Accelerated discovery of two crystal structure types in a complex inorganic phase field

The discovery of new materials is hampered by the lack of efficient approaches to the exploration of both the large number of possible elemental compositions for such materials, and of the candidate structures at each composition1. For example, the discovery of inorganic extended solid structures has relied on knowledge of crystal chemistry coupled with time-consuming materials synthesis with systematically varied elemental ratios2,3. Computational methods have been developed to guide synthesis by predicting structures at specific compositions4,5,6 and predicting compositions for known crystal structures7,8, with notable successes9,10. However, the challenge of finding qualitatively new, experimentally realizable compounds, with crystal structures where the unit cell and the atom positions within it differ from known structures, remains for compositionally complex systems. Many valuable properties arise from substitution into known crystal structures, but materials discovery using this approach alone risks both missing best-in-class performance and attempting design with incomplete knowledge8,11. Here we report the experimental discovery of two structure types by computational identification of the region of a complex inorganic phase field that contains them. This is achieved by computing probe structures that capture the chemical and structural diversity of the system and whose energies can be ranked against combinations of currently known materials. Subsequent experimental exploration of the lowest-energy regions of the computed phase diagram affords two materials with previously unreported crystal structures featuring unusual structural motifs. This approach will accelerate the systematic discovery of new materials in complex compositional spaces by efficiently guiding synthesis and enhancing the predictive power of the computational tools through expansion of the knowledge base underpinning them

Crystal structure of Ba27Fe16Ti33O117

Single-crystal X-ray diffraction studies indicate that the compound Ba27Fe16Ti33O117 crystallizes in the rhombohedral space group R3m, with a hexagonal unit cell a = 5.7400(8), c = 127.11(3) Angstrom: Z = 1.5. The arrangement may be described as a 54-layer (54L) close-packed structure (stacking sequence (cchcchhhhcchhhhcch)(3)) built from oxygen and {Ba, O} layers, with Ti4+ occupying octahedra and Fe3+ occupying both octahedral and tetrahedral interstices. The 54L structure contains hexagonal 6L BaTiO3-type (cch)(2) units via a 9-fold repeat of the 6L stacking sequence, with iron preferentially occupying layers centered a-round z = 1/6, Ba27Fe16Ti33O117 melts incongruently at 1270 degreesC and is difficult to purify in polycrystalline form, although crystals are easily obtained from partial melts. The new compound is a member of a family of ternary Ba-Fe-Ti-O phases that may be considered as dielectric-magnetic hybrids of barium-polytitanate and barium-hexaferrite crystal chemistries. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved

Synthesis and characterization of bismuth zinc niobate pyrochlore nanopowders

Bismuth zinc niobate pyrochlores Bi1.5ZnNb1.5O7 (alpha-BZN), and Bi2(Zn1/3Nb2/3)2O 7 (beta-BZN) have been synthesized by chemical method based on the polymeric precursors. The pyrochlore phase was investigated by differential scanning calorimetry, infrared spectroscopy, and X ray diffraction. Powder and sintered pellets morphology was examined by scanning electron microscopy. The study of alpha-BZN phase formation reveals that, at 500 °C, the pyrochlore phase was already present while a single-phased nanopowder was obtained after calcination at 700 °C. The crystallization mechanism of the beta-BZN is quite different, occurring through the crystallization of alpha-BZN and BiNbO4 intermediary phases. Both compositions yielded soft agglomerated powders. alpha-BZN pellets, sintered at 800 °C for 2 hours, presented a relative density of 97.3% while those of beta-BZN, sintered at 900 °C for 2 hours, reached only 91.8%. Dielectric constant and dielectric loss, measured at 1 MHz, were 150 and 4 x/10-4 for a-BZN, and 97 and 8 x 10-4 for beta-BZN