title: Modeling nuclear volume isotope effects in crystals.
abstract: Mass-independent isotope fractionations driven by differences in volumes and shapes of nuclei (the field shift effect) are known in several elements and are likely to be found in more. All-electron relativistic electronic structure calculations can predict this effect but at present are computationally intensive and limited to modeling small gas phase molecules and clusters. Density functional theory, using the projector augmented wave method (DFT-PAW), has advantages in greater speed and compatibility with a three-dimensional periodic boundary condition while preserving information about the effects of chemistry on electron densities within nuclei. These electron density variations determine the volume component of the field shift effect. In this study, DFT-PAW calculations are calibrated against all-electron, relativistic Dirac-Hartree-Fock, and coupled-cluster with single, double (triple) excitation methods for estimating nuclear volume isotope effects. DFT-PAW calculations accurately reproduce changes in electron densities within nuclei in typical molecules, when PAW datasets constructed with finite nuclei are used. Nuclear volume contributions to vapor-crystal isotope fractionation are calculated for elemental cadmium and mercury, showing good agreement with experiments. The nuclear-volume component of mercury and cadmium isotope fractionations between atomic vapor and montroydite (HgO), cinnabar (HgS), calomel (Hg2Cl2), monteponite (CdO), and the CdS polymorphs hawleyite and greenockite are calculated, indicating preferential incorporation of neutron-rich isotopes in more oxidized, ionically bonded phases. Finally, field shift energies are related to Mössbauer isomer shifts, and equilibrium mass-independent fractionations for several tin-bearing crystals are calculated from (119)Sn spectra. Isomer shift data should simplify calculations of mass-independent isotope fractionations in other elements with Mössbauer isotopes, such as platinum and uranium.
authors: Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095.
authors: Edwin A
authors: EA
authors: Schauble
mesh_terms: D002104:Cadmium / Q000737:chemistry*; D019187:Cadmium Compounds / Q000737:chemistry; D007554:Isotopes / Q000737:chemistry*; D008628:Mercury / Q000737:chemistry*; D017669:Mercury Compounds / Q000737:chemistry; D008956:Models, Chemical; D010087:Oxides / Q000737:chemistry; D015204:Spectroscopy, Mossbauer / Q000379:methods; D014001:Tin / Q000737:chemistry*
chemical_list: D019187:Cadmium Compounds; D007554:Isotopes; D017669:Mercury Compounds; D010087:Oxides; D002104:Cadmium; C029663:cadmium oxide; D014001:Tin; D008628:Mercury; C019468:mercuric oxide; C015728:calomel; C034211:cinnabar
keywords: Mössbauer spectroscopy; mass independent fractionation; nuclear field shift
references: J Chem Phys. 2010 Jul 28;133(4):044309
references: 20687652
references: Phys Rev Lett. 2008 Apr 4;100(13):136406
references: 18517979
references: Phys Rev Lett. 1992 Nov 30;69(22):3204-3207
references: 10046757
references: J Phys Chem A. 2010 Apr 1;114(12):4238-45
references: 20192261
references: Environ Sci Technol. 2010 Jun 1;44(11):4191-7
references: 20443581
references: J Phys Condens Matter. 2009 May 13;21(19):195501
references: 21825488
references: Anal Chim Acta. 2009 Jan 26;632(2):234-9
references: 19110099
references: J Chem Phys. 2009 Mar 28;130(12):124121
references: 19334822
references: Phys Rev B Condens Matter. 1994 Dec 15;50(24):17953-17979
references: 9976227
references: J Chem Phys. 2008 Oct 28;129(16):164309
references: 19045268
references: Phys Rev B Condens Matter. 1992 Jun 15;45(23):13244-13249
references: 10001404
references: Phys Rev Lett. 1996 Oct 28;77(18):3865-3868
references: 10062328
references: J Chem Phys. 2008 Apr 14;128(14):144309
references: 18412447