Mathias Gogolin , M. Mangir Murshed , Lkhamsuren Bayarjargal , Detlef Klimm and Thorsten M. Gesing

Zeitschrift für Kristallographie - Crystalline Materials (2021), 236, 283-292

https://doi.org/10.1515/zkri-2021-2046

Pb₂Sc₂Si₂O₉ and Pb₂In₂Si₂O₉, respectively, the scandium and indium containing structural analogues of the mineral kentrolite are grown by spontaneous crystallization from a PbO flux. The corresponding polycrystalline powder samples are synthesized by conventional solid-state approach. The compounds are thoroughly characterized using temperature-dependent single crystal and powder X-ray diffraction, heat capacity measurements, second harmonic generation experiments and Raman spectroscopy. At ambient conditions, both compounds crystallize in the non-centrosymmetric Pna2₁ space group and undergo phase transitions to the centrosymmetric Pbcn space group at elevated temperatures. The Pbcn into Pna2₁ phase transitions are complemented by the signals of the temperature-dependent second harmonic generation. The specific heat capacity exhibits distinct cusp, supporting the λ-type second-order phase transition. The temperature dependency of some selective Raman modes further complements the findings, showing softening and hardening of the phonons across the phase transitions.