Hydrothermal synthesis, structural and thermal characterizations of three open-framework gallium phosphites

Journal of Solid State Chemistry 255 (2017) 8–12

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Hydrothermal synthesis, structural and thermal characterizations of three open-framework gallium phosphites

MARK

Farida Hamchaouia, Véronique Alonzob, Isabelle Marlartb,1, Sandy Augustec, Cyrille Galvenc, ⁎ Houria Rebbaha, Eric Le Furb, a Laboratoire des Sciences des Matériaux, Faculté de Chimie, Université des Sciences et de la Technologie Houari Boumediène, BP 32 El-Alia, 16111 BabEzzouar, Alger, Algeria b Institut des Sciences chimiques de Rennes, UMR6226, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, Avenue de Beaulieu, CS 50837 Rennes Cedex 7, France c Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Université du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France

A R T I C L E I N F O

A BS T RAC T

Keywords: Hydrothermal synthesis Crystal structure Gallium phosphite

Three new gallium phosphites A[Ga(HPO3)2], where A = K (1), NH4 (2), Rb (3), have been synthesized by using mild hydrothermal conditions under autogeneous pressure. Their structures have been determined by singlecrystal X-ray diffraction. These compounds crystallize in the hexagonal P63mc space group with a = 5.2567 (2) Å and c = 12.2582 (3) Å for 1, a = 5.2576 (2) Å and c = 12.9113 (4) Å for 2, a = 5.27020 (10) Å and c = 12.7619 (5) Å for 3, with Z = 2 in the three phases. The three compounds are isostructural and exhibit the same framework topology, consisting of a layered structure stacked along the c-axis with the A+ cations located in the interlayer spaces. The [Ga(HPO3)2]- sheets contain GaO6 octahedra interconnected by phosphite units through sharing vertices. Thermal analysis under air atmosphere shows a large range stability for alkali cations containing compounds with decomposition starting around 750 K leading to phosphate phases. Under nitrogen, a disproportionation of the phosphite into red phosphorus and phosphates is expected, accompanied by a release of H2.

1. Introduction Since the discovery of the microporous crystalline aluminophosphates in the 1980s [1], considerable efforts have been directed towards the synthesis of new open-framework metal phosphates due to their wide structural chemistry and potential applications in the fields of catalysis, biology, optical and magnetism [2]. Among metal phosphates, gallium phosphates constitute an important family that exhibits a rich structural and compositional diversity which is due to the flexible structural coordination of metal Ga as GaO4 tetrahedra, GaO5 trigonal bipyramids and GaO6 octahedra. During the last decades, a large number of gallium phosphates with 1-dimensional (1D) chains, 2D layers, and 3D open-frameworks have been reported in the literature [2,3 and references therein]. More recently, it became apparent that the (HPO3)2- pseudo tetrahedral species can be incorporated in place of the traditional tetrahedral phosphate group (PO4)3- as a basic building unit to construct open structures. Both oxoanion units have similarities but

⁎

compared to the phosphate group, the hydrogenphosphite group has fewer available coordination centers and low-average charges per oxygen, which can lead to the formation of more open interrupted or intriguing frameworks with unusual metal/phosphorus ratios. In contrast to the large family of gallium phosphates, the gallium phosphites are very less developed and only few examples of pure gallium phosphites have been synthesized and characterized [4–14]. With the aim of searching novel metal phosphites, we conducted our study on the hydrothermal synthesis in alkali-metal/ammonium ion – gallium – phosphite acid systems. During our investigations, we obtained three novel pure gallium phosphites namely A[Ga(HPO3)2] (A = K, NH4, Rb). It is worth mentioning that these materials are isostructural with the A[M(HPO3)2] (A = K, NH4, Rb and M = V, Fe, In) phases reported in the literature [14–17]. In order to improve the understanding about this type of microporous materials, we report here the syntheses, single-crystal structure determinations and thermal studies of the isostructural series obtained with gallium.

Corresponding author. E-mail address: [email protected] (E. Le Fur). Current address: Laboratoire Matière Molle et Chimie, UMR7167 CNRS-ESPCI Paris, Ecole Supérieure de Physique et de Chimie Industrielles de la Ville de Paris, PSL Research University, 10 rue Vauquelin, 75005 Paris, France. 1

http://dx.doi.org/10.1016/j.jssc.2017.07.030 Received 24 April 2017; Received in revised form 21 July 2017; Accepted 26 July 2017 Available online 27 July 2017 0022-4596/ © 2017 Elsevier Inc. All rights reserved.

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The powder X-ray diffraction patterns for 1–3 were consistent with the simulated ones on the basis of their single-crystal structures, as shown in Fig. S1. The diffraction peaks on both patterns correspond well in position, indicating the phase purity of the as-synthesized products.

2. Experimental 2.1. Synthesis The title compounds were synthesized by using mild hydrothermal conditions under autogeneous pressure. The pure phases of these materials can be prepared from a reaction mixture of 0.4 g Ga2(SO4)3 (Fluka, 99.99%), 1.2 g H3PO3 (Aldrich, 99%), 0.8 g (NH4)2CO3 (Fluka, 30–33% of NH3) or 1.1 g K2CO3 ( Merck, 99%) or 1.8 g Rb2CO3 (Aldrich, 99%) and 5 mL deionized water (molar ratio of 0.1:1.5:0.8:28 for the starting reagents) heated at 453 K for 3 days and then cooled to room temperature. In all the cases, colorless resulting products consisting of well formed medium crystals were recovered by vacuum filtration, washed with deionized water and dried in a desiccator. The yields of the reaction were approximately 67% in weight based on gallium for the three phases.

2.3. Thermal behavior Coupled thermogravimetric and DSC measurements were performed with a TA instruments SDT Q600 thermobalance under both pure nitrogen and air from room temperature to 973 K with a heating rate of 5 K/min. Thermo Gravimetric Analysis-Mass Spectroscopy (TGA-MS) was performed using a Netzch STA 449 F3 coupled with a QMS 403 C mass spectrometer (1–200 amu mass range). The thermoanalytical curves were recorded together with the ion current (IC) curves in the multiple ion detection (MID). A constant purge gas flow of 80 mL/min nitrogen and a constant heating rate of 5 K/min were applied.

2.2. Crystal structures determinations

2.4. FTIR spectroscopic analysis

A suitable colorless single-crystal of each compound was carefully selected under an optical microscope and glued to a thin glass fiber. Crystal structures determined by X-ray diffraction were performed using a Nonius Kappa CCD diffractometer at CDIFX (University Rennes1). The experiments were conducted at room temperature with graphite monochromated MoKα (λ = 0.71073 Å) radiation. The intensity data collections were performed through the program COLLECT [18]. Empirical corrections were applied using the SADABS program [19]. The compound structures were solved by direct methods (SIR97 [20]) and refined by the full matrix least-squares procedure based on F2, using the SHELXL-97 [21] computer program belonging to the WINGX software package [22]. The crystallographic details for compounds 1–3 are summarized in Table 1. The gallium and phosphorus atoms were first located, whereas the oxygen, nitrogen atoms were found in the successive difference Fourier maps. The hydrogen atoms residing on the phosphorus were located by Fourier maps and the remaining hydrogen atoms were placed geometrically. All non-hydrogen atoms were refined anisotropically. All drawings were made with the DIAMOND program (version 3.0 e) [23]. Fractional atomic coordinates with equivalent displacement parameters for all atoms are listed in Table S1. Selected bond distances and angles for compounds 1–3 are presented in Table S2. The structural data were deposited in the Inorganic Crystal Structure Database: CSD numbers are as follows: CSD-432716 for compound 1, CSD-432718 for 2 and CSD-432717 for 3.

The infrared spectra were recorded using a Thermo Scientific Nicolet iS5 FT-IR spectrometer with iD7 ATR accessory. A small amount of the grinded samples was laid down on the diamond crystal. Infrared spectra were recorded from 4000 to 400 cm−1. Omnic software was used to display spectra. The FTIR spectra of compounds 1–3 (Fig. S2) exhibit the bands corresponding to the vibrations of the phosphite anions (ν vibrations around 2400–2540 cm−1 for P-H and 1000–1100 cm−1 for P-O). Selected bands obtained from IR spectroscopy are given in Table 2. Two P-H stretching bands indicate the presence of two crystallographically independent phosphite groups in the structure of all compounds. In addition, the existence of NH4+ groups in ammonium compound is clearly shown by three typical bands (3229, 1477 and 1417 cm−1). 3. Results and discussion 3.1. Structural description The crystal structures of the three compounds are isotypic. Their asymmetric units contain 6 non-hydrogen atoms, of which one gallium and two phosphorus atoms are crystallographically distinct, as shown in Fig. 1. Ga adopts six-coordination geometry to construct an octahedron by six oxygen atoms. Both P atoms bond to three oxygen atoms and one hydrogen atom leading to a pseudo-tetrahedral environment. In the three compounds, each GaO6 octahedron shares its six oxygen atoms with adjacent phosphorus atoms with the Ga–O bond distances ranging from 1.963(2) to 1.973(2) Å. The cis and trans O– Ga(1)–O bond angles are in the range of 87.51(9)–91.97(6)° and 179.03(9)–197.39(11)°, respectively. Reciprocally, each P atom connects to three different metallic centers through vertex-sharing O atoms, leading to layers which propagate in the ab plane (Fig. 2). As a result, each HPO3 tetrahedron exhibits three identical P–O distances

Table 1 Crystal data and structure refinement parameters for compounds 1–3. Struct param

1

2

3

Empirical formula Formula weight Crystal system Space group a (Å) c (Å) V (Å3) Z ρcalc (g cm−3) μ (mm−1) θrange (deg) Reflections collected Unique reflections Number of parameters Goodness of fit R factors [I > 2σ(I)]

H2KO6P2Ga 268.78 Hexagonal P63mc (no. 186) 5.2567 (2) 12.2582 (3) 293.348 (17) 2 3.043 5.91 4.5–41.9 6120 745 27 0.8 R1 = 0.023 wR2 = 0.052 − 0.79; 0.34

H6NO6P2Ga 247.72 Hexagonal P63mc 5.2576 (2) 12.9113 (4) 309.083 (19) 2 2.662 4.94 4.5–35.0 4617 522 29 1.04 R1 = 0.024 wR2 = 0.056 − 0.92; 0.42

H2RbO6P2Ga 315.15 Hexagonal P63mc 5.27020 (10) 12.7619 (5) 306.973 (15) 2 3.409 12.85 4.5–35.0 4432 562 28 0.81 R1 = 0.016 wR2 = 0.045 − 0.37; 0.41

Largest diff. peak and hole (e Å−3)

Table 2 Selected bands for IR spectra (cm−1) for compounds 1–3. Assignment

K[Ga (HPO3)2]

NH4[Ga (HPO3)2]

Rb[Ga (HPO3)2]

ν(PH)

2535 2481 1116 1072 569 503

2539 2420 1116 1062 567 503

2534 2416 1114 1071 561 501

νas(PO3) νs(PO3) δs(PO3) δas(PO3)

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Fig. 3. Projection along the [010] direction showing the two-dimensional framework in K[Ga(HPO3)2]. Fig. 1. The asymmetric unit and symmetry-related atoms of K[Ga(HPO3)2] shown with 50% probability displacement ellipsoids. [Symmetry codes: (i) x, y-1, z; (ii) -x+y-2, -x-2, z; (iii) -x+y-3, -x-2, z; (iv) -y-1, x-y+1, z; (v) -x+y-1, -x-1, z; (vii) -x+y-2, -x-1, z; (viii) -y-2, x-y, z; (ix) -y-1, x-y, z].

alkali-metal cations occupy the interlayer spaces and are in nine fold coordination sites (see Fig. S3). The coordination environments adopted by them are supported by BVS calculations, which give values of 1.02 and 1.16 v.u. for A in 1 and 3, respectively. The A–O bond distances ranging from 2.906(2), 3.0195(11) Å to 2.951(1), 3.020(2) Å for 1 and 3, respectively are near the values reported in the literature [26–29]. The ammonium cation, also displayed between the layers of the host framework, exhibits N–H bond distances in the range usually found for this cation and the angles are similar to those expected for sp3 hybridization. Thus, the [Ga(HPO3)2]- sheets of the compound 2 are held together by means of nine N–H···O hydrogen bonds formed between NH4+ and the oxygen atoms of the framework (Fig. S4). The hydrogen bond length and angle details are listed in Table S3. The BVS calculations made for NH4+ cations using the parameters ro = 2.219 Å and B = 0.37 Å [30], gave values of 1.01 v.u. for 2. This result indicates that its valence is satisfied by the described H-bonding environment. The A[Ga(HPO3)2] (A = K, NH4, Rb) studied phases are isostructural with the recently reported (H3O)In(HPO3)2 [15], RbIn(HPO3)2 [14], A[M(HPO3)2] [16] ( A = K, NH4, Rb and M = V, Fe) and NH4[In(HPO3)2] [17]. The Table 3 allows us to compare their refined unit-cell parameters. For each A+ intercalated ion, we note an evolution of the cell volume with the M3+ radius [31]. In the case of iron and vanadium, these volumes are very close in accordance with their ionic radii. The structural model described is also related to the yavapaiite alums type [32] and the mixed selenite-selenate RbFe(SeO4)(SeO3) [33].

Fig. 2. Projection along the [001] direction showing the [Ga(HPO3)2]- layer in K[Ga(HPO3)2].

and a shorter bond corresponding to the P–H bond. In the three gallium phosphites, the P–O and P–H bond distances range from 1.513(2) to 1.532(2) Å and from 1.22(7) to 1.50(7) Å, respectively. The O–P–O and H–P–O angles show values usually found in tetrahedral coordination. The bond lengths and angles in the crystal structures of 1, 2 and 3 are similar to those observed in gallium phosphites reported in the literature [4–14]. The bond-valence-sum (BVS) calculations [24] for Ga center give the values of 3.16, 3.16 and 3.14 v.u. for 1, 2 and 3, respectively. These results are close to the expected + 3 valence. As there no tabulated bond distances for phosphorus in oxidation state + 3, (PH)4+ groups have been considered to made BVS calculations [25]. The results are (P(1)H)4+: 3.87, 4.04, 4.07 v.u. and (P(2)H)4+: 4.05, 3.88, 3.88 v.u. for 1, 2 and 3, respectively. These values are in good agreement with the oxidation state of the (PH)4+ group. Finally, the arrangement of the layers leads to a bidimensional framework formed by anionic sheets of formula [Ga(HPO3)2]- stacked along the c axis (Fig. 3). The negative charge of this network is balanced by K+, NH4+ and Rb+ ions for 1, 2 and 3, respectively. Thus, both the

3.2. Thermal behavior Thermogravimetric analyses were performed to investigate the thermal behavior of compounds 1–3. These experiments have been Table 3 Unit-cell parameters of the NH4[M(HPO3)2] (M = V, Fe, Ga and In) compounds, and ionic radii of the metallic cations in hexacoordination. Compound NH4[M (HPO3)2]

a (Å) c (Å) V (Å3) Ionic radius (Å)

10

M Ga (this work)

V [16]

Fe [16]

In [17]

5.2576 (2) 12.9113 (4) 309.083 (19) 0.620

5.3330 (2) 12.8760 (4) 317.143 (19) 0.640

5.3229 (2) 12.8562 (4) 315.457 (19) 0.645

5.4705 (1) 13.0895 (4) 339.24 (1) 0.800

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Table 4 Experimental and calculated weight variations from TGA experiments and X-ray powder diffraction analysis obtained after thermal treatment of A[Ga(HPO3)2] phosphites. Phosphite A/Ga

Under air atmosphere XRD result

Under nitrogen atmosphere Weight variation (%)

XRD result

Calculated

Observed

Weight variation (%) Calculated

Observed − − − − −

K[Ga(HPO3)2]

KGaP2O7

+ 5.2

+ 2.2

KGaP2O7

+ 5.2

Rb[Ga(HPO3)2]

RbGaP2O7

+ 4.4

+ 3.1

RbGaP2O7

+ 4.4

NH4[Ga(HPO3)2]

GaPO4 + amorphous phase(s)

/

− 11.7

GaPO4 + amorphous phase(s)

/

0.7 1.7 0.6 2.7 12.8

AGa(HPO3)2 → 2 P + 2 AGaP2O7 + “A3Ga3(PO4)4”+ 5H2 This is in agreement with the observations made on the decomposition of sodium phosphite for which red phosphorus has been identified by a red coloration of the materials and Raman spectroscopy [34]. X-ray diffraction performed on compounds 1 and 3 heated at 800 K revealed that the materials are amorphous at this stage but the reddish color supports the assumption of the presence of elemental phosphorus. The second weight loss observed above 850 K is followed by the crystallization of AGaP2O7 phases accompanied by an unidentified material, as evidenced by X-ray powder diffraction after thermal treatment. Several hypotheses have been made to explain this loss of weight, mainly with the evolution of phosphorus species but no signals were detected on the mass spectrometer during this second weight loss. A small negative signal occurs for the Ion Current curve m/z = 32 (O2) (Fig. S6), which can be explained by a capture of O2 which may be contained in the nitrogen gas flow as a small impurity. Under air atmosphere, the structures of alkali cations containing compounds remain stable up to about 750 K (Fig. 5). From this temperature and up to about 770 K, while the mass starts to increase, a small weight loss occurs and immediately a weight gain is again observed. Finally an abrupt mass gain accompanied with a sharp and intense exothermic signal is observed. Powder XRD measurements performed after TGA reveal the presence of AGaP2O7 with A = K or Rb, indicating oxidation of phosphite groups to pyrophosphate. The small mass loss at 770 K may be interpreted regarding the thermal behavior of the compounds under nitrogen (vide supra): a partial dehydrogenation of the phosphite group may occur at this stage. In both cases, the observed weight increases are less than the expected values notably for potassium compound (Table 4). This could be the result of expulsion of matter out of the crucible during the TGA

Fig. 4. Thermogravimetric curves for gallium phosphites 1 and 3 under nitrogen atmosphere.

conducted under nitrogen and air to compare the influence of atmosphere. Weight variations observed and information obtained from Xray powder diffraction performed on the samples after thermal treatment are summarized in Table 4. Under nitrogen atmosphere, only weight losses are observed for K and Rb compounds (Fig. 4). These weight losses occur in two stages: at 760 K, a first loss of 0.7% for 1 and 0.6% for 3 is observed, followed by a second step with a weight loss of 1.7% for 1 and 2.7% for 3. To understand the first weight loss, the compound 1 has been heated at 800 K, and cooled down to room temperature and analyzed by infrared spectroscopy (Fig. S5). The P-H band at 2530 cm−1 observed in the compound 1 is no longer present showing that at this stage the decomposition of the phosphite group has occurred. The broadening of the bands corresponding to the P-O vibrations is related to the amorphous state of the sample at this temperature. The classical scheme for the thermal decomposition of phosphite is a disproportionation of the phosphite (PIII) leading to the formation of phosphine (P-III), phosphates (PV) and water according to the following equation [34]: HPO32- → 2 PH3 + P2O74- + 4 PO43- + H2O But as already observed in the decomposition of sodium phosphite, the weight losses are smaller than expected. Since compounds 1 and 3 exhibit the same behavior during heating, the compound 3 was chosen to perform a coupled TGA-MS study in order to show an example of typical behavior. The results reveal that H2 is the main evolved gas accompanied with traces of water (Fig. S6). The proposed mechanism for the decomposition of the phosphite group observed at 760 K is the disproportionation of the phosphite into red phosphorus, phosphates and a release of H2 according to the following equation:

Fig. 5. Thermogravimetric curves for gallium phosphites 1 and 3 under air atmosphere.

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Journal of Solid State Chemistry 255 (2017) 8–12

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For the three gallium phosphites studied, phosphate compounds identified at the end of the analyses reveal the oxidation reaction of phosphite to phosphate anions. This reaction occurs with H2 release as evidenced by TGA-MS experiment. Full comprehension of thermal decomposition of phosphites still remains a challenge. Acknowledgment This work was supported by the Algerian–French Program CMEPPHC Tassili 10MDU819. The authors are indebted to T. Roisnel for Xray data collection (Centre de DIFfractométrie des rayons X, CDIFX) from the Université de Rennes1, France. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2017.07.030. Fig. 6. Thermogravimetric curves for ammonium compound 2 under air and nitrogen atmospheres.

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experiment because of the highly exothermic reaction occurring at 850 K as evidenced by DSC measurements (Fig. S7). For ammonium compound, the behaviors observed under air and nitrogen atmospheres show strong similarities: the thermogravimetric curves reveal one stage for the lost mass, as shown in Fig. 6. This weight loss may be associated to the release of NH3 molecules followed by water elimination. The analysis by X-ray powder diffraction of the remaining residue for each sample reveals the presence of a mixture of unidentified amorphous phase(s) and the orthorhombic GaPO4 dense phase. Thermal behavior of phosphites based compounds is in general quite complex to explain because of the numerous phenomena which can occur [35]. For example we may cite hydrogen or phosphorus species releases, complex redox processes and condensation or highly exothermic reactions leading to ejection of matter. In the present study the predominant observations for the thermal behavior of the phosphite group are i) a thermal stability up to 750 K, ii) under nitrogen an hydrogen release and iii) under air the oxidation of phosphites into pyrophosphates resulting in frameworks of higher dimensionality (from bi-dimensional to three dimensional) and an increase of the density of the materials upon heating.

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4. Conclusion In summary, three open-framework gallium phosphites, A[Ga(HPO3)2] (A = K, NH4, Rb), have been successfully prepared as good quality crystals under hydrothermal conditions. Structural analyses indicate that these compounds are isostructural and possess a similar two-dimensional network constructed from corner-sharing GaO6 and HPO3 polyhedra with A+ cations filling the interlayer spaces. The layered structural framework described here has been observed in some metal phosphites A[M(HPO3)2] (A = K, NH4, Rb and M = V, Fe, In). This structural model exhibits close structural relationships with some of the previously reported compounds with AMXO4 formula (A = alkaline, M = transition metal and X = S, Mo). Thermogravimetric analyses carried under air atmosphere indicate that alkali cations containing compounds are stable up to 750 K. AGaP2O7 (with A = K or Rb) is identified as major constituent after thermal treatment. For NH4 phosphite compound, the decomposition leads to amorphous phase(s) accompanied with orthorhombic GaPO4.

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