Raman Scattering of YB₆

Abstract

Temperature and polarization dependencies of Raman scattering spectra of YB 6 have been measured in the temperature region from room temperature to 2.4K. All Raman active modes have been observed and well assigned by the polarization dependence. The observation of the doublet structure of T 2g and E g peaks suggests the crystallographic symmetry of P4/mmm for YB 6 , not Pm3m from group theoretical consideration. In the lower energy region than 200cm −1 , the peaks due to the Y vibration have been observed. Within the present experimental accuracy, any anomalies of phonon spectra have not been observed at the superconducting transition temperature.

Spectroscopy of metal hexaborides: Phonon dispersion models

“…The Raman spectra of metal hexaborides have been intensely investigated by many researchers . The dominant Raman‐active phonons in MB 6 (where M = Ca, Sr, Ba, Ce, La, Pr, Nd, Sm, Eu, Gd, Dy, and Yb, respectively) show A 1g , E g , and T 2g (or F 2g as used in article) symmetries that correspond to the breathing and deforming modes of the B 6 octahedral clusters .…”

“…These modes are observed at about 1,300, 1,200, and 700 cm −1 , respectively, and completely satisfy the polarization selection rule for cubic symmetry. In this symmetry, all Raman‐active phonons appear in the (x + y, x + y) polarization geometry, whereas A 1g is particularly apparent in the (x, x) geometry, E g in (x, x) and (x + y, x − y), and T 2g in (x, y), where x and y correspond to the [100] and [010] axes, respectively . The relatively high energies of these modes are caused by strong covalent B–B bonds that are also largely responsible for the hardness of many hexaboride compounds.…”

“…These peaks are in addition to the more readily assignable Raman‐active phonons and crystal electric‐field excitations. These modes are associated with the movement of the metal ion inside the truncated cube, sometimes called the “boron cage” (the movement is often referred to as “rattling”), and are assigned to second‐order excitations of the acoustic branch and of the 700 cm −1 T 2g modes, respectively. In addition, the line shape of the T 2g phonon for the trivalent metal is asymmetric, but for divalent metal, the line shape is symmetric [29].…”

“…Differences in structural parameters are also expected to induce variations in lattice dynamics and vibration properties of metal hexaborides. In general, vibration properties of solids are reflected in Raman and Infrared (IR) spectra and metal hexaborides are no exception . However, several earlier Raman investigations of metal hexaborides observe peaks that are additional to those predicted for the symmetry .…”

Аномальные низкочастотные фононы в ZrB-=SUB=-12-=/SUB=-

“…Эти коле-бания в принципе могут наблюдаться в однофононных и двухфононных спектрах комбинационного рассеяния света (КРС) боридов. Кроме разрешенных правилами отбора для идеальных структур пиков в гексаборидах и додекаборидах наблюдается ряд особенностей, относи-тельно идентификации которых нет единого мнения [12][13][14][15][16][17][18][19]. Часть из них связана с двухфононным рассеянием, на что указывает наблюдение пиков вблизи удвоенной частоты мод E g и A 1g (2000−2500 cm −1 ) во всех боридах.…”