Rhombohedron

The common angle at the two apices is here given as ${\displaystyle \theta }$ . There are two general forms of the rhombohedron, oblate (flattened) and prolate (stretched.

In the oblate case ${\displaystyle \theta >90^{\circ }}$  and in the prolate case ${\displaystyle \theta <90^{\circ }}$ . For ${\displaystyle \theta =90^{\circ }}$  the figure is a cube.

Certain proportions of the rhombs give rise to some well-known special cases. These typically occur in both prolate and oblate forms.

For a unit (i.e.: with side length 1) rhombohedron,^[4] with rhombic acute angle ${\displaystyle \theta ~}$ , with one vertex at the origin (0, 0, 0), and with one edge lying along the x-axis, the three generating vectors are

e₁ : ${\displaystyle {\biggl (}1,0,0{\biggr )},}$ 

e₂ : ${\displaystyle {\biggl (}\cos \theta ,\sin \theta ,0{\biggr )},}$ 

e₃ : ${\displaystyle {\biggl (}\cos \theta ,{\cos \theta -\cos ^{2}\theta \over \sin \theta },{{\sqrt {1-3\cos ^{2}\theta +2\cos ^{3}\theta }} \over \sin \theta }{\biggr )}.}$ 

The other coordinates can be obtained from vector addition^[5] of the 3 direction vectors: e₁ + e₂ , e₁ + e₃ , e₂ + e₃ , and e₁ + e₂ + e₃ .

The volume ${\displaystyle V}$  of a rhombohedron, in terms of its side length ${\displaystyle a}$  and its rhombic acute angle ${\displaystyle \theta ~}$ , is a simplification of the volume of a parallelepiped, and is given by

${\displaystyle V=a^{3}(1-\cos \theta ){\sqrt {1+2\cos \theta }}=a^{3}{\sqrt {(1-\cos \theta )^{2}(1+2\cos \theta )}}=a^{3}{\sqrt {1-3\cos ^{2}\theta +2\cos ^{3}\theta }}~.}$ 

We can express the volume ${\displaystyle V}$  another way :

${\displaystyle V=2{\sqrt {3}}~a^{3}\sin ^{2}\left({\frac {\theta }{2}}\right){\sqrt {1-{\frac {4}{3}}\sin ^{2}\left({\frac {\theta }{2}}\right)}}~.}$ 

As the area of the (rhombic) base is given by ${\displaystyle a^{2}\sin \theta ~}$ , and as the height of a rhombohedron is given by its volume divided by the area of its base, the height ${\displaystyle h}$  of a rhombohedron in terms of its side length ${\displaystyle a}$  and its rhombic acute angle ${\displaystyle \theta }$  is given by

${\displaystyle h=a~{(1-\cos \theta ){\sqrt {1+2\cos \theta }} \over \sin \theta }=a~{{\sqrt {1-3\cos ^{2}\theta +2\cos ^{3}\theta }} \over \sin \theta }~.}$ 

Note:

${\displaystyle h=a~z}$ ₃ , where ${\displaystyle z}$ ₃ is the third coordinate of e₃ .

The body diagonal between the acute-angled vertices is the longest. By rotational symmetry about that diagonal, the other three body diagonals, between the three pairs of opposite obtuse-angled vertices, are all the same length.

Four points forming non-adjacent vertices of a rhombohedron necessarily form the four vertices of an orthocentric tetrahedron, and all orthocentric tetrahedra can be formed in this way.^[6]

The rhombohedral lattice system has rhombohedral cells, with 6 congruent rhombic faces forming a trigonal trapezohedron^{[citation needed]}:

• Lists of shapes

• Weisstein, Eric W. "Rhombohedron". MathWorld.
• Volume Calculator https://rechneronline.de/pi/rhombohedron.php