Low temperature phase transition and structure of CsMgPO₄

CsMgPO4 doped in radioisotopes is a promising compound for usage as a radioactive medical source. However, a low temperature phase transition at the temperatures close to ambient conditions (-37⁰C) was observed. Information about structural changes is important in order to understand whether it can cause any problem for medical use of this compound. Structural changes have been investigated in detail using synchrotron powder diffraction methods, Raman spectroscopy and DFT calculations. The structure undergoes transformation from orthorhombic modification, sp. gr. Pnma (RT phase) to monoclinic modification, sp.gr P21/n (LT phase). New LT modification adopts similar to RT but slightly distorted unit cell: a=9.58199(2)Å, b=8.95501(1) Å, c=5.50344(2)Å, β=90.68583(1)⁰, V=472.198(3) Å3. The framework is made up of alternating magnesia and phosphate tetrahedra sharing vertices with caesium counter cations located in the channels formed. Upon the transformation a combined rotation of PO4 and MgO4 tetrahedral takes place. A comparison with other phase transition in ABW-type framework class compounds is given.

Nanoindentation and ellipsometry of Li₂ZnTi₃O₈ & Zn₂TiO₄ single crystals

Spinel-type Li₂ZnTi₃O₈ and Zn₂TiO₄ are useful for various industrial applications due to their interesting chemical and physical properties, for example, as promising anode materials in Li-ion batteries or as components in dielectric devices [1]. Since Li₂ZnTi₃O₈ and Zn₂TiO₄ are expected to have high refractive indices (n calc. = 2,33 and 2,26) we tried to characterize these materials in more detail including single-crystal X-ray diffraction, nanoindentation, spectroscopic ellipsometry and electron microprobe analysis. Single crystals of Li₂ZnTi₃O₈ and Zn₂TiO₄ were grown directly from melt at 1723 K and 1873 K, respectively. Fragments of sintered polycrystalline Li₂ZnTi₃O₈ and Zn₂TiO₄ precursors were placed on an iridium sheet and fired in a muffle furnace from 1273 to 1723/1873 K with a heating ramp of 15 K/min. After a dwell time of 3 min the melt was cooled down to 1473 K with a ramp of 10 K/min and subsequently quenched in water. Structural investigations of the Li₂ZnTi₃O₈ and Zn₂TiO₄ single crystals resulted in the following basic crystallographic data: cubic, P4₃32, a = 8.3697(2) Å, V = 586.31(3) ų, Z = 4 and Fd-3m, a = 8.46230(17) Å, V = 605.99(2) ų, Z = 8, respectively. Nanoindentation experiments were performed with a Berkovich diamond indenter tip to determine the hardness and elastic modulus of Zn₂TiO₄ and Li₂ZnTi₃O₈. For sample preparation the single crystals were embedded in resin and polished to a mirror-like surface finish. More than 150 indents with a distance of 10 µm were made with a maximum load of 20 mN. Analysis of the load-displacement curves for Zn₂TiO₄ revealed a hardness of 10.51 ¹ 0.39 GPa and a reduced elastic modulus of 180.90 ¹ 3.92 GPa. Atomic force micrographs displayed indents with a max. depth of 288 ¹ 5 nm. Li₂ZnTi₃O₈ exhibited a hardness of 6.86 ¹ 0.45 GPa and a reduced elastic modulus of 148.88 ¹ 6 GPa. Zn₂TiO₄ showed a dispersion of 0.09 due to the variation of the refractive index from 2.24 (430,8 nm, Fraunhofer G line) and 2.15 (686,7 nm, B line).