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bohedral phase, 118 K). The base vectors for the O60 domain wall (see Eqn. 3) at 208 K are 

The right four columns of the Table III contain corre- sponding numerical values of the domain wall coefficients g , a$_{1}$ , a$_{11}$ , and a$_{111}$ derived from GLD parameters and spontaneous order-parameter values using above derived analytical expressions, mostly given explicitly in Table II. The left part of the Table III contains the key domain wall properties such as wall thickness 2 ξ and energy density Σ. 

Clearly, T90 and O60 domain walls are considerably broader then others. T90 wall in the tetragonal phase is predicted to be 3.59 nm thick at 298 K (as already cal- culated in Ref. 4) and S-type O60 domain wall in the orthorhombic phase is predicted to be 3.62 nm thick at 208 K. Since the wall thickness is greater than the lat- tice spacing, the pinning of the walls is weak 21 and they 

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can be easily moved. Moreover, in case of O60 wall, the pinning is further suppressed due to ’incommensurate’ character of Miller indices of the wall normal. 

As follows from Eqn. (23), domain wall thickness is determined by quantities g , U , and p$_{∞}$ . By inspec- tion of their values in Table III, we see that the most important factor is the coefficient g . Indeed, the do- main walls R71, R109, O90, O180 { 001 } , T180 with g = 2 × 10 − 11 kg m $^{5}$s − $^{2}$C − 2 are all very narrow (thickness below 1 nm), and the thickness of various walls mono- tonically increases with increasing value of g . Let us stress that coefficients a$_{1}$ , a$_{11}$ , a$_{111}$ , and g depend on the direction of the domain wall normal. For exam- ple, O180 { 1 ¯ 10 } with g = 26 . 5 × 10 − 11 kg m $^{5}$s − $^{2}$C − 2 is almost four times broader than O180 { 001 } with g = 2 × 10 − 11 kg m $^{5}$s − $^{2}$C − $^{2}$. 

In the absence of other constraints, the probability of appearance of domain wall species should be determined by the surface energy density Σ. Therefore, in the or- thorhombic phase, the thinner O180 { 001 } wall is more likely to occur than the 0180 { 1 ¯ 10 } one. Interestingly, 

