Thesis Summary
Nature of chemical bonds in double perovskite-type oxide BaBiO3 and related oxides visualized bysynchrotron-radiation X-ray diffraction(ᨺᑕග㹖⥺ᅇᢡ࡟ࡼࡾྍど໬ࡉࢀࡓ஧㔜࣌ࣟࣈࢫ࢝࢖ࢺᆺ㓟໬≀BaBiO3࠾ࡼࡧ㛵㐃㓟໬≀ࡢ໬Ꮫ⤖ྜࡢᛶ㉁)                    Zhao Qing

Perovskite-type oxide BaBiO 3 has attracted extensive research interests since thediscovery of superconductivity in the doped solid solutions (Ba, K) (Pb, Bi) O 3 .
Considering the charge balance, the Bi ion normally will take average 4+ valence
state in the compound. However, as the Bi ion is referred to be a “valence skipper”,
hence, the Bi ions are suggested to exhibit charge disproportionation from average
Bi4+ valence state to Bi3+ and Bi 5+ valence states with equal ratio. BaBiO 3 is
supposed to be not a simple perovskite, but a B-site ordered double perovskite withtwo different B-sites in the compound. The charge ordering accounts for the factthat BaBiO3 is electrically semiconductor with a band gap. However, clear
experimental evidence of Bi charge order is lacking.
Accurate electron charge density distributions of BaBiO 3 have been obtainedby the high-resolution synchrotron-radiation powder X-ray diffraction data using
the maximum entropy method/Rietveld method. The charge ordered Ba 2 Bi3+ Bi5+O 6
double perovskite adopts four structures, i.e., monoclinic P2 1 /n, monoclinic I2/m,
rhombohedral R , and cubic Fm m phases upon heating progress. We investigate
these phase transitions at charge density level, such as the change of chemicalbonding nature and possible modifications in charge density. We found BaBiO 3forms a two-dimensional covalent layer with P21 /n symmetry, a three-dimensional

covalent network with I2/m and R

symmetry, and an ionic structure consisting ofisolated Bi 3+ ions and [Bi 5+ -O6] 7- octahedron upon heating progress.
In order to discover the novel oxides with intriguing properties, engineered
local environments of cations coordinated with oxygen ions in oxides is an effective
pathway. Here we present the successful synthesis of BaBiO2.5 with layered
structure enabled by a non-topotactic phase transformation from the perovskite-typeoxide BaBiO3 . Using the maximum entropy method/Rietveld method based on thehigh-resolution

powdersynchrotron-radiation

X-ray

diffraction

data,

wesuccessfully found that Bi ions form unusual chemical bonds with four coordinatedoxygen ions that results in an asymmetric coordination geometry. One ionic bondand three covalent bonds of Bi-O bonds are successfully visualized in this layered
structure that is owing to the hybridization of O 2p and Bi 6p orbitals. Specifically,
we find that this unusual structure shows the near-infrared photoluminescencedifferent from the ordinary Bi-containing systems. Experimental and quantum
chemistry calculations motivate us to propose the excitonic nature of near-infrared
photoluminescence. Our work highlights that creating materials with un usual Bi-O
bondings and Bi coordination geometry will provide a pathway to discover thematerials with new physical properties. We believe this work may stimulateinterests in exploring a series of materials containing heavier p-block elements, thus
providing prospects for discovering systems with unconventional properties .

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