The Moissanite-9R CSi Crystal Structure

Last modified 9 July 2001

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This is a hypothetical alternate stacking (ABCBCACAB) for tetrahedral structures. Compare this to Zincblende (ABCABC), Wurtzite (ABABAB), and 6H (ABCACB). The 9R refers to the fact that there are 9 CSi dimers in a hexagonal unit cell (which contains 3 rhombohedral unit cells). Zincblende is denoted 3C, and Wurtzite is 2H.

This structure is related to the aSm (C19) structure in the same way that Zincblende (B3) is related to the fcc (A1) lattice.

The spacings in structure file assume that the local environment is similar to Zincblende CSi. Thus the internal parameters have the values given in the table below, and c/a = (9/2) (8/3)^1/2.

• Prototype: CSi
• Pearson Symbol: hR6
• Space Group: R3m (Cartesian and lattice coordinate listings available)
• Number: 160
• Primitive Vectors:

  a and c are the lattice constants of the equivalent hexagonal unit cell.

  • A₁ = + ½ a X + ½ 3^-1/2 a Y + 1/3 c Z
  • A₂ = - ½ a X + ½ 3^-1/2 a Y + 1/3 c Z
  • A₃ = - 3^-1/2 a Y + 1/3 c Z
• Basis Vectors:

  All atoms are on (1a) Wyckoff positions. The 'ideal' structure parameters, those that get us closest to the Zincblende structure are:

  x2 = - 2/9

  x3 = + 2/9

  x4 = 3/36

  x5 = 11/36

  x6 = -5/36

  B1	=		0		(C-I)
  B2	=	x2 (A1 + A2 + A3)	=	x2 c Z	(C-II)
  B3	=	x3 (A1 + A2 + A3)	=	x3 c Z	(C-III)
  B4	=	x4 (A1 + A2 + A3)	=	x4 c Z	(Si-I)
  B5	=	x5 (A1 + A2 + A3)	=	x5 c Z	(Si-II)
  B6	=	x6 (A1 + A2 + A3)	=	x6 c Z	(Si-III)

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