Calcium carbonate is a relevant constituent of the Earth’s crust that is transferred into the deep Earth through the subduction process. Its chemical interaction with calcium-rich silicates at high temperatures give rise to the formation of mixed silicate-carbonate minerals, but the structural behavior of these phases under compression is not known. Here we report the existence of a dense polymorph of Ca 5 (Si 2 O 7 )(CO 3 ) 2 tilleyite above 8 GPa. We have structurally characterized the two phases at high pressures and temperatures, determined their equations of state and analyzed the evolution of the polyhedral units under compression. This has been possible thanks to the agreement between our powder and single-crystal XRD experiments, Raman spectroscopy measurements and ab-initio simulations. The presence of multiple cation sites, with variable volume and coordination number (6–9) and different polyhedral compressibilities, together with the observation of significant amounts of alumina in compositions of some natural tilleyite assemblages, suggests that post-tilleyite structure has the potential to accommodate cations with different sizes and valencies.

The physicochemical properties of rare-earth zirconates can be tuned by the rational modification of their structures and phase compositions. In the present work, La 3+ -, Nd 3+ -, Gd 3+ -, and Dy 3+ -zirconate nanostructured materials were prepared by different synthetic protocols, leading to powders, xerogels, and, for the first time, monolithic aerogels. Powders were synthesized by the co-precipitation method, while xerogels and aerogels were synthesized by the sol−gel technique, followed by ambient and supercritical drying, respectively. Their microstructures, thermogravimetric profiles, textural properties, and crystallographic structures are reported. The co-precipitation method led to dense powders (S BET < 1 m 2 g −1 ), while the sol−gel technique resulted in large surface area xerogels (S BET = 144 m 2 g −1 ) and aerogels (S BET = 168 m 2 g −1 ). In addition, the incorporation of lanthanide ions into the zirconia lattice altered the crystal structures of the powders, xerogels, and aerogels. Single-phase pyrochlores were obtained for La 2 Zr 2 O 7 and Nd 2 Zr 2 O 7 powders and xerogels, while defect fluorite structures formed in the case of Gd 2 Zr 2 O 7 and Dy 2 Zr 2 O 7 . All aerogels contain a mixture of cubic and tetragonal ZrO 2 phases. Thus, a direct effect is shown between the drying conditions and the resulting crystalline phases of the nanostructured rare-earth zirconates.

The search for material hosts being able to incorporate Er 3+ impurities with a thermally stable structure and a high melting temperature is priority in optical thermometry. In this work, we report on the structural and spectroscopic characterization of Er 3+ -doped and Yb 3+ /Er 3+codoped LaGdO 3 nanocrystals synthesized via the sol−gel Pechini method. X-ray diffraction and Raman spectroscopy unequivocally show that the synthesis method provides nanocrystals with a single-phase B-type monoclinic structure (space group, C2/m). Intensity decay curves, I(t), were measured to investigate the efficiency of upconversion processes yielding green emission. We showed that an energy transfer upconversion (ETU) process involving Yb−Er pairs governs visible emission upon near-infrared (NIR) excitation. The temperature dependence of the thermalized green luminescence at 525 nm ( 2 H 11/2 → 4 I 15/2 ) and 549 nm ( 4 S 3/2 → 4 I 15/2 ) was checked for thermometric applications in the room temperature (RT) to 900 K range. We demonstrate that the B-type monoclinic phase of LaGdO 3 is stable from low temperatures up to 900 K. Doped with Er 3+ , it shows suitable thermometer capabilities with a maximum sensitivity of S = 4.3 × 10 −3 K −1 at 554 K and a relative sensitivity decreasing from its maximum value at 0 K to S R = 1.2 × 10 −2 K −1 at 298 K. The results suggest that LaGdO 3 in its B-type monoclinic phase is a promising material as a wide-range temperature sensor, without any further surface protection.

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