Solid solubility in the system NaLREETi2O6-ThTi2O6 (LREE, light rare-earthelements): experimental and analytical data

The existence of an incomplete solid solution series between loparite (NaLR EETi2O6), a member of the perovskite mineral group, and thorutite (ThTi2O6) is established on the basis of experimental and mineralogical data. The pr oducts of low- and high-pressure synthesis in the system NaLaTi2O6 - ThTi2O 6 were studied by energy-dispersive spectrometry, X-ray diffractometry and Rietveld analysis. At atmospheric pressure, Th is incorporated in loparite as both ThTi2O6 and Na2ThTi3O9. In synthetic systems, the maximum determine d ThTi2O6 content of loparite is 18 mol%, with a corresponding A-site catio n deficiency of 9%. The structure of such loparite is tetragonal and presum ably derived from the cubic aristotype by octahedral rotation [14/mcm, a=5. 4652(1) Angstrom, c=7.7476(2) Angstrom]. At a pressure of 6 GPa, no solubil ity between loparite and ThTi2O6 is observed, and Th is accommodated in the loparite structure entirely as Na2ThTi3O9 (up to 30 mol%). Naturally occur ring loparite contains up to 29 mol% ThTi2O6, based on the conventional met hod of analysis recalculation, or 23.5 mol% ThTi2O6, assuming the presence of protons at the vacant A-sites. ThTi2O6 synthesized by the solid-state re action, crystallizes with monoclinic metry [C2/m, a=9.8140(2) Angstrom, b=3 .8228(1) Angstrom, c=7.0313(2) Angstrom, beta=118.82(1)degrees]. Atomic coo rdinates for ThTi2O6 obtained in this study from X-ray powder data, as well as structural parameters derived from the new data, are in a good agreemen t with those known from single-crystal refinement. ThTi2O6 does not crystal lize at high pressure, and Th is accommodated in perovskite-type compounds and cubic ThO2 that provide a twelve- and eight-fold coordination site for Th, respectively.