Preparation

Numerous layered phosphates of the elements of the groups 4, 5, 14 and 15 of the periodic table have been synthesized and many of them are listed in Table 1, together with their interlayer distance, the free area

Table 1 Formulae and some properties of layered phosphates of groups 4, 14 and 5, 15 elements

Formula

Density (g cm 3)

Ion exchange

Interlayer distance

Free area

capacity

(A)

(A2)*

(mmolH+g-1)

a-Ti(HPO4)2 • H2O

2.61

7.76

7.56

21.6

a-Zr(HPO4)2 • H2O

2.72

6.64

7.56

24.0

a-Hf(HPO4)2 • H2O

-

5.15

7.60

23.7

y-Ti(PO4)(H2PO4) • 2H2O

2.37

7.25

11.60

16.5

y-Zr(PO4)(H2PO4) • 2H2O

2.43

6.27

12.20

17.8

a-Si(HPO4)2

-

8.90

7.4

-

a-Ge(HPO4)2 • H2O

-

7.07

7.75

-

a-Sn(HPO4)2 • H2O

3.12

6.08

7.80

21.4

a-Pb(HPO4)2 • H2O

-

4.79

7.95

21.5

ZrPO4Cl(CH3)2SO

-

-

10.2

-

VOPO4 • 2H2O

2.4

-

7.41

38.5

VO(HPO4) 0.5H2O

2.8

5.81

5.70

35.7

NbOPO4 • 3H2O

-

-

8.04

-

HNb(PO4)2

-

3.52

-

-

HTa(PO4)2 • 2H2O

-

2.45

9.48

46.0

HAs(PO4)2

2.88

3.52

7.98

37.1

KSb(PO4)2

3.50

-

8.47

19.6

HSb(PO4)2

-

-

-

-

SbOPO4

4.42

-

6.34

-

*Area associated to each -OH group on the plane.

*Area associated to each -OH group on the plane.

surrounding the surface phosphate groups, density and calculated ion exchange capacity. It may be seen that, except for carbon, a-type phosphates of all the elements of groups 4 and 14 have been obtained. They are prepared with procedures similar to those used to obtain Zr(HPO4)2 • H2O, that is, by refluxing the amorphous precipitates in phosphoric acid (10-12 mol dm~3). An alternative procedure, especially used for Zr and Ti hydrogenphosphates, involves direct precipitation from solutions containing phosphoric acid and Zr (or Ti) fluoro-complexes. The degree of crystallinity of the precipitates may be controlled by modifying the velocity of removal of the complexing agent, as gaseous HF. With this method, crystals of millimetre dimensions have been obtained. Note that only Zr(IV) and Ti(IV) can form phosphate dihydrogenphosphates of y-type. The preparation involves the slow decomposition of fluoro-complexes in an NH4H2PO4 solution. The precipitate, e.g. [Zr(PO4)(NH4HPO4)], is then converted into its hydrogen form by treatment with HCl solution.

Tetravalent elements with large dimensions, such as Ce(IV) and Th(IV) do not give rise to layered phosphates of a- or y-type. The acid phosphates of these elements have been obtained in fibrous form suitable for the preparation of fully inorganic, self-consistent papers, thin films or membranes. The acid phosphates of groups 5 and 15 elements have been obtained by dissolving the oxides in concentrated phosphoric acid and heating to 270°C. HSb(PO4)2 can be obtained by treating the potassium salt with a strong acid solution. Group 5 elements also produce non-acid layered phosphates of formula XOPO4 (X = V, Nb, Ta) and structure similar to that of Zr(PO4)Cl(CH3)2SO. Vanadyl phosphate is one of the rare examples of a layered phosphate which has a low electronic conductivity and is capable of redox intercalation reactions similar to those shown by graphite or layered dichalcogenides. Generally speaking, layered phosphates possess good chemical and thermal stability. Layered Zr(HPO4)2 • H2O is a very insoluble compound, stable even in highly concentrated non-complexing acid solutions. The interlayer water is lost after prolonged heating at 110°C while the condensation water of monohydrogenphosphates to pyrophosphates is lost at 450-500°C. Molybdenum and some divalent cations such as Fe, Cd and Mn also form layered phosphates but their physical and chemical properties have not been investigated thoroughly.

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