Polymorphisms in M2+AlF5(H2O)7 (M2+ = Fe2+, Co2+, or Ni2+): Syntheses, Crystal Structures, and Characterization of New Mixed Metal Fluoride Hydrates

Seo, Jun Min; Jo, Hongil; Choi, Myung Ho; Ok, Kang Min; Chang, Hong Young; Kim, Sun Woo

doi:10.3390/cryst14020197

Open AccessArticle

Polymorphisms in M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺): Syntheses, Crystal Structures, and Characterization of New Mixed Metal Fluoride Hydrates

¹

Department of Chemistry Education, Chosun University, Gwangju 61452, Republic of Korea

²

Department of Chemistry, Sogang University, Seoul 04107, Republic of Korea

³

Department of Computer, Mathematical and Physical Sciences (CMPS), Sul Ross State University, Alpine, TX 79832, USA

^*

Author to whom correspondence should be addressed.

Crystals 2024, 14(2), 197; https://doi.org/10.3390/cryst14020197

Submission received: 22 January 2024 / Revised: 14 February 2024 / Accepted: 16 February 2024 / Published: 17 February 2024

(This article belongs to the Section Inorganic Crystalline Materials)

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Abstract

:

Three new mixed metal fluoride hydrates, M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺), were synthesized and characterized. The crystals of M²⁺AlF₅(H₂O)₇ were obtained using a hydrothermal method with a CF₃COOH aqueous solution. The crystal structures displayed polymorphisms in C2/m (No. 12) or P-1 (No. 2) space groups, depending on temperature variations. The observed polymorphisms in M²⁺AlF₅(H₂O)₇ are associated with changes in the bonding environment of [M(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻ octahedra, along with changes in hydrogen bonds and unit cell volumes. Infrared spectra and thermogravimetric analyses confirmed the presence of water molecules. The ultraviolet–visible spectra of M²⁺AlF₅(H₂O)₇ revealed distinctive absorption bands dependent on the [M(H₂O)₆]²⁺ complex. This work provides a detailed account of the synthetic procedure, crystal structures, and spectroscopic characterization of M²⁺AlF₅(H₂O)₇.

Keywords:

polymorphisms; fluorides; hydrothermal; hydrogen bonding; mixed metal

1. Introduction

Mixed metal fluoride materials have captured considerable attention due to their varied and intriguing physical properties [1], including their multiferroic behavior (BaMnF₄ [2,3,4], BaCoF₄ [5,6,7], BaNiF₄ [8,9], K₃Fe₅F₁₅ [10,11,12], K₃Cr₂Fe₃F₁₅ [13,14], and Pb₅Cr₃F₁₉ [15,16]), non-linear optical (NLO) behavior (BaMgF₄ [17,18], KNa₂ZrF₇ [19], and CsNaTaF₇ [20]), and luminescent features (Yb³⁺/Er³⁺-co-doped NaYF₄ [21], Mn⁴⁺-doped K₂LiAlF₆ [22], and Tb³⁺-doped Na₅Lu₉F₃₂ [23]). Despite the allure of these properties, research on mixed metal fluoride materials has been limited compared to oxide materials, primarily due to challenges in their synthesis.

Traditionally, solid-state reactions for the synthesis of fluoride materials have relied on HF, F₂, and CF₄ gases as fluorination sources to prevent hydrolysis into oxides [24]. However, the use of these gases is inherently hazardous and toxic, and it requires meticulous precautions for safe handling. An alternative approach for fluoride material synthesis involves hydrothermal or solvothermal methods. Nevertheless, these methods introduce HF to regulate the reaction acidity, posing difficulties due to its toxicity and its tendency to dissolve glass materials in synthetic experiments. Furthermore, many fluoride materials lack well-defined crystal structures and comprehensive characterizations. Therefore, there is a pressing need for innovative synthetic methods and strategies to facilitate the creation and in-depth study of these materials, unraveling their unique physical properties.

Kim et al. previously presented an innovative synthetic method for making complex fluoride materials. Instead of utilizing an aqueous HF solution via the hydrothermal route, they employed a CF₃COOH aqueous solution, resulting in materials such as BaMF₄ (M = Mg, Mn, Co, Ni, and Zn) [25], RbFe₂F₆ [26], and K₄Fe₃F₁₂ [27]. Consequently, this method opens the door for the synthesis of various new mixed metal fluoride materials.

In previous studies, mixed metal fluorides hydrates, denoted as M²⁺M’³⁺F₅(H₂O)₇ (M = Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, or Zn²⁺; M’ = Al³⁺, Cr³⁺, Mn³⁺, Fe³⁺, Ga³⁺, or In³⁺) have been reported, with crystal structures identified as P-1 (No. 2), C2/m (No. 12), or C2/c (No. 15) space groups depending on their chemical compositions (See Table 1) [28,29,30,31]. Notably, the crystal structure of NiFeF₅(H₂O)₇ exhibits polymorphisms (P-1 or C2/c) dependent on the temperature [30]. Furthermore, the crystal structure of NiGaF₅(H₂O)₇ also displays polymorphisms (P-1 or C2/m) contingent on synthetic conditions [31]. These findings suggest that the polymorphic characteristics of the crystal structures in M²⁺M’³⁺F₅(H₂O)₇ also may be influenced by specific chemical compositions and/or synthetic conditions. Building on these insights, we aim to further explore compositional modifications and controlled synthetic conditions of M²⁺M’³⁺F₅(H₂O)₇ to uncover novel crystal structures, including polymorphisms with intriguing physical properties.

In the current study, utilizing a hydrothermal method with a CF₃COOH aqueous solution, we delve into the detailed syntheses, crystal structures at both room temperature and 100 K, and characterization of three noble mixed metal fluorides hydrates, specifically M²⁺AlF₅(H₂O)₇, where M represents Fe, Co, or Ni. While the room temperature crystal structure of CoAlF₅(H₂O)₇ has been previously reported [28], its crystal structure at 100 K has not been explored.

2. Materials and Methods

The following reagents were used without purification: FeF₂ (Alfa Aesar, Haverhill, MA, USA, 99.5%), CoF₂ (Alfa Aesar, Haverhill, MA, USA, 98%), NiF₂ (Alfa Aesar, Haverhill, MA, USA, 97%), AlF₃ (Alfa Aesar, Haverhill, MA, USA, 99.5%), and CF₃COOH (Alfa Aesar, Haverhill, MA, USA, 99%).

M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) were synthesized using a mild hydrothermal method using aqueous CF₃COOH solution (see Figure 1).

Crystals of FeAlF₅(H₂O)₇ were grown by mixing 0.1473 g (1.57 mmol) of FeF₂, 0.1318 g (1.57 mmol) of AlF₃, 3 mL of CF₃COOH (39 mmol), and 5 mL of H₂O. The resultant solution was placed in a 23 mL Teflon-lined stainless autoclave that was subsequently closed. The autoclave was gradually heated to 230 °C, held for 24 h, and cooled slowly to room temperature at a rate of 6 °C/h. The mother liquor was decanted from the products, then the products were recovered via filtration and washed with distilled water and acetone. Slightly yellowish-colored block-shaped crystals, the only product of the reaction, were recovered with a ~50% yield based on AlF₃. The powder X-ray diffraction pattern of the synthesized phase was in good agreement with the generated pattern from the single crystal data (see Supporting Information, Figure S1).

Crystals of CoAlF₅(H₂O)₇ were grown by mixing 0.1521 g (1.57 mmol) of CoF₂, 0.1318 g (1.57 mmol) of AlF₃, 3 mL of CF₃COOH (39 mmol), and 5 mL of H₂O. The resultant solution was placed in a 23 mL Teflon-lined stainless autoclave that was subsequently closed. The autoclave was gradually heated to 230 °C, held for 24 h, and cooled slowly to room temperature at a rate of 6 °C/h. The mother liquor was decanted from the products, then the products were recovered via filtration and washed with distilled water and acetone. Orange-colored block-shaped crystals, the only product of the reaction, were recovered with an ~80% yield based on AlF₃. The powder X-ray diffraction pattern of the synthesized phase was in good agreement with the generated pattern from the single crystal data (see Supporting Information, Figure S1).

Crystals of NiAlF₅(H₂O)₇ were grown by mixing 0.1518 g (1.57 mmol) of NiF₂, 0.1318 g (1.57 mmol) of AlF₃, 3 mL of CF₃COOH (39 mmol), and 5 mL of H₂O. The resultant solution was placed in a 23 mL Teflon-lined stainless autoclave that was subsequently closed. The autoclave was gradually heated to 230 °C, held for 24 h, and cooled slowly to room temperature at a rate of 6 °C/h. The mother liquor was decanted from the products, then the products were recovered via filtration and washed with distilled water and acetone. Blue–green-colored block-shaped crystals, the only product of the reaction, were recovered with an ~80% yield based on AlF₃. The powder X-ray diffraction pattern of the synthesized phase was in good agreement with the generated pattern from the single crystal data (see Supporting Information, Figure S1).

Single crystal X-ray diffraction (SCXRD) data for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) were collected using a Bruker D8 QUEST diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and a PHOTON-II CPAD detector at room temperature and 100 K, respectively. For data collection, a slightly yellowish-colored block-shaped crystal of FeAlF₅(H₂O)₇ (0.051 mm

\times

0.154 mm

\times

0.212 mm), an orange-colored block-shaped crystal of CoAlF₅(H₂O)₇ (0.051 mm

\times

0.154 mm

\times

0.212 mm), and a blue–green-colored block-shaped crystal of NiAlF₅(H₂O)₇ (0.088 mm

\times

0.150 mm

\times

0.151 mm) were selected and mounted on a glass fiber. The data were integrated using the SAINT program [32] with the intensities corrected for Lorentz, polarization, and air absorption. Multiple scans were used for the absorption correction across the hemisphere of the data. The crystallographic space group for the structures of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) under ~100 K or room temperature were determined as P-1 (No. 2) and C2/m (No. 12), respectively. The systematic forbidden reflections (h + k = 2n + 1) attributable to the C-centered cell of the C2/m space group were observed as the reflection conditions of the P-1 space group. Thus, two space groups, P-1 and C2/m, were clearly distinguished [33]. The structures were solved via the direct method (SHELXS-2014) [34,35], and all the atomic positions are thermal parameters were refined using the program SHELXL (2019/3) [34,35]. Notably, the crystal structures of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at 100 K were refined as pseudo-merohedral twins. The data were converged for I > 2σ (I). All calculations were performed using the WINGX-98 crystallographic software package [36]. Relevant crystallographic data and selected bond distances for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) are given in Table 2, Table 3 and Table 4. Further details are available in the Supporting Information, specifically, in Tables S1 and S2. Additionally, the local coordination environments of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) are illustrated in the Supporting Information, Figure S2. The structural figures for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) were drawn using the VESTA crystal structure drawing package [37].

The powder X-ray diffraction (PXRD) data for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) were collected using a PANalytical X’Pert pro diffractometer using CuKα radiation in the 2θ range of 5–65°. A step size of 0.008 degrees (deg) with a scan time of 0.3 s/deg was used. No impurities were observed, and the calculated and experimental PXRD patterns were in good agreement (see Supporting Information Figure S1).

Fourier-transform infrared spectroscopy (FT-IR) spectra of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) were obtained using a Thermo Scientific Nicolet 6700 FT-IR spectrometer in the range of 400–4000 cm⁻¹. Thermogravimetric analyses (TGA) for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) were carried out using a TA Instruments TGA 2050. Approximately ~10 mg of each sample was placed into a Pt crucible and heated under a nitrogen atmosphere at a rate of 10 °C min⁻¹ to 900 °C. The UV–Vis diffuse reflectance spectra of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) were collected using a Shimadzu SolidSpec-3700 Diffuse Reflectance UV–VIS-NIR spectrophotometer in the range of 300–1400 nm.

3. Results and Discussion

3.1. Synthesis

Previously, various researchers reported the synthesis and crystal structures of mixed metal fluoride hydrates prepared via a hydrothermal method using HF as a mineralizer. For example, crystals of Sr₂Fe₂F₁₀(H₂O) were prepared via a high-temperature (700 °C) hydrothermal method using an aqueous solution of SrF₂, Fe₂O₃, and HF [38]. Additionally, M(II)M’(III)F₅(H₂O)₇ (M = Co or Ni; M’ = Mn, Ga, or In) were obtained via a hydrothermal method using an aqueous solution of Co(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O, Ga(NO₃)₃·xH₂O, In(NO₃)₃·xH₂O, Mn₂O₃, CoCO₃, tetrapropylammoniumhydroxide (TPAOH), and HF at 170~200 °C [31].

However, HF is very toxic and corrosive, and it has a propensity to dissolve glass materials, posing serious health risks. Proper personal protective equipment and expertise in handling HF are essential. In a previous study, Kim et al. reported a new synthetic method to synthesize mixed metal fluorides using a CF₃COOH aqueous solution, providing a safer alternative [25]. Using this method, crystals of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) can be easily and safely prepared without using an aqueous HF solution (see Figure 1). The hydrothermal conditions are believed to facilitate the in situ generation of aqueous HF from a CF₃COOH aqueous solution in the reaction mixture. The potential reaction mechanisms are as follows:

MF₂ (s) (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) + AlF₃ (s) + 5CF₃COOH (aq) →
(CF₃COO)₂M (aq) (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) + (CF₃COO)₃Al (aq) + 5HF (aq)

(1)

(CF₃COO)₂M (aq) + (CF₃COO)₃Al (aq) + 5HF (aq) →
M²⁺AlF₅(H₂O)₇ (s) (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) + 5CF₃H (g) + 5CO₂ (g)↑

(2)

Initially, the hydrothermal reaction of metal fluorides, M²⁺F₂ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) and AlF₃, with a CF₃COOH aqueous solution led to the in situ generation of metal trifluoroacetate salts and aqueous HF. Subsequently, the in situ intermediates are expected to further reactions, ultimately resulting in the formation of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺).

3.2. Crystal Structure

The crystal structures of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) exhibit both C2/m (No. 12) or P-1 (No. 2) space groups depending on the temperature (see Table 2). At room temperature, M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) reveals a zero-dimensional structure. Two crystallographically unique M²⁺ sites give rise to octahedral hexa-aqua complexes, [M(H₂O)₆]²⁺ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) alongside octahedral [AlF₅(H₂O)]²⁻ complexes, where O and F atoms are disordered (see Figure 2). These cationic and anionic complexes, [M(H₂O)₆]²⁺ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) and [AlF₅(H₂O)]²⁻, are interconnected through hydrogen bonding, as illustrated in Figure 3. Thus, it exhibits a pseudo ABAB layered structure along the b-axis. The corresponding hydrogen bonding distances between H and O or O/F vary from 1.702 to 2.536 Å (see Table 2). In addition, the M²⁺ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) – O bond distances in the octahedral complexes of [M(H₂O)₆]²⁺ range between 2.021(4) and 2.126(4) Å, and the Al–F or O/F bond distances of the [AlF₅(H₂O)]²⁻ complex range from 1.775(3) to 1.867(3) Å, respectively. Selected bond distances for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at RT are also summarized in Table 3. The bond valence sum (BVS) [39,40] calculations resulted in values of 2.05~2.06, 2.08, 2.09, 3.06~3.12, 1.64~2.01, and 0.85~1.10 for Fe²⁺, Co²⁺, Ni²⁺, Al³⁺, O²⁻, and F⁻, respectively (see Supporting Information Table S3).

Similar to the room temperature structure, M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at 100(2) K adopts a zero-dimensional structure (see Figure 4). This structure features two distinct octahedral cations, [M(H₂O)₆]²⁺ and two distinct octahedral anions, [AlF₅(H₂O)]²⁻, with disorder observed in the positioning of both O and F atoms. These different components bind to each other through hydrogen bonding and electrostatic interactions, contributing to a pseudo ABAB layered structure, as illustrated in Figure 5. The corresponding hydrogen bonding distances between H and O or O/F vary from 1.680 to 2.639 Å. The M²⁺–O bond distances in the octahedral [M(H₂O)₆]²⁺ complexes range from 2.014(4) to 2.138(4) Å, and the Al–F or O/F bond distances in the octahedral [AlF₅(H₂O)]²⁻ complexes range from 1.769(4) to 1.904(4) Å, respectively. Selected bond distances for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at 100 K are summarized in Table 4. The BVS [39,40] calculations resulted in values of 2.04~2.06, 2.07~2.11, 2.07~2.10, 2.96~3.02, 1.63~1.97, and 0.69~1.24 for Fe²⁺, Co²⁺, Ni²⁺, Al³⁺, O²⁻, and F⁻, respectively (see Supporting Information Table S3).

The observed polymorphism in M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) is associated with alternations in the local coordination environment of [M(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻ octahedra, accompanied by changes in the M²⁺–O (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) and Al³⁺–F or O/F bond lengths. Despite the expected tilting of these octahedra, indicative of out-of-center distortion in octahedral structures, such effects were not discernible in M²⁺AlF₅(H₂O)₇ (see Supporting Information, Table S4). Initially, as the temperature decreased, the unit cell volume reduced by approximately 2.03 times. This led to increased strength in hydrogen bonding interactions between [M(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻ octahedra, accompanied by an augmentation in the number of hydrogen bonds. Furthermore, the local coordination environments of [M(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻ exhibited subtle changes with temperature variations. For instance, the average bond distances of Fe–O in the axial position of [Fe(H₂O)₆]²⁺ increased as the temperature decreased (see Figure 6). Additionally, the number of crystallographic Al sites increases to two under ~100 K temperature conditions. In summary, this led to reduced crystal symmetry, ultimately inducing a polymorphism between monoclinic C2/m (No. 12) and triclinic P-1 (No. 2).

3.3. FT-IR Spectroscopy

The FT-IR spectra of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) revealed distinctive O–H vibrations around 3400 cm⁻¹ and 1650 cm⁻¹, attributed to the presence of water molecules [41]. The bands occurring between 730–700 cm⁻¹ and 670–450 cm⁻¹ can be assigned to Al–F or Al–O/F and M²⁺–O (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) vibrations, respectively. These assignments are consistent with previous reported results [41]. Detailed IR spectra and corresponding assignments are provided in the Supporting Information (see Figure S3).

3.4. Thermal Analysis

The thermal behavior of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) was investigated using a thermogravimetric analysis (TGA) under a nitrogen atmosphere. All three materials exhibited two distinct weight loss steps (see Figure 7 and Table 5).

For FeAlF₅(H₂O)₇, the initial weight loss (found: ~35.6%) observed between 100 °C and 200 °C can be attributed to the release of six coordinated water molecules from the crystal structure (calculated: 35.6%). Subsequently, a second weight loss between 200 °C and 800 °C is expected due to the sample decomposition (found: ~55.3%). The powder XRD pattern of the final residue product after TGA revealed the presence of Fe₂O₃ and unidentified phases (possibly Al₂O₃ or AlFeO₃, etc.) (see Supporting information, Figure S4).

In the case of CoAlF₅(H₂O)₇, the initial weight loss (found: ~34.10%) that occurred between 100 °C and 220 °C corresponded to the release of six water molecules from the crystal structure (calculated: 35.2%). A subsequent weight loss between 200 °C and 800 °C suggested the occurrence of sample decomposition (found: ~56.3%). The powder XRD pattern of the final residue product after TGA displayed the formation of CoAl₂O₄ (see Supporting Information, Figure S4).

For NiAlF₅(H₂O)₇, the initial weight loss (found: ~34.07%) observed between 100 °C and 220 °C was attributed to the release of six water molecules from the crystal structure (calculated: 35.2%). A subsequent weight loss between 200 °C and 800 °C was indicative of sample decomposition (found: ~56.5%). The powder XRD pattern of the final residue product after TGA showed the presence of NiO and NiAl₂O₄ (see Supporting Information, Figure S4).

It is noteworthy that a prior TGA investigation focused on specific M²⁺M’³⁺F₅(H₂O)₇ materials revealed the existence of dihydrate intermediates, namely M²⁺M’³⁺F₅(H₂O)₂, with an inverse weberite structure. These intermediates have been successfully isolated or synthesized in previous studies [42,43]. However, in the current study, we were unable to isolate such intermediates. Indeed, the ability to obtain these intermediates may be influenced by the specific chemical compositions and crystal structures of M²⁺M’³⁺F₅(H₂O)₇.

3.5. UV–Vis Diffuse Reflectance Spectroscopy

UV–vis diffuse reflectance spectra of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) in the range of 300–1400 nm are presented in Figure 8. Notably, the crystal structure of all three materials contains [M(H₂O)₆]²⁺ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) complexes, implying the anticipated appearance of distinct absorption bands for each material (see Figure 8).

For FeAlF₅(H₂O)₇, absorption with two broad peaks between 800 nm and 1300 nm was observed, corresponding to the ⁵T₂ → ⁵E transition. The splitting of this absorption band is attributed to the Jahn–Teller distortion of the excited state t_2g³e_g³ in Fe²⁺ (d⁶) [44,45].

In the case of CoAlF₅(H₂O)₇, the broad band near 500 nm can be attributed to the ⁴T_1g (F) → ⁴T_1g (P) transition, and a slightly higher energy transition (the shoulder) is predicted to be the ⁴T_1g → ⁴A_2g transition. The small energy difference results in the overlap of these two bands. Additionally, a broad band near 1200 nm was observed, assigned as the ⁴T_1g → ⁴T_2g transition [44,45].

For NiAlF₅(H₂O)₇, three principal absorption bands were identified, with one of the bands exhibiting signs of further splitting. The band near 400 nm was assigned to the ³A_2g → ³T_1g(P) transition. A two-humped absorption band near 700 nm was assigned to the spin-forbidden ³A_2g → ¹E_g transition (~620 nm, weak) and the ³A_2g → ³T_1g(F) transition (~725 nm), respectively. The splitting of these bands was a result of Jahn–Teller distortion in the excited state. The last band near 1200 nm was assigned to the ³A_2g → ³T_2g transition [44,45].

4. Conclusions

The synthesis and characterization of three novel mixed metal fluorides hydrates, M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺), have been successfully achieved using a hydrothermal method employing a CF₃COOH aqueous solution. The crystal structures exhibited polymorphisms in C2/m (No. 12) or P-1 (No. 2) space groups, with these polymorphisms linked to variations in the local environment of [M(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻ octahedra in response to temperature changes. The presence of water molecules in the synthesized materials was confirmed through FT-IR and TGA analyses, highlighting the significance of hydration in the structural framework. UV–Vis spectra of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) showed a distinctive absorption band, dependent on the [M(H₂O)₆]²⁺ complex, providing valuable insights into the electronic transition within the material.

This comprehensive study not only contributes to the understanding of the synthetic procedure and crystal structures of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) but also sheds light on the intricate relationship between the two polymorphs and the evolving coordination environments of the M²⁺ and Al³⁺ cations. The presented findings provide a robust exploration and applications of mixed metal fluoride hydrates in the field of material sciences.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst14020197/s1, Table S1: Atomic coordinates and equivalent isotropic displacement parameters (Å²) for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at RT. Table S2: Atomic coordinates and equivalent isotropic displacement parameters (Å²) for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at 100 K. Table S3: Bond valence calculation for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at RT and 100 K, respectively. Table S4: Out-of-center distortion of octahedral structures in [Fe(H₂O)₆]²⁺ at RT and 100 K. Figure S1: Experimental and calculated X-ray powder diffraction patterns for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺): (a) FeAlF₅(H₂O)₇, (b) CoAlF₅(H₂O)₇, and (c) NiAlF₅(H₂O). Figure S2: ORTEP representation (50% probability ellipsoid) for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺). Figure S3: FT-IR Spectra of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺): (a) FeAlF₅(H₂O)₇, (b) CoAlF₅(H₂O)₇, and (c) NiAlF₅(H₂O)₇. Figure S4: Powder X-ray diffraction pattern for final residuals after TGA experiments: (a) FeAlF₅(H₂O)₇, (b) CoAlF₅(H₂O)₇, and (c) NiAlF₅(H₂O)₇.

Author Contributions

Conceptualization, J.M.S. and S.W.K.; validation, J.M.S., H.J., M.H.C., K.M.O., H.Y.C., and S.W.K.; formal analysis, J.M.S., H.J., M.H.C., K.M.O., H.Y.C., and S.W.K.; investigation, J.M.S., H.J., M.H.C., K.M.O., H.Y.C., and S.W.K.; data curation, J.M.S., H.J., M.H.C., K.M.O., H.Y.C., and S.W.K.; writing—original draft preparation, J.M.S. and S.W.K.; writing—review and editing, J.M.S., H.J., M.H.C., K.M.O., H.Y.C., and S.W.K.; visualization, J.M.S. and S.W.K.; supervision, S.W.K.; project administration, S.W.K.; funding acquisition, S.W.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received funding from the National Research Foundation of Korea (NRF), Ministry of Education (2022R111A3063132). H.Y.C. also thanks the Robert A. Welch Foundation for support (Gant AK-0050-20201025).

Data Availability Statement

The corresponding crystallographic information files (CIFs) have been deposited in the Cambridge Crystallographic Centre (CCDC) database and can be obtained free of charge at: http://www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on 22 January 2024) (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033; E-mail: [email protected]) with the following depository number: 2327066~2327071. The data presented in this study are available from the corresponding author upon request. The data are not publicly available due to privacy.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Photographs of crystals of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺): (a) FeAlF₅(H₂O)₇, (b) CoAlF₅(H₂O)₇, and (c) NiAlF₅(H₂O)₇.

Figure 2. Ball-and-stick and polyhedral representations of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at room temperature in the ab-plane. Octahedral complexes, [M(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻, are balanced by electrostatic charges. Hydrogen atoms bonded to disordered O/F sites were excluded to enhance clarity.

Figure 3. Hydrogen bonding interactions between two structural components, [M(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻ within the M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at room temperature. Hydrogen bonding interactions produce a pseudo ABAB layered structure.

Figure 4. Ball-and-stick and polyhedral representations of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at 100 K in the bc-plane. Electrostatic charges are balanced between octahedral complexes, [M(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻. Hydrogen atoms bonded to disordered O/F sites were excluded to enhance clarity.

Figure 5. Polyhedral representations of hydrogen bonding interactions between [M(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻ within the M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at 100 K in the bc-plane. A pseudo ABAB layered structure in the bc-plane is shown with hydrogen bonds.

Figure 6. Ball-and-stick representations of the local coordination environments of [Fe(H₂O)₆]²⁺ and [AlF₅(H₂O)]²⁻ within FeAlF₅(H₂O)₇ at two different temperatures: (a) room temperature and (b) 100 K.

Figure 7. Thermogravimetric analysis (TGA) diagrams for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺).

Figure 8. UV–Vis diffuse reflectance spectra of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺).

Table 1. Crystallographic parameters of M²⁺M’³⁺F₅(H₂O)₇ (M²⁺ = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, or Zn²⁺; M’³⁺ = Al³⁺, Cr³⁺, Mn³⁺, Fe³⁺, Ga³⁺, or In³⁺).

Compound	Space Group, T	Lattice Parameter (Å)	V(Å³)	Reference
CoAlF₅(H₂O)₇	C2/m, 296 K	a =10.917(7), b = 13.863(14), c = 6.525(5), β = 100.30(6)°	971.60	[28]
MnFeF₅(H₂O)₇	C2/c, 296 K	a = 11.182(3), b = 14.168(4), c = 13.264(3), β = 100.51(2)°	2066.12	[28]
CuInF₅(H₂O)₇	P-1, 100 K	a = 6.5659(5), b = 8.8631(6), c = 9.1548(6), α = 103.16(1)°, β = 97.66(1)°, γ = 97.31(1)°	507.24(6)	[29]
ZnGaF₅(H₂O)₇	P-1, 296 K	a = 6.5659(5), b = 8.8631(6), c = 9.1548(6), α = 103.16(1)°, β = 97.66(1)°, γ = 97.31(1)°	486.18(3)	[29]
NiMnF₅(H₂O)₇	P-1, 100(2) K	a = 6.4149(2), b = 8.9609(2), c = 8.9609(2), α = 104.1985(10)°, β = 95.9379(10)°, γ = 96.6946(10)°	491.22(2)	[30]
NiCrF₅(H₂O)₇	P-1, 100(2) K	a = 6.4661(4), b = 8.6504(5), c = 8.8679(5), α = 104.6606(19)°, β = 96.584(2)°, γ = 94.423(2)°	473.67(5)	[30]
NiFeF₅(H₂O)₇	C2/c, 301(2) K	a = 10.9229(3), b = 13.9060(3), c = 13.0252(4), β = 100.2400(10)°	1946.93(9)	[30]
NiFeF₅(H₂O)₇	P-1, 100(2) K	a = 6.4783(3), b = 8.699(3), c = 8.9131(4), α = 104.5130(10)°, β = 96.9280(10)°, γ = 94.5090(10)°	479.31(4)	[30]
CoInF₅(H₂O)₇	C2/c, 300(2) K	a = 11.1238(4), b = 14.2361(5), c = 13.3064(5), β = 100.7280(14)°	2070.37(13)	[31]
NiInF₅(H₂O)₇	P-1, 300(2) K	a = 6.61530(10), b = 8.9621(2), c = 8.9961(2), α = 102.9620(10)°, β = 98.3470(10)°, γ = 96.3110(10)°	508.609(18)	[31]
NiGaF₅(H₂O)₇	C2/m, 300(2) K	a =10.8563(4), b = 13.8331(5), c = 6.4968(2), β = 99.9610(10)°	960.96(6)	[31]
NiGaF₅(H₂O)₇	P-1, 300(2) K	a = 6.4800(2), b = 8.6957(3), c = 8.8694(3), α = 103.9584(12)°, β = 96.8177(13)°, γ = 95.1076(12)°	477.98(3)	[31]
FeAlF₅(H₂O)₇	C2/m, 300(2) K	a = 10.9189(7), b = 13.8020(7), c = 6.5210(4), β = 100.154(2)°	967.26(10)	This work
FeAlF₅(H₂O)₇	P-1, 100(2) K	a = 6.4864(9), b = 8.7190(11), c = 8.7917(11), α = 103.934(3)°, β = 96.364(3)°, γ = 95.400(3)°	476.05(11)	This work
CoAlF₅(H₂O)₇	C2/m, 300(2) K	a = 10.8749, b = 13.7822(16), c = 6.4894(8), β = 100.397(5)°	956.7(2)	This work
CoAlF₅(H₂O)₇	P-1, 100(2) K	a = 6.4434(5), b = 8.6691(6), c = 8.8031(6), α = 104.019(2)°, β = 96.749(2)°, γ = 95.254(2)°	470.08(6)	This work
NiAlF₅(H₂O)₇	C2/m, 296(2) K	a = 10.7880(4), b = 13.6952(4), c = 6.4554(2), β = 100.1507(14)°	938.82(6)	This work
NiAlF₅(H₂O)₇	P-1, 101(2) K	a = 6.4022(6), b = 8.5874(7), c = 8.7691(7), α = 104.255(3)°, β = 96.638(3)°, γ = 94.816(3)°	460.96(7)	This work

Table 2. Crystallographic data for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺).

	FeAlF₅(H₂O)₇		CoAlF₅(H₂O)₇		NiAlF₅(H₂O)₇
F.W.	303.94		307.02		306.80
T(K)	300	100(2)	300(2)	100(2)	296(2)	101(2)
Crystal system	Monoclinic	Triclinic	Monoclinic	Triclinic	Monoclinic	Triclinic
Space group	C2/m (No. 12)	P-1 (No. 2)	C2/m (No. 12)	P-1 (No. 2)	C2/m (No. 12)	P-1 (No. 2)
a (Å)	10.9180(7)	6.4864(9)	10.8749(16)	6.4434(5)	10.7880(4)	6.4022(6)
b (Å)	13.8020(7)	8.7190(11)	13.7822(16)	8.6691(6)	13.6952(4)	8.5874(7)
c (Å)	6.5210(4)	8.7917(11)	6.4894(8)	8.8031(6)	6.4554(2)	8.7691(7)
Angle (deg)	β = 100.154(2)	α = 103.834(3) β = 96.364(3) γ = 95.400(3)	β = 100.397(5)	α = 104.019(2) β = 96.749(2) γ = 95.254(2)	β = 100.1507(14)	α = 104.255(3) β = 96.638(3) γ = 94.816(3)
V (Å)³	967.26(10)	476.05(11)	956.7(2)	470.08(6)	938.82(6)	460.96(7)
Z	4	2	4	2	4	2
ρ_cald (g/cm³)	2.087	2.120	2.132	2.169	2.171	2.210
μ (mm⁻¹)	1.742	1.770	1.979	2.014	2.255	2.296
2θ_max(deg)	56.84	56.64	66.18	56.82	56.66	56.66
R(int)	0.0487	0.0336	0.0350	0.0424	0.0214	0.0435
GOF	1.085	1.129	1.111	1.148	1.116	1.121
R(F) ^a	0.0424	0.0425	0.0434	0.0329	0.0407	0.0512
R_w(F_o²) ^b	0.0997	0.1133	0.1031	0.0743	0.0945	0.1408

^a R (F) = Σ||F_o| − |F_c||/Σ|F_o|, ^b R_w (F_o²) = [Σw(F_o² − F_c²)²/Σw(F_o²)²]^1/2.

Table 3. Selected bond distances for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at RT.

Bond	Distance (Å)	Bond	Distance (Å)
FeAlF₅(H₂O)₇
Fe(1)–O(2)	2.093(4) × 2	Al(1)–O(1)/F(1)	1.867(3) × 2
Fe(1)–O(3)	2.105(3) × 2	Al(1)–F(2)	1.784(3) × 2
Fe(1)–O(4)	2.126(4) × 2	Al(1)–F(3)	1.787(2) × 2
Fe(2)–O(5)	2.087(4) × 2
Fe(2)–O(6)	2.123(3) × 4
H(1B) · · · O(2)	2.536 × 2	H(1B) · · · O(4)	2.267 × 2
H(2) · · · F(3)	1.732	H(3) · · · O(1)/F(1)	2.316
H(3) · · · F(2)	2.054	H(4) · · · O(1)/F(1)	1.963
H(5) · · · F(2)	1.729	H(6A) · · · F(2)	1.761
H(6A) · · · F(3)	1.704
CoAlF₅(H₂O)₇
Co(1)–O(2)	2.045(4) × 2	Al(1)–O(1)/F(1)	1.863(3) × 2
Co(1)–O(3)	2.081(3) × 2	Al(1)–F(2)	1.779(2) × 2
Co(1)–O(4)	2.109(4) × 2	Al(1)–F(3)	1.786(2) × 2
Co(2)–O(5)	2.055(3) × 2
Co(2)–O(6)	2.088(2) × 4
H(1B) · · · O(2)	2.508 × 2	H(1B) · · · O(4)	2.172 × 2
H(2) · · · F(3)	1.716	H(3) · · · O(1)/F(1)	2.349
H(3) · · · F(2)	2.053	H(4) · · · O(1)/F(1)	1.949
H(5) · · · F(2)	1.731	H(6A) · · · F(2)	1.780
H(6A) · · · F(3)	1.718
NiAlF₅(H₂O)₇
Ni(1)–O(2)	2.019(4) × 2	Al(1)–O(1)/F(1)	1.860(3) × 2
Ni(1)–O(3)	2.036(3) × 2	Al(1)–F(2)	1.775(3) × 2
Ni(1)–O(4)	2.079(4) × 2	Al(1)–F(3)	1.782(2) × 2
Ni(2)–O(5)	2.021(4) × 2
Ni(2)–O(6)	2.057(3) × 4
H(2) · · · F(3)	1.710	H(3) · · · O(1)/F(1)	2.291
H(3) · · · F(2)	2.114	H(4) · · · O(1)/F(1)	1.915
H(5) · · · F(2)	1.729	H(6A) · · · F(2)	1.783
H(6A) · · · F(3)	1.702

Table 4. Selected bond distances for M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺) at 100 K.

Bond	Distance (Å)	Bond	Distance (Å)
FeAlF₅(H₂O)₇
Fe(1)–O(3)	2.102(4) × 2	Al(1)–O(1)/F(1)	1.904(4) × 2
Fe(1)–O(4)	2.099(3) × 2	Al(1)–F(3)	1.769(4) × 2
Fe(1)–O(5)	2.138(4) × 2	Al(1)–F(4)	1.793(3) × 2
Fe(2)–O(6)	2.084(3) × 2	Al(2)–O(2)/F(2)	1.872(4) × 2
Fe(2)–O(7)	2.105(4) × 2	Al(2)–F(5)	1.794(3) × 2
Fe(2)–O(8)	2.138(4) × 2	Al(2)–F(6)	1.809(3) × 2
H(1A) · · · O(2)/F(2)	1.768 × 2	H(1B) · · · O(3)	2.270 × 2
H(1B) · · · O(5)	2.346 × 2	H(2A) · · · O(5)	1.814 × 2
H(2B) · · · O(1)/F(1)	1.789 × 2	H(3A) · · · O(2)/F(2)	2.639
H(3A) · · · O(5)	2.324	H(3B) · · · F(4)	1.713
H(4A) · · · O(1)/F(1)	1.824	H(4B) · · · O(2)/F(2)	2.652
H(4B) · · · F(6)	1.802	H(5A) · · · O(1)/F(1)	2.069
H(5B) · · · O(8)	1.964	H(6A) · · · F(3)	1.716
H(6B) · · · F(6)	1.769	H(7A) · · · F(4)	1.711
H(7B) · · · F(3)	1.740	H(8A) · · · F(6)	1.875
H(8B) · · · F(5)	1.680
CoAlF₅(H₂O)₇
Co(1)–O(3)	2.055(3) × 2	Al(1)–O(1)/F(1)	1.892(3) × 2
Co(1)–O(4)	2.083(2) × 2	Al(1)–F(3)	1.776(2) × 2
Co(1)–O(5)	2.099(3) × 2	Al(1)–F(4)	1.788(2) × 2
Co(2)–O(6)	2.051(2) × 2	Al(2)–O(2)/F(2)	1.866(2) × 2
Co(2)–O(7)	2.081(3) × 2	Al(2)–F(5)	1.791(2) × 2
Co(2)–O(8)	2.085(3) × 2	Al(2)–F(6)	1.802(2) × 2
H(1A) · · · O(2)/F(2)	1.753 × 2	H(1B) · · · O(3)	2.267 × 2
H(2A) · · · O(5)	1.833 × 2	H(2B) · · · O(1)/F(1)	1.925 × 2
H(3A) · · · F(5)	1.713	H(3B) · · · F(4)	1.699
H(4A) · · · O(1)/F(1)	1.854	H(4B) · · · F(6)	1.793
H(5A) · · · O(1)/F(1)	2.046	H(5B) · · · O(7)	2.420
H(5B) · · · O(8)	2.083	H(6A) · · · F(3)	1.704
H(6B) · · · F(6)	1.755	H(7A) · · · F(4)	1.720
H(7B) · · · F(3)	1.762	H(8A) · · · F(6)	1.796
H(8B) · · · F(5)	1.690
NiAlF₅(H₂O)₇
Ni(1)–O(3)	2.039(4) × 2	Al(1)–O(1)/F(1)	1.884(4) × 2
Ni(1)–O(4)	2.039(4) × 2	Al(1)–F(3)	1.793(4) × 2
Ni(1)–O(5)	2.064(4) × 2	Al(1)–F(4)	1.789(3) × 2
Ni(2)–O(6)	2.014(4) × 2	Al(2)–O(2)/F(2)	1.867(4) × 2
Ni(2)–O(7)	2.055(4) × 2	Al(2)–F(5)	1.789(3) × 2
Ni(2)–O(8)	2.058(4) × 2	Al(2)–F(6)	1.802(3) × 2
H(1B) · · · O(5)	2.031 × 2	H(2A) · · · O(5)	1.983 × 2
H(2B) · · · O(1)/F(1)	1.751 × 2	H(3A) · · · O(2)/F(2)	2.557
H(3A) · · · O(5)	2.351	H(3B) · · · F(4)	1.703
H(4A) · · · O(1)/F(1)	1.820	H(4B) · · · F(6)	1.806
H(5A) · · · O(1)/F(1)	2.110	H(5B) · · · O(7)	2.400
H(5B) · · · O(8)	2.024	H(6A) · · · F(3)	1.726
H(6B) · · · F(6)	1.742	H(7A) · · · F(4)	1.710
H(7B) · · · F(3)	1.817	H(8A) · · · F(6)	1.782
H(8B) · · · F(5)	1.687

Table 5. Weight loss% of M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺).

Sample	^a First Weight Loss% (Found, Calculated)	^b Final Weight Loss% (Found)
FeAlF₅(H₂O)₇	35.6, 35.6	55.3
CoAlF₅(H₂O)₇	34.1, 35.2	56.3
NiAlF₅(H₂O)₇	34.1, 35.2	56.5

^a Released six water molecules (−6H₂O). ^b The composition of the residue for samples, determined using XRD.

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Seo, J.M.; Jo, H.; Choi, M.H.; Ok, K.M.; Chang, H.Y.; Kim, S.W. Polymorphisms in M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺): Syntheses, Crystal Structures, and Characterization of New Mixed Metal Fluoride Hydrates. Crystals 2024, 14, 197. https://doi.org/10.3390/cryst14020197

AMA Style

Seo JM, Jo H, Choi MH, Ok KM, Chang HY, Kim SW. Polymorphisms in M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺): Syntheses, Crystal Structures, and Characterization of New Mixed Metal Fluoride Hydrates. Crystals. 2024; 14(2):197. https://doi.org/10.3390/cryst14020197

Chicago/Turabian Style

Seo, Jun Min, Hongil Jo, Myung Ho Choi, Kang Min Ok, Hong Young Chang, and Sun Woo Kim. 2024. "Polymorphisms in M²⁺AlF₅(H₂O)₇ (M²⁺ = Fe²⁺, Co²⁺, or Ni²⁺): Syntheses, Crystal Structures, and Characterization of New Mixed Metal Fluoride Hydrates" Crystals 14, no. 2: 197. https://doi.org/10.3390/cryst14020197

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