Denticity and Mobility of the Carbonate Groups in AMCO3F

Oct 18, 2017 - Synopsis. The crystal structures of a novel KMnCO3F fluorocarbonate and high temperature polymorph of KCaCO3F have been determined. Rev...
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Denticity and Mobility of the Carbonate Groups in AMCO3F Fluorocarbonates: A Study on KMnCO3F and High Temperature KCaCO3F Polymorph Gwenaelle Rousse,†,‡,§ Hania Ahouari,§,∥,⊥ Vladimir Pomjakushin,# Jean-Marie Tarascon,†,‡,§ Nadir Recham,§,∥,⊥ and Artem M. Abakumov*,¶ †

UMR 8260 “Chimie du solide et énergie”, Collège de France, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France Sorbonne UniversitésUPMC Univ Paris 06, 4 Place Jussieu, F-75005 Paris, France § Réseau sur le Stockage Electrochimique de l’Energie (RS2E), FR CNRS 3459, 80039 Amiens Cedex, France ∥ Laboratoire de Réactivité et Chimie des Solides, UMR CNRS 7314, 33 Rue Saint Leu, 80039 Amiens Cedex, France ⊥ ALISTORE-European Research Institute, FR CNRS 3104, 80039 Amiens, France # Laboratory for Neutron Scattering, Paul Scherrer Institut, CH-5232 Villigen, Switzerland ¶ Center for Electrochemical Energy Storage, Skolkovo Institute of Science and Technology, Nobel Street 3, 143026 Moscow, Russia ‡

S Supporting Information *

ABSTRACT: We report on a thorough structural study on two members of layered fluorocarbonates KMCO3F (M = Ca, Mn). The Ca-based member demonstrates a phase transition at ∼320 °C, evidenced for the first time. The crystal structure of the high temperature phase (HT-KCaCO3F) was solved using neutron powder diffraction. A new Mn-based phase KMnCO3F was synthesized, and its crystal structure was solved from electron diffraction tomography data and refined from a combination of X-ray synchrotron and neutron powder diffraction. In contrast to other members of the fluorocarbonate family, the carbonate groups in the KMnCO3F and HT-KCaCO3F structures are not fixed to two distinct orientations corresponding to mono- and bidentate coordinations of the M cation. In KMnCO3F, the carbonate group can be considered as nearly “monodentate”, forming one short (2.14 Å) and one long (3.01 Å) Mn−O contact. This topology provides more flexibility to the MCO3 layer and enables diminishing the mismatch between the MCO3 and KF layers. This conclusion is corroborated by the HT-KCaCO3F structure, in which the carbonate groups can additionally be tilted away from the layer plane thus relieving the strain arising from geometrical mismatch between the layers. The correlation between denticity of the carbonate groups, their mobility, and cation size variance is discussed. KMnCO3 orders antiferromagnetically below TN = 40 K.

1. INTRODUCTION The AMCO3F (A = K, Rb, Cs, M = Mg, Ca, Sr, Pb, Zn, Cd) fluorocarbonates are promising wide band gap non-centrosymmetric materials capable of large second harmonic generation and potentially considered for generating coherent deepultraviolet light.1−4 The crystal structure of this family is built of MCO3 and AF layers alternating along the c axis of the hexagonal unit cell. The triangles of the carbonate groups are strictly confined to the ab plane, and their oxygen atoms form an equatorial oxygen environment of the M cations. The carbonate groups can be oriented toward the M cations with either one or two oxygens, acting as a monodentate or bidentate chelating ligand, respectively. Switching denticity of the carbonate group occurs through its 60° rotation around the c axis. The coordination polyhedron of the M cation is completed with two apical fluorine atoms. Among the known structures, variants exist where the M cations are coordinated © XXXX American Chemical Society

by 3 bidentate, 2 bidentate and 1 monodentate, 1 bidentate and 2 monodentate, and 3 monodentate carbonate groups giving rise to MO6F2 hexagonal bipyramid, MO5F2 pentagonal bipyramid, MO4F2 distorted tetragonal bipyramid, and MO3F2 trigonal bipyramid, respectively. Somewhat aside of this general picture is the KCuCO3F structure, where the carbonate groups adopt an orientation intermediate between those characteristic of the mono- and bidentate ligands.5 Each oxygen atom forms only one strong bond to the Cu2+ cation, which is situated in a distorted CuO3F2 tetragonal pyramid. This distortion can be attributed to the Jahn−Teller effect, which is also responsible for the symmetry lowering from hexagonal to orthorhombic. Received: July 28, 2017

A

DOI: 10.1021/acs.inorgchem.7b01926 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

measurement. Isothermal field-dependent magnetization was measured at T = 2 K. Electrochemical tests on KMnCO3F were conducted in Swageloktype cells assembled in an argon-filled glovebox and cycled in a galvanostatic operating mode using a VMP system (Biologic S.A., Claix, France). Lithium metal was used as the negative electrode, and the working electrode consisted of a composite of the active material and carbon SP (Csp) (80:20 wt %), prepared by ball-milling for 15 min in a Spex 8000 miller. The negative and the positive electrodes were separated by a Whatman GF/D borosilicate glass fiber sheet saturated with 1 M LiPF6 in EC:DMC (1:1 weight ratio) (LP30). Cells tested in oxidation show some capacity around 5 V which arises from copious electrolyte degradation, and not from the material itself, as no reversible capacity was observed during the subsequent discharge. This indicates a total absence of electrochemical activity until 5 V.

Besides nonlinear optical properties, the layered fluorocarbonates can be considered as potential positive electrode (cathode) materials for the metal-ion batteries. Feasibility of this application has been studied with first-principles calculations for the AMCO3F (A = Li and Na, M = Mn, Fe, Co, Ni) fluorocarbonates.6 In this contribution, we initially aimed to explore KMCO 3F (M = Mn, Fe, Co, Ni) fluorocarbonates to be used as cathodes for the Li-, Na-, and even K-ion batteries. We applied a solid state synthesis procedure, but only the Mn-based material has been obtained, which, however, does not demonstrate any electrochemical activity. In this contribution, we report on the crystal structure of this novel KMnCO3F compound, and we complement its crystallographic characterization with an investigation of a phase transition in KCaCO3F in order to obtain a more comprehensive picture of the structural behavior of the carbonate groups in this family of materials.

3. RESULTS 3.1. The KMnCO3F Crystal Structure. In order to determine the KMnCO3F crystal structure model, an electron diffraction tomography experiment has been conducted. In this experiment, the reciprocal lattice sections are collected with small angular step, preferably out of zone axis orientation. The electron diffraction intensities collected this way do not suffer from dynamical scattering so much as the intensities in the zone-axis selected area electron diffraction patterns; thus they can be considered as quasi-kinematical and used for the structure solution and refinement. The 3D reconstruction of the scanned reciprocal lattice volume is shown in the Supporting Information movie. The observed reflections were indexed on a hexagonal lattice with the unit cell parameters a ≈ 5.1 Å, c ≈ 8.4 Å. The crystal structure solution has been performed using a charge flipping algorithm implemented in the SUPERFLIP program.10 The procedure quickly converged, and the symmetry analysis of the resulting scattering density revealed P6̅c2 space symmetry. In the obtained 3D scattering density map, all atoms are easily identified (Figure 1). The

2. EXPERIMENTAL SECTION KMCO3F (M = Mn, Ca) compounds were prepared using a solid state method. The procedure consists of mixing the stoichiometric amounts of MCO3 and KF under argon for 20 min using a Retsch miller. The recovered powders were pressed into pellets and annealed under argon for 24 h in alumina crucibles placed in a stainless steel reactor. Annealing temperatures were 600 and 400 °C for M = Ca and M = Mn, respectively. Powder X-ray diffraction (PXRD) patterns were recorded using a Bruker D8-Advance diffractometer with Cu Kα radiation source (λ1 = 1.54056 Å, λ2 = 1.54439 Å) equipped with a LynxEye detector. Temperature-dependent PXRD data were collected using an AntonPaar HTK 1200N furnace, on both heating and cooling between room temperature (RT) and 500 °C in 2θ range of 10−55°. Synchrotron Xray powder diffraction (SXPD) patterns were recorded in transmission mode (0.5 mm diameter capillary) with a wavelength of 0.4137 Å on the 11-BM beamline at Argonne National Laboratory. Neutron powder diffraction (NPD) study at RT and 500 °C was performed at SINQ-PSI (Villigen, Switzerland) on the HRPT powder diffractometer using a wavelength of λ = 1.494 Å obtained with a germanium (511) monochromator. The powder was placed in a 6 mm diameter vanadium container, and the measurements were performed at RT and 500 °C under primary vacuum. The PXRD and NPD patterns were refined using the Rietveld method with the FullProf and JANA2006 programs, respectively.7,8 Transmission electron microscopy (TEM) study was performed with a FEI Tecnai G2 electron microscope operated at 200 kV and equipped with an EDAX detector. The specimen was prepared in air by dispersing the materials in dry ethanol and depositing a drop of suspension on a holey carbon grid. For the electron diffraction tomography (EDT) experiment reciprocal lattice sections were collected by tilting the goniometer with 1° step over the angular range of 73°. More details are presented in Table S1. The data handling, namely, difference vector space analysis, indexing, and integration of the reflection intensities, were performed with the PETS and JANA2006 software.8,9 Energy dispersive X-ray (EDX) analysis of KMnCO3F revealed a K:Mn atomic ratio of 1.00(9):1.00(9). Thermogravimetric analyses (TGA) were conducted employing a simultaneous thermal analyzer coupled with a quadrupole mass spectrometer (STA 449C Jupiter, QMS Aëolos 32, Netzsch Inc.). Around 20 mg of sample was loaded in an alumina crucible and heated from RT to 700 °C under argon flow at a constant rate of 5 °C/min. Differential scanning calorimetry (DSC) analysis was performed under argon by heating the sample from RT until 400 °C followed by cooling to RT at a constant rate of 5 °C/min. Temperature-dependent magnetic susceptibility was measured using a SQUID (Quantum design) magnetometer in zero-field-cooled (ZFC) and field-cooled (FC) modes, under an applied magnetic field of 1 kOe. About 50 mg of powder was placed into gel caps for the

Figure 1. 3D scattering density map after KMnCO3F structure solution with the charge flipping algorithm. Note that in the final structure the unit cell origin is at the K atom.

structure refinement from the EDT data revealed a reliability factor RF = 0.184. For EDT data without correction for dynamical effects, this indicates a good agreement between the observed and calculated structure factors (also shown in the Fobs − Fcalc plot in Figure S1). The EDT experiment and refinement parameters, atomic coordinates, and atomic displacement parameters (ADPs) after refinement from the EDT data are listed in Tables S1 and S2, respectively. The space group and atomic coordinates obtained after the EDT structure analysis were used for the Rietveld refinement of the KMnCO3F structure from synchrotron powder X-ray B

DOI: 10.1021/acs.inorgchem.7b01926 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry diffraction and neutron powder diffraction data. The simultaneous Rietveld refinement from both data sets converged smoothly, confirming the correctness of the EDT structure solution. For the final refinement, the KMnF3 and KHCO3 admixture phases were included and the refinement was performed with an isotropic approximation for individual ADPs for every atomic position. The crystallographic parameters, refined atomic coordinates and main interatomic distances are listed in Tables S2, S3, and S4, respectively. The experimental, calculated, and difference diffraction profiles after the Rietveld refinement are shown in Figure 2.

Figure 3. Crystal structure of KMnCO3F shown as clinographic (top) and [001] (bottom) projections. The MnO3F2 polyhedra are shown.

adjacent trigonal bipyramids adopt the staggered configuration: their basal MnO3 triangles are rotated with respect to each other by 45.3°, so that the oxygen atoms of the neighboring MnO3F2 pyramids avoid being positioned on top of each other. The chains of the trigonal bipyramids are linked together by the carbonate groups. The K atoms reside in the 9-fold polyhedra, where the top and bottom triangular faces are composed of the oxygen atoms, and three fluorine atoms are at the triangular equatorial plane. 3.2. Thermal Properties of KMCO3F (M = Mn, Ca) and Phase Transition in KCaCO3F. At atmospheric pressure KMnCO3F displays a large weight loss with copious CO2 release from ∼300 to 350 °C, indicating the total decomposition of KMnCO3F with the formation of KMnF3 and MnxOy phases. KCaCO3F is more thermally stable and decomposes above 550 °C according to the reaction 3KCaCO3F → KCaF3 + 2 CaO + K2O + 3CO2. No phase transition was observed for KMnCO3F in the studied temperature interval. DSC measurements on KCaCO3F (Figure 4) reveal two peaks not related to the weight loss: an endothermic one at 330 °C (19 J/g) on heating and an exothermic one at 230 °C (−24 J/g) on cooling. These features indicate the occurrence of a phase transition. In order to get deeper insight into the nature of this phase transition, temperature-dependent PXRD measurements were carried out in air between RT and 500 °C (Figure 5). The room temperature PXRD pattern is indexed with a hexagonal unit cell with the space group P6̅m2 and lattice parameters a = 5.09338(4) Å and c = 4.44935(4) Å, and the Rietveld refinement from the NPD pattern at RT (Figure S2, Tables S5−S8) confirms full agreement of the RT-KCaCO3F structure with the previous reports.1,11,12 Heating the sample up to 300

Figure 2. Experimental, calculated, and difference SXPD (a) and NPD (b) diffraction profiles after the Rietveld refinement of the KMnCO3F structure. The vertical bars mark the reflection positions for (from bottom to top) the KMnCO3F, KMnF3, and KHCO3 phases.

The crystal structure of KMnCO3F is shown in Figure 3. The Mn2+ cations are situated in the MnO3F2 trigonal bipyramids. However, this first coordination sphere provides underestimated bond valence sum (BVS) of 1.83 for the Mn cations. There are three longer, almost nonbonding Mn−O distances of 3.008 Å, but their contribution to BVS for Mn is not negligible and amounts to 0.11, bringing the total BVS to 1.94, much closer to the Mn nominal valence of +2. Thus, the coordination number of the Mn cations is better described as “5 + 3”. The MnO3F2 trigonal bipyramids form infinite chains running along the c axis. The trigonal bipyramids are linked into the chains through corner-sharing via the apical fluorine atoms. Two C

DOI: 10.1021/acs.inorgchem.7b01926 Inorg. Chem. XXXX, XXX, XXX−XXX

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discrepancies between the experimental and calculated profiles were pronounced. Performing the refinement with anisotropic approximation for ADPs greatly improved the quality of fit resulting in RF = 0.032. Very significant anisotropy of the ADPs has been observed for the F1 position (U11, U22 ≫ U33), for the O1 position (U11, U22 ≪ U33), and for the O2 position (U11 ≪ U22, U33). These ADP values may indicate dynamic or static cooperative tilt and rotations of the carbonate groups and CaO5F2 polyhedra. Additionally, abnormally short C2−O2 interatomic distance of 1.206 Å was observed in this model. Such compression of the carbonate group looks unreasonable being compared to the room temperature geometry of the carbonate group in the LT-KCaCO3F phase (d(C−O) = 1.283 Å). Thus, the symmetry lowering was tested by refining the structure in the P31m and P321 subgroups of the P6̅2m space group. This, however, provided only marginal improvement of the fit. Next, a possibility of incommensurate modulation was considered. Careful inspection of the NPD pattern reveals the presence of three very weak reflections at 2θ = 21.07°, 33.78°, and 41.86°, which can be indexed with a modulation vector q = (1/3, 1/3, 0.132) and P31m(1/3,1/3,γ)000 superspace group. These reflections are absent in the NPD pattern of LTKCaCO3F, which confirms their appearance in the course of the phase transition. For the refinement, the atomic displacements were modeled with first order harmonic modulation functions, but stable refinement was not achieved because of very low intensity and small number of the observed satellites. In order to get a clue on the displacements of the carbonate groups, a “disordered” model was introduced. In this model, the O1 and O2 atoms are shifted to a 12l general position from the special positions 6j (x,y,0) and 3f (x,0,0) of the P6̅2m space group, respectively. For these positions isotropic ADPs were refined, whereas anisotropic approximation was kept for all other positions. This model also provides reasonably good fit (RF = 0.036) and restores the C2−O2 interatomic distance to the more reasonable value of 1.28 Å. The crystallographic parameters for the HT-KCaCO3F structure are provided in Table S9. The atomic coordinates and ADPs for the “ordered” (i.e., with O1 and O2 at the 6j and 3f positions, respectively) and “disordered” models are listed in Tables S10 − S12. Main interatomic distances for the “disordered” model are given in Table S13. The experimental, calculated, and difference diffraction profiles after the Rietveld refinement are shown in Figure 6. The crystal structure of HT-KCaCO3F (Figure 7) is close to that of RbCaCO3F. The Ca atoms are situated in CaO5F2 pentagonal bipyramids. In the equatorial plane they are bonded to five oxygen atoms: four from two bidentate carbonate groups and one from a monodentate carbonate group. The CaCO3 layers are separated by the KF layers; the K cations are surrounded by eight oxygens from the CaCO3 layers above and below and three fluorines in the equatorial plane. In the RbCaCO3F structure the carbonate groups are strictly confined to the ab plane. By selecting possible local configurations from the pattern of random oxygen displacements in HT-KCaCO3F, one can identify that the C1−O1 carbonate group tilts away from the ab plane by ∼9.1°, whereas the C2−O2 carbonate group is tilted by ∼15° and rotated around the 6̅ axis by ∼11.3°. A remark should be made on obvious discrepancy between our observation of the temperature-dependent structure evolution of KCaCO3F and the report from Sun et al., where no structural transition has been observed by neutron powder

Figure 4. DSC curves for KCaCO3F.

Figure 5. Temperature dependent PXRD patterns of KCaCO3F.

°C (Figure 5, blue patterns) does not change the crystal structure. However, at around 320 °C we observe the coexistence of two phases (Figure 5, black pattern). Above this temperature and up to 500 °C, the peaks of the low temperature phase (LT) fully disappear and a new well crystallized high temperature (HT) phase is formed (Figure 5, green pattern). On cooling, the peaks of the LT phase appear again at around 200 °C and the LT structure is fully restored at 180 °C. Thus, both PXRD and DSC demonstrate the reversibility of the phase transition, although with noticeable hysteresis between heating and cooling. The PXRD pattern of the HT-KCaCO3F phase can be indexed with a primitive hexagonal unit cell with the lattice parameters related to those of the LT phase as aHT = aLT√3, cHT = cLT. These lattice parameters are characteristic of the RbCaCO3F structure with the space group P6̅2m. In order to get precise structural information on the HT-KCaCO3F phase, high temperature neutron powder diffraction study was undertaken. 3.3. High Temperature Crystal Structure of KCaCO3F. The crystal structure of the high temperature KCaCO3F polymorph (HT-KCaCO3F) has been refined from neutron powder diffraction data at T = 500 °C. The atomic coordinates of the RbCaCO3F structure were used as a starting model for the Rietveld refinement.1 The first attempt to refine the crystal structure with an isotropic approximation for ADPs was not fully successful: the reliability factor was high (RF = 0.083), and D

DOI: 10.1021/acs.inorgchem.7b01926 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Experimental, calculated, and difference NPD diffraction profiles after the Rietveld refinement of the HT-KCaCO3F structure at T = 500 °C.

Figure 8. (a) Temperature dependence of magnetic susceptibility, (b) inverse magnetic susceptibility in field-cooled (FC) and zero-fieldcooled (ZFC) regimes, and (c) field dependence of isothermal magnetization at T = 2 K for KMnCO3F. The dashed lines indicate Curie−Weiss fit.

strate kinks at TN ≈ 40 K that might indicate the onset of antiferromagnetic (AFM) ordering. Above T = 100 K the magnetic susceptibility follows the modified Curie−Weiss law (Figure 8a,b): χ = χ0 +

Figure 7. Crystal structure of HT-KCaCO3F: orientation variants of the carbonate groups in the disordered model (top) and one of the possible local configurations (bottom).

C T−Θ

where χ0 is a temperature-independent term, C is the Curie constant, and Θ is the Weiss temperature. The fit provides χ0 = 1.08(7) × 10−3 emu mol−1 Oe−1, C = 4.36(2) emu K mol−1, and Θ = −147(1) K. The negative Θ points to predominant AFM interactions with moderate frustration as indicated by the |Θ|/TN = 3.7 ratio. The effective magnetic moment per one Mn atom is estimated to 5.88(2) μB, which is in excellent agreement with the spin-only value for high spin Mn2+ (5.916 μB). The field-dependent magnetization measured at T = 2 K (Figure 8c) does not show any remnant magnetization, consistent with the proposed AFM ordering.

diffraction in the 295−673 K temperature range.12 However, the DTA curve of KCaCO3F provided by Sun et al. clearly demonstrates an endothermic peak at ∼330 °C (Figure 3 in ref 12), in agreement with our findings. We can speculate that the source of these disagreements might originate from slight difference in chemical composition of the samples: Sun et al. have reported uncontrolled contamination of their samples by silicon during the synthesis. In our work this contamination can be totally excluded. 3.4. Magnetic Properties of KMnCO3F. Temperature dependence of magnetic susceptibility and field dependence of magnetization for KMnCO3F are shown in Figure 8. The magnetic susceptibility increases on cooling with a subsequent drop below T ∼ 35−40 K and very slight increase below T ∼ 5 K, which can be attributed to paramagnetic impurities. Both field-cooled (FC) and zero-field-cooled (ZFC) curves demon-

4. DISCUSSION KMnCO3F further enriches the series of the AMCO3F (A = K, Rb, Cs, M = Mg, Ca, Sr, Pb, Cu, Zn, Cd) fluorocarbonates, in which the carbonate groups can coordinate the divalent M cations with one or two oxygen atoms playing a role of either monodentate or bidentate ligand. Among this family, E

DOI: 10.1021/acs.inorgchem.7b01926 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. Different variants of the oxygen coordination of the M cations in the AMCO3F series and denticity of the carbonate groups. The representatives of each variant are listed along with the supercells with respect to the simplest basic ab, cb hexagonal cell of the LT-KCaCO3F structure.

= Mn, Zn, Cd) and other members of the AMCO3F family. The correlation between the denticity and cation size variance is based on the necessity to eliminate the mismatch between the A−F interatomic distances in the AF layers (having a fixed triangular geometry) and the M−O distances in the MCO3 layers, which can change their geometry by changing the ratio between the monodentate and bidentate carbonate groups. However, if bidentate groups are present, the flexibility of the MCO3 layers is significantly restricted: the M−O−C bond angles adopt values of either ∼90° for bidentate groups or ∼180° for monodentate groups. In KMnCO3F this angle is 116.9°, in between the two above-mentioned limiting values. Apparently, the absence of bidentate carbonate groups enables the cooperative rotation of the MnO3F2 bipyramids and CO3 triangles in the ab plane thus providing a possibility to establish the commensurability between the MnCO3 and KF layers. In fact, the carbonate groups in KMnCO3F cannot be considered as purely monodentate. As the Mn−O−C bond angle deviates significantly from 180°, the second O atom of the carbonate group approaches closer to the Mn cation, making a nonnegligible, albeit small, contribution to its coordination sphere with long Mn−O contact of 3.008 Å (marked with dashed lines in Figure 9). This structural arrangement is similar to that in KCuCO3F, where the oxygen atoms of two out of three “monodentate” carbonate groups also form long Cu−O contacts of 2.81 and 3.08 Å.5 The third oxygen is at a definitely nonbonding distance of 3.47 Å. The carbonate groups are also significantly rotated, with Cu−O−C angles ranging between 113.5° and 129.5°. Similar consideration is seemingly valid for the AMCO3F (A = K, Rb, M = Zn, Cd) materials. For instance, if one assumes purely monodentate carbonate groups and CdO3F2 trigonal bipyramidal coordination for Cd2+ cations in KCdCO3F and RbCdCO3F, the Cd cations appear to be strongly underbonded (BVS of 1.857 and 1.849, respectively).4 Including longer Cd−O contacts of 2.919 Å (A = K) and 3.018 Å (A = Rb) completes the coordination sphere and provides more realistic BVS of 2.083 and 1.995, respectively. The phase transition in KCaCO3F can also be rationalized in terms of size mismatch between CaCO3 and KF layers. Thermal expansion of the chemical bonds can be estimated from the BVS method, which establishes a semiquantitative correlation between BVS of a particular bond at room temperature and the thermal expansion of the bond dR/dT, where R is the bond length and T is the temperature.13 To perform this estimate we used the empirical bond valence−dR/

KCaCO3F, ASrCO3F (A = K, Rb) and APbCO3F (A = Rb, Cs) adopt the hexagonal cell of smallest volume, which can be selected as the basic unit cell for the whole series (Figure 9).1,2,11,12 In these compounds all carbonate groups are crystallographically identical and coordinate the M cations with two oxygens acting as bidentate ligands. In ACaCO3F (A = Rb, Cs) one-third of the carbonate groups are rotated by 60° around the c axis facing one of its oxygens toward the M cation, thus becoming a monodentate ligand.1 Ordering of one monodentate and two bidentate carbonate groups causes the a = ab√3, c = cb superstructure. The same superstructure is realized in the RbMgCO3F structure with one bidentate and two monodentate carbonate groups.3 In KMnCO3F the carbonate groups become crystallographically identical again restoring the a = ab periodicity in the ab plane. However, the staggered configuration of the MnO3F2 trigonal bipyramids and the carbonate groups doubles the c parameter compared to cb. Among the AMCO3F family KMnCO3F is isostructural to the A = K, Rb, M = Zn, Cd compounds.4 Tran et al. have proposed to formalize the tendency in changing denticity of the carbonate groups with respect to the size of the A and M cations in AMCO3F using a variance of the ionic radii.3 If we calculate the variance as s2 = (1/2)((rA − r)̅ 2 + (rM − r)̅ 2), where rA, rM, and r ̅ are the ionic radii of the A and M cations and the average ionic radius, respectively, the fully bidentate coordination corresponds to the smallest variance ranging between 0.020 and 0.046 Å2 (for the sake of comparison, here and further we use the ionic radii for the CN = 9 and 8 for the A and M cations, respectively). Increasing fraction of monodentate groups raises the variance to 0.065− 0.109 Å2 for a bidentate-to-monodentate ratio of 2:1 and then to 0.137 Å2 for a ratio of 1:2. Following this tendency, one may expect that the variance for KMnCO3F will be even higher as in this structure all carbonate groups can be considered as monodentate ligands (i.e., the bidentate-to-monodentate ratio is 0:3). However, the calculated variance of 0.087 Å2 places KMnCO3F into the range of materials with a bidentate-tomonodentate ratio of 2:1. A similar observation can be made for the AMCO3F (A = K, Rb, M = Zn, Cd) compounds, which also display the structures with monodentate carbonate groups only: their variance ranges between 0.051 and 0.133, thus smaller than that for RbMgCO3F, which has bidentate-tomonodentate ratio of 1:2 (see Figure S3). This obvious discrepancy can tentatively be ascribed to a drastic difference in flexibility of the MCO3 layers between AMCO3F (A = K, Rb, M F

DOI: 10.1021/acs.inorgchem.7b01926 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

consideration of the KMnCO3F crystal structure. One can expect that tilting of the carbonate groups out of the ab plane should also cause concomitant tilting of the CaO5F2 pentagonal bipyramids that will be reflected in the lateral displacements of the F atoms and, subsequently, K atoms in the KF layers. Although we could not directly localize these displacements in our NPD experiment, large U11 and U22 components of the anisotropic ADP tensor indirectly confirm that displacements take place. Bending the Ca−F−Ca bond caused by fluorine displacements explains an unusual contraction of the c parameter in the HT-KCaCO3F phase (4.44935(4) Å and 4.4169(2) Å for the LT and HT phases, respectively). We should also mention that phase transitions could be expected for other AMCO3F compounds with the cation size variance at the border between the stability areas of the structures with different fractions of the bidentate and monodentate carbonate groups. One of the possible candidates is CsPbCO3F, which, as LT-KCaCO3F, has all bidentate carbonate groups and cation size variance of 0.046 Å.2 Finally, a comment should be made on negligible electrochemical activity of KMnCO3F. We have estimated the diffusion pathways and associated diffusion barriers for K+ and Li+ in the “MnCO3F” framework using the bond valence energy landscape (BVEL) method.14 Within this approach, soft bond valence parameters developed by S. Adams were used,15 and counterions were taken into account up to a distance of 10 Å around the probe ion (K+ or Li+). This approach allows visualizing conduction pathways in the structure while giving hints to possible conduction mechanisms. The BVEL maps shown in Figure S4 were generated using the program BondSTR of the FullProf Suite.7 The lowest energy percolated diffusion path for K+ is two-dimensional, being confined to the ab planes between the MnCO3 layers (Figure S4a). The diffusion barrier for this pathway is 3.24 eV, which looks prohibitive for potassium deintercalation and explains absence of electrochemical response for this compound. In contrast to that, the Li+ diffusion should occur through 1D channels along the c-axis (Figure S4b) with low diffusion barrier of 0.46 eV. Thus, LiMnCO3F, once synthesized, could be a promising cathode material for Li-ion batteries. However, one also has to keep in mind that oxidation of Mn2+ to Mn3+ upon alkali metal deintercalation might be an additional obstacle due to structure distortion caused by Jahn−Teller effect. In conclusion, the two new structures, KMnCO3F and high temperature KCaCO3F, expand the family of the layered AMCO3F fluorocarbonates and provide new insight into the denticity and mobility modes of the carbonate groups, related to the geometrical mismatch between the MCO3 and AF layers. Increasing the mismatch with temperature triggers a structural phase transition in KCaCO3F that is accompanied by a change in denticity for 1/3 of the carbonate groups. The monodentate carbonate groups may have both in-plane rotations and out-ofplane tilts, whereas only tilting degree of freedom seems to be realized in the bidentate carbonate groups. Taking into account that mutual orientation of the carbonate groups is critically important for the second harmonic generation properties of the AMCO3F materials,4 we can speculate that the out-of-plane tilting mode may become an important, yet unforeseen, structural factor to be taken into account for the design of the nonlinear optical crystals for UV frequency conversion. This hypothesis, however, needs experimental verification. KMnCO3F is an example of a frustrated antiferromagnet with a triangular lattice. The magnetic structure solution and

dT correlation provided in Figure 5 of ref 13. In the LTKCaCO3F structure, the BVS values for the K−F, Ca−O, and C−O bonds (all aligned in the ab plane) amount to 0.076, 0.203, and 1.332, which provides dR/dT of >150, ∼75, and ∼5 (×10−6 Å/K), respectively. It indicates that the carbonate group behaves almost as a rigid unit, whereas the K−F bond is at least twice more expandable than the Ca−O bond. The LTKCaCO3F structure with its cation size variance of 0.046 Å is already at the upper limit of the stability region of the AMCO3F structures with all bidentate carbonate groups, and the thermal expansion should increase the variance even further, rendering the structure with 2 bidentate and 1 monodentate carbonate groups more stable at elevated temperature. The rising mismatch between the CaCO3 and KF layer cannot be relaxed by a progressive rotation of the bidentate carbonate groups, as they all form two bonds to the Ca cations in the LT-KCaCO3F structure. Thus, the phase transition has a discontinuous character, at which 1/3 of the carbonate groups abruptly change their orientation by rotation over 60° around the c-axis. This rotation breaks one out of two Ca−O bonds and transforms the rotated carbonate group from a bidentate to a monodentate ligand. Indeed, the phase transition in KCaCO3F is clearly first order discontinuous phase transition, as confirmed by coexistence of the LT and HT phases in the two-phase regions, large heating/cooling hysteresis, and absence of group− subgroup relationship (the symmetry of the LT phase is higher than the symmetry of the HT phase). Moreover, the Rietveld refinement of all PXRD data sets recorded at different temperatures clearly shows a discontinuity in unit cell parameters and unit cell volume at the transition from LT to HT-KCaCO3F (Figure 10), further supporting its first order

Figure 10. Temperature dependence of the lattice parameters and unit cell volume for KCaCO3F. For sake of comparison, the supercell lattice parameters of the high temperature polymorph have transformed into the simplest basic ab, cb hexagonal cell of the LTKCaCO3F structure.

character. The fine-tuning of the commensurability between the CaCO3 and KF layers occurs due to tilting of both bidentate and monodentate carbonate groups out of the ab plane and rotation of the monodentate group around the c axis. This difference in behavior of the monodentate and bidentate carbonate groups confirms the conclusion on larger degree of freedom of the monodentate groups, driven from the G

DOI: 10.1021/acs.inorgchem.7b01926 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(5) Mercier, N.; le Blanc, M. Synthesis, characterization and crystal structure of a new copper fluorocarbonate KCu(CO3)F. Eur. J. Solid State Inorg. Chem. 1994, 31, 423−430. (6) Tian, M.; Gao, Y.; Ouyang, C.; Wang, Z.; Chen, L. Design and Properties Prediction of AMCO3F by First-Principles Calculations. ACS Appl. Mater. Interfaces 2017, 9, 13255−13261. (7) Rodriguez-Carvajal, J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys. B 1993, 192, 55− 69. (8) Petricek, V.; Dusek, M.; Palatinus, L. Crystallographic computing system JANA2006: General features. Z. Kristallogr. - Cryst. Mater. 2014, 229, 345−352. (9) Palatinus, L. PETS − program for analysis of electron diffraction data; Institute of Physics of the AS CR: Prague, 2011. (10) Palatinus, L.; Chapuis, G. SUPERFLIP − a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions. J. Appl. Crystallogr. 2007, 40, 786−790. (11) Chen, X.-L.; He, M.; Xu, Y.-P.; Li, H.-Q.; Tu, Q.-Y. KCaF(CO3) from X-ray powder data. Acta Crystallogr., Sect. E: Struct. Rep. Online 2004, 60, i50−i51. (12) Sun, Y.-P.; Huang, Q.-Z.; Wu, L.; He, M.; Chen, X.-L. A neutron powder investigation of the structure of KCaCO3F at various temperatures. J. Alloys Compd. 2006, 417, 13−17. (13) Brown, I. D.; Dabkowski, A.; McCleary, A. Thermal Expansion of Chemical Bonds. Acta Crystallogr., Sect. B: Struct. Sci. 1997, 53, 750−761. (14) Adams, S. From Bond Valence Maps to Energy Landscapes for Mobile Ions in Ion-Conducting Solids. Solid State Ionics 2006, 177, 1625−1630. (15) Adams, S. Relationship between Bond Valence and Bond Softness of Alkali Halides and Chalcogenides. Acta Crystallogr., Sect. B: Struct. Sci. 2001, 57, 278−287.

construction of a microscopic model for magnetic exchange interactions in this material are in progress and will be published elsewhere.

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ASSOCIATED CONTENT

S Supporting Information *

The following files are available free of charge. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01926. Tabulated crystallographic data, electron diffraction structure factor plot, and Rietveld refinement profiles (PDF) 3D reciprocal lattice reconstruction for KMnCO3F (AVI) Accession Codes

CCDC 1565415−1565417 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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AUTHOR INFORMATION

Corresponding Author

*Skoltech Center for Electrochemical Energy Storage, Skolkovo Institute of Science and Technology, Nobel Street 3, 143026 Moscow, Russia. Tel: +7 (495) 280 14 81 ext 3429. E-mail: [email protected]. ORCID

Artem M. Abakumov: 0000-0002-7135-4629 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M. Courty is acknowledged for thermal analysis experiments. We thank M. Leblanc for fruitful discussions. This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. Use of the 11-BM mail service of the APS at Argonne National Laboratory was supported by the U.S. Department of Energy under Contract No. DE-AC0206CH11357 and is greatly acknowledged. A.M.A. acknowledges financial support from the #2016-1/NGP project.

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REFERENCES

(1) Zou, G.; Ye, N.; Huang, L.; Lin, X. Alkaline-Alkaline Earth Fluoride Carbonate Crystals ABCO3F (A = K, Rb, Cs; B = Ca, Sr, Ba) as Nonlinear Optical Materials. J. Am. Chem. Soc. 2011, 133, 20001− 20007. (2) Tran, T. T.; Halasyamani, P. S.; Rondinelli, J. M. Role of Acentric Displacements on the Crystal Structure and Second-Harmonic Generating Properties of RbPbCO3F and CsPbCO3F. Inorg. Chem. 2014, 53, 6241−6251. (3) Tran, T. T.; He, J.; Rondinelli, J. M.; Halasyamani, P. S. RbMgCO3F: A New Beryllium-Free Deep-Ultraviolet Nonlinear Optical Material. J. Am. Chem. Soc. 2015, 137, 10504−10507. (4) Yang, G.; Peng, G.; Ye, N.; Wang, J.; Luo, M.; Yan, T.; Zhou, Y. Structural Modulation of Anionic Group Architectures by Cations to Optimize SHG Effects: A Facile Route to New NLO Materials in the ATCO3F (A = K, Rb; T = Zn, Cd) Series. Chem. Mater. 2015, 27, 7520−7530. H

DOI: 10.1021/acs.inorgchem.7b01926 Inorg. Chem. XXXX, XXX, XXX−XXX