Structural Aspects of Phase Transitions in Na2CO3

1
Structural Aspects of Phase Transitions in Na2CO3

• A.V. Arakcheeva G. Chapuis
• Ecole Polytechnique Fédérale de Lausanne,
  Institut de Physique de la matière complexe, BSP
  1015 Lausanne, Switzerland

Synopsis In Na2CO3, 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.
Analysis of published data
C - Na distances, Å
The (C,Na3)-layers are stable in all phases.
b
g
?
d
Crystal Structure The Na3 atoms and CO3
groups form graphite-like layers stacked in the
third dimension. The Na1,2 atoms are located in
the hexagonal channels .
The T-dependent
C - Na2 distances in ?
-phase are reproduced as t-dependent
C - Na1 distances in g -phase
b
g
Structural aspect of the phase transitions
The phase transitions are the response on the
temperature-driven stabilisation of the C - Na
interactions.

• ?-phase 3D-hexagonal
  sp.gr. P63/mmc 754 lt Tlt 1000 K
• The main T-driven parameter is
  the C-Na3 distance along c

• ? -phase 3D-monoclinic
  sp.gr. C12/m1 605 lt T lt 754 K
• The main T-driven parameter is
  the C -Na2 distance

C - O distances, Å
The mobility of O atoms and tilts of CO3
triangles depend on the number of Na atoms in the
vicinity of C

• g -phase, 4D-monoclinic
  super sp.gr. C12/m1(a0g)0s
  170 lt T lt 605 K
• The main T-dependent parameter is
  the vector q q1aq2c
• The Incommensurate acoustic wave of mass density
  affects the C -Na1 distances

b

• d -phase, 3D-monoclinic
  super sp.gr. C12/m1(1/6 0 1/3 )0s Tlt 170 K
• The acoustic wave of mass density is commensurate
  

Infrared spectra of the CO3 internal modes
g
Evolution of the mass density concentration in
the vicinity of the C atom (R 3.1 Å)
g
a
b
d
TC 754
TC 605
TC 170
T, K
705
530
Projection on the mM-plane
Projection on the mV-plane
Environment of the C atom
3 O 7 Na
3 O 6 Na
3 O 5 Na
3 O 4 Na
3 O 3 Na
g
a
b
CNa6 - 7
CNa6
CNa5
CNa4
CNa3
d

• Reinterpretation of published data
• Both a - b and b - g appear to be first order
  phase transitions
• The g - d is a second order phase transition.
  The primary order parameter is Dq/q0, where q0
  1/6 0 1/3

• Predictions
• A second order phase transition exists at 705
  K. The primary order parameter is
  D(C-Na3)c/(C-Na3)c, where D(C-Na3)c is the
  splitting of the C-Na3 distance in phase b
• A first order phase transition exists at 530 K
  between two incommensurate modifications in the
  temperature range of phase g

• New fact
• The decrease of the C - Na distances is
  responsible for the phase transitions in Na2CO3

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)