Substitution-Induced Structure Evolution and Zn2+/Ga3+ Ordering in "114" Oxides MAZn2Ga2O7 ( M = Ca2+, Sr2+; A = Sr2+, Ba2+)

Temperature-driven order-disorder structural transition in the oxygen sub-lattice and the complex superstructure of the high-temperature polymorph of CaSrZn2Ga2O7

Structural order-disorder plays a decisive role in the physical properties of materials, such as magnetism, second-order harmonic generation, and ionic conductivity, and it is thus widely utilized to manipulate the crystal structure and understand structure-property correlations. Herein, we report the structural polymorphism, complex crystal structure and temperature-driven irreversible order-disorder phase transition of the polar oxides (Sr_1-xCa_x)SrZn₂Ga₂O₇. The low-temperature (LT) structure crystallizes in Pna2₁ with partial Zn/Ga ordering. Upon heating, (Sr_1-xCa_x)SrZn₂Ga₂O₇ undergoes an irreversible phase transition from orthorhombic Pna2₁ to hexagonal P6₃. Interestingly, the high-temperature (HT) P6₃ structure possesses an unexpected 3/2-fold superstructure rather than a substructure of the low-temperature (LT) Pna2₁ structure, which is a rare structural phenomenon in solid-state chemistry. This new HT superstructure is the most complex one in this series of oxides with 21 crystallographically independent sites determined accurately by a combination of the maximum entropy method and Rietveld refinement against high-resolution neutron powder diffraction data. In terms of the mechanism, this is a temperature-driven order-to-disorder transition in the oxygen sublattice. A careful structural analysis revealed that the oxygen disordering mainly occurs in the [SrO₃] layers of the HT structure and it can be understood as respective clockwise and anticlockwise rotations of distinct GaO₄-tetrahedra along the c-axis. Alternating current electrochemical impedance spectroscopic analysis revealed that the oxygen disordering in the HT structure is incapable of giving rise to oxide ionic conductivity but does lead to increased electronic conduction compared to the LT structure. The optical properties of the CaSrZn₂Ga₂O₇ and Sr₂Zn₂Ga₂O₇ representatives are also investigated in-depth via diffuse reflectance spectroscopy and theoretic calculations.

Complex crystal structure and photoluminescence of Bi3+-doped and Bi3+/Eu3+ co-doped Ca7Mg2Ga6O18

The multiple and site-selective occupancies of Bi³⁺ activators in Ca₇Mg₂Ga₆O₁₈:xBi³⁺ lead to a continuous red-shift of the broad emission band upon increasing the Bi³⁺-content.

Unprecedented lattice volume expansion on doping stereochemically active Pb2+ into uniaxially strained structure of CaBa1-xPbxZn2Ga2O7

Lone pair cations like Pb²⁺ are extensively utilized to modify and tune physical properties, such as nonlinear optical property and ferroelectricity, of some specific structures owing to their preference to adopt a local distorted coordination environment. Here we report that the incorporation of Pb²⁺ into the polar “114”-type structure of CaBaZn₂Ga₂O₇ leads to an unexpected cell volume expansion of CaBa_1-xPb_xZn₂Ga₂O₇ (0 ≤ x ≤ 1), which is a unique structural phenomenon in solid state chemistry. Structure refinements against neutron diffraction and total scattering data and theoretical calculations demonstrate that the unusual evolution of the unit cell for CaBa_1-xPb_xZn₂Ga₂O₇ is due to the combination of the high stereochemical activity of Pb²⁺ with the extremely strained [Zn₂Ga₂O₇]⁴⁻ framework along the c-axis. The unprecedented cell volume expansion of the CaBa_1−xPb_xZn₂Ga₂O₇ solid solution in fact is a macroscopic performance of the release of uniaxial strain along c-axis when Ba²⁺ is replaced with smaller Pb²⁺.

Lone pair cations can impart interesting features in some structures, such as noncentrosymmetry. Here the authors show unexpected cell volume expansion in a polar “114”-type oxide upon replacing Ba²⁺ with a smaller Pb²⁺, and attribute it to high stereochemical activity of Pb²⁺ with the strained framework.

Chemical-substitution-induced successive symmetry descent and structure-property correlation for "114" oxides CaBa1-xSrxZn2Al2O7

The crystal structures, photoluminescence properties, and transport properties of a series of new "114" oxides CaBa1-xSrxZn2Al2O7 (x = 0-1) were investigated in detail. Careful Rietveld refinements performed on solid solution samples revealed that the structural symmetry of CaBa1-xSrxZn2Al2O7 evolves from hexagonal P63mc (x < 0.2) to trigonal P31c (0.2 ≤ x ≤ 0.6) and then to orthorhombic Pna21 (x > 0.6) with an increase of the Sr2+-content, which is cooperative with the rotation of T1O4 tetrahedra around the c-axis. Eu3+ was used as a local structural probe to gain an insight into the structure, which further corroborated the correctness of the observed structural symmetry descending sequence in CaBa1-xSrxZn2Al2O7. More importantly, the reduction of structural symmetry is also associated with a tendency from layered ordering to complete charge ordering transition for Zn2+/Al3+ cations, which was revealed to have a significant influence on the transport properties. These findings are expected to offer a route to manipulate the physical properties of "114" oxides containing magnetic cations.