Stereochemically Active Lone Pairs: Unraveling the Electronic Origin of Dielectric Response in Nonlinear Optical TeO₂

Tellurite glasses have emerged as a promising candidate for nonlinear optics (NLO) due to their high refractive index and third-order optical susceptibility (χ3). In this work, we focus on the electronic structure of crystalline TeO2, a precursor to numerous TeO2-based glasses. All three crystalline TeO2 polymorphs are wide-gap semiconductors and have electronic contributions from the Te4+ 5s2 lone pairs near the Fermi level, which affect optical properties. Based on the formation energies, the formation of oxygen vacancies is strongly favorable during synthesis. Introduction of an oxygen vacancy induces notable changes in the chemical environment in the pristine α-TeO2 structure and the response of the Te 5s2 lone pairs. Comparative analysis to the measured refractive indices and static dielectric constant (a second-rank tensor) shows that the calculated values of α-TeO2 with the presence of an oxygen vacancy (α-TeO2:Ov) agree well with experimental data. The high distortion index and stereochemical activity ratio of α-TeO2:Ov contribute to its polarizable asymmetric electron density. High polarization and faster orientation of the dipoles in α-TeO2 with the presence of an oxygen vacancy make it an excellent NLO material for ultrafast optical switch applications. This study can help to guide design principles and improve our understanding of the electronic origin of NLO materials containing lone pairs

Vacancy-Driven Disorder and Elevated Dielectric Response in the Pyrochlore Pb_1.5Nb₂O_6.5

Lone pair-driven distortions are a hallmark of many technologically important lead (Pb)-based materials. The role of Pb2+ in polar perovskites is well understood and easily manipulated for applications in piezo- and ferroelectricity, but the control of ordered lone pair behavior in Pb-based pyrochlores is less clear. Crystallographically and geometrically more complex than the perovskite structure, the pyrochlore structure is prone to geometric frustration of local dipoles due to a triangular arrangement of cations on a diamond lattice. The role of vacancies on the O′ site of the pyrochlore network has been implicated as an important driver for the expression and correlation of stereochemically active lone pairs in pyrochlores such as Pb2Ru2O6.5 and Pb2Sn2O6. In this work we report on the structural, dielectric, and heat capacity behavior of the cation- and anion-deficient pyrochlore Pb1.5Nb2O6.5 upon cooling. We find that local distortions are present at all temperatures that can be described by cristobalite-type cation ordering, and this ordering persists to longer length scales upon cooling. From a crystallographic perspective, the material remains disordered and does not undergo an observable phase transition. In combination with density function calculations, we propose that the stereochemical activity of the Pb2+ lone pairs is driven by proximity to O′ vacancies, and the crystallographic site disorder of the O′ vacancies prohibits long range correlation of lone pair-driven distortions. This in turn prevents a low-temperature phase transition and results in an elevated dielectric permittivity across a broad temperature range

Structures and Phase Transitions of CePd_3+<i>x</i>Ga_8‑<i>x</i>: New Variants of the BaHg₁₁ Structure Type

New distorted variants of the cubic BaHg₁₁ structure type have been synthesized in Ga flux. Multiple phases of CePd_3+<i>x</i>Ga_8‑<i>x</i>, which include an orthorhombic <i>Pmmn</i> structure (<i>x</i> = 3.21(2)), a rhombohedral <i>R</i>3̅<i>m</i> structure (<i>x</i> = 3.13(4)), and a cubic <i>Fm</i>3̅<i>m</i> superstructure (<i>x</i> = 2.69(6)), form preferentially depending on reaction cooling rate and isolation temperature. Differential thermal analysis and <i>in situ</i> temperature-dependent powder X-ray diffraction patterns show a reversible phase transition at approximately 640 °C between the low temperature orthorhombic and rhombohedral structures and the high temperature cubic superstructure. Single crystal X-ray diffraction experiments indicate that the general structure of BaHg₁₁, including the intersecting planes of a kagomé-type arrangement of Ce atoms, is only slightly distorted in the low temperature phases. A combination of Kondo, crystal electric field, and magnetic frustration effects may be present, resulting in low temperature anomalies in magnetic susceptibility, electrical resistivity, and heat capacity measurements. In addition to CePd_3+<i>x</i>Ga_8‑<i>x</i>, the rare earth analogues REPd_3+<i>x</i>Ga_8‑<i>x</i>, RE = La, Nd, Sm, Tm, and Yb, were successfully synthesized and also crystallize in one of the lower symmetry space groups

Structures and Phase Transitions of CePd_3+<i>x</i>Ga_8‑<i>x</i>: New Variants of the BaHg₁₁ Structure Type

New distorted variants of the cubic BaHg₁₁ structure type have been synthesized in Ga flux. Multiple phases of CePd_3+<i>x</i>Ga_8‑<i>x</i>, which include an orthorhombic <i>Pmmn</i> structure (<i>x</i> = 3.21(2)), a rhombohedral <i>R</i>3̅<i>m</i> structure (<i>x</i> = 3.13(4)), and a cubic <i>Fm</i>3̅<i>m</i> superstructure (<i>x</i> = 2.69(6)), form preferentially depending on reaction cooling rate and isolation temperature. Differential thermal analysis and <i>in situ</i> temperature-dependent powder X-ray diffraction patterns show a reversible phase transition at approximately 640 °C between the low temperature orthorhombic and rhombohedral structures and the high temperature cubic superstructure. Single crystal X-ray diffraction experiments indicate that the general structure of BaHg₁₁, including the intersecting planes of a kagomé-type arrangement of Ce atoms, is only slightly distorted in the low temperature phases. A combination of Kondo, crystal electric field, and magnetic frustration effects may be present, resulting in low temperature anomalies in magnetic susceptibility, electrical resistivity, and heat capacity measurements. In addition to CePd_3+<i>x</i>Ga_8‑<i>x</i>, the rare earth analogues REPd_3+<i>x</i>Ga_8‑<i>x</i>, RE = La, Nd, Sm, Tm, and Yb, were successfully synthesized and also crystallize in one of the lower symmetry space groups