Crystal Chemistry of the Noncentrosymmetric Eulytites: A₃Bi(XO₄)₃ (X = V, A = Pb; X = P, A = Ca, Cd, Sr, Pb)

Eulytite compounds, A3Bi(XO4)3 (X = P, A = Ca, Cd, Sr, Pb), belong to the noncentrosymmetric space group I4̅3d (No. 220) as determined by single-crystal X-ray diffraction studies. The crystals were grown from the melt−cool technique with considerable difficulty as the compounds melt incongruently at their melting temperature, except for the compound Pb3Bi(PO4)3. The unit cell parameter a is 9.984(5), 9.8611(3), 10.2035(3), and 10.3722(2) Å for Ca3Bi(PO4)3, Cd3Bi(PO4)3, Sr3Bi(PO4)3, and Pb3Bi(PO4)3 respectively, and there are four formula units in the unit cell. The structure of Pb3Bi(VO4)3, a unique eulytite with vanadium substitution, is compared with all these phosphorus substituted eulytites. The A2+ and Bi3+ cations occupy the special position (16c) while the O anions occupy the general Wyckoff position (48e) in the crystal structure. Only one O position has been identified for Pb3Bi(PO4)3 and Pb3Bi(VO4)3, whereas two O atom sites were identified for Ca3Bi(PO4)3, Cd3Bi(PO4)3, and Sr3Bi(PO4)3. The UV−vis diffuse reflectance spectra indicate large band gaps for all the phosphate eulytites while a lower band gap is observed for the vanadate eulytite. The feasibility of the use of these compounds in optoelectronic devices has been tested by measuring the second-harmonic generation (SHG) values which have been found to be of a magnitude equivalent to the commercially used KDP (KH2PO4).