Discrete Polyoxovanadate Cluster into an Organic Free Metal-Oxide-Based Material: Syntheses, Crystal Structures, and Magnetic Properties of a New Series of Lanthanide Linked-POV Compounds [{Ln(H 2 O) 6 } 2 As 8 V 14 O 42 (SO 3 )]·8H 2 O (Ln = La 3+ , Sm 3+ , and Ce 3+ ) †