Solid solution of rare earth elements in synthetic titanite: a reconnaissance study

Titanite varieties doped with rare earth elements (REE) have been prepared by ceramic synthesis and quenching in air. Their crystal structure was determined by Rietveld analysis of the powder X-ray diffraction patterns. Two different substitution schemes, Ca1− x Na x /2Sm x /2TiSiO5 and Ca1− x Dy x Ti1− x \(Fe_{x^{3+}}\) SiO5, are studied at x = 0.2. Both synthetic varieties of titanite adopt space group A2/a. This implies that both single-site and complex multivalent substitutional schemes destroy the coherence of the off-centering of octahedral chains in the titanite structure resulting in a P21/a→A2/a phase transition. Unit cell dimensions obtained for the REE-bearing titanite varieties are as follows: a = 7.0541(1)A; b = 8.7247(1)A; c = 6.5664(1)A; β = 113.732(1)° for Ca0.8Na0.1Sm0.1TiSiO5; and a = 7.0021(1)A; b = 8.7256(1)A; c = 6.5427(1)A; β = 113.294(1)° for \({\rm Ca}_{0.8}{\rm Dy}_{0.2}{\rm Ti}_{0.8}{\rm Fe}_{0.2}^{3+} {\rm SiO}_5\). Both REE-doped titanite samples and a control sample of the pure titanite end member have similar unit cell parameters and consist of polyhedra distorted to a similar extent with the exception of more-distorted SiO4 tetrahedron in CaSiTiO5. The structural data suggest that the Ca1− x Na x /2Sm x /2TiSiO5 and Ca1− x Dy x Ti1− x Fe x SiO5 solid solutions adopting the titanite structure might extend to x sufficiently greater than 0.2 and involve both heavier and lighter trivalent rare earth elements.