Kenneth Poeppelmeier

Abstract:

Paraphrasing Nobel Laureate Arno Penzias who famously titled one of his many lectures “Logical Machines for Rational People”, this talk will show that while most racemates crystalize in centrosymmetric structures, some do not, and these form an important and often overlooked class of Chiral and/or Polar Matter with broken inversion symmetry. This talk is based in part on a paper that we published several years ago with the title “Machine-learning-assisted Synthesis of Polar Racemates,” the authors were M. L. Nisbet, I. M. Pendleton, G. M. Nolis, K. J. Griffith, J. Schrier, J. Cabana, A. J. Norquist, and K. R. Poeppelmeier, J. Am. Chem. Soc., 142(16), 7555–7566 (2020) that describes our efforts to synthesize other new members of a peculiar class of noncentrosymmetric (NCS) materials first reported in “From Racemic Units to Polar Materials,” R. Gautier, A. Norquist, and K. R. Poeppelmeier, Cryst. Growth Des., 12, 6267–6271 (2012). These interesting NCS phases contain equal numbers of oppositely handed chiral species forming an important, often overlooked class of materials with useful properties. 

We prepared a series of NCS racemic compounds with the formula [Cu(bpy)₂(H₂O)]₂[MF₆]₂•3H₂O (M = Ti⁴⁺, Zr⁴⁺, Hf⁴⁺_; bpy = 2,2’bipyridine), denoted as CBM-0D.   The polarity of the anion leads to inversion symmetry breaking, By following a machine learning-augmented composition space approach, we were able to synthesize the two missing members of the CBM-0D family (M = Ti⁴⁺, Zr⁴⁺) and learned that each polar CBM-0D phase forms in competition with a unique nonpolar coordination polymer (denoted as CBM-1D) based on [Cu(bpy)(H₂O)₂]²⁺ and [MF₆]^2- units. Phase stability of the CBM-0D compounds depends strongly on the choice of M, with the 1D structure predominating when M = Ti while the 0D structure is favored for M = Zr, Hf. To understand these differences in structure-directing properties between [TiF₆]^2- and [ZrF₆]^2-/[HfF₆]^2-, we performed x-ray absorption spectroscopy measurements and density functional theory calculations to probe local differences in electronic structure and describe them in terms of the second-order, or pseudo-, Jahn-Teller effects present in each [MF₆]^2- anion.