Pal Anirban, Gohil Smita, Sengupta Surajit, Poswal H K, Sharma Surinder M, Ghosh Shankar, Ayyub Pushan
Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400005, India.
J Phys Condens Matter. 2015 Oct 21;27(41):415404. doi: 10.1088/0953-8984/27/41/415404. Epub 2015 Sep 29.
Arguments based on the Mermin-Wagner theorem suggest that the quasi-1D trigonal phase of Se should be unstable against long wavelength perturbations. Consisting of parallel Se-Se chains, this essentially fragile solid undergoes a partial transition to a monoclinic structure (consisting of 8-membered rings) at low temperatures (≈50 K), and to a distorted trigonal phase at moderate pressures (≈3GPa). Experimental investigations on sub-millimeter-sized single crystals provide clear evidence that these transitions occur via a novel and counter-intuitive route. This involves the reversible formation of an intermediate, disordered structure that appears as a minority phase with increasing pressure as well as with decreasing temperature. The formation of the disordered state is indicated by: (a) a 'Boson-peak' that appears at low temperatures in the specific heat and resonance Raman data, and (b) a decrease in the intensity of Raman lines over a relatively narrow pressure range. We complement the experimental results with a phenomenological model that illustrates how a first order structural transition may lead to disorder. Interestingly, nanocrystals of trigonal Se do not undergo any structural transition in the parameter space studied; neither do they exhibit signs of disorder, further underlining the role of disorder in this type of structural transition.
基于默明 - 瓦格纳定理的论证表明，硒的准一维三角相对于长波长扰动应该是不稳定的。这种本质上易碎的固体由平行的硒 - 硒链组成，在低温（约50K）下会部分转变为单斜结构（由八元环组成），在中等压力（约3GPa）下会转变为畸变三角相。对亚毫米尺寸单晶的实验研究提供了明确证据，表明这些转变是通过一种新颖且违反直觉的途径发生的。这涉及到一种中间无序结构的可逆形成，该结构在压力增加以及温度降低时以少数相的形式出现。无序态的形成由以下两点表明：（a）在低温下比热和共振拉曼数据中出现的“玻色子峰”，以及（b）在相对较窄的压力范围内拉曼线强度的降低。我们用一个唯象模型对实验结果进行补充，该模型说明了一级结构转变如何可能导致无序。有趣的是，三角相硒的纳米晶体在所研究的参数空间中没有经历任何结构转变；它们也没有表现出无序的迹象，这进一步强调了无序在这种类型结构转变中的作用。
