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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study presents an investigation on the axial negative thermal expansion of α-PbAlBO4 and β-PbAlBO4. Polycrystalline and single-crystal samples were prepared by solid-state synthesis method, characterized by temperature- and pressure-dependent X-ray and neutron diffraction experiments. The axial negative linear compressibility (NLC) is known for the α-polymorph, although structurally different the β-polymorph also shows similar NLC phenomenon. The lattice thermal expansion was described using the first-order Grüneisen equation of state, where the vibrational energy was calculated using the Debye–Einstein-Anharmonicity model. The density functional theory (DFT)-based phonon density of states and Raman spectra helped to choose the characteristic frequency to model the metric parameters. Lattice thermal expansion was additionally simulated in the quasi-harmonic approximation using the plane-wave DFT approach at the PBEsol level. The apparent departure of the quasi-harmonic approximation model has been discussed in terms of associated thermodynamic functions. The interplay between the topology-induced negative cross-compliance and anisotropic Grüneisen parameter has been identified as the driving force for the axial negative linear compressibility and axial negative thermal expansion. [ABSTRACT FROM AUTHOR]