Antimony orthophosphate

Antimony phosphate, (also called antimony orthophosphate, or antimonous phosphate) is a chemical compound of antimony and phosphate with formula SbPO4. Antimony is in the form Sb(III) with +3 oxidation state. Antimony atoms have a lone pair of electrons.

Layered form
SbPO4 occurs as a layered compound. Two-dimensional layers are weakly held together by electrostatic forces. SbPO4 is one of the most compressible materials, and under pressure compresses more perpendicular to the layers. At standard conditions SbPO4 crystallises in a monoclinic form with space group P21/m. Antimony phosphate has been investigated for use in lithium ion and sodium ion batteries.

Antimony atoms are attached to four oxygen atoms. These atoms are arranged as a squarish pyramid with antimony at the apex. Antimony atoms form the top and bottom of the layers. Four oxygen atoms are arranged tetrahedrally around phosphorus. Antimony to oxygen bond lengths are 1.98 2.04 2.18 and 2.93 Å. the O-Sb-O angles are 87.9 164.8 84.1 and 85.0°. The structure of SbPO4 differs from two forms of BiPO4, where bismuth associates with five or eight phosphate groups.

In SbPO4 the 31P chemical shift is −18 ppm. The binding energy of the 2p electrons of phosphorus atom as determined by XPS is 133.9 eV.

When the pressure exceeds 3 GPa, bonds form between the layers, but it retains the monoclinic system. But when the pressure is between 9 and 20 GPa, it transitions to a triclinic form with space group P. 10.1021/acs.inorgchem.9b02268

The infrared spectrum shows absorption bands at 1145, 1052, and 973, 664, 590, 500, 475, and 372 cm−1. These are due to vibrations in P-O and Sb-O bonds and also bending in O-P-O bonds.

Antimony(V) phosphate
Antimony(V) phosphate SbOPO4 has monoclinic crystals. It has space group C2c. The unit cell has dimensions a = 6.791 Å, b = 8.033 Å, c = 7.046  Å, and β = 115.90°, with number of formula per unit cell of Z = 4. It is formed by heating Sb2O5*xH2O and (NH4)H2PO4. At 1218 K it loses oxygen to become antimony(III) phosphate.

Formation
SbPO4 may be formed by soaking antimonous oxide in pure phosphoric acid and then filtering the solid, and heating to 600 °C.


 * Sb2O3 + 2H3PO4 → 2SbPO4 + 3H2O

A related method involves heating a water solution of phosphoric acid with antimonous oxide at about 120 °C.

Yet another procedure involves heating diammonium phosphate with antimonous oxide at 600 °C.

Reactions
SbPO4 reacts with bases such as ammonia, hydrazine and ethylenediamine to form Sb2O3 and hydrogenphosphate salts.

However intercalation is also claimed with amines. Intercalation of amines expands the a axis of the crystals, but leaves c, and c dimensions unaltered. The β angle is reduced. This is due to a bilayer of molecules inserting between each layer in the original crystal.

There are also double salts where phosphate groups are joined to antimony.