy and low-energy areas. Increasing this

Table 1 Calculated lattice parameters (a/b), bond lengths of Bi–Se (lBi – Se)and Bi-O (lBi–O), and band gaps (Eg) based on the HSE06 method for ML,BL, and bulk Bi2O2Se

Type a/b (Å) lBi–Se (Å) lBi–O (Å) Eg (eV) Ref.

ML 4.01 3.03 2.37 2.12 This workBL 3.90 3.26 2.32 1.08 This workBulk 3.92 3.28 2.31 1.01 This workBulk 3.88 3.27 2.31 0.80 Expt30

Fig. 2 Orbital-resolved band structures together with total and partialelectronic DOSs of (a), (b) ML, (c), (d) BL, and (e), (f) bulk Bi2O2Se.

Fig. 3 Phonon dispersions and PhDOS of (a), (b) ML, (c), (d) BL, and (e),(f) bulk Bi2O2Se.

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gap will reduce the scattering phase space and lead to longerphonon relaxation time and larger kl.

57 The low-energy opticalbranches mix and overlap with acoustic branches, leading tostrong interactions and highly nonlinear dispersion curves.Similar to phosphorene-like crystal SnSe,58 Bi2O2Se deservesstrong anharmonic effects and ultralow kl. Compared with the3D bulk, 2D Bi2O2Se possesses relatively flat phonon spectrumcurves and spiculate PhDOS in acoustic branches and low-energy optical modes, indicating small phonon group velocities.This feature is beneficial to the low thermal conductivity.

3.3 Lattice thermal conductivity

Based on phonon kinetic theory, the thermal transport in asolid can be computed by

kl ¼X3Ni

ðq

Ci;qvi;q2ti;qdq; (3)

where Ci,q, vi,q, and ti,q are the mode specific heat capacity, thephonon group velocity, and the phonon relaxation time, respectively,for the phonon mode i at the wavevector q. The total latticethermal conductivity can be calculated by kl = (ka + kb + kc)/3 andkl = (ka + kb)/2 for 3D and 2D forms, respectively. Our results forBi2O2Se in the temperature range from 300 to 800 K are plottedin Fig. 4, together with available experimental data59,60 andtheoretical results.37 Obviously, the kl of the bulk at high tem-perature is consistent with the experimental data while at lowtemperature, our calculated results are slightly larger than theexperimental values. This kind of deviation may arise from theimpurities, point defects or grain boundary scattering in realcrystals, which are not included in our present calculations. Also,such a deviation has been observed for BiCuSeO61 and manyothers. Here, we also present the kl of bulk Bi2O2Se along a (ka)and c (kc) axes. The kc is much smaller than ka, which mainlyoriginates from the characteristic atomic configuration along thec axis. The weak bonding between the Se layers and the Bi–Olayers greatly suppresses the phonon transport. Meanwhile, the kl

of the ML Bi2O2Se is consistent with previous theoreticalprediction.37 Interestingly, contrary to graphene,62 the value ofthe lattice thermal conductivity for Bi2O2Se increases graduallyfrom the ML to BL and then to the bulk. To further investigate theorigin of the low intrinsic kl and the difference among them, wecalculate the phonon transport properties and discuss them indetail in next sections.

3.4 Phonon velocity

The phonon group velocities are closely relevant to the thermaltransportation. Using our phonon dispersions, we can calculatethe phonon group velocities by

viðqÞ ¼@oiðqÞ@q

; (4)

where oi(q) is the phonon frequency. Fig. 5(a)–(c) show thephonon group velocity for the ML, BL, and bulk Bi2O2Se,respectively. The different shapes of the acoustic phonon dis-persion will lead to different group velocities near the zonecenter. Just as we predicted above, the average values ofthe acoustic group velocities of the ML (0.76 km s�1) and BL(0.87 km s�1) are relatively smaller than that of the bulk(1.03 km s�1), especially for the low-frequency phonons, whichleads to a decreasing behavior of the lattice thermal conductivityfrom bulk to the 2D limit. For the ML, the two transverse andthe one longitudinal acoustic phonon velocities at the G pointare 1.59, 2.35, and 3.04 km s�1, respectively, which are smallerthan the values of 2.24, 2.37, and 3.35 km s�1 for the BL, and1.17, 2.56, and 4.48 km s�1 for bulk (see Table 2).

3.5 Gruneisen parameters and phonon relaxation time

In order to gain more insights into the thermal transport propertiesof Bi2O2Se, we introduce the Gruneisen parameters and phononrelaxation time for each of the phonon branches. The Gruneisenparameters can be used to quantitatively analyze the anharmonicinteractions of a crystal. It can be calculated according to

giðqÞ ¼ �V

oiðqÞ@oiðqÞ@V

; (5)

where V is the volume. Generally, large g indicates the existence of astrong phonon–phonon anharmonic scattering process64 and canlargely suppress the thermal transportation at high temperature.From Fig. 5(d)–(f) we surprisingly find that the ML and BL Bi2O2Sepossess ultrahigh values of Gruneisen parameters (the maximumvalues up to 72 and 53). These values are prominently larger thanthe maximum value of 16 for the bulk. This implies stronganharmonicity and low thermal conductivity in 2D Bi2O2Se. Thelarge Gruneisen parameters, mainly existing in the low-frequencyregion, are crucial to the phonon transport. The average values ofthe acoustic Gruneisen parameters,18,65

�gi ¼P ffiffiffiffiffiffiffiffi

gi;q2qP

q; (6)

�g ¼ 1

3�gTA1

þ �gTA2þ �gLA

� �; (7)

Fig. 4 Calculated lattice thermal conductivity. The solid lines are ourcalculated results, the squares and triangles are the experimental data,59,60

and the pentagrams are the theory data.37,63

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are calculated to be 5.69, 5.32 and 3.68 for the ML, BL, and bulkBi2O2Se. These values are comparable to that of low kl compounds,such as SnSe (2.83),18 PbTe (1.45),66 and GeSe (0.73).56

The phonon relaxation time can be obtained by the summationof various scattering processes, such as phonon–phonon Umklappscattering, boundary scattering, and defect scattering.67 Here,we calculate it from the anharmonic 3rd IFCs and plot it inFig. 5(g)–(i). It can be clearly seen that the phonon relaxationtime of the ML is distinctly lower than that of the bulk. Thisindicates that the ML has stronger phonon scattering than thebulk. The average acoustic phonon relaxation time is 5.41 ps forbulk, about five times that for the ML (1.04 ps) and slightlylarger than that of the BL (4.96 ps). Thus, the enhancement of

the phonon–phonon scattering, combined with the decrease inphonon group velocity, leads to the reduction of the kl for 2DBi2O2Se with respect to its 3D bulk.

In order to further understand the phonon relaxation time,we calculate the three phonon scattering phase space (P3), whichcan qualitatively characterize the magnitude of the phononscattering, and plot it in Fig. 6. P3 is defined by57,68

P3 ¼2

3OPðþÞ3 þ 1

2Pð�Þ3

� �; (8)

Pð�Þ3 ¼

Xj

ðdqD

ð�Þj ðqÞ; (9)

where O is a normalization factor and D(�)j stands for the

absorption and emission processes. We can see clearly thatcompared to 2D Bi2O2Se, the bulk has smaller scattering phasespace in the whole frequency region. In other words, 2D Bi2O2Seexists in larger phase space allowing phonon–phonon scattering,also responsible for smaller values of the kl, than that of the bulk.

3.6 Cumulative lattice thermal conductivity

Nanostructuring is one major approach for ZT enhancement,since it effectively hinders phonon transport.69–71 If the size of

Fig. 5 Calculated phonon group velocities (a)–(c), Gruneisen parameters (d)–(f), and phonon relaxation time (g)–(i) for ML (left), BL (middle), and bulk(right) at 300 K. The average values of acoustic group velocity, Gruneisen parameters and phonon relaxation time are shown in insets.

Table 2 Summary of the lowest optical frequency at the G point (oo,min inTHz), lattice thermal conductivity (kl in W m�1 K�1), transverse (TA1/TA2) andlongitudinal (LA) acoustic phonon group velocities (nTA1/TA2/LA in km s�1),average acoustic Gruneisen parameters (�gTA1/TA2/LA) and acoustic phononrelaxation time (�tTA1/TA2/LA in ps) for ML, BL and bulk Bi2O2Se

Type oo,min kl nTA1nTA2

nLA �gTA1�gTA2

�gLA �tTA1�tTA2

�tLA

ML 0.54 0.74 1.59 2.35 3.04 5.15 5.57 6.35 0.34 1.31 1.47BL 0.31 1.29 2.24 2.37 3.35 5.76 6.46 3.75 5.29 4.75 4.86Bulk 0.72 2.32 1.17 2.56 4.48 3.53 4.47 3.22 5.38 6.61 4.52

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the device is as small as the phonon mean free path (MFP), the kl

would be significantly depressed by the frequent phonon-surfacescattering. Thus, knowledge of MFP is essential to understandthe size effects and also important for designing nanoscale TEdevices.72,73 To get the characteristic MFP, we fit the cumulativethermal conductivity as a single parametric function

kðlÞ ¼ kmax

1þ l0

l

; (10)

where kmax and l0 are the ultimate thermal conductivity andevaluated characteristic phonon MFP. The cumulative latticethermal conductivity with respect to MFP at room temperaturefor 2D and 3D forms of Bi2O2Se is shown in Fig. 7. Thecumulative k(l) keeps increasing as l increases, until reachingthe thermodynamic limit. The values of the l0 for the ML, BL andbulk Bi2O2Se at 300 K are 4.6, 5.4 and 6.9 nm, respectively.Generally, nanostructures with l0 smaller than 10 nm are beneficialto their TE performance. Compared to the l0 of 110 nm for TaAs,74

Bi2O2Se shows fundamentally smaller l0 values. Thus, the sizedependence of the kl for Bi2O2Se is not that significant.

3.7 Atomic vibration mode

To further study phonon thermal transport, we investigatecarefully the atomic motions based on our calculated 2nd IFCs,especially at high symmetry points in BZ. Here, we show inFig. 8 the vibration modes of the lowest optical branch for Bi2O2Seat the G point. Similar to graphene,75 these optical branches areeasily thermally activated at room temperature and greatly affectthermal conductivity via phonon–phonon scattering. For the ML,we find that the Bi atoms move in opposite directions with respectto the other atoms, including both intralayer O atoms and inter-layer Se atoms. Such short distance (monoatomic layer) of thealternation of their atomic vibration modes would greatly reducethe heat transport ability.34 Many phonon energies are dispersedby this kind of opposite atomic mode. For the BL and bulk, theircentral Se atoms stay fixed, while the upper Bi–O layer has counteratomic vibration modes with respect to the lower one. Theopposite atomic vibration modes within the Bi–O layer have notbeen observed for both the BL and bulk. The alternating distanceof their atomic vibration modes is up to four-atomic layers in the caxis, greatly larger than that in the ML. This difference can explainwell why the ML exhibits that small kl. Besides, the atomicvibration modes are all within the ab plane, not along the c axis,which is responsible for the smaller values of ka in comparisonwith kc, for bulk.

4 Conclusion

In summary, we report on the lattice thermal properties of theML, BL, and bulk Bi2O2Se using DFT and an iterative solutionof the phonon Boltzmann transport equation. The results showthat the intrinsic lattice thermal conductivity of the ML form islargely lower than that of the BL and bulk in the temperaturerange of 300–800 K. The ML Bi2O2Se shows good thermaltransport features, such as soft optical modes, small valuesof phonon velocity, large Gruneisen parameters, and shortphonon relaxing time. Our results shed light on the inherentphysical nature of the phonon transport and indicate thatBi2O2Se is a good candidate to investigate layer-dependentperspectives of thermal properties.

Fig. 6 Comparison of three phonon scattering phase space for the ML,BL, and bulk Bi2O2Se. The total three phonon scattering phase space isshown in the inset.

Fig. 7 Cumulative lattice thermal conductivity of Bi2O2Se along the a axisas a function of the MFP at 300 K.

Fig. 8 The atomic vibration modes of the lowest optical branch at the Gpoint for the ML (a), BL (b), and bulk (c).

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Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The calculations were performed at the Supercomputer Centrein the China Spallation Neutron Source. This work was financiallysupported by the National Natural Science Foundation of China(NSFC) (Grant No. 11874145) and the PhD Start-up Fund ofNatural Science Foundation of Guangdong Province, China(No. 2018A0303100013).

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