Abstract
A glass ceramic from the MgO–Al 2 O 3 –SiO 2 system containing additives of ZrO 2 , TiO 2 , La 2 O 3 and P 2 O 5 was investigated. Via x-ray powder diffraction (XRPD) and Transmission electron microscopy(TEM) the crystalline phases present could be identified as MgAl 2 O 4 (Spinel), orthorhombic ZrTiO 4 and a polymorph of LaPO 4 with a previously unknown crystal structure. The crystal structure of this LaPO 4 phase was solved ab initio via 3D ED Data. The polymorph occurs in a distorted barite structure type in the same space group (P2 1 /n) as the stable monazite type polymorph, however the density is higher and the La has a higher coordination number. Furthermore the LaPO 4 is present in a spinodal relict microstructure. Potential explanations for the presence of this metastable polymorph are pressure due to residual stresses from the crystallization or surface effects due to the high surface area resulting from the spinodal relict structure.
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