|Prescott, Hillary A.: The Crystal Structures and Thermal Behavior of Hydrogen Monofluorophosphates and Basic Monofluorophosphates with Alkali Metal and N-containing Cations |
Strong acids such as H3PO4, H2SO4, and HClO4 are known to supply hydrated protons for proton conductivity. Even in the absence of H2O, proton conductivity has been observed in these acids due to self-dissociation . Acid salts of these and other acids have been examined for proton conductivity, because they offer a solid form of the acid, which is easier to handle and less corrosive. Only small proton conductivities were observed in acid salts, until a systematic search found that acidic iodates and CsHSO4 had particularly high conductivities [1, 2]. Most acid salts undergo phase transitions to produce temperature-dependent modifications with different physical properties. Proton conductivity is largely dependent on the hydrogen bonding and the geometry of the tetrahedra in the structure. The conductivity of the H3O+ and OH- ions (350 and192 \|[OHgr ]\|-1cm2mol-1, respectively) in a hydrogen-bonded media is higher than other ions because of proton-transfer mechanisms. The processes leading to proton conductivity are described by the Grotthus and vehicle mechanisms [3, 4].
The acid salts, MHXO4 (X =S, Se), have been investigated indepth crystallographically and thermally for phase transitions and their consequent physical properties, such as ferroelectricity and superionicity. Some of them have successive phase transitions and superionicity has been found in their high temperature modifications . These phase
2transitions are often irreversible and affected by moisture . CsHSO4 undergoes several phase transitions from a low temperature phase through an intermediate phase at 333-370 K to a superionic phase at 410-414 K based on the dynamic reorientational disorder of the sulfate tetrahedra . Other salts with this composition also exhibit ferroelectric activity. RbHSO4 and NH4HSO4 both have ferroelectric phases with transitions below 260 K. The ammonium salt goes from being nonferroelectric above 270 K through a ferroelectric phase and back to being nonferroelectric below 155 K . RbHSO4 is paraelectric at room temperature and ferroelectric at lower temperatures with Tc = 265 K ; the corresponding KHSO4 is not ferroelectric . Other compositions, such as M3H(XO4)2 (M = Na, K, Rb and X = S, Se), have also been studied . The structures and thermal behavior of acid salts with both sulfate and phosphate tetrahedra  and acid adducts of sulfuric acid [10, 12] have also been examined. Although the hydrogen sulfates, selenates, and these salts have been investigated extensively, the acid salts of the monofluorophosphoric acid, H2PO3F, have hardly been studied at all. The monofluorophosphate and sulfate anions are isoelectronic due to the replacement of one of the oxygens by fluorine on phosphorus to form a PO3F2- ion. Often isosterism between compounds results in similar chemical and physical properties . Thus, it could be speculated that the acid salts of H2PO3F and H2SO4 have similar crystal structures (hydrogen bonding) and consequently common physical properties. Therefore, the
of the hydrogen monofluorophosphates with alkali metal cations and cations containing nitrogen were studied. Structural correlations and differences between the hydrogen monofluorophosphates and hydrogen sulfates were then established. The investigations led to conclusions on the hydrogen bonding and the influence of fluorine on the bonding in the acid salts of monofluorophosphoric acid.
Monofluorophosphoric Acid and the Monofluorophosphates
Monofluorophosphoric acid, H2PO3F, is an oxo acid, in which one of the OH groups on phosphorus has been substituted by fluorine. The monofluorophosphates have been known for over 100 years. The first "hydrogen monofluorophosphates" with rubidium and
3potassium were synthesized by the reaction of the phosphate with the hydroxide and hydrofluoric acid (Reaction 1) [14,15, 16].
K3PO4 + KOH + aq. HF (40%) KHPO3F
The compounds were characterized by elemental analysis. The constitution of these salts was suggested to be similar to that of the phosphates with one of the OH groups replaced by a fluorine atom to form a PO3F2- anion [14, 15], but it was not proven at the time. The general constitution of the PO3F2- anion was later confirmed by 31P and 19F NMR spectroscopy. Both the 31P and 19F NMR spectra show 1-1 doublets for the tetrahedral orthophosphate group with one oxygen atom substituted by a fluorine atom . This substitution of one O/OH group on phosphorus creates an anion isoelectronic to the sulfate anion, SO42-. Similarities between the basic monofluorophosphates and sulfates were noted in . The monofluorophosphoric acid, H2PO3F, like sulfuric acid, is a strong, diprotic acid. It is commercially available, but not in pure form. This is due to the hydrolysis and decomposition of the monofluorophosphoric acid to orthophosphoric acid (Reaction 2). The hydrolysis is complete in dilute solution . The rate of hydrolysis is pH-dependent with the monofluorophosphate anion hydrolyzing rapidly at very low and high pH values . The equilibrium of the hydrolysis of H2PO3F (Reaction 2) has been studied in detail .
H2PO3F + H2O H3PO4 + HF
Basic salts of the monofluorophosphoric acid are stable in a neutral or weakly alkaline aqueous solution , which is reflected in the literature. A variety of basic monofluorophosphates have been published in the last century; some of which are mentioned in [16, 17, 20]; the thermal behavior of CaPO3F·2H2O [21, 22], SrPO3F·H2O [23, 24], and Mg(NH4)2(PO3F)2·2H2O  has also been investigated. On the other hand, the hydrogen monofluorophosphates have remained practically unknown. In 1968, J. Neels and W. Grunze published the synthesis of the following hydrogen monofluorophosphates: NaHPO3F, KHPO3F, and NH4HPO3F . The products were characterized by Guinier exposures and NMR spectroscopy (31P and 19F); their crystal structures were not determined. The thermal behavior of KHPO3F was later investigated by paper chromotography . Since then, only the crystal structure of anilinium hydrogen monofluorophosphate, [C6H5NH3]HPO3F , has been determined. Consequently, very little is known about the hydrogen bonding in the hydrogen monofluorophosphates.
Hydrogen bonding, which involves weak interactions between hydrogen atoms and electronegative atoms, influences the structure and properties of compounds. The hydrogen bond is defined as the interaction of a hydrogen atom with two of its nearest electronegative neighbors, such as oxygen, nitrogen, and/or fluorine. The atom, X, with the shorter distance to hydrogen below 1.0 Å is defined as the hydrogen donor; whereas, the second neighbor, Y, with the weaker interaction with hydrogen is referred to as a hydrogen acceptor. This interaction forms a bond with the X···Y interatomic distance ranging from about 2.26-3.20 Å [29, 30], when X and Y are oxygen, nitrogen, and/or fluorine. Physical properties of compounds are dependent on the strength of the hydrogen bond, which is determined by the length of the X···Y distance. The hydrogen bonds can be classified as very strong (< 2.50 Å), strong (2.50-2.65 Å), medium (2.65-2.80 Å), or weak (>2.80 Å) based on O···O distances . The geometry of the hydrogen bond can be asymmetrical, as described above, or symmetrical, when very short separations are found between the two electronegative elements and the hydrogen atom is involved in two equivalent bonds to X and Y. The geometry of the hydrogen bond is dependent on the potential surfaces of the possible positions for the hydrogen atom. Hydrogen bonds with two equivalent hydrogen positions (two minima on the potential curve) are disordered, either statistically or dynamically. Theoretically, the hydrogen atom lies on a line between the two neighbors forming a linear bond, X-H···Y with an XHY of 180°. However, angles found in structures tend to deviate to lower values. Some example lengths found for the different hydrogen bonds are: F-H···F 2.27-2.49 Å in NaHF2, KH4F5, and HF; O-H···O 2.40-2.63 in acid salts and 2.7-2.9 Å in ice, hydrates, and hydroxo compounds; N-H···O 2.86 in (NH4)2H3IO6; and N-H···F 2.6-2.96 Å in NH4F and (N2H6)SiF6 . The mixed hydrogen bond, O-H···F, has lengths of 2.56 and 2.87 Å found in the hydrates of metal fluorides . No length is given for this type of hydrogen bond in an inorganic acid salt, in which the fluorine atom is covalently bonded to another atom, such as P.
Structural Features of the Hydrogen Sulfates
A variety of structural patterns have been found for hydrogen-bonded HSO4 tetrahedra in the crystal structures of the hydrogen sulfates on the basis of the strong O···O hydrogen bonds [10, 32]. These patterns or structural features include isolated dimers, cyclic dimers, infinite chains, branched chains, cyclic tetramers, and layer structures depending on the H/SO4 ratio. The type of structure formed is also dependent on the cation. In the case of a H/SO4 ratio of 1:1, infinite chains, cyclic dimers, and/or branched chains have been found.
5Structures with this H/SO4 ratio have one hydrogen donor, OD, and one hydrogen acceptor, OA, for every HSO4 tetrahedron. Infinite chains have been observed in the following structures: RbHSO4 [8, 33], CsHSO4 , CsHSeO4 , and [C(NH2)3]HSO4 . Two types of geometry are seen in the structures with infinite zigzag chains of HSO4 depending on the angle between the tetrahedra (SSS). Tetrahedral angles were found in RbHXO4 [8, 33] and CsHXO4 [6, 34] with X = S or Se with smaller angles observed in structures with larger cations . Cyclic dimers formed by two HSO4 tetrahedra (2OA+2OD/dimer) were observed in the isotypic structures of -NaHSO4  and NaHSeO4  and a superprotonic phase of CsHSO4 [2, 38], but according to  they are rather rare for the structures of the hydrogen sulfates. The cyclic dimers in these structures have two different hydrogen bonds holding them together. The structure of KHSO4 [9, 39] and KHSeO4  consists of separate units of cyclic dimers and infinite chains. In comparison with the cyclic dimers in the -modification of NaHSO4, branched chains were found in the -NaHSO4 . Two types of branched chains were observed in hydrogen sulfates depending on the linear or zigzag symmetry of the chain : the branched tetrahedra are situated on one side of the linear chains and alternate sides of zigzag chains. Whether infinite or branched chains form in the structure seems to be dependent on the size of the metal cation . An additional structural motif is the cyclic tetramers found along with separate infinite chains in the structure of AgHSO4 [42, 43]. This type of tetrameric bonding had not previously been observed in the hydrogen sulfates. Isolated dimers are formed in structures with the composition: M3[H(SO4)2], where the H/SO4 ratio is less than one. Such dimers were found in the sulfate and selenate structures with M = K [44, 45] and Rb [46, 47, 48], Cs3H(SeO4)2 [49, 50], and the sulfate structures with Na  and NH4 . All of which are isostructural except for the sodium salt. The composition, [H(SO4)2], has a H/SO4 ratio of ½ which implies: ½D + ½A. The hydrogen atom is shared by two SO4 tetrahedra connected to each other by either a symmetrical or an asymmetrical hydrogen bond. The symmetrical hydrogen bond in the NH4, K, Rb, and Cs structures was formed by disordered hydrogen atoms; an asymmetrical hydrogen bond was found in Na3H(SO4)2. Disorder was observed in strong O···O bonds with two minima and, for the most part, an O···O interatomic distance of 2.45-2.55 Å in the M3H(SO4)2 structures .
In comparision to the alkali metal hydrogen sulfates, the hydrogen sulfates with N-containing cations other than ammonium have hardly been studied crystallographically. One exception is the hydrogen sulfate with guanidinium, already mentioned, based on the discovery of ferroelectricity in the guanidinium sulfates, [C(NH2)3]Al(SO4)2·6H2O 
6and [C(NH2)3]UO2(SO4)2·3H2O .
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