Borophosphates (BPO) are ternary and higher intermediate phases of the systems MxOy – B2O3 – P2O5 (– H2O). The systematic research on borophosphates has been carried in recent years. In contrary, the heavier homologues of boron (Al, Ga, In) were already in focus of research (e.g. alumophosphates and metal-substituted derivatives) because of their ability to form microporous materials. The same potential is expected for borophosphates. Furthermore, borophosphates might be promising candidates for applications in nonlinear optics.
The crystal chemistry of borophosphates exhibits a larger variety of patterns, compared with purely tetrahedral structures such as alumophosphates or alumosilicates because of the fact that boron can either be three-fold (trigonal-planar) or four-fold (tetrahedral) coordinated by oxygen. A first approach to the development of a structural chemistry of borophosphates was done based on linking principles of the primary building units following the general line of silicate crystal chemistry [2]. A classification was made according to dimensionality, water content and molar B:P ratio of the borophosphate anions. Later, this approach was modified and a distinction between purely tetrahedral and mixed-coordinated borophosphate anions was proposed [3].
The so far known crystal structures of borophosphates comprise a broad variety of connection patterns including isolated species, rings, chains, layers and even frameworks. Furthermore, syntheses of borophosphates with chiral crystal structures [4-6] 
(Figure 1) and metallo-borophosphates with tetrahedral frameworks were successful showing the close structural relationships with zeolites and feldspars [7,8] (Figure 2).
Borophosphates can be synthesized under mild hydrothermal conditions using common starting materials. The use of organic templates lead to open-framework structures [9,10] (Figure 3). Solid state reactions and microwave assisted reactions are the other less frequently adopted synthetic routes in the preparation of borophosphates. A comprehensive characterization of the products includes X-ray diffraction methods, EDX and SEM, Chemical Analyses, Thermal Analyses and if necessary Magnetic Measurements.