Article Open Access
Received: 27 September 2024 Accepted: 12 November 2024 Published: 14 November 2024
© 2024 The authors. This is an open access article under the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/).
As the first element of the f-block of the periodic table, lanthanum is a very abundant and active element and has been widely studied for its fascinating physical and chemical properties [1]. Lanthanum exists in various compounds such as lanthanum hydride [2,3,4], lanthanum oxide [5,6,7] and lanthanum hydroxide [8,9,10], which possess excellent properties like superconductivity [2], superionic conduction [4], catalysis [5], as well as refining grains in the iron industry [6], adsorption of hazard dyes [10] and phosphate removal from wastewater [11]. For the compounds mentioned above and their corresponding unique properties, the crystal structures often play a pivotal role in tailoring the performances. For instance, Laniel et al. [2] have presented seven lanthanum hydrides synthesized in the range of 50 to 180 GPa by single-crystal X-ray diffractions and DFT calculations for the purpose of guiding the search of candidate high-temperature superconductors. Luo et al. [7] reported four types of hexagonal La₂O₃ with varying degrees of oxygen vacancies and crystal characteristics synthesized by ball milling, finding that these oxygen vacancies influence the photocatalytic degradation of tetracycline. On the contrary, Jonderian et al. [10] found that the native crystalline phase of La(OH)3 remains unaltered and dominant by the azo-dye Congo Red adsorption.
There is also considerable research focused on the transformation between La₂O₃ and La(OH)₃ [12,13,14]. Fleming et al. [12] discovered that the La2O3 will rapidly hydroxylate to form stable La(OH)3 when exposed to an open atmosphere. Further study suggest that there exists a intermediate LaO(OH)-type phase involved in the hydroxylation and de hydroxylation process. Hu et al. [13] reported that La2O3 nanobelts were obtained when the La(OH)3 nanobelts were calcined in air at 690 °C for 6 h. Very recently, Ismail et al. [14] proposed a five-step decomposition pathway La(OH)3, which is firstly converted into LaOHCO3, followed by LaOHCO3·xH2O (LaOHCO3·yH2O), then converted into La2O2CO3 and finally into La2O3 nanorods through annealing.
This work was prompted by simple observations that a piece of single-crystal LaH3 transforms into powder during single-crystal X-ray diffraction measurements, and a specimen of LaH3 on a glass sliderapidly turn into white powders within 24 h. In the remaining sections of the paper, the experimental and computational methods will be briefly introduced, followed by the main results on the crystal structures determination on LaH3 and La(OH)3, as well as the reaction pathway discrimination by self-designed hydrogen detection approach combined with formation and adsorption enthalpies calculations within the scheme of density functional theory (DFT) method [15,16,17]. Finally, the main conclusions will be suggested.
The fragment of lanthanum hydride used for SXRD diffraction was selected from the factory product as is (more than 99.5% purity, Jiangxi Province,China ,zhongxi Metal Materials Co., Ltd., particle size of 100–200 mesh). The lanthanum hydroxide material used for simulated powder X-ray diffraction (PXRD) measurements was the reaction product after the lanthanum hydride material was exposed to the air for 24 h. During the SXRD measurements, lanthanum hydride was placed on a clean glass sheet, washed the surface impurities with anhydrous ethanol and fixed on a thin glass fiber with glue and mounted on the sample stage. All the SXRD measurements were performed using a four-circle single-crystal X-ray diffractometer (Bruker D8 Venture, Bruker AXS GmbH, Karlsruhe, Germany). The hydron detection was carried out with the hydrogen test tape (DX-2106H, Beijing, China,Nitto), with the detected concentrations as low as 1%.
All DFT-based first-principles calculations were performed using the CASTEP [18] program as integrated in the Materials Studio software(2020 verision). The Perdew-Burke-Ernzerhof (PBE) [19,20] exchange correlation functional with generalized gradient approximation (GGA) is used to describe the interaction between the nucleus and the electron and the interaction potential between the ion, and the valence electron is realized by Norm conserving potential. Periodic boundary conditions are applied, and the wave function of crystals is expanded through the plane wave base set. The plane-wave cut-off energy Ecut was selected as 830 eV. It has been tested that the PBE-sol method does not change the result, indicating that the top position is the most stable adsorption site. Spin polarization has not been considered as the system is diamagnetism [2]. The Brillouin region integral was divided into K-point grids according to the Monkhorst-Pack scheme, where the density of K-point grids was set as 2 × 2 × 1. In the structure optimisation process, the convergence standards of force and energy were 0.01 eV/Å and 5 × 10−6 eV, which have been performed convergence tests for La(OH)3 system. LaH3 crystal is adopted as a cubic structure with space group Fmm (No.225). Considering calculation accuracy and efficiency, a 7-layer atomic supercell model is constructed to simulate the surface of LaH3. The vacuum layer is set to be 15 Å, and the atomic coordinates of the bottom five atomic layers are fixed during construction optimization. The top surface atoms relax together with the adsorbed molecules.
The lanthanum hydride single crystal sample with a size of 0.09 mm × 0.08 mm × 0.05 mm was selected. The reciprocal diffractions projected along three main directions are shown in Figure 1. It is indexed to be a = b = c = 5.62 Å, α = β = γ = 90°. The detailed diffraction information during collection and the structural refinement parameters of the LaH3 phase are illustrated in Table 1. From Table 1, one can find that Fmm -LaH3 phase fit the SXRD extremely well with the final R indices of 0.4%. The refined crystal structure of Fmm -LaH3 is also in accordance with the previous structural model in which the hydrogen atoms occupy the tetrahedral and octahedral interstices [21].
The La(OH)3 phase was discovered during SXRD measurements of LaH₃ single-crystal samples.It was found that the LaH3 sample will change into white powder when exposed to the air within 24 h. The black LaH3 changes to white La(OH)3, and the density is enhanced significantly after phase transformation (see Figure S1 in the Supplementary Materials). To determine its crystal structure, a piece of white powder measuring 0.2 mm × 0.18 mm × 0.14 mm was selected and mounted on the sample stage of a four-circle single-crystal X-ray diffractometer. A different strategy named the Phi360 test (the sample is rotating around the φ axis for 360°, which is equivalent to PXRD powder diffraction) was adopted for the powder sample. Figure 2a shows the obtained Phi360 diffraction pattern with multiple Debye rings, indicating that the sample is polycrystalline rather than single-crystalline. The crystallographic information for La(OH)3 is shown in Table 2.
The diffraction peaks were obtained by integrating the Debye rings and compared with the available La(OH)3 standard PDF card, as shown in Figure 2b. The good agreement between PDF#36-1481 and the integrated Debye rings suggests that the phase transition product is primarily La(OH)3, formed from LaH₃ upon exposure to air.The crystal structure model of La(OH)3 is illustrated in Figure 3.
As shown in Figure 4, it was found that the LaH3 is constructed by tetrahedrons with 9 oxygen atoms centered on one La atom, while La(OH)3 is constructed by octahedrons with 6 H atoms centered on the La atom.
Three possible transition paths from LaH3 to La(OH)3 have been proposed in accordance with the fact that LaH3 might react with oxygen, or with water vapor, or with both of them:
Reaction route 1:
Reaction route 2:
Reaction route 3:
In order to discriminate the most suitable reaction route, first-principles calculations have been used to calculate the formation enthalpy of these three reaction routes, as shown in Table 3. Since reaction route 2 has a positive formation enthalpy, it was initially excluded. For the other two reaction routes, route 1 seems more suitable than route 3. However, the possibility of reaction route 3 cannot be excluded extensively. Therefore, a simple hydrogen detection method was designed using special hydrogen detection tape, as shown in Figure 5. The concept is that the tape will change from amber to black if hydrogen gas is produced during reaction route 3, while the container remains partially open to allow oxygen to participate in the reaction. During our experiment, within 48 h, the tape color remained the original color, as shown in Figure 5. Therefore, the reaction route 3 has also been excluded.
After the reaction route 1 has been confirmed, first-principles calculations have been utilized to explore the adoption process of oxygen at LaH3 (100), (110) and (111) surfaces with a hydrogen atom as the plane model by considering the top, bridge and hollow (fcc and hcp) positions.
Surface adsorption energy is adopted to describe the surface adsorption capacity of oxygen molecules. It is the change of total energy caused by the chemical bond reorganization of the whole system after the adsorption of small molecules on a solid surface. The formula is as follows:
In the equation, is the total energy of the adsorption system, is the total energy of LaH3 surface structure before adsorption, is the total energy of spin polarized oxygen molecules in the supercell. The surface adsorption energy can be used to evaluate the adsorption capacity of the surface to oxygen molecules. If the adsorption energy of oxygen molecules is low (i.e., the absolute value of the adsorption energy is high), this indicates a strong adsorption capacity of the surface for oxygen molecules, and vice versa. Surface energy is an important physical quantity for characterizing structural stability and studying the surface of hydrogen storage materials. Surface energy is defined as:
In the equation, is the energy of the material surface system, is the total energy of the slab model cut in different directions along the cell, N is the number of LaH3 units in the surface system, is the total energy of the LaH3 unit in the cell, A is the system’s surface area. The lower the surface energy, the more stable the system.
Table 4 shows the calculated surface energy values of LaH3’s three low exponential surfaces. Obviously, the surface energy of LaH3’s three low exponential surfaces has the following relationship: Esurf (100) > Esurf (110)> Esurf (111). The lowest and most stable surface energy is LaH3 (111) surface. Therefore, LaH3 (111) surface is selected as the adsorption surface of oxygen on LaH3 in the simulation calculation.
The adsorption of oxygen molecules on the (111) surface is considered. Figure 6 shows the initial adsorption configuration of the LaH₃ (111) surface, with four highly symmetric adsorption positions: the top position, bridge position, fcc position, and hcp position. After optimizing the four adsorption sites for the oxygen molecule on the surface, it was found that the top and bridge sites remained unchanged while the other sites shifted. This shows that oxygen molecules have unstable adsorption at the fcc and hcp sites, so the subsequent study only analyzed the adsorption energy at the top and bridge sites. The configurations before and after adsorption of the optimized top and bridge adsorption position have been obtained according to the abovementioned approach and are shown in Figure 7 and Figure 8.
First is the top position of adsorption configuration; after adsorption, O–O bond length increases, tend to have a dissociation and oxygen molecules adsorbed on the surface LaH3 and with the surrounding La atomic bonding, compared before and after adsorption oxygen molecules and adsorption to the left deviation about 34 degrees. According to the adsorption energy value calculated in Table 5, the top position is the most stable adsorption site for oxygen on the LaH3 (111) surface.
At the bridge position of adsorption configuration, after adsorption, the length of O-O bond is significantly increased, and the dissociation tendency is greater. Oxygen molecules are adsorbed on the surface of LaH3 and bond with the surrounding La atoms. The orientation of the oxygen molecules remains unchanged before and after adsorption, and their position shifts closer to the nearby hydrogen atoms.
Motivated by the observations that large-grain of LaH3 will decompose into powders when exposed to the air within 24 h, both the crystal structures of the LaH3 and the decomposed product have been solved by SXRD and PXRD, respectively. Meanwhile, the optimal reaction route from LaH3 to powdered La(OH)3 has been clarified by combining the formation enthalpies and a self-designed hydrogen detection experiment. In addition, the preferred adsorption of oxygen atoms on the (111) surfaces has been extensively investigated by calculating the adsorption energy for all candidate adsorption sites. The conclusions are as follows:
The following supporting information can be found at: https://www.sciepublish.com/article/pii/336, Figure S1: Morphology of the sample before and after the phase transition.
Not applicable.
The National Natural Science Foundation of China (grant No. 52173231; grant No. 51925105); Hebei Natural Science Foundation (grant No. E2022203182); The Innovation Ability Promotion Project of Hebei supported by Hebei Key Lab for Optimizing Metal Product Technology and Performance (grant No. 22567609H).
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
