English

• 0.   Introduction

  Proton exchange membrane fuel cells (PEMFCs) are considered to be the most promising candidates for energy applications because of their high energy conversion efficiency and low emissions^[1-4]. The proton conductor, as the core component of a PEMFC, directly affects its performance and durability. Therefore, the development of proton-conductive materials exhibiting remarkable stability in water, acidic, and basic environments is an important and challenging research topic.

  Recently, metal-organic frameworks (MOFs) have gained significant attention because of their porous structures with modifiable channels and rich hydrogen-bonding networks or hydrophilic features, making them highly suitable as novel proton conductors^[5-7]. Excellent thermal and chemical stabilities are crucial for the practical implementation of proton-conductive materials. The construction of high‑conductivity MOFs requires high-temperature and high-humidity conditions, making it essential to evaluate their stability in water, as most MOFs tend to be unstable in these environments. Hence, designing a material with exceptional thermal, water, and chemical stabilities, as well as excellent proton conduction performance, is highly desirable to meet application requirements. As a significant branch of MOFs, lanthanide metal-organic frameworks (Ln-MOFs) have garnered substantial interest owing to their exceptional functionalities and properties^[8-11]. Lanthanides are characterized by their large radii and high positive charges, which allow them to bind to organic ligands, thereby exhibiting high coordination numbers and flexible coordination geometries^[12-14]. These behaviors make Ln-MOFs far superior to their transition-metal counterparts in terms of stability. Therefore, Ln-MOFs have promising applications in proton conduction^[15-16]. For instance, {H[(N(CH₃)₄)₂] [Gd₃(NPA)₆]}·3H₂O (H₂NPA=5-nitroisophthalic acid) demonstrates remarkable water-stable and superior proton conduction [σ=7.17×10^-2 S·cm^-1, 75 ℃, and 98% of relative humidity (RH)]^[17]. Oxalate-based metal-organic frameworks (MOFs) possess characteristics such as high crystallinity, tunable structures, and ease of functionalization of their internally ordered pores. These materials exhibit excellent proton‑conduction performance^[18]. Furthermore, according to research reports, the incorporation of oxalate groups into frameworks contributes to proton conductivity. For instance, Kitagawa reported the initial example of a metal oxalate material, Fe(ox)·2H₂O, showing an outstanding conductivity of 1.3×10^-3 S·cm^-1 (25 ℃ and 98% of RH). In the same year, Kitagawa et al. synthesized a material named (NH₄)₂(adp)[Zn₂(ox)₃]·3H₂O (apd=adipic acid) by introducing NH₄⁺ ions as counterions into the pores of oxalate-based MOFs, achieving a superior proton conductivity of 10^-2 S·cm^-1 at 25 ℃^[19-20].

  Metal phosphate/phosphite oxalates (MPOs), a new class of crystalline porous materials, have superior water and thermal stabilities, which is beneficial for in-depth studies of the proton transport mechanism. To date, studies on proton-conducting MPOs have mainly focused on transition and main-group metals^[21], but there are limited reports on rare-earth MPOs in the literature. Based on the above considerations, by choosing cerium as a metal source, we have synthesized several lanthanide phosphite-oxalate compounds showing high proton conductivity at 75 ℃ and 98% of RH, such as Ce₂(H₂O)₂(H₂PO₃)₂(C₂O₄)₃·C₆H₁₁N₃O₂·H₂O (σ=3.67×10^-4 S·cm^-1) using amino acids as templates, [Ce₂(H₂PO₃)(C₂O₄)₄](C₆N₂H₁₆)·H₃O·3H₂O (σ=9.17×10^-4 S·cm^-1) exhibiting excellent chemical stability and water stability^[22-23]. As part of our ongoing studies, we report two 3D lanthanum phosphite-oxalates, named [La(HPO₃)(C₂O₄)_0.5(H₂O)₂] (La-1) and (C₆H₁₆N₂)(H₃O) [La₂(H₂PO₃)₃(C₂O₄)₃(H₂O)] (La-2), where C₆H₁₄N₂=cis-2, 6-dimethylpiperazine. The results of the alternating current (AC) impedance measurements indicate that both compounds exhibited proton conductivity, and the σ can reach 10^-4 S·cm^-1 at 75 ℃ and 98% of RH (La-1: σ=2.71×10^-4 S·cm^-1, La-2: σ=4.97×10^-4 S·cm^-1). The time-dependent conductivities of La-1 and La-2 were also examined at 75 ℃ and 98% of RH. The proton transport mechanism was also discussed by combining the structure and activation energy. Notably, La-1 possesses excellent water and acid-base stability.

  1.   Experimental

  1.1   Materials and instrumentation

  All the reagents were purchased commercially without further purification. Powder X-ray diffraction (PXRD) patterns were obtained using a Bruker D8 VENTURE diffractometer with Cu radiation (Kα, λ=0.154 18 nm, 40 kV, 40 mA) over a 2θ range of 3°-50°. Fourier transform infrared (FTIR) spectra were recorded on a Perkin Elmer Spectrum GX spectrometer (4 000-400 cm^-1) using KBr pellets. CHN analyses were performed using a Euro Vector EA 3000 CHNS analyzer. Thermogravimetric (TG) analysis was performed using a TA Q600 TGA thermogravimetric analyzer at a scanning rate of 10 ℃·min^-1 in an air atmosphere.

  1.2   Synthesis

  La-1 was synthesized hydrothermally by heating a mixture of LaCl₃·4H₂O (0.10 g), H₃PO₃ (0.70 g), H₂C₂O₄·2H₂O (0.20 g), C₆H₁₄N₂ (0.20 g), and H₂O (1.00 mL) in a 25 mL Teflon-lined autoclave. The autoclave was heated to 130 ℃ for 3 d under autogenous pressure, and then slowly cooled to room temperature. Colorless plate-like crystals of La-1 were obtained by filtration and washed with water. When the mass of H₃PO₃ was reduced to 0.60 g and H₂O was changed to CH₃CH₂OH (0.50 mL), colorless plate-like crystals of La-2 were obtained. The yields of La-1 and La-2 were 48.7% and 43.2%, respectively (based on La). FTIR (cm^-1): 3 417(w), 2 378(w), 1 641(s), 1 505(m), 1 033(m)for La-1; 3 446(m), 3 193(w), 2 984(w), 2 390(m), 1 616(s), 1 474(w), 1 309(m), 1 220(w), 1 163(m), 1 057(m) for La-2 (Fig.S1, Supporting information). Elemental analysis for CH₅LaO₇P (La-1) yielded C: 4.15% and H: 1.78% (Calcd. C: 4.01%; H: 1.67%); for C₁₂H₂₇La₂N₂ O₂₃P₃ (La-2) yielded C: 15.45%, N: 2.89%, and H: 3.10% (Calcd. C: 15.36%, N: 2.99%, H: 2.90%).

  1.3   Crystal structure determination

  A diffraction-quality single crystal from each sample (dimensions: 0.14 mm×0.13 mm×0.13 mm for La-1 and 0.16 mm×0.15 mm×0.15 mm for La-2) was selected, and data were recorded at 296(2) K on a Bruker D8 Quest diffractometer with Mo Kα radiation (λ=0.071 073 nm). The structure was solved using a direct method and refined using full-matrix least squares on F ² with SHELXTL-2014^[24]. The hydrogen atoms of the P—H groups and H₂O molecules were initially located from the difference Fourier maps, and the remaining hydrogen atoms were geometrically replaced. All non-hydrogen atoms were treated as anisotropic. Details of the crystal data and refinement for La-1 and La-2 are provided in Table 1. The selected bond distances and angles are listed in Table S1 and S2.

  Table 1

  

  Table 1.  Crystal data and structure refinement for La-1 and La-2

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  Parameter	La-1	La-2
  Empirical formula	CH5LaO7P	C12H27La2N2O23P3
  Formula weight	298.63	938.08
  Crystal system	Triclinic	Monoclinic
  Space group	P1	P21/n
  a / nm	0.687 05(10)	1.048 43(3)
  b / nm	0.719 66(10)	2.492 83(6)
  c / nm	0.815 55(12)	1.977 5(3)
  α / (°)	111.415(4)
  β / (°)	113.903(4)	97.519 0(10)
  γ / (°)	91.839(4)
  V / nm3	0.335 44(8)	2.844 36(13)
  Z	2	4
  Dc / (g·cm-3)	3.098	2.191
  Absorption coefficient / mm-1	6.601	3.230
  F(000)	292	1 824
  θ range for data collection / (°)	3.001-25.043	2.042-25.029
  Index ranges	-8 ≤ h ≤ 8, -8 ≤ k ≤ 8, -9 ≤ l ≤ 9	-12 ≤ h ≤ 12, -29 ≤ k ≤ 29, -13 ≤ l ≤ 13
  Reflection collected	5 629	59 361
  Independent reflection	1 175 (Rint=0.069 6)	5 025 (Rint=0.072 5)
  Completeness to θ=25.052° / %	98.7	99.9
  Data, restraint, number of parameters	1 175, 0, 92	5 025, 1, 389
  Parameter	La-1	La-2
  Goodness-of-fit on F 2	1.057	0.992
  Final R indices [I > 2σ(I)]	R1=0.034 4, wR2=0.080 6	R1=0.032 5, wR2=0.075 9
  R indices (all data)	R1=0.036 4, wR2=0.082 1	R1=0.037 9, wR2=0.079 2

  1.4   Proton conductivity measurement

  Using a CHI660E electrochemical workstation, electrochemical impedance spectroscopy (EIS) measurements were undertaken across a frequency range of 1 to 10⁶ Hz and under an applied voltage of 200 mV. The powder samples were compressed into pellets with thicknesses of 0.78 mm for La-1 and 0.94 mm for La-2 and a diameter of 7.00 mm under a pressure of 6 MPa. The detailed data processing methods are described in reference^[25-28].

  2.   Results and discussion

  2.1   Structural description

  La-1 belongs to the triclinic system with the P1 space group. As depicted in Fig. 1a, the asymmetric unit contains 10 crystallographically independent non-H atoms, including one La atom, one HPO₃^2- group, two coordinated H₂O, and half a C₂O₄^2- group (Table S1). The La atom adopts a nine-coordination pattern with two O atoms from one oxalate ligand, five O atoms from the surrounding four phosphite groups, and two O atoms from two coordinated H₂O molecules to form a distorted trigonal dodecahedron. The O—La—O bond angles vary from 54.29(13)° to 146.59(14)°, and the La—O bond distances are in a range of 0.239 4(5)-0.274 7(4) nm. The P atom is bonded to three bridging O atoms with neighboring La atoms, and the fourth position is occupied by a terminal H atom. The P—O bond lengths range from 0.149 2(5) to 0.153 4(4) nm, and the bond angles of the O—P—O bonds range from 106.4(2)° to 114.8(3)°, which is similar to those found in other metal phosphites. La-1 displays a 3D structure built from LaO₉ octahedra, HPO₃^2-, and C₂O₄^2-. The edge-sharing LaO₉ polyhedra are connected to form a 1D chain. Subsequently, HPO₃^2- anions bridge two adjacent chains to build a 2D network with four‑ membered ring (4-MR) windows (Fig. 1b). Finally, the layers are connected by oxalate ligands to form a 3D structure (Fig. 1c). To better demonstrate the structural features of La-1, La and P atoms were used as connected nodes. Topological analysis revealed that La-1 exhibits a 4, 7-c net network (Fig. 1d).

  Figure 1

  

  Figure 1.  (a) 50% ellipsoid representation of the asymmetric unit of La-1; (b) Polyhedral view of 2D layer; (c) 3D open-framework view along the a-axis; (d) 4, 7-c net

  Symmetry codes: a: 1+x, y, z; b: 1-x, 2-y, 1-z; c: 1-x, 1-y, -z.

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  La-2 belongs to the triclinic system with a P2₁/n space group. As depicted in Fig. 2a, the asymmetric unit contains 42 crystallographically independent non-H atoms, of which two are La atoms, three are H₂PO₃^2- groups, one is a coordinated H₂O, three are C₂O₄^2- groups, and one is a free H₃O⁺ ion (Table S1). The La atoms adopted a nine-coordination pattern, forming a distorted trigonal dodecahedron with nine surrounding atoms. However, the coordination is different. For La1, six O atoms are from three oxalate ligands, two O atoms come from the surrounding two phosphite groups, and the last O atom is the terminal-coordinated H₂O. For La₂, six O atoms are from three oxalate ligands, and the remaining atoms are from the surrounding three phosphite groups. The O—La—O bond angles vary from 62.08(11)° to 148.48(12)°, and the La—O bond distances range from 0.242 4(4) to 0.267 3(4) nm. All P atoms adopt pseudo-pyramidal coordination with three oxygen atoms and one H atom. For P1 and P2, two oxygen atoms are bridging oxygen atoms, and the third is a terminal hydroxyl oxygen atom. For P3, one O atom is a bridging O atom, one O atom is a terminal O atom, and the last one is a terminal hydroxyl O atom. The average P—O bond length is 0.152 8 nm, and the bond angles of the O—P—O bonds range from 108.3(2)° to 114.5(2)°. La-2 exhibits a 3D structure composed of lanthanum oxalate 2D layers and H₂PO₃^2- units. As depicted in Fig. 2b, the LaO₉ polyhedra and C₂O₄^2- units are connected to form a 2D layer with 12-MR windows. The H₂P3O₃^- anions decorated the 2D layers and pointed toward the center of the windows. The H₂P1O₃^- and H₂P2O₃^- anions pillar the 2D layers to construct the final 3D structure (Fig. 2c). To better demonstrate the structural features of La-2, the La atom was used as a 5-connected node, and oxalate and H₂PO₃^- groups were used as sticks. Topological analysis revealed that La-2 has a 5-connected sqp network with a {4⁴, 6⁶} point symbol (Fig. 2d). The protonated cis-2, 6-dimethylpiperazine and H₃O⁺ cations reside in the channels along the b- and a-axes, respectively, and interact with the O atoms of the framework through extensive hydrogen bonds. The average N…O distance is 0.285 8 nm, and the average O…O distance is 0.289 2 nm. The details of the H-bonds are listed in Table S3.

  Figure 2

  

  Figure 2.  (a) 50% ellipsoid representation of the asymmetric unit of La-2; (b) Polyhedral view of the La-oxalate 2D layer; (c) 3D open framework constructed with 2D layers and H₂PO₃^- groups viewed along the b-axis; (d) sqp network with a {4⁴, 6⁶} point symbol

  Symmetry codes: a: 1-x, 1-y, -z; b: -1/2+x, 3/2-y, -1/2+z; c: 2-x, 1-y, 1-z; d: 1+x, y, 1+z.

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  2.2   Stability test

  The experimental PXRD patterns of La-1 and La-2 were compared with the simulated patterns obtained from the single-crystal diffraction data (Fig. 3). We discovered that the peak positions were virtually identical. La-1 and La-2 were demonstrated to be pure. To evaluate the water stability of La-1 and La-2, the samples were immersed in ultrapure water at 25 ℃ for a week. After this treatment, the samples were examined using PXRD to assess structural changes. The resulting diffraction patterns indicated that La-1 exhibited good stability in aqueous solution, whereas sample La-2 was unstable. Additionally, La-1 was soaked in aqueous solutions with pH values of 1 and 13 at 25 ℃ for 24 h. La-2 was soaked in aqueous solutions with pH values of 5 and 9 at 25 ℃ for 24 h. After immersion, the samples were filtered, dried, and analyzed using PXRD. The structural integrity of La-1 was well preserved, and it exhibited excellent pH stability; however, La-2 was unstable. Compared to several MOFs that are unstable in aqueous and strong acidic or basic solutions^[29], La-1 exhibited excellent water and acid-base stability.

  Figure 3

  

  Figure 3.  Simulated and experimental PXRD patterns of La-1 (a) and La-2 (b) under different conditions

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  2.3   TG analysis

  TG analysis under an air flow revealed that compounds La-1 and La-2 underwent a two-step weight loss as the temperature increased (Fig.S2). For La-1, the curve showed the first significant weight loss of 11.17% from 100 to 235 ℃, indicating that the coordinated water molecule (Calcd. 12.04%) leaves this framework. Subsequently, the weight loss (9.25%) of La-1 between 348 and 630 ℃ is attributed to the release of the oxalate ligand (Calcd. 14.72%). For La-2, the first weight loss of 11.66% occurred between 122 and 257 ℃ owing to the loss of guest (C₆H₁₄N₂)²⁺ cations and free and coordinated H₂O units (16.33%). The second weight loss of 27.33% from 280 to 335 ℃ is attributed to the release of the oxalate ligand (Calcd. 28.15%). The observed total weight losses are lower than the expected values, which may be explained by the retention of carbon in the solid residue. In brief, the thermal stabilities of La-1 and La-2 fully satisfied the requirements of AC impedance testing up to 75 ℃.

  2.4   Proton conductivity

  To the best of our knowledge, the inherent water molecules in the structure facilitate proton transfer. Coordinated and free water molecules were present in the structures of La-1 and La-2, respectively. La-1 exhibited outstanding water and pH stability. The above structural characterizations provide the prerequisites for proton conduction. Therefore, we investigated the proton-conduction properties of both compounds. AC impedance tests were performed at variable temperatures (30-75 ℃) and RHs (44%-98%) to express the proton conductivities of La-1 and La-2 (Fig.S3-S7 and Fig. 4).

  Figure 4

  

  Figure 4.  Nyquist plots of La-1 (a) and La-2 (b) at 98% of RH; Temperature-dependent σ values for La-1 (c) and La-2 (d); Plots of lg σ against RH of La-1 (e) and La-2 (f) at different temperatures

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  The Nyquist diagrams exhibited incomplete curves at high frequencies and skewed tails at low frequencies, which are attributed to the bulk and grain boundary resistances characteristic of proton conductors. First, the proton conductivities of La-1 and La-2 were measured at 44% of RH and various temperatures. As depicted in Fig.S3, an increase in temperature corresponded to a gradual reduction in the size of the arcs, suggesting an incremental increase in the value of σ. As the test temperature increased from 30 to 75 ℃, the value of σ improved from 10^-9 S·cm^-1 (La-1: σ=2.07×10^-9 S·cm^-1; La-2: σ=1.37×10^-9 S·cm^-1) to 10^-8 S·cm^-1 (La-1: σ=2.25×10^-8 S·cm^-1; La-2: σ=1.2×10^-8 S·cm^-1). Similar behavior was observed at other humidities, indicating the temperature dependence of the proton conductivity (Fig.S4-S7). At 98% of RH and 75 ℃, the maximum values of σ were obtained (La-1: σ=2.71×10^-4 S·cm^-1; La-2: σ=4.97×10^-4 S·cm^-1), as shown in Fig. 4a and 4b. To discuss the relationship between temperature and proton conductivity, plots of lg σ against temperature of La-1 and La-2 are given in Fig. 4c and 4d, showing an increase of an order of magnitude as the temperature rose. It is demonstrated that σ slowly increased with increasing temperature at a fixed humidity. The impact of temperature on proton conduction can be attributed to the fact that the activation energy for the formation of H₃O⁺ and H⁺ ions is not easily reached at low temperatures^[30]. It is noted that the maximum value was compared to those of metal phosphates like V₂O₂(HPO₄)(HPO₃)(C₂O₄)₂(C₄N₂H₁₂)₂·2H₂O (σ=4.25×10^-4 S·cm^-1, 75 ℃ and 98% of RH)^[31] and the proton-conducting Ln-MOFs such as {[Dy(MA)(OX)(H₂O)]_n·1.5H₂O} (H₂MA=mucic acid; OX=oxalate) (σ=9.06×10^-5 S·cm^-1, 80 ℃ and 95% of RH), {[Gd(MA)(OX)(H₂O)]_n·3H₂O} (σ=4.7×10^-4 S·cm^-1, 80 ℃ and 95% of RH)^[32]. The detailed σ values of both compounds at 58%, 67%, 76%, and 86% of RHs (30-75 ℃) are provided in Table S4 and S5. Subsequently, to determine the humidity dependence of the proton conductivity of the two compounds, plots of lg σ against RH for La-1 and La-2 are shown in Fig. 4e and 4f, respectively. As expected, La-1 and La-2 demonstrated humidity-dependent conduction behaviors at a fixed temperature. For example, at 75 ℃, σ increased from 10^-8 S·cm^-1 (44% of RH) to 10^-4 S·cm^-1 (98% of RH), enhancing by a factor of 10⁴. The same trend was observed at different temperatures. This phenomenon can be ascribed to the fact that, as the humidity increases, a substantial quantity of water molecules enters the pores. The adsorbed water molecules interact with the inherent H₂O molecules, creating a robust hydrogen-bonding network that aids in efficient proton transport. The σ value of La-2 was higher than that of La-1. This may be because the guest cations ((C₆H₁₄N₂)²⁺ and H₃O⁺ cations), which not only provide carriers but also construct H-bond networks for proton transport, exist in the channels of La-2.

  To further investigate the proton transport mechanism, we estimated the activation energy (E_a) of both samples at 30-75 ℃ according to the Arrhenius plot slopes (Fig. 5a and 5b). As shown in Fig. 5c, with increasing humidity, the overall trend of E_a exhibited a consistent decrease. It is suggested that the proton conduction of both compounds mainly follows the vehicle mechanism (E_a > 0.4 eV) at low RH, while proton transport follows the Grotthuss mechanism (E_a < 0.4 eV) at high RH. However, when the RH was 67%, the E_a reached the maximum value (0.56 eV for La-1, 0.55 eV for La-2). This phenomenon can be ascribed to the fact that, as the humidity increases, a substantial quantity of water molecules enters the pores and impedes the mobility of water molecules, thereby increasing the value of E_a. When the humidity continued to increase, the number of water molecules entering the framework increased. The value of E_a of La-1 and La-2 dropped to 0.20 and 0.14 eV at 98% of RH, respectively. The lower activation energy of La-2 is attributed to its structural characteristics. A schematic of proton transport in La-1 (a) and La-2 (b) is shown in Fig. 6. For La-1, because of the lack of guest molecules, the adsorbed water molecules in the pore channels can only interact with the host framework, such as coordinated H₂O, to form a hydrogen-bond network, providing efficient conducting pathways. For La-2, the adsorbed water molecules not only interact with the host framework but also interact with the guest cations, such as inherent free H₃O⁺ cations and the protonated cis-2, 6-dimethylpiperazine cations, thereby favoring the formation of an effective continuous hydrogen bond network^[33-35].

  Figure 5

  

  Figure 5.  Arrhenius plots for the proton conductivity of La-1 (a) and La-2 (b) under different RH conditions; (c) Values of E_a under different RH conditions; (d) Time-dependent σ values of La-1 and La-2 at 75 ℃ and 98% of RH

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  Figure 6

  

  Figure 6.  Schematic representation of proton transport in La-1 (a) and La-2 (b)

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  To assess the conductivity stability of La-1 and La-2, a prolonged conductivity stability test was performed. Fig. 5d shows that both compounds maintained a constant proton conductivity over a continuous testing period of up to 10 h at 75 ℃ and 98% of RH. To the best of our knowledge, the stability of the material structure during electrochemical testing is crucial for future application research. After determining the AC impedance, we selected a sample that had undergone 10 h of continuous testing for PXRD analysis (Fig. 3). The results show that the peaks of the sample after the test of prolonged conductivity stability could still be fully matched with the original peaks, indicating that the sample exhibits good stability in electrochemical impedance experiments.

  3.   Conclusions

  In summary, two 3D open-framework lanthanum phosphite-oxalates (La-1 and La-2) were successfully synthesized under hydrothermal and solvothermal conditions. La-1 displayed high stability in water and acid-base solutions. Under the condition of 75 ℃ and 98% of RH, the σ values of both compounds reached 10^-4 S·cm^-1. Moreover, tests of prolonged conductivity stability demonstrated that both samples exhibited good durability at 75 ℃ and 98% of RH. The proton transport mechanism is also discussed by combining the structure and activation energy. This study provides insights into the synthesis of novel metal phosphate/phosphite-oxalate materials with proton conductivity and predicts their potential applications in fuel cells.

  
  Conflicts of interest: There are no conflicts to declare.
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Figure 1  (a) 50% ellipsoid representation of the asymmetric unit of La-1; (b) Polyhedral view of 2D layer; (c) 3D open-framework view along the a-axis; (d) 4, 7-c net

  Symmetry codes: a: 1+x, y, z; b: 1-x, 2-y, 1-z; c: 1-x, 1-y, -z.

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  Figure 2  (a) 50% ellipsoid representation of the asymmetric unit of La-2; (b) Polyhedral view of the La-oxalate 2D layer; (c) 3D open framework constructed with 2D layers and H₂PO₃^- groups viewed along the b-axis; (d) sqp network with a {4⁴, 6⁶} point symbol

  Symmetry codes: a: 1-x, 1-y, -z; b: -1/2+x, 3/2-y, -1/2+z; c: 2-x, 1-y, 1-z; d: 1+x, y, 1+z.

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  Figure 3  Simulated and experimental PXRD patterns of La-1 (a) and La-2 (b) under different conditions

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  Figure 4  Nyquist plots of La-1 (a) and La-2 (b) at 98% of RH; Temperature-dependent σ values for La-1 (c) and La-2 (d); Plots of lg σ against RH of La-1 (e) and La-2 (f) at different temperatures

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  Figure 5  Arrhenius plots for the proton conductivity of La-1 (a) and La-2 (b) under different RH conditions; (c) Values of E_a under different RH conditions; (d) Time-dependent σ values of La-1 and La-2 at 75 ℃ and 98% of RH

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  Figure 6  Schematic representation of proton transport in La-1 (a) and La-2 (b)

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  Table 1.  Crystal data and structure refinement for La-1 and La-2

  Parameter	La-1	La-2
  Empirical formula	CH5LaO7P	C12H27La2N2O23P3
  Formula weight	298.63	938.08
  Crystal system	Triclinic	Monoclinic
  Space group	P1	P21/n
  a / nm	0.687 05(10)	1.048 43(3)
  b / nm	0.719 66(10)	2.492 83(6)
  c / nm	0.815 55(12)	1.977 5(3)
  α / (°)	111.415(4)
  β / (°)	113.903(4)	97.519 0(10)
  γ / (°)	91.839(4)
  V / nm3	0.335 44(8)	2.844 36(13)
  Z	2	4
  Dc / (g·cm-3)	3.098	2.191
  Absorption coefficient / mm-1	6.601	3.230
  F(000)	292	1 824
  θ range for data collection / (°)	3.001-25.043	2.042-25.029
  Index ranges	-8 ≤ h ≤ 8, -8 ≤ k ≤ 8, -9 ≤ l ≤ 9	-12 ≤ h ≤ 12, -29 ≤ k ≤ 29, -13 ≤ l ≤ 13
  Reflection collected	5 629	59 361
  Independent reflection	1 175 (Rint=0.069 6)	5 025 (Rint=0.072 5)
  Completeness to θ=25.052° / %	98.7	99.9
  Data, restraint, number of parameters	1 175, 0, 92	5 025, 1, 389
  Parameter	La-1	La-2
  Goodness-of-fit on F 2	1.057	0.992
  Final R indices [I > 2σ(I)]	R1=0.034 4, wR2=0.080 6	R1=0.032 5, wR2=0.075 9
  R indices (all data)	R1=0.036 4, wR2=0.082 1	R1=0.037 9, wR2=0.079 2

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