Studies of ²⁷Al NMR in SrAl₄

A charge density wave (CDW) transition at T_CDW = 243 K and a structural phase (SP) transition at approximately 100 K occur in SrAl₄ with the BaAl₄-type body center tetragonal structure, which is the divalent and non-4f electron reference compound of EuAl₄. To understand the behaviors of the CDW and SP transitions, the ²⁷Al NMR measurements using a single crystal and a powder sample of SrAl₄ have been carried out. The line width below T_CDW is modulated by an electrical quadruple interaction between ²⁷Al nucleus and CDW charge modulation. The incommensurate CDW state below T_CDW changes into a different structure below T_SP. The temperature dependences of Knight shifts of ²⁷Al(I) and ²⁷Al(II) show the different behaviors. The temperature variation of ²⁷Al(I) Knight shift shows anomalies at the CDW and SP transition temperatures, revealing the shift to negative side below T_CDW, which is attributable to the core polarization of the d-electrons. However, ²⁷Al(II) Knight shift keeps almost constant except for the small shift due to the SP transition. The 1/T₁T of ²⁷Al(I) indicates the obvious changes due to the CDW and SP transitions, while that of ²⁷Al(II) takes a constant value. The density of state at the Fermi level at Al(I) site below 60 K would be about 0.9 times less than that above T_CDW.