Calcium orthophosphates (CaPs) are an important class of minerals in geology, as well as biomineralization and biomedicine. They play a crucial role for many organisms, besides humans, as they are the essential constituents of bone and tooth enamel among vertebrates. To date, CaPs have been found to occur in less than a dozen different crystalline forms, where apatite is the most abundant in vertebrate organisms. Most of the known CaPs were identified during the 18th and 19th century, and later, their atomic structures were determined with X-ray diffraction techniques in the 20th century. During the last century of research on CaPs, no new crystalline phases have been uncovered.  
In this study we identify a new member of the family of crystalline CaPs, dicalcium phosphate monohydrate (DCPM). This finding was possible through a careful investigation of the synthesis regime in which metastable CaPs form, as well as the development of new methods for atomic structure determination based on electron diffraction.

Figure 1. The crystal structures of dicalcium phosphate (a), the new phase dicalcium phosphate monohydrate (b) and dicalcium phosphate dihydrate (c) viewed along c-, c- and a-axes, respectively. The three phases all show different packing of calcium- (shown in green) and phosphate ions (containing one phosphorous atom surrounded by four oxygen atoms, shown in magenta and red respectively). Dicalcium phosphate monohydrate (b) and dicalcium phosphate dihydrate (c) are both layered structures with water molecules (represented in blue) linking the layers. The new phase, however, contains half the amount of water compared to dicalcium phosphate dihydrate and the layer itself has a different atomic structure. (Hydrogen atoms are omitted for clarity).

The newly discovered phase DCPM is metastable, that is, it transforms into other known CaP phases under different conditions. In water, dicalcium phosphate monohydrate transforms into hydroxyapatite. It can, however, be stabilized by organic molecules such as citrate salts and sodium polyacrylate, which are biocompatible organic molecules. DCPM has a higher uptake capacity of organic therapeutic compounds, such as ibuprofen and doxorubicin hydrochloride, than other metastable CaPs currently used in applications. These findings indicate that DCPM could lead to innovations, resulting in new and/or improved applications, e.g. as medical implants, drug carriers or pollutant treatments.

Read more about the discovery here: