The preparation of inorganic compounds, exhibiting open-framework structures, by hydrothermal methods has been presented. To illustrate the efficacy of this approach, few select examples encompassing a wide variety and diversity in the structures have been provided. In all the cases, good quality single crystals were obtained, which were used for the elucidation of the structure. In the first example, simple inorganic network compounds based on phosphite and arsenate are described. In the second example, inorganic-organic hybrid compounds involving phosphite/arsenate along with oxalate units are presented. In the third example, new coordination polymers with interesting structures are given. The examples presented are representative of the type and variety of compounds one can prepare by careful choice of the reaction conditions.

The metal ions (Ti⁺⁴, Mg⁺², Zn⁺² and Co⁺²) have been substituted in place of Al$^{+3}$ in aluminophosphates (AlPOs). These compounds were used for the first time as possible photocatalysts for the degradation of organic dyes. Among the doped AlPOs, ZnAlPO-5, CoAlPO-5, MgAlPO-11, 18 and 36 did not show any photocatalytic activity. MgAlPO-5 showed photocatalytic activity and different loading of Mg (4, 8, 12 atom % of Mg) were investigated. The activity can be enhanced by the increasing of concentration of the doped metal ions. TiAlPO-5 (4, 8, 12 atom % of Ti) showed the highest photocatalytic activity among all the compounds and its activity was compared to that of Degussa P25 (TiO₂). The activity of photocatalysts was correlated with the diffuse reflectance and photoluminescence spectra.

A series of lanthanide(III) based coordination polymers have been synthesized using chelidamic acid (CA) as a ligand, [Ln(C₇H₂NO₅ (H₂O)₃]. H₂O (Ln = Eu, Gd, Tb, and Dy). All the compounds were prepared by solvothermal technique using H₂O-DMF as solvents (4:1, volume ratio). Compounds were characterized through various instrumental techniques such as single-crystal X-ray diffraction, powder X-ray diffraction, thermogravimetric analysis, IR spectroscopy, etc. In all the cases lanthanide ion adopts distorted dodecahedrongeometry. And these dodecahedrons are connected through chelidamic acid ligand to form the two-dimensional structure. These layers are H-bonded to form the three-dimensional supramolecular structure. The optical bandgap energy measurements exhibit that the variation of the band gap energy is independent of the f -electrons. This is due to the weak bonding connectivity between the metal ion and ligands, which could not perturb the density ofstates significantly. The variable-temperature magnetic measurements exhibit the paramagnetic behavior of all the compounds though the effective magnetic moments rise with increasing number of f -electrons. The present study illustrates the usefulness of the lanthanides for the structure building as well as the role of f -electrons for opto-electronic behaviors.