Table 1 Total energy of various CDW structures, grouped per periodicity, at three different values of the in-plane lattice constant.

	2 × 3		3 × 3			\(\bf \sqrt{13}\,\times \,\sqrt{13}\)			4 × 4
	MM	MW	HX	CC	HC	CCccws	HXHC	HC	1q′	3q	1q″
∣a∣ = 3.49	−1.9	−0.9	−3.0	−3.5	−4.4	−0.3	−0.4	−0.4	−2.9	–	−3.0
∣a∣ = 3.45	−1.9	−0.0	−2.8	−3.4	−4.0	−1.4	−0.6	−1.4	−4.4	−2.5	−4.3
∣a∣ = 3.41	−2.3	−0.0	−2.8	−3.6	−3.6	−3.4	−2.4	−3.6	−6.2	–	−6.2

The energy of each structure is given as the difference, expressed in meV/f.u., with the energy of the symmetric structure, and it is negative for favoured structures. The names of the structures are explained as follows: MM and MW in the 2 × 3 cells are two stripe phases with a different Nb–Nb pattern; HX, CC and HC in the 3 × 3 cells are the known hexagonal, triangular chalcogen centred and triangular hollow centred structures, respectively; CCccws, HXHC and HC in the \(\sqrt{13}\times \sqrt{13}\) cells are phases with triangular chalcogen centred plus counterclockwise vortex, hexagonal with triangular chalcogen centred merged and just triangular chalcogen centred, respectively; finally, the 1q′, 3q and 1q″ in the 4 × 4 cells are a single ordering vector CDW structure, a triple ordering vector CDW structure and a single ordering vector structure, respectively, as also illustrated in Fig. 2.

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