Synthesis and Characterization of Perovskite-Type [K_1−xNa_x]MgF₃ Mixed Phases via the Fluorolytic Sol-Gel Synthesis

Abstract

The focus of this article is the synthesis of perovskite-type [K_1−xNa_x]MgF₃ mixed phases via the room-temperature fluorolytic sol-gel approach. Different molar ratios of K/Na were examined and analyzed by ¹⁹F MAS NMR and X-ray powder diffraction. Starting from pure KMgF₃, a systematic substitution of potassium by sodium was evidenced when replacing K by Na. As long as the amount of sodium is less than 80% as compared to potassium, spectra just show [K_4−xNa_xF] environments in a [K_1−xNa_x]MgF₃ mixed phase but separate structures appear when the amount of sodium is further increased. Moreover, colloidal dispersions of nanoscaled KMgF₃ particles were obtained, which were used to fabricate coatings on glass slides. Thin films showed antireflective behavior and high transmittance.

1. Introduction

In 2003, our group explored the fluorolytic sol-gel synthesis [1]. In contrast to the common aqueous sol-gel synthesis which leads to the formation of nanosized metal oxides, the fluorolytic sol-gel synthesis yields metal fluoride nanoparticles [2,3]. Because of the low refractive indexes of metal fluorides, obtained metal fluoride sols can be used to fabricate antireflective thin films on different substrates, e.g., glasses or polymers, via dip coating. Additionally, these sols can be subsequently processed to xerogels as well. This is achieved by evaporating the solvent and yields solid metal fluorides with high surface areas. Diversification of the general fluorolytic synthesis approach resulted in the access of a wide variety of fluoride based materials [4,5]. In addition, in this case, the powerful room temperature synthesis method is used to access homodispersed perovskite-type mixed phases of [K_1−xNa_x]MgF₃ on a nano-scaled dimension under ambient conditions. For many decades, scientists are dealing with perovskite-type compounds like KMgF₃ and NaMgF₃ for different purposes, especially because of their interesting physical properties [6,7], synthetic accessibility [8,9], and application-focused demands [10,11]. Furthermore, the synthesis of mixed phases was investigated and will be used for direct comparison [12]. In general, compounds like KMgF₃ act as high-performance ceramics and luminescence host matrices [13,14,15]. Hence, the synthetic access of nanosized [K_1−xNa_x]MgF₃ mixed phases via the fluorolytic sol-gel synthesis under ambient conditions, their characterization, and probing their applicability for antireflective coatings is the main focus of this article.

2. Methods and Experimental

2.1. Methods

¹⁹F MAS NMR: ¹⁹F MAS NMR data were measured with a Bruker Advance 400 MHz spectrometer (Billerica, MA, USA). All dried powder samples were filled into 2.5 mm ZrO₂ rotors and the main rotation frequency was applied at 20 kHz. Referred to δ = 0 ppm of CFCl₃ (using α-AlF₃ as the secondary standard for calibration) and with a D₁ time of 5 s, every spectrum consists of 32 scans.

XRD: Powder diffraction analysis was done by a Seifert XRD 3003 TT (Schnaittach-Hormersdorf, Germany) with rotating sample holder using Cu radiation (Cu K_α1,2; λ = 1.54 Å; U = 40 kV; I = 40 mA; Ni filter). All samples were measured in Bragg-Bretano geometry and results compared to powder diffraction files of COD-inorganic.

Dip-Coating: To fabricate thin films from nanoscopic dispersions, float glasses (40 mm × 20 mm × 3 mm) were coated by dip coating. To ensure a layer thickness of d > 100 nm, the dip-coating process was repeated three times. Obtained layers were heated up to 450 °C with a heating rate (HR) of 10 K·min⁻¹, and kept at this temperature for 15 min.

Transmission-Spectroscopy: Layer thickness (d), reflectance (R_min), transmission (T_max), and refractive index n, using a FILMetrics Thin Film Analyzer F10-RT (Unterhaching, Germany) were determined. Spectra were measured in a wavelength range of λ = 400–1050 nm.

2.2. Experimental

Materials:

Compound	Formula	Concentration/Purity	Supplier
Magnesium Ethoxide	Mg(OEt)2	99.8%	EVONIK Industries
Magnesium Chloride	MgCl2	anhydrous, ≥98%	Aldrich
Potassium Methoxide	KOMe	≥95%	Aldrich
Hydrogen Fluoride	HF	anhydrous, pure	Solvay
Methanol	MeOH	≥99.6%	Aldrich
HF solution in Methanol	HFMeOH	20.53 mol·L−1	-

Synthesis of HF_MEOH: The alcoholic HF solution was prepared according to our previous report by dissolving HF in methanol [4].

Synthesis of KMgF₃ (coating sol): 100 mL MeOH, 2.99 mL HF_MeOH (60 mmol), and 0.1 mL nitric acid (2 mmol) were added in a 250 mL PP bottle. In a schlenk flask, 1.40 g KOMe (20 mmol) and 2.29 g Mg(OEt)₂ (20 mmol) were dissolved in 90 mL MeOH. The grey solution of the potassium and magnesium precursors were transferred dropwise into the HF/HNO₃ solution under heavy stirring. After one day, the grey dispersion turned into a milky sol and 0.3 mL TMOS (2 mmol) were added. After 7 days of stirring, a clear, colorless sol was obtained.

Synthesis of NaMgF₃: 100 mL MeOH, 2.99 mL HF_MeOH (60 mmol), and 0.1 mL nitric acid (2 mmol) were added in a 250 mL PP bottle. In a schlenk flask, 0.46 g Na (20 mmol) and 2.29 g Mg(OEt)₂ (20 mmol) were dissolved in 90 mL MeOH. The grey solution of the sodium and magnesium precursors were transferred dropwise into the HF/HNO₃ solution under heavy stirring. After one day, an additive (

Table 1. Amounts of Additives in [K_(1−x)Na_x]MgF₃ syntheses.

Additive	nAdditive [mmol]	vAdditive [mL]
TFA	2	0.15
Al(OsBu)3	2	0.50
Zr(OiPr)4	2	0.62
TMOS	2	0.30

TFA: Trifluoro acetic acid, TMOS: Tetramethoxysilane. s: sec-, i: iso-.

) was added. After 7 days of stirring, a grey opaque dispersion was obtained.

Xerogel: 40 mL of NaMgF₃ sol were put into a glass flask. The solvent was removed under vacuum and the obtained powder was divided into three parts, whereas two were thermally treated at different temperatures.

Synthesis of [K_1−xNa_x]MgF₃: 100 mL MeOH, 2.99 mL HF_MeOH (60 mmol), and 0.1 mL nitric acid (2 mmol) were added in a 250 mL PP bottle. In a schlenk flask, Na (

Table 2. Amounts of reactants in [K_(1−x)Na_x]MgF₃ syntheses.

Compound	nNa [mmol]	mNa [g]	nKOMe [mmol]	mKOMe [g]
[K0.2Na0.8]F3	16	0.37	4	0.28
[K0.4Na0.6]F3	12	0.28	8	0.56
[K0.6Na0.4]F3	8	0.18	12	0.84
[K0.8Na0.2]F3	4	0.09	16	1.12

), KOMe (Table 2) and 2.29 g Mg(OEt)₂ (20 mmol) were dissolved in 90 mL MeOH. The grey solution of the sodium/potassium and magnesium precursors were transferred dropwise into the HF/HNO₃ solution under heavy stirring. After one day, an additive (Table 1) was added. After 7 days of stirring, a grey opaque dispersion was obtained.

Xerogel: 40 mL of [K_1−xNa_x]MgF₃ sol were put into a glass flask. The solvent was removed under vacuum and the obtained powder was divided into three parts, whereas two were thermally treated at different temperatures.

3. Results and Discussion

[K_1−xNa_x]MgF₃ phases of varying K to Na ratios were synthesized according the following general reaction stoichiometry (Scheme 1).

In addition to the reaction presented in the scheme above, Mg(OEt)₂ was partly replaced by MgCl₂ (10–30%). In analogy to our previous report, the idea was to generate a catalytic cycle in which HCl, formed by the reaction of MgCl₂ with HF, would increase the reaction rate significantly due to the higher acidity of HCl as comapred to HF [4]. Unfortunately, the usage of MgCl₂ within this reaction leads to the formation of [K/Na]Cl, and hence, is not recommended for this reaction system. Thus, we tried to create a new catalytic cycle based on nitric acid: comparing the acidity of nitric acid sand hydrogen fluoride, nitric acid is a better proton donator towards metal–oxygen bonds than HF. Fortunately, metal nitrates react subsequently with HF forming [K_1−xNa_x]MgF₃ mixed phases without any side products.

With the focus on easily access clear sols, which indicates the formation of mainly homodispersed nanoparticles inside the sol, we investigated the influence of temperature on clearing up rate during KMgF₃ syntheses (

Table 3. Parameters of KMgF₃ syntheses.

#	Concentration [mol/L]	Temperature [°C]	Additive	Appearance
1	0.2	23	TFA	opaque
2			Al(OsBu)3	opaque
3			Zr(OiPr)4	opaque
4			TMOS	slightly opaque
5		40	TFA	opaque
6			Al(OsBu)3	opaque
7			Zr(OiPr)4	opaque
8			TMOS	opaque
9		0	TFA	slightly milky
10			Al(OsBu)3	opaque
11			Zr(OiPr)4	opaque
12			TMOS	clear

TFA: Trifluoroacetic acid, TMOS: Tetramethoxysilane. s: sec-, i: iso-.

). Thus, it turns out that clear sols can only be obtained by performing the synthesis under cooling. To ensure a long-time stability of the obtained sols, different additives were tested in order to avoid particle agglomeration and water induced gelation. Moreover, the addition of zeta-potential affecting electrolytes allows a faster clearing up of the KMgF₃-sols. Especially tetramethoxysilane (TMOS) and trifluoroacetic acid (TFA) turned out to cause a fast clearing up of the reaction systems resulting in very fast formation of clear sols. Comparing the visual appearance of different samples, the influence of TMOS and TFA is conspicuous (Figure 1). Based on dynamic light scattering (DLS) investigations, a mean particle size diameter of d = 69 nm within sol 12 was determined (Figure 2). It can be seen that the majority of the particles exhibit diameters less than 50 nm. Moreover, the correlation curve appears to be sigmoidal, which basically indicates a monodispers contribution.

To understand the action of the sol-stabilizing additives TMOS and TFA more precisely, IR-measurements of xerogel 9 and 12 (obtained from sol 9 and 12) were performed (Figure 3). Comparing the IR spectra of obtained xerogels to pure additives, it can be seen that xerogel 9 shows similar stretching bands to pure TFA. Especially the shifting of the C=O band from1760 cm⁻¹ (purple) to 1685 cm⁻¹ (orange) supports the supposition of a coordinative bonding between the Lewis acidic sites of the nanoparticle and the trifluoro acetic acid molecules. Thus, electrons of the C=O HOMO drain off, leading to a weaker bonding and a red-shifted band within the IR spectrum. In contrast to this, no traces of TMOS within xerogel 12 were found, which may be taken as an indication for the zeta-potential affecting properties of TMOS.

To receive structural information, obtained X-ray amorphous xerogel 12 was thermally treated at 450 °C to increase crystallinity and to allow analysis by XRD. The ¹⁹F MAS NMR spectra of non-annealed sample of xerogel 12 show the main signal of KMgF₃ at −184.6 ppm (Figure 4) [12]. Moreover, a small signal for MgF₂ can be found at −198.2 ppm [4]. Thermally induced changes in the structure were not confirmed by comparing ¹⁹F MAS NMR spectra, but by XRD. While the non-annealed perovskite-type mixed phases of [K_1−xNa_x]MgF₃. xerogel is totally amorphous, the annealed xerogel shows good agreement with the powder diffraction file of KMgF₃ (Figure 5, pdf: 18-1033, star). Hence, just amorphous KMgF₃ can evidently be obtained not only via hydrothermal synthesis as described in literature but also via the fluorolytic sol-gel synthesis very easily under ambient conditions [9].

Based on the results described above, we tried to obtain pure NaMgF₃ and [K_1−xNa_x]MgF₃ mixed phases the same way by cooling down the reaction mixtures and adding TMOS subsequently. Unfortunately, all sols obtained this way were not fully clear but opaque (

Table 4. Parameters of NaMgF₃ and [K_(1−x)Na_x]MgF₃ syntheses.

#	Compound	Concentration [mol/L]	Temperature [°C]	Additive	Appearance
13	NaMgF3	0.2	0	TMOS	Opaque
14	[K0.2Na0.8]F3				Opaque
15	[K0.4Na0.6]F3				Opaque
16	[K0.6Na0.4]F3				Opaque
17	[K0.8Na0.2]F3				Opaque

TMOS: Tetramethoxysilane.

).

To receive structural information of obtained [K_(1−x)Na_x]MgF₃ mixed solid phases, ¹⁹F MAS NMR of annealed xerogels 9, 13–17 were compared (Figure 6). In line with literature reports, the replacement of K-cations by Na-cations can be confirmed [12]. Up to an amount of 20% of Na-doping, the cation mixed perovskite phases remain in a KMgF₃ structural environment; the formation of a separate NaMgF₃ phase can be excluded. As already mentioned before, ¹⁹F MAS NMR of xerogel 9 shows the main signal of KMgF₃ at −184.6 ppm. With increasing Na⁺ doping concentration, the spectra show a high field shifted widening. At a Na⁺ doping level of 40% (xerogel 16) a signal at −202.9 ppm appears, pointing out the formation of [Na₄F] moieties. While this signal increases with higher amounts of Na⁺ within the samples (xerogel 15 and 14), the corresponding KMgF₃ signal decreases until complete disappearance (xerogel 13). With the help of the dmfit program, separate signals in the range of −180 ppm and −200 ppm can be determined and allow the assignment of different fluoride ion environments within a cubic KMgF₃ structure (Figure 7) [16]. The dmfit simulation shows four almost equidistant (Δδ_ave = 4.3 ppm) signals with different intensities at invariable peak-widths (

Table 5. ¹⁹F MAS NMR signals of xerogel 17 by dmfit simulation [16].

n(Na+)	δ (19F)exp. [ppm]	Δδexp. [ppm]	Proportionexp. [%]	Proportioncalc. [%]	nexp(K+)	ncalc. (K+)
0	−184.6	3.8 (0,1)	64.8	41.0	2.59	1.64
1	−188.4	4.6 (1,2)	24.4	41.0	0.73	1.23
2	−193.0	4.5 (2,3)	7.2	15.4	0.14	0.31
3	−197.5	-	3.6	2.6	0.04	0.03
4	-	-	0	0.2	0	0
				ntotal(K+)	3.50	3.21
				ntotal(K+)/4	0.88	0.80

). The equidistant shifts to a higher field can be explained by a step-by-step substitution of K⁺ by Na⁺. Although it can be assumed that a statistic substitution occurs, the experimentally determined proportion for end-member environments [K₄F] and [Na₄F] is favored, and therefore, results given in the literature can be confirmed.

This cation-replacement can also be evidenced by XRD measurements complying with the law of Vegard (Figure 8) [17]. With increasing amounts of the smaller Na-cations (r_Na⁺ = 153 pm, r_K⁺ = 178 pm) [18], the cell volume decreases leading to right-side shifting of peaks in the diffractograms. In comparison to the ¹⁹F MAS NMR spectra, a pure NaMgF₃ phase cannot be found at x = 0.4. This may confirm the formation of [Na₄F] moieties within a [K_(1−x)Na_x]MgF₃ solid solution, but does not indicate the formation of pure a NaMgF₃ phase. The varying numbers for calculated and experimentally determined n_total(K⁺) are in line with this assumption (Table 5). Even at Na-doping level of 60%, reflections of a NaMgF₃ phase are difficult to identify, whereas reflections of both compounds show up at Na-levels ≥80% [K_0.2Na_0.8]MgF₃.

Concerning the fact that sol 12 represents a water clear colorless sol, it was used for dip coating on glass-slides. After dip coating, glass slides were thermally treated at 450 °C for 15 min. The obtained thin films show nearly perfect antireflective behavior with a remaining reflectance of just R = 0.15% and transmission as high as T = 96.1% (Figure 9,

Table 6. Overview of layer properties.

Layer	T [°C]	t [min]	dT [°C/min]	d [nm]	R [%]	T [%]	n632,8nm
1	450	15	10	116.7	0.15	96.1	1.27

).

4. Summary

Employing the room-temperature fluorolytic sol-gel synthesis, the formation of KMgF₃, NaMgF₃, and [K_1-xNa_x]MgF₃ mixed phases was evidenced. However, only with pure KMgF₃ colorless and water clear sols were they obtained when the synthesis was performed under ice bath cooling and with the addition of TMOS as a stabilizer. With the help of ¹⁹F MAS NMR and XRD, we obtained evidence for the consecutive occupancy of potassium by sodium sites within a cubic KMgF₃ structure. If the Na dopant concentration exceeds 20% in relation to K, the formation of [Na₄F] environments takes place and can be observed in ¹⁹F MAS NMR. In addition, the substitution does not take place statistically, instead, end-membered environments are favored. The appearance of a pure NaMgF₃ phase within samples with less than 80% Na⁺ can be denied as evidenced by XRD measurements. Thin films, which were obtained by dip coating with clear KMgF₃ sols showed very low remaining reflectance and high transmission.