Both Dielectrics and Conductance Anomalies in an Open-Framework

Article Cite This: Inorg. Chem. 2017, 56, 13998-14004

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Both Dielectrics and Conductance Anomalies in an Open-Framework Cobalt Phosphate Hong-Bin Luo,†,‡ Mei Wang,†,‡ Shao-Xian Liu,†,‡ Wen-Long Liu,*,§ Yang Zou,†,‡ Zheng-Fang Tian,∥ and Xiao-Ming Ren*,†,‡ †

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Molecular Engineering and College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P. R. China § College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China ∥ Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang 438000, P. R. China ‡

S Supporting Information *

ABSTRACT: Switchable conducting or dielectric materials, as the key component, show important technological applications in modern electrical and electronic devices, including data communication, phase shifters, varactors, and rewritable optical data storage. To explore new types of switchable conducting or dielectric materials could significantly accelerate the development of efficient electrical and electronic devices. Herein we present the first example of switchable conducting and dielectric material, which is based on an open-framework phosphate, (C2N2H10)0.5CoPO4. A reversible isostructural phase transition occurs at ∼348 K in this open-framework phosphate, to give both dielectrics and conductance anomaly around the critical temperature of phase transition. This study will provide a roadmap for searching new switchable conducting or dielectric materials as well as new applications of open-framework phosphates.

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dynamics of counter cations.21−25 For such kinds of CPs, a symmetry-breaking phase transition commonly occurs owing to the deformation of flexible framework and orientation movement of guests residual in the framework, which gives the switchable dielectrics. In some cases, the switchable dielectric CPs even exhibit intriguing ferroelectric/anti-ferroelectric and multiferroic properties.26−29 Open-framework phosphates are a class of crystalline porous materials, and such materials, besides the low-cost preparation, are endowed with distinguished features of rich structural diversity, pore-size tenability, and high surface areas. These advantages compare favorably with recently burgeoning CPsbased porous materials. Particularly, the open-framework phosphate-based materials show more excellently water and thermal stabilities regarding the CPs-based materials; this is because of their solidly inorganic frameworks. In the past three decades, the open-framework phosphate-based materials have been demonstrated possessing vast applications in gas adsorption, ion exchange, separation process, heterogeneous catalysis, and proton conductor.30−33 Nonetheless, there is still rising interest in probing for new functionalities and new applications for the open-framework phosphate-based materials. Most open-framework phosphates possess the inherent anionic open framework, and the ammonium or alkylammo-

INTRODUCTION Phase transitions in materials occurring widely in nature can frequently impart functionality to the system, resulting in important technological innovations. The physical properties, such as magnetism, optics, electrics, etc., of a material probably change massively across a phase transition; consequently, the materials endowed with rapid and reversible phase transitions show practical applications in switching devices.1−5 For instance, the switchable dielectric materials show the important potential application in the fields of electrical and electronic devices, including phase shifters, varactors, data communication, and rewritable optical data storage.6−8 Recently, enormous efforts have been dedicated to the study of structure−property relationship, which finally aims to lay a theoretical foundation for rational design of efficient switchable dielectric materials9,10 and even for enriching multifunctional materials.11,12 In this context, the lately burgeoning switchable dielectric materials can be mainly separated into two categories. One is the amphidynamic crystal of organic salts, where the structural phase transition undergoes owing to the orientational motion of the moieties in the lattice, which leads to the switchable dielectric feature of an organic salt.13−20 The other one is the perovskite-type structure coordination polymers (CPs), where the guest components residual in the cavities or channels have rotatable and motionable freedom, as well as the structural flexibility and small lattice energy of such type of CP result in the lattice being easily deformed to match well the © 2017 American Chemical Society

Received: August 18, 2017 Published: November 2, 2017 13998

DOI: 10.1021/acs.inorgchem.7b02137 Inorg. Chem. 2017, 56, 13998−14004

Article

Inorganic Chemistry

Figure 1. (a) Plots of ε′ vs T at selected frequencies and (b) plots of M″ vs f at the selected temperatures. (c) Arrhenius plot of dielectric relaxation.

nium cations locate in the pores of the framework for charge balance, and usually, these counter cations show significant degree of freedom in the lattice, and this type of structural feature is rather analogous to the perovskite-type structural CPs. Therefore, open-framework phosphates have prospectively switchable dielectrics; however, this functionality has aroused little concern. There is no doubt that the investigation of open-framework phosphates with dielectric phase transition will open new perspectives for open-framework phosphates with new applications, more importantly, to give a renewed impetus to develop new switchable dielectric materials. In this study, we present the structural, dielectric, and conducting properties for an open-framework phosphate, (C2N2H10)0.5CoPO4 (1). Our results demonstrate that 1 exhibits arresting both dielectric and conducting anomaly natures associated with the isostructural phase transition.

Accordingly, it is possible that a structural phase transition gives rise to dielectric anomaly or switchable dielectrics34 in 1. Another striking dielectric feature of 1 is that the dielectric permittivity strongly depends on the alternating-current (ac) frequency, indicating the existence of dielectric relaxation in the 223−423 K range. Usually, there are several different mechanisms to give rise to the dielectric relaxation, which include the molecular dipole orientation and the ion migration, all of which are the intrinsic nature, in a dielectric material, as well as the space charge injection and the electrode polarization effects, which belong to the extrinsic effects and undergo especially in the case at the low frequency and the high temperature. The extrinsically dielectric relaxation effects can be efficiently suppressed in dielectric modulus analysis; such a method could lead to the inherent dielectric relaxation behavior being more clearly observed. The electrical modulus (M*) is related to the dielectric permittivity (ε*) by eq 135,36

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RESULTS AND DISCUSSION Dielectrics, Conductance, and DSC Analysis. The plots of dielectric permittivity (ε′) versus temperature in the 223− 423 K range are shown in Figure 1a. When heated, the dielectric permittivity steadily increases at the temperature below 352 K and subsequently jumps with a knee in shape at ca. 352 K, and then the dielectric permittivity slightly grows with increasing temperature. In the successively cooling process, the kneelike dielectric anomaly is also observed; however, the critical temperature of dielectric anomaly shifts to the low temperature a little and locates at ca. 348 K, with a thermal hysteresis of ca. 4 K in the heating−cooling cycle. In a dielectric material, the dielectric permittivity is related to the polarizable dipole moment and the density of polarizable dipoles, and these natures depend on the crystal structure.

M *(ω) =

ε ′ + jε ″ 1 = 2 = M′ + jM″ ε*(ω) ε′ + ε′′2

(1)

The electrical modulus imaginary parts M″ are plotted as a function of frequency in the range from 1 to 1 × 107 Hz and displayed in Figure 1b at the selected temperatures for 1, and in these curves, a broad maximum appears in the frequencydependent M″ plots at the temperature above 268 K. The Arrhenius equation, eq 2, is used to estimate the activation energy of the dielectric relaxations. ⎛ E ⎞ τ = τ0 exp⎜ a ⎟ ⎝ kBT ⎠

(2)

In eq 2, the macroscopic relaxation time (τ) is defined as τ = 1/f max, where the f max is the maximum in the M″−f plot at the 13999


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Inorganic Chemistry certain temperature. The symbol τ0 is the pre-exponential factor, which is the inverse of the frequency factor. The symbol Ea denotes the activation energy of dielectric relaxation, and kB represents Boltzmann constant. The plot of ln(τ) against 1000/ T in the temperature range of 268−423 K (heating run) is displayed in Figure 1c for 1, which shows two intersecting lines. It is mentioned that the temperature at the point of two intersecting lines locates at ca. 348 K, which coincides well with the critical temperature of the dielectric anomaly. The best fit for the plot of ln(τ) versus 1000/T gave Ea = 0.35 eV in the temperature range of 273−343 K and Ea = 0.76 eV in the temperature range of 363−423 K for 1. The distinct relaxation behaviors in the temperature regions above and below ca. 348 K further confirm that the dielectric anomaly occurring at ca. 348 K in the ε′−T plots is the intrinsic nature of 1. The plots of conductivity against temperature at the selected frequencies are shown in Figure 2 and Figure S3. On the one

Figure 3. DSC plots with successive heating and cooling cycles for 1.

ΔH/TC, where TC is the critical temperature of the phase transition, and TC = 348 K for 1. Single-Crystal Structure. The crystal structure of openframework cobalt phosphate (C2N2H10)0.5CoPO4 at room temperature was first reported by J. Chen and co-workers,43 who presented the monoclinic system with a nonstandard setting of I2/b as well as unit cell parameters of a = 14.719(6), b = 14.734(5), c = 17.891(6) Å, and γ = 90.02(2)° because of the pseudotetragonal nature of the structure; thus, the crystal structure of (C2N2H10)0.5CoPO4 can be also refined in the tetragonal space group I41/a, while the model required a disordered arrangement of the template molecule.43 Later, the crystal structure of (C2N2H10)0.5CoPO4 was reported by C. N. R. Rao et al. again;44 however, these authors only presented the space group (P42/n) and unit cell parameters (a = b = 10.420 Å, c = 8.956 Å) in the study and did not give the structure data in detail. In this study, we redetermined the single-crystal structure of (C2N2H10)0.5CoPO4 at room temperature (293 K), although the X-ray single-crystal diffraction data can be solved and refined in the tetragonal space group I41/a with a = b = 14.7028(7) and c = 17.8214(16) Å, the “nonpositive definite” appears in more of the atoms in both framework and template molecules. This issue can be completely solved if the X-ray single-crystal diffraction data are refined in the tetragonal space group P42/n with a = b = 10.3993(3) and c = 8.9421(5) Å. Additionally, as shown in Figure S1, the experimental and simulated patterns of (C2N2H10)0.5CoPO4 in this work show almost the same as the simulated powder X-ray diffraction (PXRD) pattern obtained from the single-crystal structure data in the literature,43 demonstrating that the crystal structures of (C2N2H10)0.5CoPO4 reported by J. Chen et al.43 are the same as that in this work. As depicted in Figure 4a, an asymmetric unit of 1 consists of one Co2+ ion, one P atom, and four crystallographically distinct O atoms together with one-half of a diprotonated ethylenediamine (H2en2+) cation. The cation shows statically disordered with two possible sites, each of which has the occupation factor of ca. 0.5. Both P atom and Co2+ ion form PO4 and CoO4 tetrahedra with four O atoms, respectively. Each PO 4 tetrahedron links four CoO4 tetrahedra by sharing vertices and vice versa; the anionic open-framework in 1 is built from the alternately connecting PO4 and CoO4 tetrahedra. The inorganic open framework contains the eight-membered ring channels along the [001] direction (Figure 4b) and the fourmembered ring channels along the [100]/[010] directions (see Figure S5), and the channels in the [100]/[010] directions intersect with those parallel to [001] giving rise to cavities, where the H2en2+ cations are accommodated.

Figure 2. Temperature-dependent conductivity for 1 at the selected ac frequencies.

hand, the conductivity has dispersion at all frequencies in the temperature range of 223−423 K. The phenomenon of frequency-dependent conductivity has been generally observed in glass and amorphous semiconducting materials;37−39 this characteristic feature is well-known in the disorder system and interpreted by the thermally activated hopping process between two sites separated by an energy barrier.40 It is worth noting that the studies also showed some crystalline materials displaying the frequency dependence of electrical conductivity.41,42 On the other hand, a peak occurs at ca. 348 K in each σ−T plot as well. Moreover, the peak temperature in the plot of conductivity versus temperature is temperature-independent and in good agreement with the critical temperature of the dielectric anomaly. Differential scanning calorimetry (DSC) plots are shown in Figure 3, clearly indicating a couple of thermal anomaly peaks in the successive heating and cooling cycles in the temperature range of 223−473 K. The peak temperatures are 348 K in the heating run and 344 K in the cooling run, and these temperatures match with the corresponding dielectric and conducting anomaly temperatures. This observation discloses that the dielectric and conducting anomaly are associated with a reversible phase transition. The enthalpy change (ΔH) of the phase transition was calculated to be 0.377 kJ·mol−1, and such a small ΔH demonstrates that this dielectric/conducting anomaly shows the character of a second-order phase transition. The corresponding entropy change of phase transition (ΔS) was estimated to be 1.083 J·mol−1·K−1, using the equation ΔS = 14000


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Inorganic Chemistry

Figure 4. (a) Asymmetric unit of 1 at 293 K with 50% thermal ellipsoid probability, where the symmetric codes No. 1 = 1.5 − x, 1.5 − y, z; No. 2 = 0.5 − y, x, 0.5 − z; No. 3 = 0.5 − x, 1.5 − y, z; and No. 4 = −0.5 + y, 1 − x, 0.5 + z. (b) Packing structure viewed along the c-axis of 1.

Figure 5. (a) Temperature-dependent cell parameters and (b) variable-temperature PXRD patterns at selected the temperatures. (c, d) The relative orientation between the cation and framework viewed along the different directions as well as the illustration for the neighboring N···O distances for 1.

Comparison of Crystal Structure in High- and LowTemperature Phases. Crystalline material, which undergoes a dielectric anomaly, usually arises from a structural phase transition; thus, the variable-temperature crystal structures of 1 were further investigated to elucidate the correlation between dielectric anomaly and crystal structure. The temperature dependence of the cell parameters is shown in Figure 5a. Albeit the changes of the parameters of a, c, and V are small with altering temperature, the discontinuous reflection point can be obviously found at the temperature around 348 K, implying the character of the second-order phase transition around this temperature region. The variable-temperature PXRD measurements were also performed in the 303−423 K range for 1. As displayed in Figure 5b and Figure S6, the patterns of 1 show high similarity within the range of 2θ = 5−50° in the temperature range of

303−423 K; however, the intensity of the diffraction peak at 34.30° decreases dramatically at the temperature above 353 K. This observation discloses that a small change indeed occurs at ca. 353 K in the crystal structure of 1, and this temperature is close to the critical temperature where the dielectric and conducting anomaly appear, further confirming that the dielectric and conducting anomaly stems from a structural phase transition. The crystallographic data at 293, 323, and 373 K are summarized in Table 1; all of them show the highly similar crystal structure with the same space group, analogous cell parameters, and asymmetric unit. The structure phase transition, which is accompanied by change in neither the crystallographic space group nor the occupied Wyckoff positions, is known as “Cowley’s type zero” transition,45 after Cowley postulated the possibility of such a transition, on the 14001


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Inorganic Chemistry

isostructural phase transition arises mainly from the small distortion of the inorganic framework and the relative displacement of the H2en2+ cations with respect to the inorganic framework as well. Origin of Dielectric Anomaly. The electric polarizability represents the tendency of a material to allow an externally applied electric field to induce electrical dipoles (separated negative and positive charges) in a material. The induced electrical dipoles, the externally applied electrical field, and the dielectric permittivity are related by the equation below

Table 1. Crystallographic Data and Refinement Parameters of 1 at 293, 323, and 373 K temperature (K) wavelength (Å) empirical formula formula weight CCDC No. crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3)/Z density (g cm−3) abs coeff (mm−1) F(000) data collect θ range index ranges

Rint independent reflections/ restraints/parameters refinement method goodness-of-fit of F2 R1, wR2a [I > 2σ(I)] R1, wR2a [all data] residual (e Å−3) a

293(2) 0.71073 CH5CoNO4P 184.96 1562554 tetragonal P42/n 10.3993(3) 10.3993(3) 8.9421(5) 90.00 90.00 90.00 967.05(7)/8 0.635 0.948 184 3.00−25.49 −12 ≤ h ≤ 12 −12 ≤ k ≤ 12 −10 ≤ l ≤ 10 0.0894 899/3/96



least squares refinement of F2 1.191 0.0330, 0.0788 0.0347, 0.0795 0.451/−0.371

1.250 0.0368, 0.0877 0.0383, 0.0886 0.497/−0.602

1.161 0.0331, 0.0810 0.0355, 0.0821 0.374/−0.429

Np = ϵ0(ϵr − 1)E

(3)

where the symbol p is the induced dipole moment, N represents the density of induced dipoles, and the symbols ε0 and εr represent the permittivity of a material in the free space and the relative dielectric constant, respectively. The relative dielectric permittivity of a material corresponds to the induced dipole moment per volume, which is generally related to the crystal structure of a material. Consequently, it is possible that a structural phase transition results in dielectric anomaly. In crystal structure of 1, the small distortion of inorganic framework and the cations being located off-center of channels result synergically in the anomalies of dielectrics and conductance when the temperature changes across phase transition. Since the open framework of cobalt phosphate is rigid and hard and deformed as well as the H2en2+ cations are also hardly movable owing to almost no accessible void in the lattice (this is checked by Platon program46), the alternations of dielectric permittivity and conductivity are undersized. This finding of gentle change of dielectric permittivity at the critical temperature is distinct from the observations in the perovskitetype open-framework compounds based on diatomic or multiatomic bridges,21 which dielectric permittivity changes sharply around the critical temperature of dielectric anomaly owing to the anion framework being easily deformed and counter-cations residual in the framework being movable.

R1 = ∑∥F0| − |Fc∥/|F0|, wR2 = [∑w(∑F02 − Fc2)2/∑w(|F02|)2)2]1/2.

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basis of the behavior of acoustic phonons within the framework of displacive structural phase transitions. Such a transition has been also categorized as “isostructural phase transition” (IPT). Accordingly, the intermolecular distances and the relative orientation between the CoO4 and PO4 tetrahedra in the inorganic framework as well as between the inorganic framework and H2en2+ cations were carefully inspected for 1. As shown in Tables S1 and S2, the typical Co−O and P−O bond distances together with the ∠O−P−O and ∠O−Co−O bond angles are not significantly changed when the temperature is elevated from 293 to 323 K in the low-temperature phase; however, the alterations of some Co−O/P−O bond lengths reach to twofold or threefold of errors, and the changes of some ∠O−P−O and ∠O−Co−O bond angles approach to five or six times of errors when the temperature changes across the phase transition from 323 to 373 K. Additionally, on the one hand, as displayed in Figure 5c,d together with Table S3, the changes of all N···O distances between H2en2+ cation and the inorganic framework fall within the ranges of error with increasing temperature from 293 to 323 K in low-temperature phase. On the other hand, the significantly nonuniform alterations of N··· O distance occur when the temperature changes across the phase transition. It is observed that the distances of d1−d5 and d8 increase, while the distances of d6 and d7 shrink from 323 to 373 K, indicating that the cations are located off-center of channels running along the [110] direction and shift toward one side of channel wall (refer to Figure S7). Consequently, the

CONCLUSION In summary, we have presented the study of structures, dielectrics, and conductance for an open-framework (C2N2H10)0.5CoPO4. The anomalies of dielectric permittivity and conductivity are observed in this open-framework phosphate, which are associated with an isostructural phase transition. Albeit both dielectric permittivity and conductivity only show gentle changes at the critical temperature of isostructural phase transition owing to the anion framework being hardly deformed and counter-cations residual in the framework being scarcely movable, this study will open new perspectives for open-framework phosphates with fascinating natures and new applications, as well as provide a roadmap to search for new switchable dielectric or conducting materials, which would have pronounced potential application in a wide range of electrical and electronic devices.

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EXPERIMENTAL SECTION

Chemicals and Materials. The solvents and chemicals, such as Co3(PO4)2·xH2O, anhydrous ethylenediamine, H3PO4, and H2O were supplied by Sinopharm Chemical Reagent Co. Ltd. of China, which are of analysis pure grade, and used as received without further purification. Sample Preparation. The crystals of open-framework cobalt phosphate, (C2N2H10)0.5CoPO4 (1), have been achieved according to our previously reported procedure.32 The blue plate crystals were dried 14002


Article

Inorganic Chemistry at 60 °C in an airflow drying oven for 12 h and then used for this study. Elemental microanalysis calculated for (C2N2H10)0.5CoPO4: Found: C, 6.48; H, 2.72; N, 7.49%. Calcd: C, 6.49; H, 2.72; N, 7.57%. Chemical and Physical Characterizations. Elemental analyses for C, H, and N were performed with an Elementar Vario EL III analytic instrument. PXRD data were collected on a Bruker D8 diffractometer, operated at 40 kV and 40 mA, with Cu Kα radiation (λ = 1.5418 Å), and the measurement was performed in the 2θ range of 5−50° with 0.01°/step. Thermogravimetric analysis (TG) was performed using a TA2000/2960 thermogravimatric analyzer in the temperature range of 293−1073 K (20−800 °C) under nitrogen atmosphere. Water adsorption experiment was performed by BelsorpMax instrument. DSC measurement was made on a DSC Q2000 V24.10 Build 122 for polycrystalline sample between 223 and 473 K, with a temperature scanning rate of 10 K·min−1. The dielectric permittivity and conductivity were measured on a concept 80 system (Novocontrol, Germany); the powdered polycrystalline pellet, with 0.92 mm in thickness and 10 mm in diameter, was made under the static pressure of 10 MPa pressure and sandwiched by two copper electrodes during the measurement. The temperatures range from 223 to 423 K, and the ac frequency spans from 1 to 1 × 107 Hz. X-ray Single Crystallography. Single-crystal X-ray diffraction data were collected for 1 at 293, 323, and 373 K using graphite monochromated Mo Kα (λ = 0.710 73 Å) radiation on a CCD area detector (Bruker-SMART). Data reduction and absorption corrections were performed with the SAINT and SADABS software packages,47 respectively. Structures were solved by a direct method using the SHELXL-1997 software package.48 The non-hydrogen atoms were anisotropically refined using a full-matrix least-squares method on F2. All hydrogen atoms were placed at the calculated positions and refined as riding on the parent atoms. The details of data collection, structure refinement, and crystallography are summarized in Table 1.

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*E-mail: [email protected]. (W.-L.L.) ORCID

Xiao-Ming Ren: 0000-0003-0848-6503 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions and by the National Nature Science Foundation of China (Grant Nos. 21671100 and 21371150).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02137. Variable-temperature PXRD patterns in the 2θ range of 5−50° at the selected temperatures, experimental and simulated PXRD patterns, TG curve, temperaturedependent conductivity at the selected ac frequencies, polyhedral packing and open-framework diagrams at 293 K, schematic illustration for the neighboring N···O distances between the cation and framework of 1; Tables of comparison of typical bond lengths and bond angles in the CoO4 and PO4 tetrahedra of inorganic framework at 293, 323, and 373 K, comparison of typical interatomic distances (Å) between N atom in H2en2+ cation and O atoms in the inorganic framework at 293, 323, and 373 K, as well as atom displacement parameters (Ueq, U11, U22, and U33/(1 × 10−2) Å2) at the selected temperatures (PDF) Accession Codes

CCDC 1562554−1562556 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

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Both Dielectrics and Conductance Anomalies in an Open-Framework

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