Compressibility of carbonophosphate bradleyite Na 3 Mg(CO 3 )(PO 4 ) by X-ray diffraction and Raman spectroscopy

Bradleyite Na 3 Mg(CO 3 )(PO 4 ) is one typical carbonophosphate, representing dual properties of both carbonates and phosphates. Compressibility of bradleyite has been investigated using synchrotron radiation X-ray diffraction and Raman spectroscopy combined with diamond anvil cells up to 41 GPa at room temperature. Experimental results clearly demonstrate that bradleyite is stable in the investigated pressure conditions. Isothermal pressure–volume relationship has been fitted to the third-order Birch–Murnaghan equation of state with K 0 = 65.9(9) GPa, K 0 ′ = 3.08(3) and V 0 = 301.4(4) Å 3 . The crystallographic axes exhibit similar and considerable compressibility up to ~16 GPa, but an increasing anisotropy can be defined thereafter because the b - axis becomes more and more rigid. Insight into the behaviors of [PO 4 ] 3− and [CO 3 ] 2− groups have been obtained using Raman spectroscopy, with the symmetrical stretching bands being observed at 970.8 and 1,078.7 cm −1 , respectively. Both modes shift to higher frequencies on compression. The pressure coefficient of Raman shifts in C–O is 3.18(2) × 10 −3 cm −1 /GPa, and the value of Grüneisen parameters ( γ ) is ~0.22. [PO 4 ] 3− groups in bradleyite exhibit a mode hardening, with a slope of 3.54(6) × 10 −3 cm −1 /GPa, γ = ~0.24 below ~16 GPa, and of 2.37(1) × 10 −3 cm −1 /GPa, γ = ~0.16 thereafter. The mode hardening may indicate correlations to the increase in symmetry of the local environment around the [PO 4 ] tetrahedra. The low γ values are compatible with the rigidness of [CO 3 ] 2− and [PO 4 ] 3− in bradleyite, which have, respectively, been documented in carbonates and phosphates. Our present study may open new perspectives on the Earth carbon and phosphorus cycle, arouse the interest in carbonophosphate minerals, and shed light on other [CO 3 ] 2− and [PO 4 ] 3− coexistent phases.