1. Crystal Structure The Na3 atoms and [CO 3 ] groups form graphite-like layers stacked in the third dimension. The Na1,2 atoms are located in the hexagonal channels. Structural Aspects of Phase Transitions in Na 2 CO 3 A.V. Arakcheeva & G. Chapuis Ecole Polytechnique Fédérale de Lausanne, Institut de Physique de la matière complexe, BSP 1015 Lausanne, Switzerland Analysis of published data Synopsis In Na 2 CO 3, the interactions between C and Na atoms are the driving forces of the  (hexagonal)   (3D-monoclinic)   (4D-monoclinic)   (lock-in) phase transitions. The general tendency of the phase transformations is an increase of the mass density concentration in the vicinity of C atoms. The C - O distances depend on this vicinity.  - phase: 3D-hexagonal; sp.gr. P6 3 /mmc; 754 < T< 1000 K The main T-driven parameter is the C-Na3 distance along c  -phase: 3D-monoclinic; sp.gr. C12/m1; 605 < T < 754 K The main T-driven parameter is the C -Na2 distance The phase transitions are the response on the temperature- driven stabilisation of the C - Na interactions.  -phase, 4D-monoclinic; super sp.gr. C12/m1(  0  )0s; 170 < T < 605 K The main T-dependent parameter is the vector q = q 1 a*+q 2 c* The Incommensurate acoustic wave of mass density affects the C -Na1 distances  -phase, 3D-monoclinic; super sp.gr. C12/m1(    0     )0s; T< 170 K The acoustic wave of mass density is commensurate Evolution of the mass density concentration in the vicinity of the C atom (R ≤ 3.1 Å ) Structural aspect of the phase transitions    Projection on the m M -plane Projection on the m V -plane [CNa 6 - 7 ] x mon 705 T C ’ = 754 T C ’’ = 605 530 T, K T C ’’’ = 170 The T-dependent C - Na2 distances in  -phase are reproduced as t-dependent C - Na1 distances in  -phase The mobility of O atoms and tilts of CO 3 triangles depend on the number of Na atoms in the vicinity of C In the m M plane In the m V planes a mon 3 O + 7 Na3 O + 6 Na3 O + 5 Na3 O + 4 Na3 O + 3 Na    New fact  The decrease of the C - Na distances is responsible for the phase transitions in Na 2 CO 3 C - O distances, Å Reinterpretation of published data  Both  -  and  -  appear to be first order phase transitions  The  -  is a second order phase transition. The primary order parameter is  q/q 0, where q 0 = [    0     ] Predictions  A second order phase transition exists at ≈ 705 K. The primary order parameter is  (C-Na3) c /(C-Na3) c, where  (C-Na3) c is the splitting of the C-Na3 distance in phase   A first order phase transition exists at ≈ 530 K between two incommensurate modifications in the temperature range of phase  [CNa 6 ][CNa 5 ][CNa 4 ][CNa 3 ] Environment of the C atom  Infrared spectra of the CO 3 internal modes x mon Na 2 CO 3 References: I. P. Swainson et al. J. Phys.: Condens. Matter, 7, 4395 (1995); M. Dusek, G. Chapuis et al. Acta Cryst., B59, 337 (2003); M. J. Harris and E.K. H. Salje. J. Phys.: Condens. Matter, 4, 4399 (1992) The (C,Na3)-layers are stable in all phases. C - Na distances, Å       