Crystal chemistry of rubidium uranyl molybdates: crystal structures of Rb/sub 6/[(UO/sub 2/)(MoO/sub 4/)/sub 4/], Rb/sub 6/[(UO/sub 2/)/sub 2/O(MoO/sub 4/)/sub 4/], Rb/sub 2/[(UO/sub 2/)(MoO/sub 4/)/sub 2/], Rb/sub 2/[(UO/sub 2/)/sub 2/(MoO/sub 4/)/sub 3/] and Rb/sub 2/[(UO/sub 2/)/sub 6/(MoO/sub 4/)/sub 7/(H/sub 2/O)/sub 2/]