Compounds exhibiting AB 2 -type tetrahedral network structures are versatile materials and of great technical importance. First of all microporous zeolites are known for their outstanding absorption properties and catalytic behavior and are therefore extensively used in industrial, agricultural, and laboratory environments. [1] However, the importance of dense AB 2 networks should not be underestimated. SiO 2 and to a minor extent GaPO 4 are applied in piezoelectric devices, such as pressure sensors and microbalances, [2] while quartz-like compounds in general, including phosphates such as AlPO 4 and BPO 4 , can also be used for second-harmonic generation (SHG) purposes. [3,4] Owing to the variety of applications, widespread research into novel AB 2 -type structures was conducted, including the prediction of more than two million unique prospective crystal structures for zeolites. [5] However, only a minute subset of possible structures has been realized to date. In this search for new structure types, we have directed our attention to the system P-O-N, which is isoelectronic to silica. The inclusion of nitrogen provides additional structural flexibility, which theoretically opens up an even wider range of possible structure types. The few known compounds in this system include the first nitridic zeolites NPO [6] and Ba 19 P 36 O 6+x N 66Àx Cl 8+x (x % 4.54) [7] as well as the nitridic clathrate P 4 N 4 (NH) 4 (NH 3 ) [8] and the polymorphs of PON exhibiting cristobalite-, [9] quartz-, [10] and moganite-type [11] structures. Glassy compounds in the system Li-Ca-P-N also exhibit desirable properties, such as a high hardness and refractive index. [12] The great potential for novel structure types in this system is offset by difficulties in preparation, such as thermal decomposition, low reactivity, and a low degree of crystallinity. To circumvent the mentioned problems, we developed a novel synthetic approach, utilizing an amorphous single-source precursor. Herein, we report on a new high-pressure phase of phosphorus oxonitride PON. Since this is the fourth known polymorph of PON, we propose the name d-PON. Unlike the quartz and moganite polymorphs, it is not directly accessible by treating cristobalite-type PON under high-pressure/high-temperature conditions. This result hints at the possibility of d-PON being thermodynamically metastable at these conditions. Instead, we prepared d-PON by carrying out the final thermal condensation step of an amorphous phosphorus oxonitride imide under high pressure by employing the multianvil technique. [13] The starting material was subjected to a temperature of around 1350 8C at 12 GPa for 120 min in a Walkertype module. The product could be obtained as a hard, colorless solid.The crystal structure of d-PON was elucidated ab initio with X-ray powder diffraction data in space group P2 1 /c (no. 14). [21] Final refinement was carried out by employing the Rietveld method (Figure 1). Energy-dispersive X-ray spectroscopy showed the presence of P, O, and N, while no other elements w...