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Kristallid


aatomeisttahkeühikrakugadifraktsioonipildiskvaasikristalleainelkristallstruktuuritemperatuuristsüsinikraudkvartspolümorfismiksIdeaalsessoojusliikumisemonokristallikspolükristallilisest kehastkorrapärase hulktahukamehaanilisteelektrilistefüüsikalisteanisotroopsedsulamidtahkudegakristallvormidsüngooniapunktigrupistmagnetilisioptilisisüsinikugrafiitelastsusanisotroopiaksElektronidetsooniteooriasoojuslikkeVääriskividpoolvääriskividehtekividkõvaduseiluvärvusePooljuhidkristallograafia










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Kristall




Allikas: Vikipeedia






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Kvartsi monokristallid.





Püriidi polükristall.





Berülli kristall.





Vaskvitrioli ja silviniidi monokristallid. Kristallide kasvatamise aeg on umbes kaks kuud.





Valkude kristallid.


Kristall on korrapäraselt paigutunud aatomeist koosnev tahke homogeenne ja regulaarselt korduva ühikrakuga struktuur.


Laiendatud definitsiooni kohaselt on kristall aine, mis ilmutab difraktsioonipildis teravaid maksimume ehk milles esineb kaugkorrastatus. See võimaldab kristalli mõiste all käsitleda ka kvaasikristalle.[1]


Sageli on ainel kindel kristalliline struktuur, kuid pole haruldane, et ainel on mitu võimalikku kristallstruktuuri ja see, missuguses aine neist on, sõltub keskkonna temperatuurist. Leidub niisuguseidki aineid (näiteks süsinik, raud ja kvarts), millel samal temperatuuril võib olla mitu kristallstruktuuri. Seda nimetatakse polümorfismiks.[2]


Kristalli osakeste tihedust iseloomustab koordinatsiooniarv, mis näitab, kui mitu lühimat osakest igal osakesel on.[2]


Kristallstruktuuri püsivus on tingitud osakestevahelisest seosest. Selle järgi eristatakse nelja liiki kristalle.


  • Aatomkristalli osakesi ühendab kovalentne side. Niisugused on näiteks teemandi, räni ja germaaniumi kristallid.

  • Ioonkristalli osakesi ühendab ioonside. Niisugused on näiteks halogeniidide, metallide oksiidide, sulfiidide ja karbiidide kristallid.

  • Metallkristallid koosnevad positiivsetest ioonidest, mida omavahel seovad elektrongaasi moodustavad juhtivuselektronid. Niisugused on näiteks vase, alumiiniumi ja naatriumi kristallid.

  • Molekulkristallide püsimise tagavad molekulide vahelised nõrgad tõmbejõud ehk van der Waalsi jõud. Niisugused on näiteks väärisgaaside ja paljude orgaaniliste ühendite kristallid. Molekulkristallide omaette rühma moodustavad vesiniksidemega kristallid. Niisugused on näiteks jää ja vesinikfluoriidi kristallid.[2]

Ideaalses kristallis on osakeste paigutus rangelt korrapärane. Reaalsetes kristallides see nii ei ole soojusliikumise ja kristallivõre defektide tõttu. Ühtse kristallivõrega terviklikku kristalli nimetatakse monokristalliks erinevalt polükristallilisest kehast, mis koosneb üksteise suhtes suvaliselt orienteeritud monokristalliteradest. Monokristallid võivad olla korrapärase hulktahuka kujulised ning mehaaniliste, elektriliste ja teiste füüsikaliste omaduste suhtes anisotroopsed. Tehnikas kasutatavad kristallid ja sulamid on harilikult polükristallilised ning nende omadusi saab muuta mehaanilise ja termilise töötlemisega.[2]


Kristallide korrapärase siseehituse välispidiseks väljenduseks on siledate ja kindlate seaduspärasuste alusel moodustunud tahkudega kristallvormid. Kõik kristallid jagatakse kuue süngoonia vahel, mis omakorda koosnevad 32 punktigrupist.


Kristallide struktuur mõjutab aine mehaanilisi, elektrilisi, magnetilisi, optilisi ja muid omadusi[2]. Polümorfsetel ainetel võivad eri kristallstruktuuride omadused väga suurel määral erineda, näiteks süsiniku vorm teemant on üks kõige kõvemaid teadaolevaid aineid, aga teine vorm grafiit on eriliselt pehme aine. Isegi ideaalse kristallstruktuuriga aine omadused, näiteks läbipaistvus ja elastsus, võivad eri suundades erinevad olla ning seda nimetatakse anisotroopiaks[3].


Elektronide liikumist kristallivõre perioodilises väljas kirjeldab tsooniteooria. See seletab tahkete ainete elektrilisi, optilisi, mehaanilisi ning osaliselt ka soojuslikke ja magnetilisi omadusi, mis tulenevad valentselektronidest.[4]


Vääriskivid, poolvääriskivid ja ehtekivid on tavaliselt kristallid. Need on väärtuslikud kõvaduse ja ilu, sageli ka värvuse tõttu.[5]


Pooljuhid on enamasti kristalsed ained.[6]


Kristallide uurimisega tegeleb kristallograafia.



Vaata ka |


  • Kristallisatsioon

  • Kristallmälu


Viited |




  1. ↑ Jaak Kikas: "Tulnukad hüpermõõtmest" Horisont, 6/11


  2. ↑ 2,02,12,22,32,4 Tehnikaleksikon, lk. 235


  3. ↑ Tehnikaleksikon, lk. 25


  4. ↑ Tehnikaleksikon, lk. 533


  5. ↑ Tehnikaleksikon, lk. 591


  6. ↑ Tehnikaleksikon, lk. 387



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