The structural, optical, and electronic properties of a series of lithium rare earth polyphosphates [LiLn(PO 3 ) 4 ] [Ln = La (1), Eu (2), Gd (3)] have been investigated by means of single-crystal X-ray diffraction, elemental analyses, and spectroscopic measurements, as well as calculations of energyband structures, density of states, and optical response functions by density functional methods. These LiLn(PO 3 ) 4 systems are monoclinic with space group C2/c and Z = 4. Their unit-cell parameters decrease as the ionic radius of Ln 3+ de-