11.3
I Sette Sistemi Cristallini: Panoramica
I cristalli con varie simmetrie di gruppi puntiformi appartengono a diverse classi cristalline, che sono termini sinonimi. Nonostante appartengano all…
I cristalli sono classificati in sette sistemi cristallini, ovvero cubici, trigonali, triclinici, monoclinici, ortorombici, tetragonali ed esagonali.
Il più semplice di questi è il sistema cubico, dove i vettori a, b e c sono uguali in lunghezza e ortogonali tra loro. Se solo due vettori condividono la stessa lunghezza, si ottiene un sistema tetragonale.
Nel sistema cristallino trigonale, chiamato anche sistema romboedrico, i tre vettori del reticolo hanno la stessa lunghezza ma sono inclinati ad angoli uguali, non a 90 gradi.
In un reticolo triclinico, i vettori definitori non hanno vincoli specifici; sono diseguali in lunghezza e formano angoli diseguali.
D'altra parte, un reticolo monoclinico richiede che due degli angoli siano di 90 gradi, mentre il terzo angolo non sia di 90 gradi.
Un reticolo ortorombico ha tre vettori reciprocamente perpendicolari di lunghezze diverse, mentre un reticolo tetragonale ha anch'esso tre angoli retti ma presenta due vettori di uguale lunghezza.
Infine, un sistema di reticolo esagonale ha due vettori di uguale lunghezza che sottendono un angolo di 120° in un piano basale, mentre il terzo vettore è perpendicolare a questo piano.
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Q1: What are the seven crystal systems and how do they differ?
The seven crystal systems are cubic, tetragonal, orthorhombic, monoclinic, triclinic, trigonal, and hexagonal. They differ based on the lengths and angles of their lattice vectors. Cubic systems have equal vectors at 90° angles, while triclinic systems have unequal vectors at unequal angles. Each system exhibits distinct symmetry properties that determine its geometric constraints.
Q2: What defines a cubic crystal system?
A cubic crystal system is the simplest system where all three lattice vectors (a, b, c) are equal in length and orthogonal to each other, with all angles at 90°. Sodium chloride (NaCl) exemplifies this system. Cubic crystals display high symmetry with multiple perpendicular axes and planes of symmetry.
Q3: How does a tetragonal lattice differ from a cubic lattice?
A tetragonal lattice has two vectors of equal length (a = b) while the third differs (c ≠ a), with all angles at 90°. Unlike cubic systems, tetragonal lattices introduce fourfold axes parallel to the c direction. This reduced symmetry compared to cubic systems creates a more elongated or compressed unit cell geometry.
Q4: What are the key characteristics of a triclinic lattice?
A triclinic lattice has no specific constraints on its defining vectors: they are unequal in length (a ≠ b ≠ c) and form unequal angles (α ≠ β ≠ γ). This system has the lowest symmetry, with only inversion centers as symmetry elements. Triclinic lattices are necessarily primitive and conventionally use the three shortest vectors.
Q5: How is a hexagonal lattice structured?
A hexagonal lattice has two equal-length vectors (a = b) that subtend a 120° angle in the basal plane, while the third vector is perpendicular to this plane. This configuration creates a six-fold rotational symmetry. The hexagonal system represents an intermediate level of symmetry between highly symmetric cubic and less symmetric triclinic systems.
Q6: What constraints define an orthorhombic lattice?
An orthorhombic lattice has three mutually perpendicular vectors of different lengths (a ≠ b ≠ c), with all angles at 90°. This system exhibits three mutually perpendicular sets of twofold axes and reflection planes. Orthorhombic lattices can be centered or primitive and represent an intermediate symmetry level between monoclinic and tetragonal systems.
Q7: Why can crystals in the same symmetry class have different external shapes?
Although crystals belong to the same point group or symmetry class, they can adopt different external shapes like cubes and octahedra because external morphology depends on growth conditions and environmental factors beyond internal symmetry. The 32 three-dimensional point groups are categorized into seven crystal systems based on unit-cell geometry, but symmetry elements alone do not determine final crystal shape.