Cs 3 LiZn 2 (WO 4 ) 4 and Rb 3 Li 2 Ga(MoO 4 ) 4 : different filled derivatives of the cation-deficient Cs 6 Zn 5 (MoO 4 ) 8 structure.

Two new compounds, namely cubic tricaesium lithium dizinc tetrakis(tetraoxotungstate), Cs ₃ LiZn ₂ (WO ₄ ) ₄ , and tetragonal trirubidium dilithium gallium tetrakis(tetraoxomolybdate), Rb ₃ Li ₂ Ga(MoO ₄ ) ₄ , belong to the structural family of Cs ₆ Zn ₅ (MoO ₄ ) ₈ (space group I-43d, Z = 4), with a partially incomplete (Zn _5/6 □ _1/6 ) position. In Cs ₃ LiZn ₂ (WO ₄ ) ₄ , this position is fully statistically occupied by (Zn _2/3 Li _1/3 ), and in Rb ₃ Li ₂ Ga(MoO ₄ ) ₄ , the 2Li + Ga atoms are completely ordered in two distinct sites of the space group I-42d (Z = 4). In the same way, the crystallographically equivalent A ⁺ cations (A = Cs, Rb) in Cs ₆ Zn ₅ (MoO ₄ ) ₈ , Cs ₃ LiZn ₂ (WO ₄ ) ₄ and isostructural A ₃ LiZn ₂ (MoO ₄ ) ₄ and Cs ₃ LiCo ₂ (MoO ₄ ) ₄ are divided into two sites in Rb ₃ Li ₂ Ga(MoO ₄ ) ₄ , as in other isostructural A ₃ Li ₂ R(MoO ₄ ) ₄ compounds (AR = TlAl, RbAl, CsAl, CsGa, CsFe). In the title structures, the WO ₄ and (Zn,Li)O ₄ or LiO ₄ , GaO ₄ and MoO ₄ tetrahedra share corners to form open three-dimensional frameworks with the caesium or rubidium ions occupying cuboctahedral cavities. The tetrahedral frameworks are related to that of mayenite 12CaO·7Al ₂ O ₃ and isotypic compounds. Comparison of isostructural Cs ₃ MZn ₂ (MoO ₄ ) ₄ (M = Li, Na, Ag) and Cs ₆ Zn ₅ (MoO ₄ ) ₈ shows a decrease of the cubic lattice parameter and an increase in thermal stability with the filling of the vacancies by Li ⁺ in the Zn position of the Cs ₆ Zn ₅ (MoO ₄ ) ₈ structure, while filling of the cation vacancies by larger Na ⁺ or Ag ⁺ ions plays a destabilizing role. The series A ₃ Li ₂ R(MoO ₄ ) ₄ shows second harmonic generation effects compatible with that of β'-Gd ₂ (MoO ₄ ) ₃ and may be considered as nonlinear optical materials with a modest nonlinearity.