Antimony orthophosphate

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^[2] with space group P2₁/m. Antimony phosphate has been investigated for use in lithium ion and sodium ion batteries.^[3][4]

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.^[2] 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°.^[2] The structure of SbPO₄ differs from two forms of BiPO₄, where bismuth associates with five or eight phosphate groups.^[2]

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.^[5]

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 P1.^[6] 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.^[7]

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.^[8]

SbPO₄ may be formed by soaking antimonous oxide in pure phosphoric acid and then filtering the solid, and heating to 600 °C.^[7]

Sb₂O₃ + 2H₃PO₄ → 2SbPO₄ + 3H₂O

A related method involves heating a water solution of phosphoric acid with antimonous oxide at about 120 °C.^[9]

Yet another procedure involves heating diammonium phosphate with antimonous oxide at 600 °C.^[9]

SbPO₄ reacts with bases such as ammonia, hydrazine and ethylenediamine to form Sb₂O₃ and hydrogenphosphate salts.^[7]

However intercalation is also claimed with amines.^[9] 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.^[9]

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