In situ high-pressure infrared spectroscopy of carbonophosphates Na3Mn(PO4)(CO3) and Na3Mg(PO4)(CO3)

Carbonophosphates are a typical proxy of [CO 3 ] and [PO 4 ] coexistent phases, and besides, they have enormous potential as rechargeable battery materials. Previous studies on stability, vibrations and electrochemical performances of carbonophosphates are mostly limited to ambient conditions, and little knowledge has been acquired on their structural modifications and compression behaviors. The present study is devoted to the high-pressure infrared spectra of carbonophosphates Na 3 Mn(PO 4 )(CO 3 ) and Na 3 Mg(PO 4 )(CO 3 ) in diamond anvil cells. We firstly confirm that Na 3 Mg(PO 4 )(CO 3 ) maintains its structural stability up to 17.3 GPa, and discover a mode softening of Na 3 Mn(PO 4 )(CO 3 ) at 14.0 GPa. This indicates a structural modification occurs in Na 3 Mn(PO 4 )(CO 3 ), which is suggested to be resulted from compression anisotropy. Another interesting finding is that the [PO 4 ] units vary much more significantly in Na 3 Mn(PO 4 )(CO 3 ), whereas the [CO 3 ] units deform a little more in Na 3 Mg(PO 4 )(CO 3 ). This different behavior results in a lower structural stability of Na 3 Mn(PO 4 )(CO 3 ) in comparison with Na 3 Mg(PO 4 )(CO 3 ), because the [PO 4 ] units play an architectural role in the structure whereas the [CO 3 ] units are just like interstitial ions. Furthermore, the mode Grüneisen parameters ( γ ) values of the [PO 4 ] and [CO 3 ] modes in carbonophosphates have been determined to be lower than those in phosphates and carbonates. This indicates that the [PO 4 ] and [CO 3 ] units in carbonophosphates are less compressed or deformed than those in phosphates and carbonates under the same pressure. Hence, carbonophosphates can be a potential carrier during phosphorus and carbon cycling in the Earth, and control their geochemistry in a certain range. The current study provides new findings on physical mechanisms and systematics of carbonophosphates, which would improve our understanding of role of carbonophosphates in both material sciences and Earth sciences. • Mode softening of carbonophosphates Na 3 Mn(PO 4 )(CO 3 ) is discovered at 14.0 GPa. • Different pressure-behavior of [CO 3 ] and [PO 4 ] are found in carbonophosphates. • Compression behaviors of carbonophosphates are compared with the analogues.