RSC Advances PAPER View Article Online View Journal | View Issue Perfect cubic La-doped boron clusters La6& +/0 [La@B24] as the embryos of low-dimensional Cite this: RSC Adv., 2020, 10,12469 lanthanide boride nanomaterials† Xiao-Qin Lu, Mei-Zhen Ao, Xin-Xin Tian, Wen-Yan Zan, * Yue-Wen Mu * and Si-Dian Li * La-doped boron nanoclusters have received considerable attention due to their unique structures and bonding. Inspired by recent experimental observations of the inverse sandwich D8h La2B8 (1) and triple- À decker C2v La3B14 (2) and based on extensive global searches and first-principles theory investigations, + 1 we present herein the possibility of the perfect cubic La-doped boron clusters Oh La6&[La@B24] (3, A1g) 2 and Oh La6&[La@B24](4, A2g) which appear to be the embryos of the metallic one-dimensional La10B32 (5) nanowire, two-dimensional La3B10 (6) nanosheet, and three-dimensional LaB6 (7) nanocrystal, facilitating a bottom-up approach to build cubic lanthanide boride nanostructures from gas-phase Creative Commons Attribution 3.0 Unported Licence. clusters. Detailed molecular orbital and bonding analyses indicate that effective (d–p)s,(d–p)p and (d–p) +/0 d covalent coordination interactions exist in La6&[La@B24] (3/4) clusters, while the 1D La10B32 (5), 2D Received 19th February 2020 La B (6), and 3D LaB (7) crystals exhibit mainly electrostatic interactions between the trivalent La Accepted 16th March 2020 3 10 6 centers and cubic B24 frameworks, with weak but discernible coordination contributions from La (5d) ) DOI: 10.1039/d0ra01616k +/0 B (2p) back-donations. The IR and Raman spectra of La6&[La@B24] (3/4) and band structures of rsc.li/rsc-advances La10B32 (5) and La3B10 (6) are computationally simulated to facilitate their future characterizations. 1. Introduction between n ¼ 16–25,11 unveiling another structural domain for This article is licensed under a boron nanoclusters. Transition-metal-doping has proven to As a prototypical electron-decient element next to carbon in induce dramatic structural and bonding pattern changes to the periodical table, boron has a rich chemistry characterized boron clusters. Typical examples include the transition-metal- À with delocalized multicenter-two-electron (mc-2e) bonds in centered monocyclic boron wheel clusters M@Bn (Co@B8 , Open Access Article. Published on 27 March 2020. Downloaded 5/25/2020 8:13:46 AM. À À 1 both bulk allotropes and polyhedral molecules. Boron-based Ru@B9 , and Ta@B10 ) with the maximum planar coordina- materials have found wide applications in eld emissions, tion number of CN ¼ 10 and transition-metal-centered boron À À À 2–6 supercapacitors, optical absorptions, photodetectors, and etc. double-ring tubular clusters M@Bn (Co@B16 , Rh@B18 , and À ¼ A bottom-up approach has received considerable attention in Ta@B20 ) with the record tubular coordination number of CN – the past two decades to investigate the structural transitions 20 known in experiments.12 18 A family of inverse sandwich di- À from small boron nanoclusters to boron nanomaterials. lanthanide boron complexes D7h La2B7 , D8h La2B8 (1), and À ¼ Persistent joint photoelectron spectroscopy (PES) and rst- D9h La2B9 with a perfect Bn ring (n 7, 8, 9) sandwiched principles theory investigations in the past two decades by between two La atoms were observed in 2018 in PES measure- Wang and coworkers have unveiled an unexpectedly rich land- ments.19,20 The rst inverse triple-decker di-lanthanide boron À scape for size-selected boron clusters from planar or quasi- cluster C2v La3B14 (2) was discovered in 2019 in a joint exper- À/0 21 planar Bn (n ¼ 3–38, 41 and 42) to cage-like borospherenes imental and theoretical investigation in which two conjoined À/0 À – D2d B40 and C3/C2 B39 . Multiple low-lying isomers appear to La2B8 (1) inverse sandwiches share a B B edge and a La vertex in À/0 – – – – compete and coexist in most gas-phase boron Bn clusters, the titled La B8 La B8 La motif. Boron clusters as typical starting from n ¼ 7.7–10 Ion-mobility measurements in combi- Wankel motor molecules with uxional bonds also exhibit nation with density functional theory (DFT) calculations, on the structural uxionalities where metal dopants play an important + 22–25 other hand, have shown that boron cluster monocations (Bn ) role. Our group very recently predicted the possibility of the possess double-ring tubular structures in the size range inverse sandwich double-ring tubular molecular rotor C2h La2& [B2@B18] which possesses the smallest core–shell structure re- ported in boron clusters.26 Nanocluster Laboratory, Institute of Molecular Science, Shanxi University, Taiyuan Bulk lanthanide hexaboride LaB (7) has been widely used as 030006, China. E-mail: [email protected]; [email protected]; [email protected] 6 † Electronic supplementary information (ESI) available. See DOI: cathode materials due to its numerous excellent properties such 10.1039/d0ra01616k as low work function, ultrahigh hardness, high chemical This journal is © The Royal Society of Chemistry 2020 RSC Adv.,2020,10,12469–12474 | 12469 View Article Online RSC Advances Paper inertness, and high melting point.27–30 It possesses a typical 500 eV. Atomic structures are fully relaxed using the conjugate 1 cubic CaB6-type lattice (Pm3m, Oh ) characterized with a 3D gradient method until the maximum force on each atom was À1 network constituted of B6 octahedrons, with the interstitial less than 0.01 eV A and the energy precision was set to 31 À5 locations lled by trivalent La atoms. Interestingly, each unit 10 eV. Chemical bonding analyses were performed for La6& + cell in LaB6 (7) contains a B24 cubic framework which possesses [La@B24] (3) using the adaptive natural density partitioning 49,50 six equivalent B8 rings on the surface sharing twelve B–B dumb- (AdNDP) approach at the PBE0 level and for 3D LaB6 (7) 51,52 bells on the edges, with each B8 ring sandwiched between two crystal utilizing the solid-state AdNDP (SSAdNDP) method. neighboring La atoms. The strong structural similarity between Natural bonding orbital (NBO) analyses were performed on + 53 the gas-phase inverse sandwich La2B8 (1) complex and cubic La7B24 (3) using the NBO 6.0 program. Born–Oppenheimer three-dimensional (3D) LaB6 (7) crystal has been previously molecular dynamics (BOMD) simulations were performed on 31 + noticed. LaB6 nanowires in cubic LaB6 (7) lattice were also La7B24 (3) for 30 ps at 300 K and 1000 K using the CP2K so - recently conrmed to be active electrochemical materials for ware package.54 32 supercapacitors. However, a cubic embryo of the LaB6 (7) lattice still remains unknown to date in a bottom-up approach starting from gas-phase clusters. Such a cubic embryo may also 3. Results and discussions be used to form one-dimensional (1D) lanthanide boride nanowires and two-dimensional (2D) lanthanide boride 3.1 Structures and stabilities À nanosheets. Starting from the experimentally observed C2v La3B14 (2) which Keep the inspiration in mind and based on extensive global possesses a tilted La–B8–La–B8–La inverse triple-decker struc- minimum (GM) searches and rst-principles theory calcula- ture with two conjoined B8 rings sharing a B–B dumb-bell, we tions, we predict herein the highly stable perfect cubic La-doped manually constructed the perfect cubic hepta-lanthanide-doped + 1 + 1 boron clusters Oh La6&[La@B24] (3, A1g) and Oh La6&[La@B24] boron clusters Oh La6&[La@B24] (3, A1g) and Oh La6&[La@B24] 2 2 À – Creative Commons Attribution 3.0 Unported Licence. (4, A2g) which consist of six equivalent La2B8 (1) inverse sand- (4, A2g) from two C2v La3B14 (2) clusters which share two B B wiches embedded in a cube sharing one La atom at center. More dumb-bells on the edges and one La atom at the center, with +/0 interestingly, La6&[La@B24] (3/4) clusters with a B24 frame- two extra La atoms added on the top and bottom, respectively. work turn out to be the cubic embryos of the metallic 1D La10B32 With one La atom endohedrally coordinated at the center (@) (5) nanowire, 2D La3B10 (6) nanosheet, and 3D LaB6 (7) nano- and six La atoms exohedrally coordinated on the surface (&), Oh +/0 crystal in a bottom-up approach. The newly obtained ferro- La6&[La@B24] (3/4) can be viewed as consisting of six equiv- magnetic 1D La10B32 (5) and nonmagnetic 2D La3B10 (6) are alent La2B8 (1) inverse sandwiches embedded in a cube sharing expected to serve as efficient electronical and optical twelve B–B dumb-bells on the edges and one La vertex at the nanomaterials. center. Interestingly and encouragingly, as shown in Fig. S1 and This article is licensed under a S2,† extensive GM searches strongly suggest that the highly + 2. Theoretical procedure stable perfect cubic La6&[La@B24] (3) and La6&[La@B24](4) with a B24 framework be the well-dened GMs of the systems + À1 Open Access Article. Published on 27 March 2020. Downloaded 5/25/2020 8:13:46 AM. Extensive GM searches were performed on La7B24 monocation with the lowest vibrational frequencies of 111 cm and 33 À1 +/0 using the TGmin2 code at the DFT level, in combination with 112 cm , respectively. La6&[La@B24] (3/4) possess the opti- manual structural constructions based on the experimentally mized La–B distances of rLa–B ¼ 2.94/2.95 A between the central À 0 ¼ : = : observed La2B8 (1) and La3B14 (2). Over 3000 trial structures La atom and its cubic B24 ligand and rLaÀB 2 70 2 69A were explored in both singlet and triplet states at PBE/TZVP.