Phase stability regions in the Na2CO3YF3H2O system at 190 °C

Phase stability domains in the Na2CO3YF3H2O system are investigated in sub-critical hydrothermal conditions with microwave heating in Teflon lined autoclaves. At T=190°C, t=1 ht=1 h and P=11×105 PaP=11×105 Pa, five phases are obtained for [YF3] ⩽ 1.0 M and [Na+]/[Y3+]<30[Na+]/[Y3+]<30: NaY(CO3)F2, Na2Y(CO3)F3, Na3Y(CO3)2F2, Na2Y(CO3)2(OH,F), and Na4Y(CO3)2F3⋅H2O. All phases are characterized by powder or single crystal X-ray diffraction, thermogravimetric or chemical analyses. NaY(CO3)F2 is the rare mineral horváthite. Na2Y(CO3)F3, and Na2Y(CO3)2(OH,F) are isostructural with Na2Eu(CO3)F3 and Na2Yb(CO3)2F, respectively; the Na3Y(CO3)2F2 and Na4Y(CO3)2F3⋅H2O phases were still unknown.The average structure of Na3Y(CO3)2F2 is determined in the monoclinic P21/n space group with a=7.047(6) Åa=7.047(6) Å, b=14.840(3) Åb=14.840(3) Å, c=7.192(3) Åc=7.192(3) Å, β=113.32(5)°β=113.32(5)°; V=690.7(6) Å3V=690.7(6) Å3; Z=4Z=4; R=0.076R=0.076 and Rw=0.278Rw=0.278 (1965 unique reflections). The structure is built up from ∞[NaY(CO3)2F2] layers; Y3+ and Na+ cations are disordered in these (010) layers which are separated by other Na+ cations.The structures of Na3Yb(CO3)2F2, previously described, and Na3Y(CO3)2F2 are correlated. Disorder between sodium and ytterbium or yttrium occurs and it is shown by transmission electron microscopy that both structures are noncommensurate.

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