1D Ceric Hydrogen Phosphate Aerogels: Noncarbonaceous Ultraflyweight Monolithic Aerogels.

Ceric hydrogen phosphate gels possess a very unique spatial organization, being nearly amorphous materials with a fibrous structure. Using a sol-gel approach, we succeeded in preparing bulky gels containing as much as 20,000 molecules of water per cerium atom. Supercritical treatment of these gels made it possible to obtain the first ultralight monolithic noncarbonaceous aerogels with a density as low as 1 mg/cm3.

Introduction

Aerogels are unique materials possessing a specific spatial structure, which gives rise to a high specific surface area, ultrahigh porosity, adjustable density ranges, low thermal conductivity and permittivity, low speed of sound propagation, and so forth.[1] To date, many aerogel materials have been obtained,[2] with the greatest attention in recent years focused on ultralight carbonaceous aerogels (density 1–10 mg/cm3), primarily graphene or graphene oxide-based aerogels, as well as ultralight renewable and biodegradable aerogel materials obtained from plant cell biomass.[3−10] Also, single representatives of extremely light carbonaceous aerogels have been reported with a density of less than 1 mg/cm3.[11−13]

Inorganic aerogels usually possess a higher density than carbonaceous aerogels. Typical examples are silica aerogels[14] and aerogels based on other metal oxides (Al2O3, SnO2, TiO2, etc.).[15−19] In most cases, the structure of inorganic aerogels obtained using template-free approaches (e.g., using a conventional sol–gel method) represents a three-dimensional open network assembled using metal oxide NPs.

In the past few years, a brand new family of ultralight noncarbonaceous inorganic aerogels, namely, 1D aerogels, has been discovered. Current examples include nanowire aerogels based on cryptomelane manganese oxide (K2–Mn8O16) and TiO2,[20] hydroxyapatite,[21] individual metals,[22,23] SiC,[24] and others with densities of less than 10 mg/cm3. Recent advances in engineering 1D aerogel materials and their specific properties are extensively discussed elsewhere.[25−27] Often, electrospinning,[28] CVD, or template-assisted approach methods are used to achieve ultralow densities of inorganic noncarbonaceous materials, but these methods are laborious and poorly scalable.[29,30] Recently, we have succeeded in the synthesis of the first 1D inorganic phosphate aerogels, namely, ceric hydrogen phosphate aerogels, by direct sol–gel synthesis, which involves mixing of a cerium-containing hydrogen phosphate solution with water followed by supercritical drying of wet gels.[31] The resulting monolithic aerogels had a relatively low density of 10 mg/cm3.

In our opinion, 1D ceric phosphate aerogels possess a significant potential for further density reduction by engineering their structures. We assume that we can obtain much looser structures by changing the packing density of ceric hydrogen phosphate fibers. Such an effect could be achieved by increasing the gelator volume, which was successfully demonstrated by Mondal et al.[32] using the example of AgVO3 gels. However, in the Ce+4–H3PO4 system, the addition of extra water as a gelator results in fast precipitation instead of gelation, which prevents the production of monolithic aerogels.

In this study, we have further analyzed the conditions for the formation of ultralight monolithic ceric hydrogen phosphate lyogels by extending the range of the inorganic gelators used. As a result, our one-pot, template-free approach has provided additional opportunities for the design of the ceric hydrogen phosphate aerogel structure and allowed us to obtain the first monolithic all-inorganic aerogel with a density as low as 1 mg/cm3.

Results and Discussion

Determination of the Conditions for the Formation of Monolithic Cerium-Containing Phosphate Gels

Earlier, we showed[31] that monolithic cerium-containing phosphate gels (CePg) were formed by the addition of distilled water to a cerium-containing phosphate solution (CePs, [Ce] = 0.1 M) at CePs: H2O volume ratios of 1:2–1:10; however, they are destroyed during ageing if the volume ratio of CePs: H2O is higher than 1:6. Supercritical drying of the gel obtained at CePs: H2O volume ratio = 1:4 afforded an aerogel with a geometric density of about 10 mg/cm3.[31] Interestingly, the formation of gels in the Ce4+–PO43––H2O system was first reported more than 50 years ago (see ref[33−35]); however, the chemical composition of these gels and the mechanism of their formation are still unclear. König and Meyn[36] suggested that a compound of the composition (Ce–O–Ce)(HPO4)3·H2O precipitates from a concentrated phosphoric acid solution of Ce4+ upon dilution with water, postulating the presence of (Ce–O–Ce)6+ ions in the solid phase that also exist in solution. Brandel et al.[37] represented the (Ce–O–Ce)(HPO4)3·H2O formula as Ce2(PO4)2HPO4·2H2O. The paper by Lebedev and Kulyako provides a comprehensive analysis of ceric and cerous phosphate complexes in concentrated orthophosphoric acid. They demonstrated that the Ce4+ ion in concentrated H3PO4 is strongly coordinated with phosphate ligands, mostly H2PO4–.[38] In the solid state, the strong coordination of Ce(III) and Ce(IV) with phosphate ligands ensures high stability of well-known cerium(III) phosphates (monazite CePO4 and rhabdophane CePO4·xH2O)[39−41] and a series of recently synthesized cerium(IV) phosphates (Ce2(PO4)2HPO4·H2O, Ce(PO4)1.5(H2O)(H3O)0.5(H2O)0.5, (NH4)2Ce(PO4)2·H2O, K2Ce(PO4)2, Na10Ce2(PO4)6, K4CeZr(PO4)4, and (NH4)[CeF2(PO4)]).[42−49]

To understand the mechanism of ceric phosphate gel formation, it is reasonable to take into account similar studies on thorium phosphate gels, since Ce(IV) is a chemical analog of Th(IV)[50−52] due to the proximity of their ionic radii (for CN = 8, the ionic radii are 97 and 105 pm for Ce(IV) and Th(IV), respectively[53]). Parmar et al.[54] conducted a detailed study of the gels obtained by mixing solutions of thorium nitrate with dilute phosphoric acid. They found that when these solutions are mixed, a thorium phosphate precipitate forms and nitric acid is released. After shaking the mixture, the precipitate was peptized. Upon ageing, this colloidal solution formed a firm transparent gel. Parmar et al. supposed that the formation of gels is associated with the tendency of thorium phosphate to hydrate and the tendency of resulting colloidal particles to agglomerate.

Herein, we have suggested that upon the addition of water, the P–O–H groups in ceric phosphate complexes dissociate, forming both free H3O+ and negatively charged ceric hydrogen phosphate moieties. Being a nucleophile, the latter can react with neutral complexes, forming Ce–[PO4]–Ce bridges. Obviously, the formation of any bridging structures is the first stage in the growth of the gel network. When water is added to ceric phosphate solutions, the formation of a gel network occurs almost instantaneously, which is most likely due to a sharp increase in the number of dissociated P–O–H groups, because of the increase in pH. We assume that the use of inorganic acid solutions instead of water would affect the acidity of the solution, reducing the concentration of nucleophilic ceric hydrogen phosphate anions in the reaction media; therefore, gel formation will occur more slowly and in a controlled manner. In a similar way, according to the early observations of Parmar et al.,[54] the addition of mineral acids to a solution of thorium phosphate at certain concentrations slowed down gel setting, which is likely to take place for cerium(IV) phosphates as well. Our preliminary studies showed that mixing concentrated orthophosphoric acid with aqueous solutions of inorganic acids (2–3 M concentration) taken in the range from a single to a 40-fold volume excess reduces the pH value by 0.1–1.5, compared to diluting concentrated H3PO4 with water in the same volumetric ratios.

To check whether the use of mineral acids instead of water will actually have an impact on the gelation process, a series of cerium-containing phosphate solutions was prepared with [Ce] = 0.01, 0.05, 0.1, 0.2, 0.4, 0.6, and 0.8 M (the highest possible concentration in accordance with our experimental data). Distilled water, or 3 M aqueous solutions of nitric/phosphoric acid, or a 2 M aqueous solution of sulfuric acid was added to cerium-containing phosphoric acid solutions; preliminary experiments showed that when more concentrated sulfuric acid was used, gelation did not take place for a long period of time.

Predictably, gelation upon the addition of acid solutions was slower than in the case of water-induced gelation. After the addition of nitric or phosphoric acid solutions, the gel network typically began to form during the first 10 min, and this time decreased with an increase in the concentration of CePs, with the gel strengthening process predominantly taking place within a day. When a solution of sulfuric acid was used, only the initial gel network formation took several hours.

We also used different volumes of the gelator to find the minimum and maximum ratios of CePs to the gelator required for the formation of a monolithic gel (Figure ). The volume ratio of solutions varied from 1:0.5 to 1:100.

Figure 1

Appearance of products obtained by mixing 0.6 M cerium-containing phosphate solution with 3 M HNO3 solution in various volume ratios; 5 min after mixing (above) and 1 day after mixing (below).

When 3 M H3PO4 or 3 M HNO3 was used, monolithic gels were formed in a wide range of concentrations of the initial solutions and CePs: acid solution volume ratios (up to 1:50 and 1:40, respectively) (Figure a,b). Note that the gelation time differed drastically when 3 M H3PO4 or 3 M HNO3 was added to the ceric phosphate solution. In particular, upon the addition of 3 M HNO3 (CePs:acid solution = 1:8 v/v), gel formation proceeded in approximately 30 min, while upon the addition of 3 M H3PO4 (with the same volume ratio), the gel was formed in approximately 1 min.

Figure 2

Conditions for the formation of monolithic cerium-containing phosphate gels depending on the molar ratios of cerium and phosphate in the final mixture. Solutions for gelation: (a) 3 M H3PO4, (b) 3 M HNO3, (c) H2O, and (d) 2 M H2SO4. Red dashed line indicates the highest possible cerium concentration in the reaction mixture.

When distilled water was added to CePs, a monolithic gel was obtained only with volume ratios CePs: distilled water = 1:3–1:8, regardless of the CePs concentration (Figure c). When 2 M H2SO4 solution was added to CePs, a monolithic gel was formed only in a narrow concentration range of the initial cerium-containing phosphate solution (Figure d). Most likely, the differences in the gelation process when using HNO3 and H2SO4 can be explained by the fact that nitrate ions are weak complexing agents,[55] while sulfate ions can compete with phosphate ions in the formation of complexes with cerium.[56,57]

Thus, it was shown that under the same conditions, using the same initial concentration of CePs but different solutions for gelation, monolithic gels of different volumes are formed. This, in turn, directly determines the density of the aerogels synthesized from the wet gels by supercritical drying. Since the use of H3PO4 and HNO3 solutions made it possible to obtain monolithic gels of the largest volume relative to the volume of CePs used, we chose these acids for the subsequent synthesis of ultralight aerogels.

Production of Ceric Hydrogen Phosphate Aerogels

Monolithic gels, obtained by mixing CePs ([Ce] = 0.1 M) with a 3 M aqueous solution of HNO3 in a volume ratio of CePs: acid solution = 1:8 and 1:20, or with a 3 M aqueous solution of H3PO4 in a volume ratio of CePs: acid solution = 1:8, 1:20, and 1:50, were used to prepare a series of aerogels. The gels were aged for 2 days and then transferred into a container with acetonitrile to replace the solvent. Acetonitrile was replaced daily for 1 week. The samples were then supercritically dried in CO2. The labeling of the obtained aerogels is presented in Table .

Table 1

Synthesis Parameters and Labeling of the Samples

labeling	CePN_8	CePP_8	CePN_20	CePP_20	CePP_50
acid	HNO3	H3PO4	HNO3	H3PO4	H3PO4
volume ratio CePs: acid solution	1:8	1:8	1:20	1:20	1:50

Supercritical drying in CO2 successfully afforded the monolithic aerogels (Figure ). The aerogels demonstrated no significant shrinkage, proving the strength of the gel network.

Figure 3

Appearance of aerogels (a) CePN_8, (b) CePN_20, (c) CePP_8, (d) CePP_20, and (e) CePP_50.

The geometric density of CePN_8 and CePP_8 aerogels (calculated as the ratio of aerogel mass to volume) amounted to about 5.5 mg/cm3, the density of CePN_20 and CePP_20 aerogels was about 2.2 mg/cm3, and the density of the CePP_50 aerogel was found to be only 1 mg/cm3, which corresponds to the density of air under ambient conditions (1.2 mg/cm3). The porosity of the aerogels, calculated by the previously published method,[58] for all samples exceeded 99%. The mechanical strength of the samples was sufficient enough to preserve their shape for at least several months and to handle them without special caution.

According to the X-ray diffraction (XRD) data, the aerogels had a nearly amorphous structure, with the most intense peak at 7.8°2θ corresponding to an interplanar distance of 1.1 nm (Figure S1). In a number of previous works, similar diffraction patterns have been observed for poorly crystalline materials based on fibrous cerium(IV) phosphates;[59−64] the presence of a broad peak at low 2θ angles was assigned to the layered structure of the materials. A detailed analysis of the XRD data (Figure S1) showed the presence of low intensity peaks that can be attributed to a rhabdophane structure (CePO4·xH2O). The estimation of rhabdophane content (using the integral intensities of rhabdophane and ceric phosphate gel diffraction patterns) resulted in <5% rhabdophane content for all the aerogel samples. Most probably, a minor admixture of rhabdophane was formed due to the partial reduction of Ce(IV) by acetonitrile at the solvent exchange stage of aerogel preparation. This assumption is confirmed by X-ray diffraction patterns of ceric phosphate gels washed with distilled water (instead of acetonitrile) and dried under ambient conditions (Figure S1), where no rhabdophane peaks are present.

The IR spectra for all aerogels were identical and were typical for rare earth phosphates[65−68] (Figure S2). The bands in the region of 1120–990 cm–1 correspond to the asymmetric stretching ν3 vibrations, and the bands in the region of 980–900 cm–1 correspond to the symmetrical stretching ν1 vibrations of P–O bonds in the phosphate groups. The splitting of these bands suggests that the PO4-groups in the structure of aerogels were directly coordinated with cerium ions.[69] The region of 650–440 cm–1 in the IR spectra correspond to the deformation vibrations δ(O–P–O), and the band in the region of 415 cm–1 was assigned to the Ce–O stretching vibrations.[43,69,70]

Absorption bands in the region of 3500(br.) cm–1 and at 1630(s.) cm–1 characterized the stretching vibrations of OH groups and the deformation vibrations of H–O–H in water molecules, respectively.[37] The band at 2390 cm–1 was assigned to CO2 vibrations.[71] The band at 1225 cm–1 most likely referred to the vibrations of P–O–H.[43]

According to EDX analysis, the average Ce:P molar ratio for all of the studied aerogels amounted to ∼1:2. Aerogels consisted of spontaneously oriented, interweaving nanofibers with an average diameter of about 40 nm (Figure ).

Figure 4

Scanning electron microscopy (SEM) data of aerogels: (a) CePN_8, (b) CePN_20, (c) CePP_8, (d) CePP_20, and (e) CePP_50; (f) size distribution of the aerogel fiber width calculated from the SEM data. Solid lines correspond to the results of data fitting using a log-normal distribution function. The mean value and standard deviation were 37.9 and 19.5 nm, 42.7 and 22.1 nm, 32.8 and 10.5 nm, 38.2 and 17.4 nm, and 35.0 and 10.4 nm for CePN_8, CePN_20, CePP_8, CePP_20, and CePP_50, respectively.

According to transmission electron microscopy (TEM) data, the aerogels were composed of fibers of various diameters, up to 5 nm (Figure S3). The observed microstructure is characteristic to 1D inorganic aerogels for which, however, the fiber diameter and uniformity, depending on the type of material and method of preparation, can vary greatly. In particular, Jung et al.,[20] by varying the precursor composition, obtained aerogels consisting of TiO2 nanofibers with either a 5–10 or 50–60 nm diameter, which, however, did not directly affect the density of the final material. Interestingly, there are reported examples of 1D aerogels with an average fiber diameter exceeding 200 nm and, at the same time, with an ultralow density of up to 0.15 mg/cm3.[28]

Figure S4 shows the full nitrogen adsorption–desorption isotherms for CePP_8, CePN_8, and CePN_20 samples. They are characterized by capillary condensation hysteresis and, according to the IUPAC classification, belong to type IV. The hysteresis corresponds to the H3 type, which may indicate the presence of slitlike pores.[72] A sharp bend of the adsorption curve at a partial pressure of about 1 is most likely due to the presence of macropores. The specific surface values for all samples differed slightly and amounted to about 60 m2/g (Table ). An increase in the volume ratio of CePs: acid solution from 1:8 to 1:20 during the synthesis of lyogels led to a significant increase in the specific pore volume in the obtained aerogels, as evidenced by an increase in the absolute values of adsorption in the region of high partial pressures and the value of hysteresis. Small values of the Brunauer–Emmett–Teller (BET) constant indicate the absence of micropores[58] (Table ).

Table 2

Microstructure Characteristics of the Samples as Derived from the Analysis of the Complete Nitrogen Adsorption–Desorption Isotherms Using the BET and Barrett–Joyner–Halenda (BJH) Models

labeling	SBET, m2/g	C constant (BET)	pore volume (BJH, desorption curve), cm3/g	pore diameter (BJH, desorption curve), nm
CePP_8	63	18	0.21	1.7
CePN_8	55	24	0.22	2.0
CePN_20	64	19	1.75	1.7

The structure of aerogels was further analyzed by small-angle neutron scattering. The experimental dependences of the macroscopic differential cross-section dΣ(q)/dΩ of small-angle neutron scattering on the momentum transfer are presented in Figure . The experimental dependence was described using a two-level scattering model including the Porod and Guinier regions:[73]

Figure 5

Small-angle neutron scattering (SANS) differential cross-section dΣ(q)/dΩ for the samples of ceric phosphate aerogels CePP_8 and CePN_8. Fitting of the experimental data is shown as solid red lines.

According to small-angle neutron scattering (SANS), the scattering curves for CePP_8 and CePN_8 aerogels contained two Guinier regions, characterized by gyration radii Rg and Rs, corresponding to the small and large values of the momentum transfer q. The approximation parameters are presented in Table . The SANS data indicated that the aggregates of the colloidal particles in the gels had a fractal or smooth surface.[74] Most likely, the gel formation mechanism does not involve mass-fractal aggregation of individual particles.

In the case of CePP_8 aerogel, the first level corresponding to the region of large values of q > 0.1 Å–1 was characterized by scattering centers with a gyration radius Rs = 1.2 nm, which formed aggregates with a gyration radius Rg = 33 nm (the region of small values of q < 0.02 Å–1). The fractal dimension of the surface Ds = 6 – P = 2.04 corresponds to a smooth surface.

The CePN_8 aerogel contained scattering centers characterized by gyration radii Rs = 9.6 nm and Rg = 73.8 nm. The fractal dimension of the aerogel surface amounted to Ds = 2.32 (Table ). The differences in the gyration radii of the primary scattering centers (which can be ascribed to the elementary building units of the gels) are in line with the general principles of the sol–gel chemistry. Gel formation includes competing processes of nucleation and growth of colloidal particles and their condensation. Rapid condensation of colloidal particles results in a gel constructed of very small elementary units (the so-called polymer gels). In the case of slow condensation, colloidal particles grow larger resulting in the so-called colloidal gels. For the CePN_8 sample, the gelation time was ∼30 times higher than that for the CePP_8 sample, so the gyration radius of the elementary units in the CePN_8 sample is consistently larger (> 9 nm) than that in the CePP_8 sample (∼1 nm).

Table 3

Approximation Parameters of Small-Angle Neutron Scattering Data for Ceric Phosphate Aerogels

parameter	CePP_8	CePN_8
gyration radius Rg, nm	33.0 ± 0.1	73.8 ± 0.7
fractal dimension Ds	2.04 ± 0.02	2.32 ± 0.02
gyration radius Rs, nm	1.2 ± 0.2	9.6 ± 0.3

Thus, SEM, TEM, BET/BJH, and SANS data together indicated that cerium-containing phosphate aerogels were composed of quasi-one-dimensional fibers. Their unique structure allowed for varying fiber packing density in a wide range by changing the interfiber distance. It is this feature that makes it possible to build ultralight, highly porous, monolithic materials.

Conclusions

Cerium-containing phosphate gels can be obtained using a one-pot, template-free method, without the use of any organic gelators due to the aggregation of flexible inorganic fibers into a strong network using a conventional sol–gel process. Optimal conditions were established for obtaining monolithic ceric hydrogen phosphate lyogels, depending on the type of the inorganic gelator and the ratio “CeP solution: gelator solution”. We have demonstrated that the CeP framework is capable of retaining a huge amount of liquid (in the wet gels, up to 20,000 water molecules per cerium atom) and serves as a perfect starting material for the production of ultralight aerogels. Supercritical drying of lyogels synthesized using 3 M aqueous solutions of HNO3 and H3PO4 results in the formation of monolithic cerium-containing phosphoric aerogels. The fibrous 1D structure of the obtained aerogels was confirmed by a set of analytical methods, which also revealed that the total pore volume in the final product increased several times with an increase in the ratio of ≪CeP solution: gelator solution≫. It was found that using 3 M orthophosphoric acid as the gelator with a ratio of CeP solution: 3 M H3PO4 solution = 1:50 at CeP concentration = 0.1 M makes it possible to obtain a purely inorganic ceric hydrogen phosphate aerogel with a density as low as 1 mg/cm3.

Experimental Section

The following materials were used as received, without further purification: Ce(NO3)3·6H2O (99%, Aldrich #238538), aqueous ammonia (25 wt %, extrapure grade, Khimmed, Russia), orthophosphoric acid (85 wt % aq, ρ = 1.689 g/cm3, analytical grade, Khimmed, Russia), nitric acid (68 wt % aq, ρ = 1.409 g/cm3, extrapure grade, Khimmed, Russia), sulfuric acid (92 wt % aq, ρ = 1.824 g/cm3, extrapure grade, Khimmed, Russia), and distilled or deionized (18 MΩ) water.

The synthesis of the initial cerium-containing phosphoric acid solutions with [Ce] = 0.01, 0.05, 0.1, 0.2, 0.4, 0.6, and 0.8 M was carried out according to a procedure reported earlier.[75] A sample of nanocrystalline (4–5 nm) cerium dioxide obtained by precipitation from Ce(NO3)3·6H2O[76] was dissolved in a concentrated orthophosphoric acid at 80 °C. The lyogels were obtained by adding gelators (distilled water or aqueous solutions of nitric (3 M), orthophosphoric (3 M), or sulfuric (2 M) acids) to a cooled, cerium-containing phosphoric acid solution.

Supercritical drying of lyogels was performed using a modified Waters RESS/SAS system. This system consisted of a high-pressure pump, an electrical preheater, a 500 mL high-pressure vessel equipped with a basket and an electrical heater, an automatic back pressure regulator (ABPR) and a cyclone gas–liquid separator. The drying procedure was as follows. Approximately 50 mL of methanol was poured into the basket of the extraction vessel. A wet gel was gently placed into this methanol layer. The vessel was sealed and heated to 40 °C and then filled with CO2. At the initial stage, the ABPR was closed; when the working pressure reached the desired value (150 bar), the back pressure regulator was opened and a steady flow regime was established. The CO2 flow rate was 5 g/min. The gel was flushed with CO2 for 8 h. After that, the ABPR was closed and its needle was manually set to a position allowing a low depressurization rate. Slow depressurization took about 6 h.

Powder X-ray diffraction patterns were recorded with a Bruker D8 Advance diffractometer using Cu Kα1,2 radiation in the 2θ range of 3–60°, at a 2θ step of 0.02°, and a counting time of 0.3 s per step.

The FTIR spectra of the samples were recorded with a Bruker ALPHA spectrometer, in a range of 400–4000 cm–1, in an attenuated total reflectance mode.

The microstructure (determined by SEM) and the chemical composition (determined by energy dispersive X-ray (EDX) analysis) of the samples were analyzed with a Carl Zeiss NVision 40 high-resolution scanning electron microscope equipped with an Oxford Instruments X-MAX (80 mm2) detector, operating at an accelerating voltage of 1–20 kV. SEM images were recorded using an Everhart–Thornley detector (SE2) at 2 kV accelerating voltage.

The structure of the samples was studied by means of TEM with a Leo912 AB Omega analytical transmission electron microscope. TEM images were recorded at an accelerating voltage of 100 kV in a bright-field mode.

The specific surface area of the aerogels was measured using the low-temperature nitrogen adsorption method with a QuantaChrome Nova 4200B analyzer. The samples were degassed at 80 °C in a vacuum for 16 h prior to analysis. Based on the data obtained, the specific surface area of the samples was calculated using the Brunauer–Emmett–Teller (BET) model. The calculation of the pore size distribution was carried out on the basis of nitrogen desorption isotherms according to the Barrett–Joyner–Halenda (BJH) method.

SANS was analyzed using a PAXY beamline (Laboratoire Leon Brillouin, CEA-CNRS, Saclay, France). The measurements were made at two neutron wavelengths, λ = 8.5 and 5 Å, with three samples of detector distances 1, 5, and 5 m, which made it possible to measure neutron scattering intensity within the momentum transfer range 1.5·10–3 < q < 3·10–1 Å–1. The scattered neutrons were detected with a two-dimensional position-sensitive BF3 detector. The acquired two-dimensional isotropic spectra were azimuthally averaged and preprocessed using PASiNET software.[77] All the measurements were made at room temperature.