What is Crystal: Definition and 427 Discussions

A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.
The word crystal derives from the Ancient Greek word κρύσταλλος (krustallos), meaning both "ice" and "rock crystal", from κρύος (kruos), "icy cold, frost".Examples of large crystals include snowflakes, diamonds, and table salt. Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks, ceramics, and ice. A third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include glass, wax, and many plastics.
Despite the name, lead crystal, crystal glass, and related products are not crystals, but rather types of glass, i.e. amorphous solids.
Crystals are often used in pseudoscientific practices such as crystal therapy, and, along with gemstones, are sometimes associated with spellwork in Wiccan beliefs and related religious movements.

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  1. E

    Inorganic crystal structure theory for linear birefringence

    There is a crystal structure relationship for circular birefringence which can tell you if a crystal structure will have circular birefringence and its handedness (left,right) (e.g. "On the origin of optical activity in crystal structures" J. Appl. Cryst. 1986. 19, 108-122, A. M. Glazer and K...
  2. E

    Inorganic crystal structure theory for linear birefringenc

    There is a crystal structure relationship for circular birefringence which can tell you if a crystal structure will have circular birefringence and its handedness (left,right) (e.g. "On the origin of optical activity in crystal structures" J. Appl. Cryst. 1986. 19, 108-122, A. M. Glazer and K...
  3. fluidistic

    Localizing an Object Using a Crystal Ball

    Homework Statement Localize the image of an object situated at 1.2 m from a crystal ball with a diameter of 0.2 m (n=1.5). Homework Equations None given. But Snell's law I believe. The Attempt at a Solution I'm totally stuck at starting. Does the size of the object matters...
  4. J

    Calculating the Schmid factor for an FCC single crystal

    The problem said: "Determine the magnitude of the Schmid factor for an FCC single crystal oriented with its [100] direction parallel to the loading axis. " The Schmid factor is the "cos(\phi)*cos(\lambda)" term in the equation for resolved shear stress. I know that the slip planes...
  5. K

    Nonlinear gain in a SHG crystal

    I'm not a native English speaker so I need some clarification about the term "nonlinear gain" concerning a laser experiment. In detail I'm working on a lab report where I'm supposed to estimate the nonlinear gain of a KTP crystal in a Nd:YAG LASER using the intracavity second harmonic...
  6. K

    How Can DFT Methods Model Twin Energy in Crystals?

    Hello all, I am attemtping to model crystal twinning using DFT methods. In particular I wish to look at the twin energy as a function of twin angle in a hcp system. I am running into difficulty however in attempting to create my repeat unit prior to my calculations. I was wondering if...
  7. Y

    Crystal field effect on level splitting

    Hello, is there any detailed table about the s,p,d,f,etc level splitting due to various crystal symmetry(like O,Oh,D4h,...). I think it should be somewhere since it is a well established material. Can anyone help?
  8. D

    Photonic Crystal Fibres and COMSOL

    Hello everyone, I am trying to understand COMSOL 3.5 and I am interested in Photonic crystal fibres and tapered optical fibres. First of all, could you please tell me what is the background of the layout? When you create a design/object, the rest area is air or I need to define another object as...
  9. N

    Metals with what type of crystal lattice less prone to brittle fracture?

    Metals with what type of crystal lattice less prone to brittle fracture? Why?
  10. M

    Does anyone know a crystal with refractive index of water?

    When I was a little kid, I met a gemologist who showed me a cool trick with a ring he'd bought for his wife. When he submerged it in water, the crystal disappeared! As I now know, this crystal had an index of refraction of 1.33. Anyone know what this crystal is and/or how I can get my...
  11. S

    Crystal Structure Lattice Problem

    1. Homework Statement Using the powder XRD data below, show that the substance has a face centred cubic structure. (xray lamda = 0.154056 nm) Peak No.------2(theta) 1 -------------38.06 2 -------------44.24 3 -------------64.34 4 -------------68.77 5 -------------73.07 2...
  12. S

    Determine crystal lattice structure from powder XRD

    Homework Statement Using the powder XRD data below, show that the substance has a face centred cubic structure. (xray lamda = 0.154056 nm) Peak No.------2(theta) 1 -------------38.06 2 -------------44.24 3 -------------64.34 4 -------------68.77 5 -------------73.07 Homework...
  13. A

    Distortion of Zr2Cu crystal structures?

    Hello I need your help! Using XRD, I detected single crystals (single peak) on the surface of a bulk specimen. My guess is that the structure is most probably Zr2Cu, but a distorted one. I know that Zr2Cu structure is MoSi2 type, spacing group of I4/mmm, tetragonal. I want to ask 1)...
  14. MTd2

    Energy Gaps in a Spacetime Crystal (paper title)

    Energy Gaps in a Spacetime Crystal L.P. Horwitz, E.Z. Engelberg (Submitted on 14 Oct 2009) This paper presents an analysis of the band structure of a spacetime potential lattice created by a standing electromagnetic wave. We show that there are energy band gaps. We estimate the effect, and...
  15. L

    Alum Crystal Growing question

    My teacher this year assigned us to do an alum crystal growing lab by ourselves, so we find the procedure and do a related theory all by ourselves. In my group i was doing half of the related theory and i basically had questions i had to answer. The question i am stuck on is, "5. Once the...
  16. K

    Understanding the Discrete Nature of k in Photonic Crystals

    Hi, I have this quite basic question regarding photonic crystals. As I understand, a periodic dielectric structure (photonic crystal) has modes (spatial profiles) at discrete frequencies from: \nabla \times \Big( \frac{1}{\epsilon(r)}\nabla \times H(r) \Big) = \frac{\omega^2}{c^2} H(r)...
  17. J

    Entangled protons in a crystal lattice

    Can entangled protons be maintained in a crystal lattice for an indefinite period of time? The following papers seem to support this contention. I need the help of the Forum’s readers to critique this research and the assertions the authors are making. Dr. Francois Fillauxa and Dr Alain...
  18. S

    Quartz crystal microbalance

    I want to know the basic principle of working for quartz crystal microbalance, how it is used to measure the thickness of the film?And why it can work at room temp (with water cooling) and at higher temp say 65deg ??
  19. fluidistic

    Potential energy of a crystal, electrity and magnetism intermediate course

    Homework Statement Calculate the potential energy, by ion, in an infinite bi-dimensional ionic crystal. Hint : Use the power series expansion of \ln (x+1). Homework Equations \ln (x+1)=\sum _{i=1}^{\infty} \frac{(-1)^{i+1}x^i}{i}. The Attempt at a Solution W=U=k\sum...
  20. V

    Calculating Bragg Angle from Unit Cell in Cubic Crystal

    Homework Statement The unit cell in a cubic crystal is 2.74Å. Determine the Bragg angle for reflections from the planes (200), (212) and (-112). [sorry i don't know how to put a bar on top of the numbers] if the x-ray wavelength used is 1.54Å Homework Equations Given Bragg's condition as 2d...
  21. G

    Potential energy between two oppositely charged ions in a crystal

    Hi, I am having trouble with the following: All I have really done is differentiate the function to give an expression for F(r), but I am a bit clueless about the rest. How can there be equilibrium if the two ions are attracted to each other? Cheers, GeoffreyThelm
  22. S

    Deriving electrical conduction in 2d crystal

    Hey. I want to derive the conductivity of graphene. I'm looking for a formula of the sort: J = \frac{e}{\hbar} n \frac{dE}{dk} D(E) \Delta E where \frac{1}{\hbar} \frac{dE}{dk} is the electron velocity, D(E) = \frac{dN}{dE} is the density of states, \Delta E might be the applied voltage...
  23. B

    COMSOL band structure of photonic crystal waveguide

    Setup: I'm trying to make the band structure for a planar photonic crystal with finite thickness, i.e., a quasi-3D problem. I only want the x-direction band structure. So, I'm using variable floquet periodic BCs for the x-direction boundaries, and 0 degree floquet periodic BCs for the...
  24. L

    CSL Boundaries between Crystal Grains: Questions & Answers

    I have some questions about CSL (coincidence site lattice) boundaries between crystal grains. 1. What is the relation between twin boundaries and CSL boundaries? Is it true that all twin boundaries are CSL boundaries, but not vice versa? Is the energy of twin boundaries even lower that the...
  25. M

    What are the properties of this unknown crystal?

    What can this crystal be? Hello, I have some crystals with the following properties which I am unfortunately unable to find out their chemical formula or name... any other material with the following characteristics will help too. Cubic with octahedral habit. RI : 1.5 - 1.6 Density...
  26. C

    Number of electrons leaving crystal surface. any help ?

    number of electrons leaving crystal surface. any help :)? [b]1. A scanning tunnelling electron microscope (STM) measures a tiny electrical current tunnelling out of the surface of a crystal. The current is 0.30 pA (0.30 x 10-12 A). Calculate the number of electrons per second leaving the...
  27. M

    Crystal structure and diffraction

    Homework Statement You wish to study a crystal's structure by diffracting thermal, non relativistic neutrons from the crystal, so you go to a nuclear reactor and set up your diffraction experiment on one of the neutron beam lines. The neutrons emerge from the reactor with a range of...
  28. H

    Single crystal and poly crystal metal

    What difference would you observe about the yield stress, young modulus,etc if your material were a single crystal? Explain the difference.
  29. L

    Measuring atom positions in a crystal

    I heard that it is possible to measure positions of individual atoms in a crystal. Is this true? I understand the principle of measuring crystal orientation and base vectors of Bravais lattice with X-ray, but I have no idea how positions of individual atoms can be obtained. I suppose it is...
  30. G

    Why Do Ionic and Inert Gas Crystals Have Different Repulsive Potential Terms?

    Hi all, I have a doubt regarding interaction energy term between atoms of crystal. 1.) Actually in interaction energy term of Inert gas crystals, we get term as R**(-12) as repulsvie potential. But on the contra while writing term for Interaction energy term for Ionic Crystal we write Repulsive...
  31. M

    Deoxidizing Fe in an Iron rich crystal

    Hello, I was wondering if we can deoxidize Iron inside a crystal like Kyanite (Natural form), which contains Fe inclusions (sometimes rusty)... Thanks in advance.
  32. G

    Can You Build a Crystal Radio with an Electro Magnet and Germanium Diode?

    I am new to this website but I would like to know how to build a homemade radio using an electro magnet and a germanium diode crystal. Are ther any other components required to complete the construction of the radio?
  33. D

    Crystal structures and Friction

    Hello, I have this ongoing argument with my professor. I read in a paper (friction and wear) that the crystal structure can affect friction. HCP crystal structures having lower friction coefficients. I have two papers that mention this in passing one, linking FCC crystal structures to have...
  34. D

    Stress and strain in crystal structures

    Are there stress and strain graphs for different crystal structures?
  35. R

    Density of an Ionic Crystal

    Homework Statement The distance between the Li+ and Cl- ions in a LiCl is .257 nm. Use this and the molecular mass of LiCl 42.4 g/mol to compute teh density of LiCl.Homework Equations The Attempt at a Solution I am surprisingly having a lot of difficultly with this problem. The LiCl crystal is...
  36. S

    What is the atomic spacing D of this crystal?

    Homework Statement X rays with wavelength 0.10 nm are incident on a crystal with a hexagonal crystal structure. The x-ray diffraction spectrum is shown in the figure. Figure: What is the atomic spacing D of this crystal? (in nanometers) Homework Equations The Bragg...
  37. P

    Measuring device used for a stressed crystal microphone

    Homework Statement Choose an instrument to measure the output from the crystal, when measuring variations in stress. The maximum voltage from the crystal is 200mV. current in circuit is 2 x 10-8 A, p.d is 4 x 10-15 full scale deflection, sensitivty...
  38. hypatia

    Giant Crystal Cave Comes to Light

    This was just the coolest thing I have ever seen. I'd love to go there and see it in person. http://news.nationalgeographic.com/news/2007/04/070409-crystal-cave.html
  39. H

    Find Energy of Mono-Energetic Neutrons Scattered off Metal Crystal

    Homework Statement A mono-energetic beam are of neutrons is directed perpendiculary at the surface of a metal crystal whose atom are a constant distance 0.1E-10m apart. The scattered beam of neutrons is found to have maximum intensity at an angle of 25degree to the initial direction. What is...
  40. R

    Cross slip problem in f.c.c crystal

    Homework Statement Because the book gives me little information about the cross slip. So I can't handle the basic problem below. I appreciate that you can give me some hints or suggestion. An f.c.c crystal is deforming by motion of disslocations on the (a/2)[101](11-1) (I use minus instead of...
  41. M

    Distance between copper atoms in cubic crystal lattice

    Homework Statement The atomic mass number of copper is A = 64. Assume that atoms in solid copper form a cubic crystal lattice. To envision this, imagine that you place atoms at the centers of tiny sugar cubes, then stack the little sugar cubes to form a big cube. If you dissolve the sugar, the...
  42. P

    Deriving equation to describe lattice vibrations of a one dimensional crystal

    When setting up this derivation one assumes a chain of identical atoms. The interatomic interaction between atoms is short ranged and so only affects neighbouring atoms (see Hook and Hall, "Solid State Physics" chapter 2.3.1). The potential V(r) is expanded as a taylor series about r = a to...
  43. D

    Crystal Structures: Finding Positions Per Unit Cell

    Hey All.. I'm having some problems trying to understand the tetrahedral and octahedral sites in face centered cubic (fcc) and body centered cubic (bcc). It is hard to visualise the structures in 3D.. I wanted to know how to find out the positions per unit cell for the tetrahedral and...
  44. S

    What Is the Nearest Neighbor Distance in an FCC Lattice of Aluminum?

    Hi all I am new here but i have a major question, iv been told by people that this question is easy but I am just going around in circles with it and i was wondering if you could help. here is the questionAluminium cystallises in the fcc structure with a single atom basis. the lattice constant...
  45. G

    Photonic crystal operation faster than electronics?

    I've just read an article about IT with light instead of electrons: http://focus.aps.org/story/v22/st15 I wonder, is there any indication that light processing will be faster than conventional electronics? Information transport is faster, but what about logic operations? What would be...
  46. S

    Salt crystal forms after 3 days of evaporation

    If a salt crystal forms after 3 days of evaporation, and the crystal mass is 30 mg, how many Na+ and Cl- ions was added to the crystal each second (average). How do I solve this?
  47. F

    Why is the CFSE for d6 not equal to -24Dq+3p in crystal field theory?

    at crystal field stabilization in strong field case d4 CFSE=-16Dq+p d5 CFSE=-20Dq+2p d6 CFSE=-24Dq+2p d7 CFSE=-18Dq+p d8 CFSE=-12Dq it was shuld be +3P and +4 P why it be +p,2p,2p,p ?
  48. M

    FCC Crystal Problem: Nearest Neighbors, Bond Angles & Packing Fraction

    Homework Statement Write down the number of nearest neighbours in a FCC structure, calculate the angle between the bonds and the packing fraction. Homework Equations Literally don't know where to start! The Attempt at a Solution Very confused at the moment. Tried to look up...
  49. A

    Free energy of solids & Einstein crystal

    Dear all, I'm coding the calculation of the free energy of an SiO2 crystal structure Using a 2-body Buckingham potential. I'm attempting to use the method of Frenkel and Ladd which consist in transform your solid reversibly into an Einstein crystal. I've read the excellent book...
  50. O

    Does anyone know about crystal cut?

    Hi all, I'm using comsol to do surface acoustic wave simulation. And all the material I read mentioned about crystal cut, like xy-cut, yz-cut, 30° rotated yx-cut. And in COMSOL piezoelectric application mode I have to specify the material orientation, like xy, yx, zx, xz, etc. I'd like to ask...
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