A chromic molybdenum phosphate with a tunnel structure: hydrothermal synthesis and structure of (NH/sub 3/CH/sub 2/CH/sub 2/NH/sub 3/)/sub 2/.(NH/sub 3/CH/sub 2/CH/sub 2/NH/sub 2/)/sub 3/ .[NaCr/sub 2/Mo/sub 12/O/sub 30/(PO/sub 4/)(HPO/sub 4/)/sub 4/(H/sub 2/PO/sub 4/)/sub 3/].6H/sub 2/O