硼的同素異形體

硼有多種同素異形體，包括晶体和无定形体。現已知（已製備）的結晶狀硼有α菱面体硼晶、β菱面体硼晶以及β四方硼晶；此外，在某些特定情況下，α四方硼晶、γ正交硼晶等同素異形體結構也能被製備出來。非結晶體的形式有兩種已知，一種為精細粉末，另外一種為玻璃狀的固體。雖然有至少14種甚至更多的硼同素異形體被發表出來，但是這些被發現的組成卻是根據不足的證據、沒有經過實驗確認、被認為是混合的同素異形體或是含有雜質來穩定其硼的結構。而硼以β菱面体硼晶形式存在者是最穩定的，其餘次之，所以在室溫下的轉化率低下，也因此五個不同型態能夠在室溫下共存。非結晶粉末狀硼和多晶體的β菱面体硼晶最為相似，後者是一種堅硬的灰料，重量卻比鋁還要輕上十個百分點，其熔點（2080 °C）比鋼高幾百度。 元素態的硼能在星塵及隕石找到，但卻不存在於地球上含氧量高的環境。因為它不容易從其化合物中被萃取出來。早先的萃取方法包含將三氧化二硼以如鎂 或 鋁的晶屬還原。但是，此方法產物容易混雜其餘金屬硼化物。現今硼純化是在高溫下以氫還原揮發性鹵化硼。而半導體產業是用之非常純的硼之製備，是在高溫下以區熔或是柴可拉斯基法分解乙硼烷。從純硼中製備硼單晶則更加困難，原因是多態现象以及硼傾向與杂质反應；典型的晶體大小為~0.1 mm。 硼相圖的截取（α 和 β 是菱面体結構；T 是β-四方硼晶） α-R 硼的結構 β-R 硼的結構 γ 硼的結構 α菱面体硼晶是含有12個硼原子的單胞體。這些B 12的單胞體結構上為每一個單胞體與鄰近的五個單胞體結合的正二十面體結構 。如果這些鍵結是常規下的共價鍵，則每個硼貢獻出五個電子。但是硼只有三個價電子，所以科學家們認為B 12正二十面体是由3-中心電子缺乏鍵結形成的，意思為電子電荷會累積在三個鄰近原子構成的三角形中心。 單一的 B 12 正二十面體不穩定；雖然硼並非分子化合物，但是它卻含有（非常規的）共價鍵。 純α四方硼晶的合成只能在等向性碳化硼（B50C2）或氮化物（B50N2）的底層基質板上發生，α四方硼晶會沉積在比層基質板上形成一層薄膜。而大部分α四方硼晶的合成實例發生在富含硼的碳化物或氮化物。 β菱面体硼晶為含有105-108个硼原子的單胞體。大多數的原子組成 B12 離散二十面體；少數組成部分相互貫穿的二十面體，其中包含兩個三角面多面體型的B10單元，以及一個中心硼原子。在以前，研究者不甚明瞭在普通環境下α或β兩個結構何者才是最穩定態，但後來對此漸漸取得了共識，β結構於熱力學上為最穩定的同素異形體。 1960年，科學家在攝氏溫度1270–1550 °C環境下於熱鎢絲、錸絲或鉭絲上以氫還原BBr3（即化學氣相沉積）合成了β四方硼晶。後來的研究更重新進行了此合成並確認產物中没有杂质。 此γ-態為一種像氯化鈉的晶體排列著的B12二十面體及B2兩種形式原子。可藉由壓縮別種型態的硼至12–20 GPa和加熱到1500–1800 °C而生成，並在環境下維持穩定。證據證明在這種結構中有明顯的電荷從B2原子對移轉至B12二十面體，由晶格動力學也可推测出此種顯著的遠距離靜電相互作用。 文托夫於1965年發表此態的硼，然而此硼的結構與化學組成皆不確定。其結構是通过“從頭計演算法”晶體結構預測計算後再以X射線晶體學手段确认的。 Sullenger等在1969年以及 McConville 等在1976年中階發表了此種於氩等离子實驗中觀察到的立方體硼同素異形體。此立方體硼為含有1705±3個原子，密度為2.367 g/cm3的單胞晶體。雖然這種同素異形體偶爾會在文獻中被提及到，但是沒有後續的正式出版、研究確認或是任何證據否決這種同素異形體的存在。多諾霍于1982年提到 這種單胞晶體內的原子數量不顯示它與正二十面體相關（正二十面體是硼同素異形體共同的結構形狀）。 將硼加壓至超過160 GPa會產生一種尚未明瞭的型態。 與其他型態不同，這種硼不是半導體，而是金屬，並且有超導現象，臨界溫度在160 GPa 下为4 K、250 GPa 時上升到11 K。這種壓力下的結構轉變發生在理論預測的二十面體分解時。揣測下的結構狀態包含面心立方（類似於Al），α-Ga，和體心四方（類似於In）。它也被認為其非金屬-金屬的轉變和碘的轉變一樣，是一個簡單的能隙關閉的結果，而非結構的轉變。 這種似球形的同素異形體分子硼球烯（B40）於2014年七月被發表。 非晶體硼包括B12 二十面體，會沒有固定順序的互相隨機結合。 純的非晶體硼以再1000 °C以下熱分解乙硼烷生成。在1000 °C 時以退火處理會將非晶體硼轉變為β菱面体硼晶。非晶體硼的奈米線（直径30-60 nm） 或纖維可由濺渡沉積和雷射輔助化學氣相沉積製備；也可經由1000 °C退火處理轉變為β菱面体硼晶奈米線。 維氏硬度試驗 comparable to that of cubic 氮化硼 Black when viewed by reflected light; red by transmitted light High purity amorphous boron powder is black whereas impure samples have a brown appearance: Lidin R. 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