Introduction

The design and synthesis of new materials with desirable properties is essential for advancing material applications and innovations, which may influence the future of technology. One example is the recent development of electride materials that significantly promote the catalytic transformation of molecular dinitrogen into ammonia at mild reaction conditions.1,2,3,4,5,6 Electride is a unique class of materials where the electrons are spatially confined in the vacant crystallographic sites and serve as anions to maintain charge neutrality.7,8,9 The intrinsic characteristics of electrides should suggest superior electronic performances. However, the first proposed organic electrides were thermally unstable and air sensitive, constraining their technological applications.10,11,12 In 2003, a room-temperature stable inorganic electride of [Ca24Al28O64]4+(4e–) was successfully synthesized by Matsuishi et al.13 and exhibited versatile applications in many areas, including catalysts,14,15 anode materials,16 and an electron-injection layer in organic light-emitting diodes (OLEDs).17 Subsequently, Ca2N was identified experimentally to be a layer-like two-dimensional electride18 and can be exfoliated into nanosheets.19 The Y2C electride was also realized experimentally and furthered our understanding of the interplay between magnetism and localized electrons.20,21,22,23 These findings of inorganic electrides provide new possibilities for both fundamental science and applications.

Considerable experimental and theoretical investigations have been carried out to discover and produce new electrides with superior functions and capabilities. Usually, researchers arbitrarily alter the elemental combinations of typical electrides but retain their crystal symmetry to extend for the new electrides, e.g., AB-type, A2B-type, and A3B-type (A = alkaline or rare earth elements, B = IV, V, VI, and VII elements) electrides.24,25,26,27 Besides, many electrides (e.g., Li,28 Na,29 Mg,30 C31, Na2He,32 and Sr5P333) have been found to reveal a generalized structure under pressure.34 Depending on the dimensionality of the anionic electrons localizations, electrides can be classified into 0D, 1D, and 2D systems,13,20,35,36 where the anionic electrons are either isolated or bonded with each other in the cage-like, channel-like, or layer-like voids. These interesting results suggest a geometrical way to obtain the diverse ‘interstitial spaces’ in a lattice that can stabilize excess electrons, which can provide a vast configuration space for computational discovery.

To explore new electrides, we propose a simple strategy for identifying them by checking the local connectivity of pre-existing compositions and structures with high open frameworks. Applying high-throughput ab intio screening strategy based on the Materials Project platforms and Inorganic Crystal Structure Database (ICSD),37,38,39 we focused on the reduced rare-earth yttrium and scandium chlorides (RxCly, R = Y, Sc and y: x < 2, i.e. YCl,40 Y2Cl3,40 ScCl,41 Sc5Cl8,42 and Sc7Cl1043). The reduced rare-earth halides were first reported in Gd2Cl3.44 Later, a series of halides were synthesized, such as, RCh (R = Sc, Y, La, Ce, Pr, Nd, Gd, Tb, and Ho, and Ch = Cl and Br),40 R2Ch3 (R = Y, Gd, Tb, Er, and Lu, and Ch = Cl and Br),40,44,45 R5Ch8 (R = Gd, Tb, and Sc, and Ch = Cl and Br),42,46 and R6I7 (R = Er and Tb).47 One of their characteristics is the octahedral (R6) framework topology composed of rare-earth elements, which constitutes edge-sharing chains, double chains, or layers. Furthermore, scandium and yttrium prefer oxidation states of 2+, instead of the common oxidation state of 3+, due to the contributions of the rare earth-5d orbitals to the chemical bonding induced by crystal field splitting.48,49 This unusual oxidation state gives rise to a unique chemical and physical understanding of rare-earth elements. Moreover, the various structures and compositions in R–Ch systems also provide a platform to understand the mutual coupling between excess electrons and geometrical topology and have great potential to accelerate electride discovery.

Results and discussion

Structural properties

The crystal structures of the reduced yttrium and scandium chlorides (RxCly, R = Y, Sc, y: x < 2) are characterized by the edge-sharing R6 octahedral layer-like (YCl and ScCl), channel-like (Y2Cl3 and Sc5Cl8), and double channel-like (Sc7Cl10) atomic voids in the host structures (Fig. 1). These voids are coated by Cl atoms and formed by variable R–Cl layers in the layer-like crystal structures. The metal octahedra are stretched in these structures and the R–R distances on the shared-edges are much shorter than the others. YCl crystallizes into a ZrCl-type structure (R-3m).50,51 The Y6 octahedral layers (Fig. 1a) were distorted and sheathed with chlorine atoms to form Cl–Y–Y–Cl close-packed layers with an interlayer distance of 3.56 Å. Two distinct Y–Y separations of 3.57 Å (shared edge) and 3.73 Å can be found in the Y6 octahedrons that are comparable with those of 3.56 and 3.80 Å in Y5Si3 electride.52 Y2Cl3 (Fig. 1b) is isostructural to Gd2Ch3 and Tb2Ch3 (Ch = Cl and Br).44,53 The Y–Y distances in the Y6 octahedra varied from 3.30 to 3.85 Å and the shorter Y–Y separation (3.30 Å in shared edges) suggests a stronger Y–Y interaction connected to its phase stability.40,54 Despite being congeners of the yttrium element, the Sc–Cl system has stable stoichiometries of ScCl, Sc5Cl8, and Sc7Cl10.40,42,43 ScCl crystallizes in the same structure as YCl, containing layer-like interstitial spaces formed by edge-sharing Sc6 octahedra. Channel-like voids in Sc5Cl8 (Fig. 1c) and Sc7Cl10 (Fig. 1d) are analogous to that of Y2Cl3. Due to the distortion of the Sc6 octahedrons, the Sc–Sc bond distances change within the range of 3.26–0.52 Å for ScCl, 3.10–3.57 Å for Sc5Cl8, and 3.13–3.55 Å for Sc7Cl10, respectively. The shared edge in the Sc6 octahedral chains has the minimum Sc–Sc distance, comparable to that of metal Sc (3.26 Å).

Fig. 1
figure 1

Crystal structures of (a) YCl, (b) Y2Cl3, (c) Sc5Cl8, and (d) Sc7Cl10. The blue and red spheres represent the Y/Sc and Cl atoms, respectively. The edge-sharing R6 octahedra in the host structures condensed into edge-sharing layers, chains and double chains, which are displayed at the bottom

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To evaluate the formation possibilities of interstitial electrons in Y–Cl and Sc–Cl compounds, we compared the configuration of interstitial voids in Y–Cl and Sc–Cl with other rare-earth-metal-containing electrides, e.g. A2B-type compounds (anti-CdCl2 structure: Y2C, Gd2C, Tb2C, Dy2C, Ho2C, Er2C, Sc2C, Y2N, La2N, and La2Bi; Cu2Sb-type structure: Sc2As, Sc2Sb, Sc2Bi, Y2Sb, and Y2Bi; others: Sc2N, Y2As, Y2O, Y2S, La2C, La2As, La2Sb, La2O, and La2S, LaH2, CeH2, and YH2)55,56,57; A5B3 with Mn5Si3-type structure (Yb5Sb3, Y5Si3)58,59; AB with ScBr-type structure (YBr, ScBr)57; La8Sr2(SiO4)635 and LaScSi.3 Metal octahedron severing as interstitial voids can be observed in most of the host structures except Y2O and A2B with Cu2Sb-type structure (Sc2As, Sc2Sb, Sc2Bi, Y2Sb, and Y2Bi). For Cu2Sb-type A2B compounds, the interstitial voids can be viewed to be pseudo-octahedron (R5X), in which the octahedron is built by five cations and one anion. To compare the interstitial volume with previously proposed electrides, the octahedral volumes of Y–X and Sc–X compounds were plotted in Fig. 2a, b. We can see that the interstitial volumes in Y–Cl (Fig. 2a) and Sc–Cl (Fig. 2b) are much higher than those of Cu2Sb-type structure (e.g. Y2Sb and Sc2As), but comparable to the anti-CdCl2-type Y2N and Sc2N. The interstitial voids in these compounds proposed here are thus believed to have large enough space to trap anionic electrons.

Fig. 2
figure 2

Interstitial volumes of R6 octahedron in YCl, Y2Cl3, ScCl, Sc5Cl8, and Sc7Cl8, comparing with other (a) Y and (b) Sc contained electrides. Interstitial volumes of R5X octahedron in Cu2Sb-type compounds: Sc2As, Sc2Sb, Sc2Bi, Y2Sb, and Y2Bi were also collected

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Electron localization function (ELF) analysis

To uncover the underlying electride characteristics of RxCly, we first carried out analysis of the ELF of each phase.60,61 Interestingly, as shown in Fig. 3a (YCl), Fig. 3b (Y2Cl3), and Fig. 3c (Sc7Cl10), two distinct ELF attractors (non-nuclear maxima in the ELF maps) off the nuclei can be observed in the center of the R4 tetrahedra (green circle in the ELF maps, site A) and R6 octahedra (red circle in the ELF maps, site B). The ELF attractors are more localized in the R4 tetrahedrons but dispersed in the R6 octahedrons. Further, the finite value of ELF between these two distinct ELF attractors suggests there is a strong interaction between them, due to their short separation (2.31 Å in YCl, 2.44 Å in Y2Cl3, and 2.11 Å in Sc7Cl10). However, the interspace in the R4 tetrahedra is not large enough to hold interstitial electrons, so these ELF attractors may originate from the hybridization of the R-d orbitals, ruling out the possibility of anionic electrons in this configuration. The origin of the ELF attractors in R4 tetrahedra is discussed in the following section. The ELF attractors observed in the metal octahedra are expected to be originated from interstitial electrons; further examinations were performed by analyzing the band structure and partial charge density.

Fig. 3
figure 3

The electron localization function (ELF) of (a) YCl, (b) Y2Cl3, and (c) Sc7Cl10. The isosurface value was set as 0.75, 0.75, and 0.7, respectively. The ELF attractors located at the center of the R4 tetrahedra (site A) and the R6 octahedra (site B) are highlighted by blue and red circles. The ELF contours in the R–Cl layer (guided by red dashed line in the host structures) illustrated in the (1 1 0), (2 0 −1), and (2 0 −1) planes are shown at the bottom

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Band structure and magnetic property

Considering the similarity in the metal frameworks of the Sc–Cl and Y–Cl systems, the band structure, and partial charge density of specified bands for YCl and Y2Cl3, and total and projected densities of states (TDOS and PDOS) of Y2Cl3 are illustrated in Fig. 4 to further uncover their electronic properties. Through analysis of the partial charges, one can unambiguously understand the bonding character of the electrons in specified bands and the band distributions of the anionic electrons. There are invariably three bands observed around the Fermi level (denoted as band 1, band 2, and band 3) in YCl (Fig. 4a) and Y2Cl3 (Fig. 4b). These bands are closely related to the multiple metal–metal bonding in the shared-edge of the R6 octahedrons (Fig. 4c), which are attributed to the stretched octahedral configurations. Such multiple metal–metal bonds can usually be observed in the heavier transition metal elements, e.g., Re–Re, Mo–Mo, and W–W.62,63 The unique divalent yttrium and scandium (Y2+: 4d15s0 and Sc2+: 3d14s0) give rise to the formation of multiple Y–Y bonds, which has not been observed in most rare earth-containing compounds (no d electrons in R3+). The partial charge density of band 1 in Y2Cl3 is shown in Fig. 4d; two lobes of atomic orbitals (dxz) in the Y atoms form a Y–Y π bonding within the nearest Y atoms. A similar case can be observed in the partial charge density of band 2 (Fig. 4e), where the Y-dyz orbitals of the nearest Y atoms develop another Y–Y π bonding. Besides, the Y-dyz orbitals (band 2) strongly overlap in the Y4 tetrahedra forming a localized electron center, which is responsible for the presence of ELF attractors in R4 tetrahedra. Decomposed Y-d orbitals also (PDOS of Y-d orbitals in Fig. 4c) show that the contributions from Y-dxz and Y-dyz are much stronger than other Y-d orbitals in the corresponding energy range of band 1 and band 2. The partial charge density of band 3 in YCl and Y2Cl3 are displayed in Fig. 4f, g, respectively. In sharp contrast with band 1 and band 2, the electrons in band 3 are loosely dispersed in the interstitial voids of the Y6 octahedron layers (YCl) and chains (Y2Cl3), these electrons correspond to the ELF attractors in R6 octahedron, which is a characteristic of electrides. The electrons in band 3 originated from the Y-5s orbital (Y: 4d15s2 convert toY2+: 4d15s0, one electron in Y-5s orbital transferred to Cl-3p orbital, another is dispersed in the interstitial spaces) and two excess Y-5s electrons are available in their primitive cells. These electrons are confined in the interstitial spaces of the R6 layers and chains, thus, band 3 can be described as ‘an interstitial band’. Oxygen atoms have been added into the center of the R6 octahedron in YCl and Y2Cl3, as shown in Fig. S1a, b, and the band 3 disappears, suggesting that it does not belong to any specific atomic orbital but an interstitial site.

Fig. 4
figure 4

Band strcrures for (a) YCl and (b) Y2Cl3, spin-polarized calculations are included. There are three invariable bands in YCl and Y2Cl3 around the Fermi level, which are denoted as band 1, band 2, and band 3. (c) The TDOS, PDOS curves, and Y-d orbitals in Y2Cl3, the bands where the anionic electrons confined are marked by ‘inter’ and green curves in band structure. (d) Orbital energy of multiple metal–metal bonding. Partial charge density of band 1 (e), band 2 (f) in Y2Cl3, the schematic diagrams for Y–Y π bonds in band 1 and band 2 are inserted. Partial charge density of band 3 in (g) YCl and (h) Y2Cl3, the partial charge density maps in (1 1 0) and (2 2 −7) planes are inserted, the white line between nearest Y atoms are the guide for the shared edge of nearest Y6 octahedron

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To identify the contributions of interstitial electrons, the PDOS of the interstitial electrons in Y2Cl3 are plotted in Fig. 4c. The PDOS curves of the interstitial electrons are derived by adding a pseudoatom20,64 with a Wigner–Seitz radius of 1.72 Å in the center of the octahedra. At the corresponding energy range of band 3, the contribution from interstitial electrons is larger than that of Y atoms. Since the size of the interstitial spaces is not large, the interstitial electron states are partially projected onto the Y-d orbitals (dominantly dz2) of the neighboring Y atoms, behaving like pseduo-σ bonds (denoted σ′). Band structure calculations reveal that YCl is ferromagnetic with a magnetic moment of 1.58 μB per unit cell (2YCl per formula unit). Y2Cl3 is a semiconductor with a calculated band gap of 0.77 eV, and no spin polarization was observed. The magnetic properties (YCl) and semiconducting behavior (Y2Cl3) can be understood by detailed band structure analysis. The band 1 and band 2 in Y2Cl3 are fully occupied by the rest of d electrons (Y2+: 4d15s0, four d electrons available in primitive cell Y4Cl6), which are attributed to the diamagnetic and semiconducting character. For YCl, analogous to Y2Cl3, band 1 and band 2 are contributed by two Y–Y π bonding. However, only two d electrons are available in the primitive cell YCl and result in the high-spin state, which leads to the ferromagnetic character with a magnetic moment of 1.58 μB.

The band structures and partial charge density of the interstitial bands in ScCl, Sc5Cl8, and Sc7Cl10 are illustrated in Fig. S2a, b, c. Ferromagnetism with a magnetic moment of 1.53, 0.83, and 1.11 μB per unit cell is predicted for ScCl, Sc5Cl8, and Sc7Cl10, respectively. For Sc7Cl10 (Fig. S2c), due to the double-chain configuration, four Sc–Sc π bands (two Sc-dyz and two Sc-dxz) appear in the band structure, which leads to a magnetic moment of 1.11 μB per unit cell (seven Sc-3d electrons are available in the primitive cell). Similar to YCl and Y2Cl3, the partial charge (green curves in the band structure) suggests that the interstitial electrons are loosely dispersed in the R6 metal layer or chains, which is a characteristic of electrides. The PDOS of ScCl, Sc5Cl8, and Sc7Cl10 were also calculated through adding a pseudoatom (Wigner–Seitz radius was set as 1.65 Å in YCl, 1.61 Å in Sc5Cl8, and 1.61 Å in Sc7Cl10) into the interstitial sites.20,34,64 It can be observed that, as plotted in Fig. S3a, b, c, the contributions of interstitial electrons are higher than those of Sc-d orbitals in the energy range where interstitial bands are distributed.

Experimental characterization has also verified the semiconducting behavior of Y2Cl3. As shown in Fig. S4a, b, c, a band gap of 1.14 eV for Y2Cl3 can be estimated (Fig. S4b). Moreover, consistent with our calculations, the magnetic measurements of the Y2Cl3 sample suggest its diamagnetic nature, which is significantly different from the other reduced rare-earth chlorides discussed above.

Electronic dimensionality of interstitial electrons

The distributions of the anionic electrons and the interaction between them or other atoms can significantly influence the physical and chemical properties of electrides. For YCl and ScCl, the layer-like anionic electrons show up in the inner layer of the Cl–Y–Y–Cl layers within the edge-sharing Y6 octahedra (Fig. 5). This feature especially differs from those of typical A2B-type 2D electrides,24 where the interstitial spaces appear between the A2B layers. Flat bands are observed along the L-A, Γ-Z, and F-B orientations of the interstitial bands in the band structure, which corresponds to the directions perpendicular to the Cl–Y–Y–Cl layers. The band dispersion is relatively smaller along the other directions and the anionic electrons are well confined in the 2D inner layer induced by the edge-sharing Y6 octahedrons, giving rise to a typical 2D electride.18,25,65 Y2Cl3, Sc5Cl8, and Sc7Cl10 have similar channel voids where the anionic electrons are confined, and the interactions between the nearest anionic electrons are quite similar. The electrons confined in the metal octahedrons are weakly bonded with each other (Figs. 4h and S2b, c), and the connected anionic electrons forming 1D chains along the b-axis are reminiscent of previously reported 1D electrides.33,35,59 Thus, YCl and ScCl can be regarded as quasi-2D electrides with chemical formula of [YCl]+∙e− and [ScCl]+∙e−; Y2Cl3, Sc5Cl8, and Sc7Cl10 can be regarded as quasi-1D electrides with a chemical formula of [Y2Cl3]+∙e−, [Sc5Cl8]2+∙2e−, and [Sc7Cl10]4+∙4e− electrides.

Fig. 5
figure 5

Charge density of interstitial electrons in (a) YCl and (b) Ca2N, the white and black spheres in Ca2N represent N and Ca atoms, respectively

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Hydrogen absorption behavior

Typical electrides usually possess a strong hydrogen affinity caused by the effective interactions between the anionic electrons and hydrogen nuclei.55,66 The insertion of hydrogen atoms usually occupies the positions where the anionic electrons are trapped and induces a pronounced effect on the electronic properties of electride materials. For example, hydrogen insertion makes the electrides transform to conventional ionic compounds, e.g., C12A7:H−.1,51 It was also revealed that hydrogen permeation results in the formation of RHalHx (R = Sc, Y, La, Ce, Pr, Nd, Gd, Tb, and Ho, Hal = Cl and Br, x < ~1.0);67,68 a similar observation was also reported in Y2Cl3.69 However, neutron diffraction studies suggest that hydrogen prefers to occupy the center of metal tetrahedrons instead of the R6 octahedron.70

To locate the position and investigate the influence of hydrogen on the yttrium and scandium chlorides electrides, YClH and Y2Cl3H were constructed with hydrogen atoms located in the center of the metal tetrahedrons (crystallographic 6c site for YCl and 4i site for Y2Cl3). Hydrogen atoms were also added in the center of the metal octahedron, however, the imaginary phonon frequency modes confirm the loss of structural integrity with H entry into the octahedron site of YCl and Y2Cl3 (see Fig. S5a, b), which concurs with the experimental observation. The calculated band structures of YClH (Fig. 6a) and Y2Cl3H (Fig. 6b) show that the introduction of the H atom leads to Fermi level reorganization. Although the H atom has been added into the center of the metal tetrahedron, band 3, where the anionic electrons are mainly confined, vanished in both YCl and Y2Cl3, transforming to a conventional ionic crystal. The interstitial electrons in band 3 are found to transfer to the H 1s orbital caused by the strong hydrogen affinity of the interstitial electrons. Since the Y–Y π bonding (dyz, band 2 in YCl, and Y2Cl3) formed electron maxima in the R4 tetrahedron, the repulsion between H− (1s2) ions and electrons in R4 tetrahedron significantly increases the energy of this Y–Y π bonding, which is responsible for the Fermi level reorganization. For YClH, the electrons in band 2 of YCl are transferred to another Y–Y π bonding (band 1 in YCl, dxz), which leads band 1 to shift to the valence region in YClH (green curves in Fig. 6a), and no spin polarization was confirmed after hydrogen insertion. Due to the band 1 in Y2Cl3 being fully occupied by two Y-4d electrons, Y2Cl3H is unperturbed in Fig. 6b (green curves) and the electrons in the Y–Y π bonding of dyz orbital move to the conduction band region, resulting in the significant transition from semiconductor to metal.

Fig. 6
figure 6

Band structures and work functions for YClH (a, c, e) and Y2Cl3H (b, d, f). a, b Band structures. c, e, d, f Calculated work functions as a function of position (Å) along c-axis for YCl and YClH in the (0 0 1) surfaces, Y2Cl3 and Y2Cl3H in the (0 1 0) surfaces. The Fermi level is set as 0 eV, the Fermi level in YCl and Y2Cl3 is also marked by a black line in the band structures. The green curves are the band 1 in YCl and Y2Cl3 which remains after H absorption. The bands contributed by hydrogen atoms are marked by red curves

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The intriguing locations of the H atoms drive the electrons in band 2 to transfer to other bands in a higher energy region, which significantly increases the energy at the Fermi level (1.0 and 0.42 eV higher than YCl and Y2Cl3, respectively, Fig. 6a, b). Such unusual variation may induce significant modification of work function (WF) (ΦWF). Thus, the WFs of YCl, Y2Cl3, YClH, and Y2Cl3H were calculated. The most stable surfaces are the [0 0 1] planes for YCl and YClH, and the [2 0 1] planes for Y2Cl3 and Y2Cl3H, which are parallel to the Y–Cl layers listed in Fig. 6c–f. Consistent with the increased energy at the Fermi level, the insertion of hydrogen atoms results in a sharp decrease of WFs; 4.57 and 3.40 eV along the [0 0 1] planes for YCl (Fig. 6c) and YClH (Fig. 6e), and 3.75 eV (relative to CBM), 3.20 eV along the [0 0 1] planes for Y2Cl3 (Fig. 6d) and Y2Cl3H (Fig. 6f), respectively. Surfaces with orientations perpendicular to the Y–Cl layers were also calculated in [1 1 0] of YCl and YClH, and [0 1 0] for Y2Cl3 and Y2Cl3H. As illustrated in Fig. S6a, b, c, d, the WFs show strong anisotropic characters for these phases, similar to the typical electrides with layered structural configuration, e.g. Y2C20 and Ca2N.71 For YClH, the calculated WF of the [1 1 0] plane decreases to 2.47 eV (3.13 eV in YCl). However, the WF of the [0 1 0] planes for Y2Cl3H remains largely unchanged (2.81 and 2.94 eV in Y2Cl3 and Y2Cl3H). Considering the layered-like structure in the host structures, these surfaces parallel to the Y–Cl layers tend to be exposed. YClH0.5 with H partially occupying the center of the R4 tetrahedron was constructed to study the effect of hydrogen. As shown in Fig. S7, band 3 remains, suggesting that YClH0.5 retains its electride nature after hydrogen absorption. The Fermi level also increased by 0.12 eV, leading to the decrease of WFs in the [0 0 1] planes. The decreased WF after hydrogen absorption found in YCl and Y2Cl3 is opposite to typical electrides, which may lead to a new chemical and physical understanding of electrides.

Extending to LaCl

We also extended our study to lanthanide halide because the lanthanide elements usually possess similar chemical properties. LaCl takes the same crystal structure as YCl,72 the band structures accompanied with an ELF and partial charge density of LaCl and LaClH (H located in the La4 tetrahedron) are displayed in Fig. S8a, b. Similar to YCl, the partial charge density of the interstitial bands (green curves in the band structure) suggest that the interstitial electrons exist in the center of the La6 octahedron, to form layer-like anionic electrons in the inner layer of the Cl–La–La–Cl layers, suggesting its 2D electride character. For LaClH, The variation of the Fermi level also lead to decreased WF in the [0 0 1] (from 4.51 eV in LaCl to 3.54 eV in LaClH) and [1 1 0] (from 2.81 eV in LaCl to 2.32 eV in LaClH) orientations.

Our calculations suggest that the electrides characters still remain, when hydrogen atoms occupied the center of R4 tetrahedron partially. LaClHx (x < 1) was thus synthesized by the conventional solid state reaction: LaCl3 + La + LaH2 > LaClHx + ((1 − x)/2)H2. X-ray diffraction (XRD) pattern refinement indicates the presence of LaClH0.66 (Fig. S9a). The amount of hydrogen incorporated in LaClH0.66 were measured by thermal desorption spectrometry (TDS). The WF of LaClH0.66 (Fig. S9) is determined by a cut-off energy of ultraviolet photoemission spectroscopy (UPS, hv: 21.2 eV) spectra; the obtained WF is ~3.2 eV, comparable to Y2C (2.9 ± 0.1 eV).20 We also calculated the WF for LaClH0.5 through the construction of the insertion of hydrogen atoms into half of the La4 tetrahedron. The magnitude of measured WF is between the calculated WF along the [0 0 1] and [1 1 0] planes, which are 4.01 and 2.52 eV, respectively. The large family of R–Ch (R = rare earth elements, Ch = Cl, Br and I) systems provides a platform to understand the mutual coupling between excess electrons and geometrical topology, which can greatly accelerate electride discovery.

In summary, we identified yttrium and scandium chlorides with various stoichiometries and compositions as a new class of quasi-2D-([YCl]+∙e− and [ScCl]+∙e−) and quasi-1D-electrides ([Y2Cl3]+∙e−, [Sc7Cl10]+∙e−, and [Sc5Cl8]2+∙2e−) based on geometrical identifications and high-throughput ab intio screening strategy. Anionic electrons are confined in the edge-shared R6 octahedron. The R6 octahedron is highly stretched, which is contributed by the multiple metal–metal bonding existing in the shared edge of the R6 octahedron. Except Y2Cl3, all the other structures are ferromagnetic. Experimental measurement reveals that Y2Cl3 is a semiconductor with a measured band gap of 1.14 eV. These new yttrium and scandium chloride electrides have distinctive features: (i) all the structures are constituted by R–Cl close-packed layers, differing from those of typical A2B-type 2D electrides, where the anionic electrons were observed in the inner R–Cl layer voids instead of the inter-layer spaces. (ii) The cation arrangement is characterized by the formation of edge-sharing octahedral framework topology. These R6 octahedra constitute layer-like (YCl and ScCl), channel-like (Y2Cl3 and Sc5Cl8), and double channel-like (Sc7Cl10) interstitial spaces where the anionic electrons are confined. (iii) Hydrogen intercalations decrease the WF in YClH and Y2Cl3H. This is due to the unique hydrogen sites increasing the Fermi level energy. Our results provide novel insights into the basic understanding of these electrides and will stimulate continuing experimental and theoretical efforts to explore new electrides.

Methods

The rare-earth chlorides were selected from the Materials Project platforms and ICSD,37,38,39 The structural relaxations and calculations were performed using the density functional theory (DFT) as implemented in the Vienna Ab initio simulation package (VASP)73 within the projector-augmented-wave (PAW) method.74,75 Generalized-gradient approximation (GGA-PBE)76 was used for the electron exchange–correlation interaction. The PAW potentials valence electrons of Y as 4s2, 4p6, 4d1, 5s2, Sc as 3p6, 3d1, 4s2, and Cl as 2s2, 3p5 were adopted. The plane-wave kinetic energy cutoffs were set to 600 eV, and a dense Brillouin zone sampling grid with a resolution of 2π × 0.03 Å−1 was chosen to ensure the enthalpy calculations converged well within 1 meV/atom. Spin-polarized calculations were considered.

To determine the WF (ΦWF) of YCl and Y2Cl3, we constructed two slab models of [0 0 1] (24 atoms) and [1 1 0] (48 atoms) crystal planes for YCl, and [0 1 0] (90 atoms) and [2 0 1] (50 atoms) crystal planes for Y2Cl3, which are perpendicular or parallel to the edge-sharing metal octahedron chains (Y2Cl3) and layer (YCl). The vacuum was set as 25 Å for those slab models. The k-point grids were set as 15 × 15 × 1 and 8 × 5 × 1 for the [0 0 1] and [1 1 0] crystal planes of YCl, and 5 × 4 × 1 and 5 × 13 × 1 for the [0 1 0] and [2 0 1] crystal planes for Y2Cl3 using the Monkhorst–Pack method.77 A similar set-up was applied to the YClH and Y2Cl3H hydrides. The ΦWF value was determined using the equation: ΦWF = Evac − EF, where Evac is the vacuum level, and EF is the Fermi level. The crystal structure, ELF, and charge density maps were made by VESTA.78

The samples used in the measurements were synthesized using a conventional solid-state reaction. The elemental yttrium (turning) and YCl3 (powder) were mixed with the ratio of Y:YCl3 = 2:1 and sealed into an Ar-filled SUS tube. The tube was sintered twice at 750 °C for 48 h to ensure homogeneity. The obtained powders were then filtered to exclude excess yttrium. The obtained samples were black in color and sensitive to air. Powder XRD patterns were collected on a Bruker D8 advance diffractometer with CuKα radiation at room temperature. To avoid air exposure, the sample was sealed into the plastic sample holder filled with Ar gas.

The optical reflectance was measured using a UV–visible spectrophotometer (Hitachi U-4000) at room temperature. The samples were pelletized and sealed into an Ar-filled sample holder during the measurements to avoid air exposure. The magnetic properties were measured using commercial SVSM (Quantum design). LaClH0.66 was synthesized by solid-state reaction from La metal, LaH3 and LaCl3. LaH2 was obtained by heating metal La in a H2 atmosphere. All starting materials and precursors were prepared in a glove box under a purified Ar Atmosphere. The mixture of starting materials was wrapped with Mo foil and placed in the sealed stainless tube; the tube was heated at 973 K for 36 h. The amount of hydrogen incorporated in LaClH0.66 were measured by TDS (ESCO TDS-1000S/W). The WF of LaClH0.66 was determined from a cut-off energy of ultraviolet photoemission spectroscopy (UPS, hv: 21.2 eV) spectra. Clean surface was prepared by cleaving the LaClH0.66 bulk sample in the globe box that is directly connected to the UPS chamber. DC bias dependence was also checked at 5, 7, and 10 V.