Phase stability regions in the Na 2CO 3[sbnd]YF 3[sbnd]H 2O system at 190 °C

Abstract

Phase stability domains in the Na 2CO 3 YF 3 H 2O system are investigated in sub-critical hydrothermal conditions with microwave heating in Teflon lined autoclaves. At T = 190 °C , t = 1 h and P = 11 × 10 5 Pa , five phases are obtained for [YF 3] ⩽ 1.0 M and [ Na + ] / [ Y 3 + ] < 30 : NaY(CO 3)F 2, Na 2Y(CO 3)F 3, Na 3Y(CO 3) 2F 2, Na 2Y(CO 3) 2(OH,F), and Na 4Y(CO 3) 2F 3⋅H 2O. All phases are characterized by powder or single crystal X-ray diffraction, thermogravimetric or chemical analyses. NaY(CO 3)F 2 is the rare mineral horváthite. Na 2Y(CO 3)F 3, and Na 2Y(CO 3) 2(OH,F) are isostructural with Na 2Eu(CO 3)F 3 and Na 2Yb(CO 3) 2F, respectively; the Na 3Y(CO 3) 2F 2 and Na 4Y(CO 3) 2F 3⋅H 2O phases were still unknown. The average structure of Na 3Y(CO 3) 2F 2 is determined in the monoclinic P2 1/n space group with a = 7.047 ( 6 ) Å , b = 14.840 ( 3 ) Å , c = 7.192 ( 3 ) Å , β = 113.32 ( 5 ) ° ; V = 690.7 ( 6 ) Å 3 ; Z = 4 ; R = 0.076 and R w = 0.278 (1965 unique reflections). The structure is built up from ∞[NaY(CO 3) 2F 2] layers; Y 3+ and Na + cations are disordered in these (010) layers which are separated by other Na + cations. The structures of Na 3Yb(CO 3) 2F 2, previously described, and Na 3Y(CO 3) 2F 2 are correlated. Disorder between sodium and ytterbium or yttrium occurs and it is shown by transmission electron microscopy that both structures are noncommensurate.