Lithium arsenate is an inorganic compound of lithium and the arsenate ion, with the formula Li3AsO4. It is a colorless crystalline solid that is only sparingly soluble in water. Several crystalline polymorphs are known, including low-temperature and high-temperature orthorhombic forms.
Preparation Lithium arsenate can be prepared by precipitation from aqueous solutions containing lithium and arsenate ions:
It can also be prepared by complete neutralization of arsenic acid with lithium hydroxide:
Properties Crystal structure and polymorphism Lithium arsenate is polymorphic. The low-temperature form crystallizes in the orthorhombic crystal system, with lattice parameters approximately a = 6.27 Å, b = 5.38 Å, c = 4.95 Å.
This phase is closely related to the low-temperature forms of Li3PO4 and Li3VO4.
A second orthorhombic modification occurs at elevated temperature, with approximate lattice parameters
a = 6.28 Å b = 10.76 Å c = 5.05 Å.
The low- and high-temperature structures are closely related. In both, arsenic occurs in isolated AsO4 tetrahedra and lithium is approximately tetrahedrally coordinated by oxygen.
A later structural refinement assigned the low-temperature phase to space group Pmn21, with lattice parameters approximately a = 6.286 Å, b = 5.390 Å and c = 4.962 Å.
High-temperature phase At still higher temperature, Li3AsO4 forms a γ-phase related to γ-Li3PO4.
Neutron-diffraction measurements at 770 and 850 °C showed that the γ-phase consists of a distorted hexagonal close-packed array of oxygen atoms in which half of the tetrahedral sites are occupied by lithium and arsenic.
The lithium sublattice becomes substantially disordered at high temperature. One lithium site splits into central and off-centre positions within its tetrahedron, while another lithium species becomes distributed over pairs of face-sharing tetrahedral sites at 850 °C.
The increased lithium disorder is characteristic of the high-temperature phase and is responsible for significant structural differences from the lower-temperature modifications.
Solubility Lithium arsenate is only sparingly soluble in water, in contrast to the corresponding sodium and potassium arsenates.
It is soluble in acidic media such as aqueous acetic acid.
Safety Lithium arsenate is toxic because it contains pentavalent arsenic. Soluble or bioavailable inorganic arsenic compounds can cause serious acute and chronic health effects.
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