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5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Revisiting the structures and phase transitions of Ba 2 NaNb 5 O 15

The room-temperature and low-temperature structure(s) of Ba 2 NaNb 5 O 15 (BNN) have been debated since the structure was proposed in the 1960s. This work revisits the structures and phase transitions of BNN, combining high-resolution X-ray and neutron powder diffraction with density functional theory calculations. Temperature-dependent high-resolution X-ray powder diffraction patterns are collected from 4 to 918 K, and sequential batch Rietveld refinement using a symmetry mode approach to describe the structure is used to extract the main structural changes as a function of temperature. The data show that the average structure of BNN is best described by the Ama2 space group, and no other structural phase transitions were observed below the ferroelastic transition. The symmetry mode analysis, combining results from diffraction and density functional theory, shows significant octahedral tilting and corrugations of both the A1 and A2 sites along the c direction. A strong correlation between the spontaneous strain and the octahedral tilting was observed, and a potential connection with emerging microstructure at low temperatures is proposed, all enabled by the symmetry mode approach used in this work.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Materials Data on NaNb(CuS2)2 by Materials Project

NaNb(CuS2)2 is Chalcostibite-derived structured and crystallizes in the orthorhombic Ama2 space group. The structure is three-dimensional. Na1+ is bonded in a 3-coordinate geometry to three S2- atoms. There are a spread of Na–S bond distances ranging from 3.41–3.45 Å. Nb5+ is bonded to four S2- atoms to form NbS4 tetrahedra that share edges with four CuS4 tetrahedra. There are a spread of Nb–S bond distances ranging from 2.30–2.34 Å. There are two inequivalent Cu1+ sites. In the first Cu1+ site, Cu1+ is bonded to four S2- atoms to form CuS4 tetrahedra that share corners with four equivalent CuS4 tetrahedra and edges with two equivalent NbS4 tetrahedra. There are two shorter (2.34 Å) and two longer (2.35 Å) Cu–S bond lengths. In the second Cu1+ site, Cu1+ is bonded to four S2- atoms to form CuS4 tetrahedra that share corners with six CuS4 tetrahedra and edges with two equivalent NbS4 tetrahedra. There are two shorter (2.32 Å) and two longer (2.35 Å) Cu–S bond lengths. There are three inequivalent S2- sites. In the first S2- site, S2- is bonded in a 5-coordinate geometry to one Na1+, one Nb5+, and three Cu1+ atoms. In the second S2- site, S2- is bonded in a 3-coordinate geometry to one Nb5+ and two Cu1+ atoms. In the third S2- site, S2- is bonded in a 4-coordinate geometry to two equivalent Na1+, one Nb5+, and one Cu1+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NaNb(CuS2)2 by Materials Project

NaNb(CuS2)2 crystallizes in the orthorhombic Ama2 space group. The structure is three-dimensional. Na1+ is bonded in a rectangular see-saw-like geometry to four S2- atoms. There are a spread of Na–S bond distances ranging from 2.84–3.37 Å. Nb5+ is bonded to four S2- atoms to form NbS4 tetrahedra that share edges with four CuS4 tetrahedra. There are a spread of Nb–S bond distances ranging from 2.28–2.35 Å. There are two inequivalent Cu1+ sites. In the first Cu1+ site, Cu1+ is bonded to four S2- atoms to form CuS4 tetrahedra that share corners with six CuS4 tetrahedra and edges with two equivalent NbS4 tetrahedra. There are two shorter (2.32 Å) and two longer (2.37 Å) Cu–S bond lengths. In the second Cu1+ site, Cu1+ is bonded to four S2- atoms to form CuS4 tetrahedra that share corners with four equivalent CuS4 tetrahedra and edges with two equivalent NbS4 tetrahedra. There are a spread of Cu–S bond distances ranging from 2.32–2.38 Å. There are three inequivalent S2- sites. In the first S2- site, S2- is bonded in a distorted trigonal non-coplanar geometry to one Na1+, one Nb5+, and one Cu1+ atom. In the second S2- site, S2- is bonded in a distorted rectangular see-saw-like geometry to one Na1+, one Nb5+, and two Cu1+ atoms. In the third S2- site, S2- is bonded in a 5-coordinate geometry to one Na1+, one Nb5+, and three Cu1+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NaNb(OF)2 by Materials Project

NaNbO2F2 crystallizes in the monoclinic P2_1/c space group. The structure is three-dimensional. Na1+ is bonded in a 6-coordinate geometry to one O2- and five F1- atoms. The Na–O bond length is 2.57 Å. There are a spread of Na–F bond distances ranging from 2.29–2.82 Å. Nb5+ is bonded to four O2- and two F1- atoms to form distorted corner-sharing NbO4F2 octahedra. The corner-sharing octahedra tilt angles range from 31–32°. There are a spread of Nb–O bond distances ranging from 1.84–2.20 Å. There are one shorter (2.05 Å) and one longer (2.08 Å) Nb–F bond lengths. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a bent 150 degrees geometry to two equivalent Nb5+ atoms. In the second O2- site, O2- is bonded in a 1-coordinate geometry to one Na1+ and two equivalent Nb5+ atoms. There are two inequivalent F1- sites. In the first F1- site, F1- is bonded in a 3-coordinate geometry to three equivalent Na1+ and one Nb5+ atom. In the second F1- site, F1- is bonded in a distorted trigonal planar geometry to two equivalent Na1+ and one Nb5+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Multiferroic ground states in free standing perovskite-based nanodots: a density functional theory study

In this work, we use density functional theory to investigate the possibility of polar and multiferroic states in free-standing, perovskite-based nanodots at the atomic limit of miniaturization: single unit cells with terminations which allow centro-symmetry. We consider both A-O and B-O 2 terminations for three families of nanodots: (i) A = Ba with B = Ti, Zr, and Hf; (ii) A = Ca and Sr with B = Ti; and (iii) A = Na and K with B = Nb. We find all A–O terminated dots to be non-polar and to exhibit cubic symmetry (except for K 8 NbO 6 ), regardless of the presence of ferroelectricity in the bulk. In contrast, all the B–O 2 terminated nanodots considered relax to a non-cubic ground state. Rather surprisingly, all of these structures exhibit polar ground states (except NaNb 8 O 12 ). We propose a new structural parameter, the cluster tolerance factor (CTF), to determine whether a particular chemistry will result in a polar ground state nanodot, analogous to the Goldschmidt factor for bulk ferroelectrics. In addition, we find that all A–O terminated (except Ca 8 TiO 6 ) and all polar B–O 2 terminated nanodots are magnetic, where none show magnetism in the bulk. As with bulk systems, multiferroicity in the B–O 2 terminated dots originates from separation between spin density in peripheral B atoms and polarity primarily caused by the off-center central A atom. Our findings stress that surface termination plays a crucial role in determining whether ferroelectricity is completely suppressed in perovskite-based materials at their limit of miniaturization.

36 MATERIALS SCIENCE↗