Single-crystal X-ray diffraction study of the incommensurately modulated structure of flamite under high pressure

The influence of pressure on the incommensurately modulated structures has been poorly studied to date, particularly on the structures modulated at ambient conditions. Among them is the structure of natural flamite with the general formula (Ca,K,Na)2(Si,P)O4. The behavior of the incommensurately modulated structure of flamite was studied by single-crystal X-ray diffraction in a diamond-anvil cell up to 5 GPa. The modulation vector of flamite remains constant in the investigated pressure range. In the range up to 2–3 GPa, the atomic modulation parameters in the flamite structure are not sensitive to applied pressure, but at higher pressures, amplitudes of the atomic modulation functions increase. The PV-curve of flamite was fitted with the second-order Birch–Murnaghan equation of state: V0 = 350.8(7) Å3, K0 = 85(9) GPa.