A new 12L-hexagonal perovskite Cs₄Mg₃CaF₁₂: structural transition derived from the partial substitution of Mg²⁺ with Ca²⁺

Abstract

A new 12L-hexagonal perovskite Cs₄Mg₃CaF₁₂ was synthesized using an open high-temperature solution method. It crystallizes in the trigonal space group R3̄m (no. 166) with lattice constants a = 6.2196(9) Å, c = 29.812(9) Å, Z = 3. Cs₄Mg₃CaF₁₂ is changed from the cubic phase CsMgF₃ by replacing 25% Mg²⁺ with Ca²⁺. The changes derived from the substitution and the structural comparison between Cs₄Mg₃CaF₁₂ and other perovskites are discussed in this paper. Thermal analysis, infrared spectroscopy, and electronic structure calculations were performed on the reported material.

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Introduction

Fluorine has the highest electronegativity of all elements, and can combine with metals to form fluorides with high chemical stability, especially for lower-electronegativity alkali metals and alkaline earth metals. In recent years, there has been much attention paid to complex metal fluorides because of their particular physical properties such as ferromagnetic,¹ nonmagnetic insulator behavior,² piezoelectric characteristics,³ and photoluminescence properties.⁴ Among these important complex metal fluorides, alkali and alkaline earth metal fluorides are particularly attractive because of their applications in UV-Deep UV optical crystals, such as LiF, MF₂ (M = Mg, Ca, Ba),⁵ KMgF₃,⁶ Li_(1−x)K_xBa_(1−y)Mg_yF₃,⁷ NaSrF₃, NaBaF₃, LiBaF₃,⁸ BaMgF₄ ⁹ crystals and so on, which can be made into high transparency and low loss optical windows, prisms, and lenses. While existed crystals cannot meet all of the application needs in UV-Deep UV wavebands. Hence, it is significant to search new fluorides used for UV-Deep UV optical materials.

Despite the simplicity of the original perovskite crystal structure, this family of compounds shows an enormous variety of structural modifications and variants. The interest in compounds belonging to this family of crystal structures arise in the large and ever surprising variety of properties exhibited and the flexibility to accommodate almost all of the elements in the periodic system. The different degrees of distortion in ABX₃ can change the perovskite symmetry from ideal cubic to rhombohedral and hexagonal.¹⁰ In the ideal form ABX₃, the A cation is divalent, the B cation is tetravalent and X is often oxygen but also other large ions such as F⁻ and Cl⁻ are possible. In recent years, many interests have been concentrated on the mixed metal fluoride perovskites. Interestingly, in this system most of perovskites were synthesized by displacing A and partial B sites with alkali metal cations, such as Cs₂QYF₆ (Q = Na and K)¹¹ and Q^′₂LiGaF₆ (Q′ = Rb and Cs).¹² However, what will happen if the B-cations are completely replaced by alkaline earth metal cations? We will make some efforts to answer this problem in this work.

Generally, closed environment or protective atmosphere is necessary for growing fluoride crystals because of the volatility of fluorine. Hydro/solvent-thermal method,¹³ Bridgman–Stockbarge method¹⁴ and Czochralski method¹⁵ are commonly used to grow fluoride crystals, but these methods have complex processes, harsh conditions and high costs. In this paper, a new alkali–alkaline earth metallic fluoride, Cs₄Mg₃CaF₁₂ (CMCF), was synthesize by open high-temperature solution method. We used low-melting B₂O₃ as flux to lower the temperature of solution and decrease the volatilization of fluorine. Herein, the syntheses, crystal structure, thermal and optical properties of CMCF are presented, and the electronic structure was calculated by the first principles method to further explore the structure–property relationship.

Experimental section

Syntheses

All commercially available chemicals (CsF, MgF₂, CaF₂ and H₃BO₃) are of reagent grade and were used as received. Small single crystals of CMCF, were grown by spontaneous crystallization with the molar ratio of CsF : MgF₂ : CaF₂ : H₃BO₃ equal to 5 : 1 : 1 : 9. A mixture of CsF (5.484 g, 36.10 mmol), MgF₂ (0.916 g, 7.35 mmol), CaF₂ (0.574 g, 7.35 mmol) and H₃BO₃ (4.540 g, 73.43 mmol) were thoroughly ground. The mixtures were then placed in a platinum crucible which was placed in a vertical, programmable-temperature furnace. The crucible were gradually heated to 800 °C and held for 30 min, then quickly cooled down to 720 °C and held for 8 h, and then slowly cooled down to 650 °C at a rate of 2 °C h⁻¹, followed by rapid cooling to room temperature. Colorless crystals of CMCF were obtained, and separated mechanically from the crucible for further characterization by single-crystal XRD measurements.

Polycrystalline samples of CMCF were synthesized via solid-state reactions. A stoichiometric mixtures of CsF, MgF₂, CaF₂ and borate acid was initially ground and placed in alumina crucibles, then heated to 720 °C under reducing atmosphere which was acquired by placing the crucible in amorphous carbon, and held for 4 days with 8 times grindings and mixings. The purity was characterized on a Bruker D2 PHASER diffractometer with Cu Kα radiation (λ = 1.5418 Å) at room temperature. The 2θ range was 10–70° with a step size of 0.02° and a fixed counting time of 1 s per step. No impurities were observed (Fig. 1).

Fig. 1 (a) The calculated XRD pattern (black) of CMCF derived from the single-crystal data, (b) the XRD pattern (red) of CMCF synthesized at 720 °C, (c) the XRD pattern (blue) of CMCF calcined at 850 °C.

Single-crystal X-ray diffraction

Block crystal of CMCF (0.145 mm × 0.130 mm × 0.097 mm) was used for single-crystal data collection. Data were collected on a Bruker SMART APEX II CCD diffractometer using monochromatic Mo Kα radiation (λ = 0.71073 Å) at 293(2) K and integrated with the SAINT program.¹⁶ The numerical absorption corrections were carried out using the SADABS program¹⁷ for area detector. All calculations were performed with programs from the SHELXTL crystallographic software package.¹⁸ The structure was solved by direct methods using SHELXS-97,¹⁹ and all of the atoms were refined using full-matrix least-squares techniques with anisotropic thermal parameters and final converged for I > 2σ. The structures were checked for missing symmetry elements with PLATON.²⁰ The crystal data and structure refinement are presented in Table 1. The selected bond distances are listed in Table S1 in the ESI.† The final refined atomic positions and isotropic thermal parameters are given in Table S2 in the ESI.†

Table 1 Crystal data and structure refinement for CMCF

Empirical formula	Cs4Mg3CaF12
a R1 = Σ||Fo| − |Fc||/Σ|Fo| and wR2 = [Σw(Fo^2 − Fc^2)^2/ΣwFo^4]^1/2 for Fo^2 > 2σ(Fo^2).
Formula weight	872.65
Temperature (K)	296(2)
Crystal system	Trigonal
Space group, Z	R3̄m, 3
a (Å)	6.2196(9)
c (Å)	29.812(9)
Volume (Å^3)	998.7(4)
Density (calculated, g cm^−3)	4.353
Absorption coefficient (mm^−1)	11.512
F(000)	1152
Crystal size (mm^3)	0.145 × 0.13 × 0.097
Index ranges	−8 ≤ h ≤ 7, −5 ≤ k ≤ 8, −38 ≤ l ≤ 37
Reflections collected/unique	2049/329 [R(int) = 0.0183]
Completeness to theta = 27.56	99.7%
Refinement method	Full-matrix least-squares on F^2
Data/restraints/parameters	329/0/27
Goodness-of-fit on F^2	1.202
Final R indices [Fo^2 > 2σ(Fo^2)]^a	R1 = 0.0110, wR2 = 0.0247
R indices (all data)^a	R1 = 0.0123, wR2 = 0.0256
Largest diff. peak and hole (e Å^−3)	0.546 and −0.576

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Thermal analysis

The thermal behavior of CMCF was investigated on thermogravimetry and differential scanning calorimeter (TG-DSC) using a NETZSCH STA 449 F3 simultaneous thermal analyzer. The sample was placed in a Pt crucible and heated at a rate of 5 °C min⁻¹ in the range of 40–1200 °C under flowing of nitrogen gas.

Infrared spectroscopy

An infrared spectrum ranging from 400 to 4000 cm⁻¹ with a resolution of 2 cm⁻¹ was recorded on a Shimadzu IR Affinity-1 Fourier transform infrared spectrometer to specify the M–F (M = Cs, Ca and Mg) bonds in CMCF. The sample was mixed thoroughly with dried KBr (5 mg of the sample and 500 mg of KBr).

Numerical calculation details

The electronic structure was calculated by using the DFT method implemented in the CASTEP package.^21,22 In the present study, the cell parameters and the atomic coordinates of all the atoms were obtained from experimental values. During the calculation, the generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional was adopted.²³ Under the norm-conserving pseudopotential (NCP),^24,25 the following orbital electrons were treated as valence electrons: F 2s²2p⁵, Mg 2p⁶3s², Ca 3s²3p⁶4s², Cs 5s²5p⁶6s¹. The kinetic energy cutoff of 940 eV was chosen, and the numerical integration of the Brillouin zone was performed using a 5 × 5 × 2 Monkhorst–Pack k-point sampling respectively. The other calculation parameters and convergent criteria were the default values of the CASTEP code.

Results and discussion

Structural transition

Although CMCF has different space group and structure with CsMgF₃ (ref. 26) and CsCaF₃ (ref. 27) (parent structure), the chemical formula can also be written as (CsMgF₃)₃·(CsCaF₃). Viewing from this formula, we deduce that CMCF can be regarded as 25% substitution of cubic CsMgF₃ with isostructural CsCaF₃. However, it is the difference of ionic radii between Mg²⁺ and Ca²⁺ cations that leads to the structural transition from cubic space group to trigonal space group. CsMgF₃ and CsCaF₃ are perovskite structures and crystallize in Pm3̄m space group. In the structure of CsMgF₃, [MgF₆] octahedra connect to form three dimensional (3D) frameworks by corner-sharing with Cs⁺ cations inserted in void space (Fig. S1 in the ESI†). Mg–F bond length in CsMgF₃ is 2.115 Å, and Ca–F bond length in CsCaF₃ is 2.262 Å. When larger Ca²⁺ cations take the place of Mg²⁺ cations, there would be three changes occurred in the parent structure. In order to illustrate the structural transition clearly, we choose a fragment of CsMgF₃ to discuss, which is shown in Fig. 2.

Fig. 2 Structural transition from CsMgF₃ to CMCF.

Firstly, lager Ca²⁺ cations extrude the occupied space of [MgF₆] octahedra and change the connecting mode of [MgF₆] from corner-sharing to face-sharing. Three face-shared [MgF₆] octahedra form an isolated [Mg₃F₁₂] rod which linked into 3D framework by [CaF₆] octahedra with Cs⁺ cations located in void space (Fig. S2 in the ESI†). Owing to the strain of Mg–F bonds, [MgF₆] octahedra distort with bond length ranging from 1.9737(18) to 2.0660(2) Å (average: 1.9973 Å), and F–Mg–F bond angles distributing in 77.20(8)–180.0°. Besides, the coordination numbers of Cs⁺ cations in CMCF and parent structures are all 12, but the Cs–F bond lengths of title compound are longer than that of parent structures.

The second change occurs in [CaF₆] octahedra: as shown in Fig. 2, when [CaF₆] octahedra replace the [MgF₆] octahedra resided in dashed box, the pressure derived from different ionic radius force [CaF₆] octahedra existing in isolation rather than connection. In this structure, [CaF₆] octahedra play an important role which connects all of isolated rods. There are also a weak distortion with F–Ca–F bond angles located in a small range: 87.27(6)–92.73(6)° (Fig. S3 in the ESI†).

The last one is about symmetry. CsMgF₃ is an optical homogeneous with seven axes of the higher order (four triadaxes along 〈111〉 crystallographic direction and three tetradaxes along 〈100〉 crystallographic direction). However, the introduction of [CaF₆] octahedra lower the symmetry of CsMgF₃ structure, which leads to trigonal space group with only one triad-rotoinversion axis along crystallographic c axis (Fig. 3).

Fig. 3 The symmetrical comparison of CMCF and CsMgF₃ (green octahedra are [MgF₆] octahedra and blue one is [CaF₆] octahedron).

Additionally, CMCF is a 12L-hexagonal perovskite which is similar to Ba₄RR^′₃O₁₂ (R = lanthanoid and Bi, R′ = Ru and Ir).^28–32 As shown in Fig. S2 in the ESI,† in the direction of the c axis, the 12L perovskite exhibits a (cchh)₃ sequence, where c and h correspond to corner-and face-sharing octahedra, respectively. In general, reported examples of metal fluoride hexagonal perovskites appear to have an A^′₂BB′F₆ composition, such as K₂LiAlF₆, Rb₂LiGaF₆,^12a Cs₂NaFeF₆, Cs₂NaCrF₆,^12b and Cs₂NaNiF₆.³³ However, the title compound has another composition A^′₂B^′_1.5B^′′_0.5F₆, which derives from ABX₃ perovskite by replacing the A and B sites with alkali metal, and alkaline earth metal cations, respectively.

In order to characterize the degree of distortion for CMCF, tolerance factor was calculated, which equal to 1.07. For comparison, the tolerance factor t for CsMgF₃ and CsCaF₃ are also obtained, which are 1.13 ³⁴ and 0.94,³⁵ respectively. Viewing from this, it is curious that CsMgF₃ with high tolerance factor adopts a cubic structure³⁶ and a large difference exists between the two isostructural parent structures. The different ratio of ionic radii between A and B cations should be responsible for these. As we can see from the Fig. S4 in the ESI,† it is suitable for Cs⁺ cations fitting into the interstices of adjacent [CaF₆] octahedra, which brings an ideal cubic perovskite with t = 0.94. However, Cs⁺ cations are too large to fit into the interstices of [MgF₆] octahedral, thus resulting in a higher tolerance factor 1.13. For CMCF, inclusion of larger Ca²⁺ cations can relieve the tension caused by too much face-sharing [MgF₆]; and decrease the contradiction brought by too large A cations versus too small B cations, which lead to t = 1.07.

Thermal analysis

Viewing from the TG-DSC curves of CMCF (Fig. 4), the title compound only appear one clear endothermic peak at 795 °C, and corresponding with obvious weight loss, which tentatively suggests that title compound melt incongruently. In order to assign the endothermic peak, the samples of CMCF were calcined at 850 °C for 10 h. As shown in Fig. 1, CMCF are partially decomposed into MgF₂ which then oxidize into MgO (JCPDS no. 45-0946), CaF₂ (JCPDS no. 35-0816) and CsCaF₃ (JCPDS no. 28-0817) with the volatilization of Cs and F elements; hence, the endothermic peak are the decomposed peaks of CMCF.

Fig. 4 TG-DSC curves of CMCF.

Infrared spectroscopy

To trace the Cs–F, Mg–F and Ca–F bonds in CMCF, the infrared spectrum measurement was performed. Fig. S5† shows the complete spectral region of the infrared spectrum between 400 and 4000 cm⁻¹. Only one sharp peak located 448 cm⁻¹ is observed. Referring to the literature,³⁷ this peak is the characteristic peak of metallic–fluorine bond. The infrared spectrum further confirms the existence of M–F bonds (Cs–F, Mg–F or Ca–F), consistent with the results obtained from the single-crystal X-ray structural analyses.

Electronic structures

Electronic structure calculations were performed in order to examine their band structures and explain the relationships between electronic structures and optical properties. The electronic structures of CMCF were determined using the plane-wave pseudopotential calculations. The calculated band structure of CMCF along high symmetry points of the first Brillouin zone are plotted in Fig. S6 in the ESI.† It is found that the lowest energy of the conduction bands (CBs) is localized at the G point, whereas the highest of the valence bands (VBs) is localized at the G point with a band gap of 7.10 eV. Therefore, CMCF is a direct bandgap insulator.

As seen from the total and partial densities of states (TDOS, PDOS) analyses of CMCF (Fig. 5), the VBs below the Fermi level are derived from F 2p orbitals, Cs 5p and 6s orbitals, Mg 2p and 3s orbitals and Ca 3p and 4s orbitals. While, the CBs above the Fermi level are derived from Cs 5p and 6s orbitals, Mg 2p and 3s orbitals, Ca 3p and 4s orbitals and F 2s and 2p orbitals. One can see that VBs are mostly determined by F 2p orbitals, the contribution of Cs 5p, Mg 3s, Mg 2p, Cs 6s, Ca 3p and Ca 4s orbitals decrease progressively; and for CBs, the p orbital of every element contributes more than its s orbital.

Fig. 5 Total DOS and partial DOS of CMCF.

Additionally, the charge density of CMCF is presented in Fig. S7 in the ESI.† Viewing from this figure, we find that electrons gather around the rods, which intuitively shows the existence of extruding force brought by lager Ca²⁺ cations.

Conclusions

A new 12L-hexagonal perovskite alkali and alkaline earth metal mixed fluoride Cs₄Mg₃CaF₁₂ was reported. The title compound can be regarded as the partial substitution of two parent structures (isostructural cubic CsMgF₃ and CsCaF₃). Larger Ca²⁺ cations taking the place of Mg²⁺ cations can change the connection mode from corner-sharing to face-sharing; make [CaF₆] octahedra existed in isolation rather than connection; and decrease the number of parent structural axes of the higher order from seven to one to lower the symmetry of parent structure. The charge density intuitively shows the existence of extruding force brought by lager Ca²⁺ cations. The Cs₄Mg₃CaF₁₂ belongs to a new hexagonal perovskite composition A^′₂B^′_1.5B^′′_0.5F₆, which derives form replacing A and B sites of cubic ABX₃ perovskite with alkali metal Cs⁺ cations, and alkaline earth metal Mg²⁺ and Ca²⁺ cations. Further research on this compound is underway.

Acknowledgements

This work is supported by Western Light of CAS (Grant no. XBBS201217), 973 Program of China (Grant no. 2012CB626803), the National Natural Science Foundation of China (Grant no. U1129301, 51172277, 21101168, 11104344), Main Direction Program of Knowledge Innovation of CAS (Grant no. KJCX2-EW-H03-03), The Funds for Creative Cross & Cooperation Teams of CAS, Major Program of Xinjiang Uygur Autonomous Region of China during the 12th Five-Year Plan Period (Grant no. 201130111), the High Technology Research & Development Program of Xinjiang Uygur Autonomous Region of China (Grant no. 201116143), the Science and Technology Project of Urumqi (Grant no. G121130002).

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Footnote

† Electronic supplementary information (ESI) available: Atomic coordinates and equivalent isotropic displacement parameters, structure of cubic CsMF₃ (M = Mg and Ca), crystal structure of CMCF, bond angles of [CaF₆] octahedra, IR spectrum, band structure and charge density. CCDC 1017082. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra07819e

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