AE2[Ni(Te4O10)Cl2] (AE = Ca, Sr, Ba, Pb): A New Type of Inorganic Compound Synthesized through the Recombination of Tellurium(IV) Oxide Anion Fragments by Introducing Post-transition-metal Halide NiCl2

Introduction of divalent metal oxides into the TeO2 matrix can produce the chemical formula Mn[TemO2m+n] ≡ (MO)n(TeO2)m, compounds in which divalent metal oxides provide oxygen ions to the parent compound TeO2, to cleave neutral Te–O net to form various tellurium(IV) oxide anion [TemO2m+n]2n− and divalent metal cations as balanced charges. [TemO2m+n]2n− can form isolated anion, discrete polyanion, one-dimensional chain and ribbon polyanion, two-dimensional (2D) layered polyanion, and 3D tunnel polyanion depending on the ratio of MO/TeO2. In this work, a new type of inorganic compound AE2[Ni(Te4O10)Cl2] ≡ [(AEO)2(TeO2)4](NiCl2) (AE = Ca, Sr, Ba, Pb) can be obtained by introducing NiCl2 into the compound AE2[Te4O10] ≡ (AEO)2(TeO2)4 (AE = Ca, Sr, Ba, Pb). In AE2[Ni(Te4O10)Cl2] ≡ [(AEO)2(TeO2)4](NiCl2), AEO acts as a structural shearing agent to cleave the neutral Te–O layered structures of parent TeO2 into [Te4O10]4– anionic fragments, and the introduced NiCl2 acts as an “adhesive” to recombine [TemO2m+n]2n− anionic fragments to form new [Ni(Te4O10)Cl2] anionic framework and does not provide additional oxygen ions to cleave the parent compound TeO2. AE2[Ni(Te4O10)Cl2] compounds feature 2D 2∞[Ni(Te4O10)Cl2] anionic layers composed by TeO4E, TeO3+1E (where E is the lone pair of electrons of TeIV) distorted trigonal bipyramid, and NiO4Cl2 octahedra, and the spaces between 2∞[Ni(Te4O10)Cl2] layers are filled with AE2+ cations (acting as charge balance) as well as E (protruding from sides of the layers). The optical band gaps of AE2[Ni(Te4O10)Cl2] (AE = Ca, Sr, Ba, Pb) were evaluated to be ∼3.69 eV through extrapolating the UV–vis–near-infrared optical absorption spectra. Thermogravimetric and differential scanning calorimetry measurements indicate that Ca-, Sr-, Ba-, and Pb-compounds are stable below 787, 716, 648, and 517 °C, respectively, under flowing N2 atmosphere. The effective moments estimated by Curie–Weiss equation show that the Ni2+ cations of AE2[Ni(Te4O10)Cl2] (AE = Ca, Sr, Ba, Pb) are all in high-spin ground states.