Structure refinement of (NH4)3Al2(PO4)3 prepared by ionothermal synthesis in phosphonium based ionic liquids – a redetermination

The crystal structure of (NH4)3Al2(PO4)3 was refined by powder XRD synchrotron data. (NH4)3Al2(PO4)3 is a member of the structural family with formula A 3Al2(PO4)3 where A is a group 1 element, of which the K and Rb forms are also known.


Chemical context
Following the discovery of the microporous AlPO 4 -n series of materials (Wilson et al., 1982), many efforts have been directed toward the synthesis of novel phases utilizing traditional hydrothermal (Wilson, 2007;Yu & Xu, 2006) and solvothermal syntheses (Das et al., 2012). Recently, ionothermal synthesis has been added to the stable of synthetic methods. Ionothermal synthesis is an extension of the solvothermal method of synthesis using an ionic liquid as the solvent (replacing, for example, water or ethylene glycol) where a portion of the organic structure-directing agent from a typical zeolite synthesis is derived from the ionic liquid (Morris, 2009). Many new materials have been synthesized by ionothermal synthesis, with new aluminophosphate materials among the most common (Parnham & Morris, 2007;Xing et al., 2008Xing et al., , 2010).
An important issue in ionothermal synthesis is control of water (Ma et al., 2008). Excess water often leads to synthesis of dense AlPO 4 phases such as the one with a tridymite-type of structure, which we observed as well during syntheses utilizing 85% wt H 3 PO 4 . To control the level of water in the synthesis, thereby allowing easy recycling of the ionic liquid solvent and to intentionally prepare ammonium aluminophosphates, we used (NH 4 ) 2 HPO 4 as the phosphorous source in the synthesis. Ammonium is a good structure-directing agent for aluminophosphate frameworks; multiple ammonium aluminum phosphates are known (Byrne et al., 2009;Vaughan et al., 2012). In the current phosphonium-based ionothermal synthesis, the presence of an ammonium cation in the relative absence of water provokes the formation of a 2/3 Al/P framework with the formula (NH 4 ) 3 Al 2 (PO 4 ) 3 . A structurally unrelated compound with the formula (NH 4 ) 3 Al 2 (PO 4 ) 3 has previously been synthesized via a solvothermal approach (Medina et al., 2004).
The aluminophosphate database at Jilin (Li et al., 2019) currently lists 21 framework structures with a 2:3 ratio of Al:P. A framework with sub-stoichiometric Al content is by necessity anionically charged and must be cation-balanced, so most of the known frameworks, such as UT-3, UT-4 and UT-5 (Oliver et al., 1996) are charge-balanced by organoammonium cations. Low-water-content syntheses clearly favor 2:3 compounds as most of the known materials are synthesized from low-water-content preparations.

Structural commentary and survey of related compounds
The (NH 4 ) 3 Al 2 (PO 4 ) 3 phase synthesized here is related to the series of A 3 Al 2 (PO 4 ) 3 materials synthesized via hightemperature solid-state methods (Devi & Vidyasagar, 2000) with varying monocations on the A site. Additionally, an independent synthesis previously yielded a (NH 4 ) 3 Al 2 (PO 4 ) 3 material called SIZ-2 whose structure was solved and refined from single-crystal data (Cooper et al., 2004) and possesses nearly the same structure as refined from the current powder data of (NH 4 ) 3 Al 2 (PO 4 ) 3 . A polyhedral representation of the crystal structure of (NH 4 ) 3 Al 2 (PO 4 ) 3 is shown in Fig. 1. SIZ-2 crystallized from a choline chloride/urea eutectic mixture where decomposition of urea was proposed to be the source of ammonium in the structure. The refinement of Cooper et al. (2004) included the ammonium N atoms, but made no attempt to find or model the corresponding H atoms. Devi & Vidyasagar (2000) utilized Li, Na, K, Rb, Cs, and Tl as the A cation and succeeded in crystallizing compounds with A = Na, K, Rb, Tl. The thallium derivative yielded a completely different structure with trigonal-bipyramidal coordination of Al. The A = Na structure was not solved, but apparently crystallizes in an unrelated orthorhombic spacegroup type from that observed for A = K, Rb in their work, and for A = NH 4 here. Devi & Vidyasagar (2000) utilized (NH 4 ) 2 HPO 4 as the phosphate source in their high-temperature preparations of A 3 Al 2 (PO 4 ) 3 , but did not obtain (NH 4 ) 3 Al 2 (PO 4 ) 3 , likely due to the volatility of NH 3 at high temperatures.
As in the K and Rb forms of the A 3 Al 2 (PO 4 ) 3 series, aluminum and phosphorus are both tetrahedrally coordinated and connected through corners throughout the (NH 4 ) 3 Al 2 (PO 4 ) 3 structure. The NH 4 + cations reside in a channel along the c-axis direction made from a 12 T-site ring of alternating AlO 4 and PO 4 tetrahedra ( Ball and stick representation of (NH 4 ) 3 Al 2 (PO 4 ) 3 showing the 12membered ring with three phosphate groups protruding inward with close contact to ammonium cations.

Figure 1
Polyhedral representation of (NH 4 ) 3 Al 2 (PO 4 ) 3 , showing the overall connectivity and ion channels in the crystal structure. Al is in the center of blue tetrahedra, P in gray tetrahedra, and N is represented by blue spheres. Table 1 Hydrogen-bond geometry (Å , ).

D-HÁ
solvent is present within the pores of the (NH 4 ) 3 Al 2 (PO 4 ) 3 framework. Without the NH 4 + groups, the structure would have 24% void volume. The framework is triply negatively charged and charge-balanced by the ammonium cations. Three of the six phosphate groups in the ring protrude inward such that the closest contact distance between the H atom of an ammonium group and the O atom of the nearest phosphate is between 1.83 and 1.87 Å , indicating significant hydrogenbonding interactions. The full range of HÁ Á ÁO hydrogen-bond lengths is between 1.83 and 1.97 Å (Table 1).
Crystallizing in space-group type Pna2 1 , (NH 4 ) 3 Al 2 (PO 4 ) 3 is isostructural to, but with a slightly larger unit cell than the K form synthesized by Devi & Vidyasagar (2000). Lattice expansion of $0.1-0.2 Å occurs along each of the three axes, leading to an overall 6.6% increase in cell volume from 1245 to 1327 Å 3 . A lattice expansion is no surprise as the ionic radius of NH 4 + is between 1.4 and 1.67 Å depending on the coordination number (Sidey, 2016). This is slightly larger than the reported 1.37 to 1.55 Å range for K + (Shannon, 1976). Much of the relative lattice expansion for (NH 4 ) 3 Al 2 (PO 4 ) 3 occurs along the a and c axes. Tilting of tetrahedra accounts for a significantly smaller expansion of the long b axis. In addition, an isostructural K/As form is also known where two-thirds of the phosphate groups have been replaced by arsenate (Boughzala et al., 1997). Arsenate included on the phosphate sites increases the cell volume to 1307 Å 3 , just smaller than that recorded here for (NH 4 ) 3 Al 2 (PO 4 ) 3 . The pure arsenate form K 3 Al 2 (AsO 4 ) 3 was reported by Stö ger & Weil (2012), which has a cell volume of 1328 Å 3 , essentially equivalent to that here.
An overlay plot of atomic positions of (NH 4 ) 3 Al 2 (PO 4 ) 3 (red) versus SIZ-2 (blue) shows that although the independent refinements of the two (NH 4 ) 3 Al 2 (PO 4 ) 3 materials were performed via different methods at different temperatures, most atom positions are similar, with no more than about 0.004 fractional position differences along the a or c axes (for these axes, about 0.03-0.04 Å , Fig. 3). One area stands out in the A 3 Al 2 (PO 4 ) 3 series. Fig. 4 shows the key area surrounding O11 where the largest position movement is observed in the two independent refinements of (NH 4 ) 3 Al 2 (PO 4 ) 3 .
The P3-O11 bond is always among the shortest P-O bonds found in the crystal structure, here at 1.487 (5) Å . Two clusters of P-O bond lengths occur; one at about 1.49 Å and another at 1.55 Å . These distances are relatively typical for aluminophosphates (Richardson & Vogt, 1992;Wei et al., 2012). Each of the O atoms protruding into the pore possess short P-O bonds and hydrogen bonds to two ammonium ions (Table 1). In particular, N2, N3, O11, and P3 are effectively in a plane so that with the hydrogen bonding present in our refined model from N3 and N2 through the attached H atoms to O11, O11 moves closer to P3 while N2 and N3 move slightly further away versus the positions in the SIZ-2 refinement. Table 2    Ball and stick representation of the key area surrounding O11 where the largest position movement takes place in the two independent refinements of (NH 4 ) 3 Al 2 (PO 4 ) 3 .  Boughzala et al. (1997) For each of the compounds, the atomic numbering scheme of the current (NH 4 ) 3 Al 2 (PO 4 ) 3 refinement has been utilized. For the first two compounds, A = NH 4 , while for the second two, A = K. For the As-containing compound, the P3 site is reported to have the highest occupancy of As at 0.86. four isostructural A 3 Al 2 (PO 4 ) 3 compounds. Other bond lengths and angles are otherwise relatively unremarkable versus other members of the structural class although we note that As/P-O distances are longer than P-O as expected. Rb 3 Al 2 (PO 4 ) 3 is structurally related to the NH 4 and K forms, but crystallizes in a higher symmetry space-group type (Cmc2 1 ), accompanied with higher overall coordination numbers around Rb + and a mirror plane perpendicular to a. The ionic radius of Rb + is similar to that of NH 4 + , reported as 1.52-1.63 Å (Shannon, 1976). Lithium and cesium forms of the series have not yet been synthesized, likely because of the relatively small and large, respectively, ionic radii versus those of the fitting A cations. Our initial attempts at ion-exchange of (NH 4 ) 3 Al 2 (PO 4 ) 3 with LiNO 3 or CsNO 3 in aqueous solution to form the Li or Cs form failed, with partial structural degradation and no ion-exchange observed.

Synthesis and crystallization
In a typical preparation, 1.65 g (NH 4 ) 2 HPO 4 was added to a 125 ml polytetrafluoroethene (PTFE) lined autoclave containing 24.02 g of ethyl tri(butyl)phosphonium diethyl phosphate. The mixture was stirred at room temperature for 2 min. To this mixture were added 0.49 g of Al(OH) 3 , and the contents were stirred at room temperature for 2 min. The contents of the autoclave were digested at 423 K for 24 h prior to isolating the product by filtration. Analytical results show this material has a molar ratio Al:P of 0.725. The X-ray diffraction pattern is shown in Fig. 5. Scanning electron microscopy (SEM) revealed agglomerated stacks of irregularly shaped blocky crystals of from 500 nm to 2-4 mm in length (Fig. 6). Calcination of (NH 4 ) 3 Al 2 (PO 4 ) 3 at temperatures of 773 K or higher causes the formation of an AlPO 4 phase with a tridymite-type structure. Ethyl tributyl phosphonium diethyl phosphate (Cyphos 169) was acquired from Cytec; aluminum hydroxide was acquired from Pfaltz and Bauer.   XRD pattern ( = 0.373811 Å ) of (NH 4 ) 3 Al 2 (PO 4 ) 3 synthesized ionothermally in ethyl tributylphosphonium diethylphosphate and Rietveld residuals following structure refinement. Part A shows the fit to the overall pattern, and inset B shows the fit to high-angle regions. Computer programs: local program at 11BM, GSAS (Larson & Von Dreele, 2000), coordinates from an isotypic structure, CrystalMaker (Palmer, 2005), publCIF (Westrip, 2010).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Following initial survey scans on in-house Cu source powder XRD instruments, final data were acquired from samples packed in thin glass capillaries on 11-BM at the Advanced Photon Source at Argonne National Laboratory. Starting atomic positions for the refinement were adapted from the literature examples. Starting positions for the ammonium cations were located in a difference-Fourier map and subsequently refined using GSAS (Larson & Von Dreele, 2000) as tetrahedral rigid bodies with N-H bond lengths held at 0.9526 Å and tetrahedrality enforced, leading to HÁ Á ÁH distances of 1.5556 Å . No soft constraints were applied to the framework positions. All atoms in the structure were refined with a common U iso parameter. Two low-intensity reflections in the region 4.00-4.22 /2 were excluded from the refinement as belonging to an impurity phase after assessment of multiple (NH 4 ) 3 Al 2 (PO 4 ) 3 batches. Refinement trials with a higher symmetry model (space-group type Cmc2 1 ) were attempted but showed poor agreement with the experimental data, with R wp > 0.16.