Vacancy-ordered double perovskites of the general formula A2BX6 are a family of perovskite derivatives composed of a face-centered lattice of nearly isolated [BX6] units with A-site cations occupying the cuboctahedral voids. Despite the presence of isolated octahedral units, the close-packed iodide lattice provides significant electronic dispersion, such that Cs2SnI6 has recently been explored for applications in photovoltaic devices. To elucidate the structure-property relationships of these materials, we have synthesized solid-solution Cs2Sn1-xTexI6. However, even though tellurium substitution increases electronic dispersion via closer I-I contact distances, the substitution experimentally yields insulating behavior from a significant decrease in carrier concentration and mobility. Density functional calculations of native defects in Cs2SnI6 reveal that iodine vacancies exhibit a low enthalpy of formation, and that the defect energy level is a shallow donor to the conduction band rendering the material tolerant to these defect states. The increased covalency of Te-I bonding renders the formation of iodine vacancy states unfavorable and is responsible for the reduction in conductivity upon Te substitution. Additionally, Cs2TeI6 is intolerant to the formation of these defects, because the defect level occurs deep within the band gap and thus localizes potential mobile charge carriers. In these vacancy-ordered double perovskites, the close-packed lattice of iodine provides significant electronic dispersion, while the interaction of the B- and X-site ions dictates the properties as they pertain to electronic structure and defect tolerance. This simplified perspective based on extensive experimental and theoretical analysis provides a platform from which to understand structure-property relationships in functional perovskite halides.

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Antiferromagnetically coupled S=1/2 spins on an isotropic triangular lattice is the paradigm of frustrated quantum magnetism, but structurally ideal realizations are rare. Here we investigate NaYbO2, which hosts an ideal triangular lattice of Jeff=1/2 moments with no inherent site disorder. No signatures of conventional magnetic order appear down to 50 mK, strongly suggesting a quantum spin liquid ground state. We observe a two-peak specific heat and a nearly quadratic temperature dependence in accord with expectations for a two-dimensional Dirac spin liquid. Application of a magnetic field strongly perturbs the quantum disordered ground state and induces a clear transition into a collinear ordered state consistent with a long-predicted "up-up-down" structure for a triangular lattice XXZ Hamiltonian driven by quantum fluctuations. The observation of spin liquid signatures in zero field and quantum-induced ordering in intermediate fields in the same compound demonstrate an intrinsically quantum disordered ground state. We conclude that NaYbO2 is a model, versatile platform for exploring spin liquid physics with full tunability of field and temperature.Exotic ground states of quantum antiferromagnets are encouraged by the combination of low dimensionality, geometric frustration, and inherent anisotropies. Planar triangular lattices have long been sought as platforms for stabilizing them 1-7 ; however, ideal manifestations that do not break crystallographic or exchange symmetries upon approaching the quantum regime are rare. The organic compounds κ-(BEDT-TTF)2Cu2(CN)3 8 and EtMe3Sb[Pd(dmit)2]2 9 are two promising examples of triangular lattices with S=1/2 moments and a dynamically disordered spin ground state. However, S=1/2 inorganic analogs such as Ba3CoSb2O9 10 , Ba8CoNb6O24 11 , and NaTiO2 12-14 either order magnetically or undergo a lattice deformation and dimerization upon cooling. A key roadblock in inorganic systems is the identification of a material with a high crystallographic symmetry, rigid structure, and minimal defect mechanisms that also contains magnetic ions possessing strong quantum fluctuations. Ideally, the magnetic ions should be located at high symmetry positions that preclude antisymmetric Dzyaloshinskii-Moriya exchange from lifting geometric frustration at low temperatures.As an alternative to S=1/2 based compounds, rare earth ions with ground state doublets may also engender enhanced quantum fluctuations when decorating a triangular lattice. Specifically, recent studies have shown that the spin-orbit entangled Jeff=1/2 moments of Yb 3+ ions on this lattice may exhibit a variety of nearly degenerate magnetic states 15-22 . Given the appropriate anisotropies and when driven close

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